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SERVICEMAN'S LOG
Re-keyboarding a Yamaha electric piano
Every now and then, I get a piece of gear from
the 1980s that’s well worth repairing. That
was certainly the case with a Yamaha electric
piano that came in recently with noisy pots
and half its keyboard not working.
Well, I just had to go and jinx myself. In a recent column, I mentioned
that we hadn’t had a good shake in
Christchurch for over four years but I
neglected to touch enough wood because the gods heard me and delivered
two very powerful aftershocks. These
have now become known as the “Valentine’s Day Quakes”.
Fortunately, the precautions I had recently taken with my workshop fittings
stood up well to the acid test. Even
though the quake measured 5.7 (relatively puny to us), the only casualties
were a couple of untethered hand tools
which fell from my workbench onto the
carpeted workshop floor. Thankfully,
all my component shelves,
drawers, racks and trays
stayed upright and closed,
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withstanding what, to be honest, was
a very scary quake.
As a result, I’m satisfied that my
workshop is now as quake-proofed
as it can be. If anything comes down
now, it’ll be along with the entire structure and I’ll have much bigger things
to worry about than a few mixed-up
components!
Anyway, as the great philosopher
Reginald Perrin once said, “time and
motion wait for no man” and so life and
service work goes on. After checking
that everything was safe and secure, I
got down to repairing a couple of musical instruments that customers had
recently dropped off to the workshop.
The first was a Yamaha electric piano
and while no doubt this conjures up a
mental image of yours truly struggling
Dave Thompson*
Items Covered This Month
•
•
•
•
Re-keyboarding a Yamaha
electric piano
Connoisseur BD2/A turntable
A tale of five oscilloscopes
Battery-powered golf cart
repair
up the driveway with a shiny, black
grand piano on his back (a feat I actually witnessed my grandfather doing
with our upright piano way back when
I was a lad), I have to disappoint you.
This Yamaha was a much smaller instrument and while it didn’t boast the
full 88 keys of a normal-sized piano
keyboard, it was none-the-less still a
comprehensive machine compared
to some.
Built back in the mid-1980s, this
keyboard looked like something from
“Back To The Future”. It used that
brushed, silver-grey plastic that was
May 2016 57
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keys made no sound at all, apart from
the dull, plastic-mechanical thud you
get with most electric piano keyboards.
Not only that but many of the numerous linear/sliding sound modification
and volume control pots that were
popular on electronic devices at that
time were now distinctly electrically
noisy in their operation.
No problem, I thought; these were all
age-related issues and most, if not all,
such keyboard instruments would likely suffer similar maladies over time.
Given that this one was now around
30 years old, it wasn’t surprising that
it was no longer working properly!
Stripping it down
so popular on commercial electronic
equipment of the time and the various stickers/menus and large function
buttons boasted all the pinks, pastels
and now-faded Day-Glo colours that
defined the era. This was the keyboard
equivalent of a human wearing a white
suit jacket over a T-shirt, baggy linen
pants and Italian loafers with no socks!
By now, most of this instrument’s
contemporaries are padding out landfills all over the globe. However, this
one had been well-loved and, outwardly at least, appeared to be in very good
nick. Unfortunately, things weren’t too
good inside the unit because it was no
longer working properly.
The owner, a baby-boomer who lives
by the same “why chuck it if it can be
fixed” ethos that I subscribe to, made
the comment that he’d had other keyboards since buying this one brandspanking new back in the day but not
one had the sound and feel that this
one possessed. As a result, he wanted
it assessed and, if possible, fixed.
The basic problem was that only
half of it worked. The in-built rhythm
machine and pre-programmed accompaniment sections still sounded spot
on (albeit with a typical 1980s’ flavour)
but many of the 25 black and 40 white
58 Silicon Chip
Disassembling it was relatively
straightforward. Once I’d placed a large
piece of foam-rubber on the workbench
(that I specifically keep for such jobs)
and flipped the keyboard over, I could
see that the top and bottom “halves”
were held together with several large
machine screws. Most of these screws
gave out a satisfying “squeak” as they
let go under the torque of my electric
screwdriver, indicating that this was
probably the first time that they had
been removed. What’s more, none
were those annoying “security” screws
that modern manufacturers seem to
be in love with, making it a breeze to
work on.
Once the case had been cracked, I
then had to disconnect several flying
leads which ran from various terminals on the circuit boards in the top
section to other connectors built into
the bottom half of the case and to the
battery holder. Only then could the
two halves be separated, so that was
a potential trap for young players too
eager to gain access.
Once the insides had been exposed,
the reason for the keyboard’s intermittent operation was immediately
obvious; dust-bunnies, hair-balls and
cobwebs choked every possible nook
and cranny of the interior of the case.
Basically, over the last 30 years, a
collection of pet hair, dust, dirt, sweat,
cigarette ash, tobacco, insects, beer
and other debris had fallen through
the gaps between the keys and into the
various open compartments beneath.
This had simply built up until it started
interfering with the electrical operation
of the keys.
It was also apparent that much the
same fate had befallen the pots. Where
once a thin, split-felt dust-cover protected the inner workings of the pots,
this had now all but gone after years of
wear and tear. Anything with gravity
on its side now had free entry into the
insides of the pots, so it was no wonder they sounded like fingernails being
dragged down a blackboard whenever
they were operated!
The keys were made from injectionmoulded plastic, formed into the shape
and colour of traditional ebony and
ivory piano keys. The quality of the
mouldings and the whole keyboard
assembly was very good and while
the main part of each key underneath
the keyboard “floated” in free air, the
upper portion of each key disappeared
between a felt-edged plastic moulding
and a PCB that ran the length of the
keyboard. Obviously, this was where
all the action took place and I’d need to
get in there in order to get a good look
at the springs, contacts and other parts
that made up the keyboard.
The keys on this type of instrument
are essentially just a line of push-tomake, release-to-break switches, with
each switch controlling an oscillator
that’s modified with various filters to
emulate the sound of a piano note (or
in some cases, used to trigger an actual
sampled piano note sound). Cheaper
keyboards typically have limited feel
and action and no matter how hard you
hit the keys, the output level remains
exactly the same.
On the other hand, the better (and
usually more expensive) keyboards are
dynamic, which means that just like on
a real piano, if you just touch the key,
you’ll get a very quiet note and if you
press the keys harder you’ll get a proportionally louder sound. Good keyboards are thus designed to mimic the
feel and action of a real piano, making
the playing (and listening) experience
far more satisfying.
This Yamaha model boasted a dynamic keyboard, so I was expecting
some complicated mechanics under
that long PCB. This PCB is about 80mm
wide and spanned the entire length
of the instrument, making it about
750mm long. It was held down by 24
equally-spaced flat-head screws and a
similar number of plastic clips, which
made me glad I had an electric screwdriver on-hand.
As an aside, while old-timers tend
to frown on mechanised screwdrivers
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(and to a point, I agree with them), the
electric screwdrivers made today are
not the clumsy devices of yesteryear
that stripped threads. Instead, they
are now smaller, easy-to-use tools that,
provided they are set properly, will not
damage anything and which make repetitive jobs far easier.
If you have all day to spend undoing
multiple screws by hand, then go for
it. However, those of us who are timepoor or on the customer’s dollar can
use all the help we can get.
Once all the screws had been removed, I carefully lifted the board
away from its locating posts and clips
and watched carefully for any springs,
magnets or ball-bearings that might fly
out and hide in the carpet. However,
all I could see falling out as I lifted it
away were bits of fluff and dust, with
most of it dropping onto the rubber mat
underneath the keyboard.
After vacuuming up all the visible
dust and dirt, I had a much clearer view
of what was going on. In fact, anyone
who has ever pulled a numeric keypad
apart would recognise the technology
used in this keyboard. Each key has a
corresponding collapsible rubber button contact shaped like a small thimble beneath it and this, when pressed,
makes contact with printed graphite
or carbon contacts etched into the PCB
beneath it.
In this case, the rubber buttons were
all part of a 750mm-long contact pad
which simply lifted out. When laid
out on the bench, it looked like a long,
narrow rubber strip mat with small,
stepped thimbles aligned along its
length, with each thimble corresponding to a key position.
When I turned this flexible mat
over, I could see a carbon contact inside each of these rubber thimbles.
In fact, each contact was composed
of three sections. First, there was a
small, round carbonised pip in the inside centre of each thimble and when
this was pressed, it sat directly down
onto the centre of its corresponding
PCB contact.
Next, on the outer-bottom edge of
each thimble, were two more carbonised pad contacts, one on each side
of the thimble but set slightly higher.
These made contact at a different spots
on the board contacts when the keys
were pressed downwards and my guess
is that these contributed to the dynamic
“feel” of the keyboard.
Obviously, the contacts on the rubber thimbles have to be clean in order
to make proper contact with the PCB.
And of course, the PCB contacts themselves must also be clean and clear of
any debris.
Years of dust & dirt
As it stood, years of accumulated
dust and dirt had coated the various
contacts and as each key was pressed,
some of this rubbish had transferred
onto the rubber pads. This very effectively prevented any connection at all
being made at those points, which was
why half the keys didn’t work.
I began by vacuuming up as much
of the mess as possible but I was already thinking that I’d have to go a
few steps further to really make sure
the keyboard was as good as new. My
next step then was to flip the rubber
contact pad over so that all contacts
faced upwards. I then went down the
line with a can of contact cleaner and
gave the first half dozen or so a good
blast. This was a little messy but it
was necessary to blow off any rubbish.
I then quickly followed that up by
pressing each thimble from the back,
thus exposing all the contacts, and going over it with a home-made contact-
cleaning wipe. This effectively gave the
inside bottom of each thimble a thorough clean, after which I repeated the
process for the next six keys.
This step-by-step process was necessary because contact cleaner evaporates pretty rapidly and I wanted to
get the wiping done while there was
still a small pool of cleaner left inside
each thimble.
Once I’d processed and cleaned all
the rubber contacts, I set the now spark
ling clean mat aside and concentrated
on the other half of the equation – the
circuit-board contacts. A potential
problem here is that if a keyboard has
seen a lot of use (eg, a home telephone
keypad or a TV remote control), there’s
a chance that its etched contacts have
worn away for the most-used keys. And
if that happens, no amount of cleaning
is going to replenish those contacts
and the only way out is to replace the
board itself.
Replacing the PCB obviously wasn’t
going to be an option here so all I could
hope for was that this keyboard hadn’t
been used enough to wear out its PCB
contacts. Fortunately, when I inspected
the board with my much-used USB
microscope, I could see that all the
contacts appeared to be in good condition underneath, although they were
covered in a thin layer of greasy dust
and dirt. Once more, my trusty can of
contact cleaner and wipes made short
work of cleaning the contacts and this
revealed they really were in excellent
condition.
In the past I’ve had good luck with
keypads by cleaning them exactly as
described above, then giving them a
very light rub over with some Scotchbrite or similar plastic scouring pad.
However, note that getting stuck into
the contacts willy-nilly with scouring
pads is never a great idea. Instead, a
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May 2016 59
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Connoisseur BD2/A turntable
G. K. of Morningside, Qld recently resurrected an old Connoisseur BD2/A turntable, so that he could listen to LPs once
more. Along the way, he discovered why he
could never get it working properly when
it was new . . .
This story was prompted by the SILICON
CHIP turntable strobe project in the December 2015 issue. It immediately reminded
me of the work I recently did restoring
my Connoisseur BD2/A turntable. Yes, the
Connoisseurs used a synchronous motor
but I have repaired an old Thorens and an
old direct-drive CDC turntable too, so the
strobe project is handy.
When I first got my BD2/A home, I was
not happy with the its sensitivity to vibration and so I bought and built a variety
of damping feet for the chassis. The was
only moderately successful because it still
wasn’t good and I could never set the antiskating bias properly.
When I started the restoration, I decided
to see if there was anything on the internet
for such an old turntable. This was a revelation; I easily found all the manuals and
I discovered that the original supplier had
assembled the chassis incorrectly from
the outset. As a result, I re-assembled it
correctly and, after fitting some new DIY
damping feet, it’s now fine.
The manuals I downloaded also referenced two bias weights for the pick-up
arm: a large one for heavy cartridge tracking weights and a small one for lighter
tracking weights. On my unit, the large
(2.5g) bias weight had been fitted to the
pick-up arm so it was no wonder I could
never get the bias right!
As well as this, the small weight necessary for low tracking-weight cartridges,
such as the Stanton 680EEE, was missing.
In fact, it was never supplied and nor was
the additional pick-up arm counter-weight
described in the manual (although it hasn’t
been missed).
I guesstimated that a 1.25g bias weight
might work for low tracking weight cartridges, so I filled the hole of a steel M3
nut with solder, thinking this wouldn’t
look too out of place with the rest of the
chrome on the pick-up arm. I then drilled
through the solder and lined it with liquid
insulation “tape” so that it would be a soft
push fit over the bias weight shaft. It turned
out to be pretty close to 1.25g and I can
now (finally) adjust the bias to match the
cartridge tracking weight.
very light rub with a (preferably used)
section of a pad can clean the contacts
and provide the rough surface area they
require for a good electrical contact.
Having given them the Scotchbrite
treatment, a final wipe down to remove
any lingering debris was all that was
needed to restore the PCB contacts to
as-new condition.
The reassembly procedure was simply the reverse the disassembly process
and after laying down the rubber mat,
lining up the circuit board and “torqueing” the screws down, I was ready to
test it out. Annoyingly, while most of
the previously non-working keys now
worked, several still didn’t, which put
a real dent in my confidence. I was sure
I’d cleaned all those contacts properly,
so there was nothing else for it but to
take it apart again and try to figure out
where I’d gone wrong.
This time, as I removed the circuit
board, I noticed that the rubber mat
was ever-so-slightly out of alignment
in some areas. This meant that the
rubber thimbles in those area did not
get compressed directly onto the PCB
contacts under key pressure.
I relaid the mat, this time making
sure that every thimble lined up perfectly with its corresponding key. I also
made absolutely sure that it was dead
flat before I screwed the PCB down.
As a precaution, I put in just the barest minimum number of screws necessary to keep it together, in case it had
to come apart again. Then, keeping in
mind that if I did a Jerry-Lee Lewis on
it I’d likely blow the board right out
from under the keys, I gently tried each
key in turn and they now all worked
correctly. I then added the remaining
screws and gave it a final workout by
playing Rachmaninoff’s Prelude in CSharp Minor (otherwise known to me
as “Chopsticks”).
60 Silicon Chip
Noisy pots
That left the noisy slider pots. I began by removing these one by one,
then disassembled them by carefully
The rubber motor suspension had perished and sagged due to Queensland’s heat
and humidity, so I made up a replacement
from a motor-cycle inner tube. This also
worked well and a search on the internet
revealed that several other owners had
also resorted to a DIY solution for the
motor suspension. Genuine kits are still
available as well.
At that stage, I took a look at replacing
the cartridge. When CDs originally began to
supersede LPs, there were some cartridge
and stylus bargains to be had as retailers
disposed of their older technology. Fortunately, I acquired a couple of spares for
very good prices at the time and had put
away them in drawer. They turned out to
be a good investment.
In 2001, I went away for a holiday
with my wife and when we returned, I found
that one channel of our stereo system was
out when listening to LPs. It was only missing when listing to LPs, so that immediately
narrowed the fault down to the turntable
and its connecting leads.
I went through the signal path checking
for continuity and ended up back at my
680EEE cartridge. One channel was open
circuit and I initially thought that this was
probably due to misuse by our adult children while we were away.
Anyhow, with nothing to lose, I tried a
“blacksmith trick” and heated the pins on
the offending channel on the cartridge. This
bending back the folded metal clips
that held the bottom section to the
main body of each pot. After removing
the knobs, the sliding assembly could
then be removed, exposing the tracks
and the sliding contacts.
Each pot was then cleaned in turn
using contact cleaner and wipes and
given a gentle rub with Scotchbrite.
They were then reassembled and that
fixed their noisy operation.
Sliding pots are becoming hard to get
these days, so being able to clean and
restore them was a lucky break. However, they’ll almost certainly need replacing when the keyboard comes back
in for another service in 30 years time!
A tale of five oscilloscopes
Despite having a good working
scope, R. B. of Denistone, NSW decided to tackle four faulty units that
were sitting unloved in his workshop.
He managed to get three of them going
again but the fourth had to be binned.
Here’s what happened . . .
siliconchip.com.au
made no difference so replacing it with
one of the spares I’d invested in was the
obvious solution. I subsequently fitted the
new unit in place and then went through all
the measurements and adjustments to get
the correct horizontal and vertical tracking
angles and the correct tracking weight, etc.
Unfortunately, when I attempted to play
some music, I now got intermittent dropouts and crackles! This led me to wonder
what I could possibly have damaged, since
during the initial diagnosis I’d had the
turntable apart to check the pick-up arm
connection (and there are very delicate
wires in there).
I went back through the signal path
again and that led me straight back to the
cartridge. Fortunately, the source of the
drop-outs turned out to be nothing more
than a coating of “gum” that had built up
on the pins of the cartridge while it had
been stored in a drawer in the sub-tropics
for many years.
So here’s a tip for analog music listeners: always clean replacement cartridge
pins before plugging them in!
This experience also convinced me that
it was old age that caused the Stanton
680EEE cartridge to fail, rather than abuse.
Finally, I wonder what my turntable’s
early history may have been like if the internet had been available back then and I
had been able to diagnose and fix some
of its problems sooner.
Over the last few years, I have acquired no less than five oscilloscopes.
Of these, three subsequently developed
faults, while the fourth, purchased
secondhand, was faulty to begin with.
They’d been sitting around for some
time, so I recently decided to tackle
them to see if they could be repaired.
I acquired two of these scopes from
a well-known local electronics chain
and another two via eBay. The fifth
(working) unit was purchased online
brand new; it was a popular Chinesemade digital scope and it proved to
be a great purchase (one that I should
have made years earlier).
Scope repair 1
The first faulty scope that I tackled was a BWD 802. This is a 25MHz,
2-channel scope and it worked when
first purchased via eBay several years
ago. However, it subsequently stopped
working a year or two later.
I put it on the workbench and applied power. The “power on” light
siliconchip.com.au
failed to illuminate and after a minute
or two I could smell something burning, so I removed the covers to see if I
could spot the offending component.
To say that the blue smoke had
been let out was an understatement!
The covers of this scope enclosed the
chassis very tightly and as I peeled the
covers off, a large plume of acrid blue
smoke wafted into the workshop. That
meant that it shouldn’t be too hard to
spot the cause of the problem, or so I
hoped.
I put the now naked scope on the
bench, turned off the power switch
on the front panel and connected the
mains power. Within a minute, a tantalum capacitor began to expel all its
remaining smoke! This in turn raised
an important question: apart from the
obviously faulty capacitor, why was
power being applied to the circuit when
the front-panel power switch was off?
I removed the power cord from the
wall socket but rather than tackle the
switch, I decided to sort out the tantalum capacitor problem first. This
device was a smoothing and stabilisation capacitor for an LM7915 regulator
and a quick check with a multimeter
indicated that it was the only one in
the vicinity of the regulator that was
faulty. I didn’t have any tantalum capacitors on hand but a quick check of
the LM7915 data-sheet indicated that
a 25µF electrolytic capacitor would do
the job. Fortunately, I did have one of
these on hand and so this was substituted for the faulty tantalum.
I then plugged the unit into the
mains again and this time the power
indicator light came on, even though
the front-panel power switch was still
off. What’s more, the scope still wasn’t
working. I checked the rail voltages
and found that I had a -15V rail but
the +15V rail was missing in action.
As it turned out, the tantalum capacitor on the +15V rail supply had
also failed. This was replaced with another 25µF electrolytic capacitor and
the scope then began operating.
So why had these two capacitors
failed? Apparently, tantalum capacitors can fail in a catastrophic way. And
when they short out, they can draw lots
of current and burn circuit boards or
even cause a fire. However, the tantalum capacitors in the BWD802 were
installed after the voltage regulators,
which by design limit the current to
just a few amps. That’s enough to let
the smoke out but low enough to pre-
vent a fire or damage to the PCB.
Now that I had this scope functioning, I turned my attention to the frontpanel power switch. This switch is
integrated with the intensity control
(a potentiometer) and the power is normally switched on by turning the knob
off zero. This was a common arrangement up to about the 1980s and basically consisted of a switch mounted on
the back of a potentiometer.
A quick check with the multimeter
showed that the switch section, which
switches both Active and Neutral, was
stuck in the “on” position. A quick
search on the internet failed to turn up
a source for this part and it appears that
they are no longer available.
The switch section was held on by
two small metal tabs. Once these had
been released, I could see the mechanism inside and this revealed that one
tiny Nylon part had broken. In the end,
rather than try to fix the switch, I decided to discard it and keep the potentiometer section.
After some thought, I decided to
make a cut-out in the rear panel and install a combined IEC male socket, fuse
and switch. This was wired directly to
the mains transformer and the unit is
now fully working.
Basically, it pays to have an open
mind when repairing old equipment,
as it’s not always possible to obtain the
parts required.
Scope repair 2
The second scope that I repaired was
a Protek 6502a, a 2-channel 20MHz
scope that I bought from a local electronics chain. It lasted perhaps 15
months before it failed, so it was outside its warranty period.
This scope came with a full set of
schematics, so that at least gave me a
good chance to repair it. The failure
was somewhat curious: the trace was
visible on the screen but could not be
brought into focus. In addition, it was
making a buzzing sound. Changing the
focus and intensity controls altered the
sound and pitch of the buzz.
This indicated a problem with the
circuitry that generates the high tube
voltages. I managed to obtain a new
EHT transformer from the place where
I purchased the scope but changing this
had no effect.
I was sure that the buzz was some
clue to the problem and that it could
possibly be due to high-voltage tracking or arcing. So I carefully checked
May 2016 61
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the PCB and all components for any
evidence of tracking or sparking but
drew a blank.
And then, just as mysteriously as it
had started, the problem disappeared
and the intensity and focus controls began working normally, as did the rest of
the scope. At this stage, I should mention that my workbench is located in
an outbuilding and is unheated. What’s
more, the night that I happened to be
working on this scope was rather cold.
Past experience told me that I hadn’t
fixed the problem and that it would almost certainly reappear. Sure enough,
when I turned the scope on the very
next day (when it was several degrees
warmer), the buzzing sound and the
focus fault were again very much in
evidence. I got a length of hose, held
one end to my ear and placed the other
end over every component around the
high-voltage section to see if I could
determine the source of the buzz but
still no luck.
In desperation, I decided to turn
off the lights in my workshop in the
hope that, if it was an arcing problem, I
could see the source. Initially, I looked
around the high-voltage section on the
main PCB but again drew a blank. And
then, just as I was starting to walk back
to the light switch, I noticed a small
spark out of the corner of my eye. It
was quite tiny and was located on the
front-panel PCB – not where I had expected a fault to be.
In turned out that this sparking was
coming from one of the terminals on
the focus potentiometer. A quick look
at the schematic confirmed that this
terminal carried a high voltage, which
explains why the front panel PCB was
shielded with a clear plastic cover.
Turning the focus and intensity controls made the spark change, so I had
found the problem.
So why had the problem disappeared the night before? Well, as I
mentioned, it was a particularly cold
night and the cold air, with its low humidity, was acting as a better insulator
than before.
In the end, the problem came down
to poor design. The clearance between
the high-voltage tracks was very small;
certainly not enough to support the
several hundred volts present.
There was no conformal coating or
any other method to improve the insulation. I applied a blob of epoxy over
the area and after waiting for the epoxy
to cure, gave it a test. The scope then
worked fine, so that was another one
out of the way.
Scope repair 3
Next on the list was a 60MHz Tektronix 2133 that I picked up a few years
ago from a deceased estate. When I subsequently tested it, channel 1 worked
OK but channel 2 didn’t operate at all.
Since I had been on a roll with the
other two scopes, I thought that I would
now give this one a shot as well. This
was a scope that I actually wanted to
keep, as it would be a good supplement to my digital scope. So onto the
bench it went.
I applied power and a trace appeared
on the screen but then the power suddenly failed. In fact, when I checked
everything out, the circuit breaker on
my distribution panel had tripped. I
reset the breaker and tried powering
the scope on a number of times but the
result was always the same – the breaker would trip after about 10 seconds.
This particular breaker has an earth
leakage function and a separate flag to
indicate when it has been tripped by an
earth leakage fault. In this case though,
it wasn’t an earth leakage fault.
I did all the usual checks, including checking the resistance between
the scope’s Active and Neutral connections and the resistance between
the supply rails and ground etc but
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62 Silicon Chip
couldn’t find anything unusual. I
then downloaded a schematic but this
didn’t give me any further clues as to
the cause.
In desperation, I tried temporarily
powering the scope via an isolation
transformer with the Earth disconnected. Surprisingly, the scope now
remained operational and there was
no indication of any excessive current
being drawn.
That led me to conclude that there
must be a fault path to ground and
the only element I could identify that
could cause this problem was the enclosed line (mains) filter built into the
scope. It took a great deal of effort to
remove and replace this item as the
original soldering was expertly done
at the factory (what else would one
expect from a Tektronix scope?). However, when I retested it, the fault was
still there.
By now, I had run out of ideas as to
what could be the cause and as I was
going to keep this scope for myself,
the option of just running it through
an isolation transformer seemed to be
the solution. After all, all the internal
supplies are working and within specification. Still, it would have been nice
to have found the problem.
Scope repair 4
I won’t name the last scope that I
tried to fix. It was one that I bought on
price (a bad mistake) when I was desperate for a scope. As before, it was a
2-channel 20MHz scope of Chinese
origin. When I opened up the case, I
saw why it was so cheap. Most of the
wiring is done via ribbon cable rather
than via PCB tracks.
The problem with this scope was
that, on power up, the trace would skip
across the screen once and then disappear. The focus and intensity controls
had no effect.
I quickly determined that neither
the tube supply nor the focus supplies were anywhere near specification. These voltages, of which there are
several, are derived from a switchmode
power supply (SMPS), which is just a
self-oscillating design with no regulation on the output side. Instead, the
regulation occurs on the input side,
where positive and negative regulators supply the switched side of the
transformer primary.
In operation, these regulators were
getting very hot and, in fact, were operating in current limit. And that exsiliconchip.com.au
Battery-Powered Golf-Cart Repair
It’s often said that golf is a good walk
ruined, which is why some people choose
to use a battery-powered golf cart. J. N. of
Tauranga, NZ recently saved a customer
the indignity of walking after that little
white ball . . .
I’m a semi-retired electrical/electronic
technician and being a keen golfer, I’ve let it
be known that I’m prepared to troubleshoot
and repair electric golf carts and trundlers.
As a result, quite a few jobs come my way
through our local club and I also often get
referrals from battery retailers.
Recently, a retailer referred a customer
who owned a 1998 Club Cart that was
manufactured by Ingersol Rand. He duly
arrived at my workshop and explained
that his cart would not go and also that
the batteries were probably flat. He had
taken it to a garage but the mechanics had
been unable to get it working. He also told
me that he had purchased a new battery
charger some two years before.
After assuring him that I would do my
best for him, I set about checking the cart
out. This particular model is the DS series
and is powered by a 48V lead-acid battery
bank running a shunt-wound motor. It has
great power for any terrain and features
dynamic braking.
In addition, the motor will act as a charger if the cart is free-running downhill. This
particular manufacturer is the only one that
provides a battery charger with the cart and
this charger is controlled by an on-board
computer mounted in the cart itself. All in
all, it’s a very well-made unit.
I began by testing all six of the 8V batteries and found that they were all in good
condition, which indicated that the charger
must be faulty. Sure enough, after plugging it in, there was no sign of any charging activity.
I then dismantled the charger and discovered that it was a switchmode type.
plained why the secondary voltages
were nonexistent.
Having obtained a circuit diagram
online, I spent an hour or two trying
to diagnose the source of the problem.
The only conclusion I could reach was
that the SMPS transformer had developed a shorted turn.
Out of curiosity, I checked out the
prices this type of scope was fetching on eBay and the answer was not
much. So the question was, should I try
and obtain a transformer and replace
siliconchip.com.au
Most of the on-board Mosfets and diodes
had fused and the main PCB looked to be
well past repair. Judging by the rust present on the screws holding it together, I
suspect that moisture had found its way
inside the unit, causing it to fail.
Fortunately, the owner had also brought
in the original Club Cart charger, so I decided to see if this could be made to run
again. However, when I checked out the
cart’s wiring, I discovered that someone
had bypassed the on-board computer in
order to get the unit working with the later
replacement charger.
Usually, the encapsulated on-board
computer has a FET to control the charging
and if this fails the unit is not repairable. As
a result, I have also carried out the same
modification to bypass the computer on
Club Carts myself.
I contacted the customer and explained
that his replacement charger would be too
costly to repair. I then told him that he
could either buy a new charger or I could
modify the original charger to make it operate automatically, this for about half the
cost of a new charger. Not surprisingly,
he opted to have the original charger
modified.
The original charger is quite simple and
uses a mains transformer with a centretapped secondary to drive a full-wave rectifier consisting of two diodes. It’s normally
operated when the on-board computer
energises a 48V DC relay in the charger
itself, to switch the incoming mains to
the transformer.
Fortunately, I had a factory-made adjustable 10-60V DC voltage controlled switch
in stock and having carried out this type
of modification before, I had previously
designed a suitable timer circuit for the
unit. It used a CMOS 4060 4-stage counter
IC to operate a second relay, to turn the
charger off after a set time. This was to
it? The vendor didn’t have any spare
parts for this model and I quickly came
to the conclusion that the necessary
investment in parts and time wasn’t
warranted. Instead, the better course
of action was to remove a handful of
useful parts and scrap the rest.
It’s a shame when a number of factors
come together and force this decision.
My first mistake was to buy cheap because I ended up with an inferior product that failed quickly. Secondly, the
vendor provides no spare parts back-
safeguard against the charger not turning
off automatically if a faulty battery stopped
the battery bank from reaching the fullycharged voltage.
Because there’s not much spare room
inside the charger, I had to relocate the
internal 48V relay and install the voltage
switch and timer in its place. The negative
lead from the cart was connected to the
NC contacts of the voltage switch and the
timer’s NC contacts to energise the original 48V relay. Now, as soon as the charger
was connected to the cart, charging would
take place until either the voltage switch
or the timer operated.
I then tested the modified charger circuit and the batteries charged up nicely.
However, nothing happened when I tried
to take the cart for a test run, much to my
frustration!
The owner had previously mentioned
that the cart had been “playing up” for
some time, either by suddenly stopping
for no apparent reason or by “juddering”
until it finally ceased working. I already had
the wiring diagram for this model, so I set
about checking the control wiring but this
was OK. I then suspected that the main
power solenoid contacts might be faulty.
This solenoid is operated via a switch as
soon as the accelerator pedal is pressed.
The solenoid was covered in dirt and
dust, so I carefully cleaned this muck
away and discovered that it looked rather
strange. As a result, I disconnected the
batteries and removed the solenoid. This
revealed that it had been broken open at
some time in the past and “repaired” by
someone. It had then been very badly reassembled, with pieces broken off, and
held together with cable ties and some
sort of glue.
No wonder the cart’s owner had been
having troubles!
I always replace a faulty solenoid since
repairs rarely last for long. The cart’s owner
is now a happy golfer again.
up and so the scope is as good as junk
if one of those specialised parts fails.
There also seems to be little interest
in secondhand scopes unless they are
a quality brand. This goes back to the
first point, where buying cheap usually
isn’t the best decision in the long run
The last scope I bought was a Rigol
DS1052e and it’s the best scope purchase that I have ever made. It’s been
completely reliable and is easy to use.
It was a bit of a stretch (for me) to buy
SC
it but it’s now my workhorse.
May 2016 63
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