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Serviceman’s Log – continued from p43
leads one to expect uncouth, mislead
ing behaviour from resistors so I
replaced it anyway.
Much to my disgust, the fault still
remained! As a result, I now decided
to remove the entire PCB assembly
and examine it for bad solder joints
and poor wiring connections etc.
This assembly actually consists
of three separate PCBs: the main
amplifier assembly PCB, a valve
base circuit PCB assembly and an
output socket PCB assembly. All
three are connected by wiring and
in particular, the valve base PCB
assembly has five heavy-duty flat
ribbon cables connecting it to the
main PCB assembly.
I made a thorough inspection of
these assemblies using a magnifying
lamp but could find no sign of bad
solder joints or poor wiring connections. This was really starting to
puzzle me by now, so I reassembled
the amplifier, carefully fitted all the
Ask SILICON CHIP
. . . continued from page 103
vember 2005 headphone amplifier to
unity by omitting resistor R2. However,
the problem is really that the valve
preamp provides too much signal. We
assume you are using the February
2004 circuit? If so, you can make the
volume control work more progressively by connecting a 100kΩ resistor
in series with the 50kΩ potentiometer.
A valve headphone amplifier is not
really practical. Your approach works
well anyway and will have authentic
valve sound.
WiFi interference to
headphone amplifier
I assembled an Altronics kit for the
Portable Headphone Amplifier (SILICON CHIP, April 2011). It works really
well except that during quiet music
or in between tracks a motor-boating
noise can be heard. I switched off my
WiFi modem/router and the noise
disappears (the response from the rest
of the family is not so quiet!)
So I moved the amplifier as far as I
could from the modem. This reduced
the motor-boating but not completely.
104 Silicon Chip
valves, reconnected my audio oscillator and tried the amplifier out once
more, this time wiggling the valves
as it warmed up. And lo and behold,
there was life in the old beast yet!
When I wiggled V1, the output appeared and disappeared, depending
on which way the valve was pushed.
Encouraged by this, I dismantled it
again and looked even more closely
at the valve base PCB assembly, especially around V1. And while I wasn’t
completely sure, it appeared that the
valve sockets pins were a little too
“open”. As a result, I tightened them
on all the valve bases, then reassembled the amplifier yet again and this
time it worked perfectly!
This exercise was a valuable
lesson re-learnt – there are more
resistors in any circuit than meets
the eye. Junction resistance can be
a real problem in electronic circuits,
so don’t forget to look at connectors,
SC
plugs and sockets, and so on.
If I hold my hand over the amplifier
the noise also stops. My main use is
playing music from my desktop PC so
I cannot separate the amplifier from
the Wifi by more than 2m.
Can I shield the unit or add any filter
capacitors etc, or would earthing help?
(M. D., Paynesville, Vic).
• The headphone amplifier already
has input filtering so if you cannot
keep the unit more than 2m away from
the Wifi modem, the only answer is
shielding. Ideally, the unit should go
in a small metal case but you might
like to try using a small piece of copper
laminate underneath the PCB. Place it
so that the copper side is away from
the PCB (to avoid shorts) and connect
the copper to the earth of CON1.
In fact, we found that simply placing the headphone amplifier case on a
sheet of aluminium or copper laminate
is quite effective in eliminating this
sort of noise.
Dog visits to letter box
not welcome
Have you ever done something to
scare dogs away from the front path?
I always have dogs coming along and
doing their business just outside my
letter-box, so I was thinking of some-
Advertising Index
Altronics.................................. 80-83
Clarke & Severn Electronics.......... 6
Element14...................................... 3
Emona Instruments...................... 12
Hare & Forbes.......................... OBC
Front Panel Express....................... 8
High Profile Communications..... 103
Icom Australia.............................. 25
Jaycar .............................. IFC,49-56
KCS Trade Pty Ltd........................ 13
Keith Rippon .............................. 103
KitStop............................................ 8
LD Electronics............................ 103
LEDsales.................................... 103
Master Instruments........................ 7
Microbee Technology..................... 9
Microchip Technology................... 37
Mikroelektronika......................... IBC
Ocean Controls............................ 11
Qualieco....................................... 39
Questronix.................................. 103
Radio, TV & Hobbies DVD.......... 101
Rohde & Schwarz.......................... 5
Sesame Electronics................... 103
Silicon Chip Binders..................... 97
Silicon Chip Online Shop............. 64
Silicon Chip PCBs...................... 103
Silicon Chip Subscriptions........... 91
Silvertone Electronics.................. 63
Tronixlabs................................... 103
Wiltronics...................................... 10
Worldwide Elect. Components... 103
thing remote-controlled inside the letter box that would go off when I press
a button from inside my place.
• We published a Barking Dog Blaster
in September 2012 and a remote control for it in October 2012. Altronics
sell the blaster (but not the UHF remote
control part) as a kit K4500 (www.
altronics.com.au). However, this may
not work as a deterrent to a casual approach by a dog for a pit stop.
We have heard that an electric fence
placed around the letter-box post and
only activated for a short period during
the “event” can quickly deter a dog.
However, the risk of electric shock
to passers-by is a definite possibility
which could have legal consequences.
A better approach may be to try a
natural dog repellent based on chilli,
SC
capsicum or ammonia.
siliconchip.com.au
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