Practically Speaking
Hands-on techniques for turning ideas into projects – by Jake Rothman
Restoring old equipment – Part 2
Fig.1. Leak Stereo 30: the UK’s best-selling, first-generation transistor amplifier.
L
ast year, in the November
issue, we discussed some of the
challenges and rewards of salvaging vintage electronic equipment. This
month, we start a short series on actually
restoring old electronics. The focus will
be on a germanium transistor Hi-Fi amplifier, in this case a Leak Stereo 30 lent
to me by loyal reader Andy Martell, as
illustrated in Fig.1. (It was so old neither
of us dared power it up!) The important
point to make from the start is that this
is just an example to demonstrate techniques. The ideas presented here could
be applied to most older transistor-based
electronics from the 1960s.
The Internet is now the main resource for
those restoring any old technology. Most
amplifier manuals and circuits, including
the Leak ones, can be easily downloaded;
for example, at leakaudio.com and other
resources at: https://bit.ly/pe-ps21-01
Another good source of information on
old Hi-Fi units are the Hi-Fi Year Books,
for example those shown in Fig.2.
poor high-frequency response. Also, the
output stage was quasi-complementary,
using single-polarity devices, because
only PNP types were available in 1963.
This circuit topology resulted in a high
distortion figure with a spectrum of high
odd-order harmonics. Malcolm Jones (of
KEF and Falcon Acoustics fame) said the
Stereo 30 destroyed many T27 tweeters,
because the distortion was about 10%
at 10W. Nevertheless, this did not stop
50,000 germanium Stereo 30s being sold
as the standard Hi-Fi amplifier, often with
a pair of KEF speakers.
Around 1967, the small-signal transistors
in the pre-amplifier and power amplifier
input stage were replaced with silicon
types. In 1969, a fully silicon derivative,
the Stereo 30+ appeared, which had
double the output power. Hi-Fi News
gave this 75% in September 2020. This
then morphed into the Delta 30 which
had a new front panel and then the Delta
70 (Fig.4) which had yet more power.
Production continued for at least 17 years.
Thus, all variants are plentiful and still
cheap, unlike the earlier valve designs.
Double trouble!
While repairing Andy’s Leak Stereo 30, I
thought it would be a good idea to acquire
Leak Stereo 30
Fig.2. Old Hi-Fi Year Books are an
excellent source of information on early
Hi-Fi. (Mike Henslow Year Books Ltd.
Later issues IPC).
56
The venerable Leak Stereo 30 was
introduced in 1964, and is recognised
as the first successful transistor Hi-Fi
amplifier in the UK. A Leak advert from
the 1967-68 Year Book is shown in Fig.3.
The price back then was £49. Multiply
that by 40 to factor in inflation, and the
modern equivalent price is nearly £2000!
By comparison, PE was 12.5p (two n’ six)
back then and it’s £4.99 today. Of course,
this is an unfair comparison, print media
has inflated more in real terms and the
cost of electronics has deflated. The new
Stereo 130 now costs around £700 and
PE is printed in high-quality colour with
superior paper.
The Stereo 30’s sound quality is not
highly regarded by the Hi-Fi fraternity. The
Hi-Fi News Vintage Review in October 2010
only gave it a 60% rating. This is because
it used germanium transistors, which have
Fig.3. The advert for the Stereo 30 in the
1967-68 Hi-Fi Year Book in Fig.2.
Practical Electronics | February | 2021
Fig.4. (left) The Delta 70, the final incarnation of the Stereo 30. Excellent, thick front panel, shame about the edge connectors on the
plug-in PCBs. Fig.5. (Right) My poor-condition (but cheap) Stereo 30 from eBay.
a really beat-up example and bring it
back to life at the same time. Typical
‘sold-item’ prices on eBay are £20 for
poor condition (‘spares only’) examples
to £60 for working ones. I bought a ‘poor
condition for spares etc’ 1967 Leak Stereo
30 for £26.98 including postage especially
for this series from eBay seller ‘classicthings’. This is shown in Fig.5. It’s going to
need a massive clean-up and I’ve decided
to do a radical repair/improvement job to
see how far I can take it. Note that the only
way I could accurately date this example
was from date codes on the rectifier diodes.
I usually add a year on to allow for delays
in manufacturing/distribution.
You might think that the lukewarm
reviews, complexity and amount of
work involved in restoring a not very
valuable item make the Stereo 30 an
unappealing candidate for restoration.
For me, though, it was a desire to have a
well-preserved example of early transistor
history (1965). For Andy it was personal.
His Leak amplifier had passed from
his grandad to his dad, who had then
left it in a loft for years. Also, they are
a good project on which to hone one’s
restoration skills before moving onto
pricier electronic classics.
Scratches
Before we get into the Leak, I forgot
to show the Novus scratch remover
kit discussed in the November 2020
Practically Speaking article – see Fig.6.
This kit is great for restoring Perspex dials
or a retro smoked acrylic turntable lid
from a charity shop find. Also, Autosol
cream from the car shop is fantastic
for burnishing metal, such as the spun
aluminium knob-caps shown in Fig.7.
They often fall off and are missing. As
part of the clean-up, take them off, polish
and re-glue with Evostik.
Starting restoration
Like all Leak products, the Stereo 30 is
well engineered with proper industrial
wiring, sound mechanical construction
and an excellent layout. I’ve never had a
dry joint in a Leak amplifier. The knobs,
although they look like plastic were die-cast
metal on Andy’s unit; but the later eBay
model exhibited ‘accountant attrition’ and
were plastic. Even the PCB is a work of
hand-drawn art, much better than normal
for the era. A full circuit diagram and
service details are given in the manual and
the PCB is labelled properly; no numbers
under components and each one with a
left or right channel suffix. Why do we
have stupid numbering today? It’s a skill
that’s been lost, like designing toilets that
flush properly! These features mean they
can be repaired forever. (There was even
a company who rebuilt them to a modern
complementary silicon design. I don’t think
that’s a good idea though, buy an old NAD
amplifier and restore that instead.)
Don’t turn it on!
The cast iron rule for newly acquired or
loft-liberated vintage electronics is, don’t
turn it on! Relegation to the loft usually
happens when equipment breaks, and
something better is bought. The huge
Fig.7. Chrome polish for metal parts; yet
more rubbing and polishing.
Let’s be clear from the get-go; restoration
is time consuming. It’s often more work to
restore than to build
from new because
the item must first
be very carefully
taken apart (see
Fig.8). I actually
enjoy pulling things
to bits – it was how I
got into engineering
in the first place.
The dust in
some 50-year-old
equipment must be
seen to be believed
(Fig.9). A good
brushing and a
blow with the air
duster does the trick
(Fig.10). It’s best done Fig.8. The eBay Stereo 30 pulled apart. Sometimes equipment is so
outside – seriously! dirty it’s the only way to clean things up to get to the components.
Practical Electronics | February | 2021
57
Fig.6. Scratch remover kit for plastic dials
– a lot of rubbing is involved.
Electric shock
Fig.9. The dust build-up in old gear can
be horrendous, and a fire hazard.
temperature extremes in roof spaces
accelerate components ageing. With
something over 56 years-old like this, it is
guaranteed not to work. Indeed, if it’s just
switched on, electrolytic capacitors that
have de-formed through lack of use may
blow-up. The resulting leakage currents
may then take out other components and
the speakers. This leads us to basic rule
number two – never connect speakers to
untested amplifiers.
Voltage selector
The Stereo 30, like most amplifiers of
the time, was designed to give optimum
results into 15Ω speakers. Back then,
this was considered the ‘Hi-Fi’ speaker
impedance. The other normal value was
3Ω for radios and other ‘lo-fi’ products.
This meant the main power rail was set
to –42V, high for a transistor amp at the
time. Note the main power rail is negative,
and the earth is positive, normal with
PNP germanium circuits. Although the
Fig.11. Old equipment often has rotary
mains voltage selectors since there were
wide local variations in mains voltages at
the time. Use the 250V setting in the UK to
minimise stress on old parts.
58
In the 1960s an electric
shock every few months was
considered par for the course for
electrical work. Not anymore
– in today’s safety-conscious
world a product such as the
Stereo 30 would be illegal.
There was bare mains on the
back of the volume control,
the fuse holder, multiple tags
on the mains transformer and
the voltage selector. All this is
shown in Fig.12 and Fig.13.
I knew it was dangerous and
would have to be properly
insulated. However, in my
Fig.10. The difficult to reach bits can be blown with an
enthusiasm to get it going,
air duster.
I forgot to do this vital
preparatory work. When feeling the heatsink
specs stated that the amplifier could drive
on the driver transistor during testing, the
down to 4Ω, doing so could easily result in
stud at the back of the mains fuse holder
damage because four-times more current
got me – Ouch! – I dropped the whole amp
was drawn through the output transistors.
off the bench onto the floor and bent the
Eventually, Hi-Fi designers settled on a
corner of the front panel. Complacency
value of 8Ω for speaker impedance. 15Ω
comes to us all eventually. The good thing
speakers are now rare, and if the Stereo 30
is that I can show you how to repair a bent
is to be used with normal 8Ω speakers it’s
front panel later.
sensible to drop the power rail to –38V.
Fig.14 shows how the mains wiring was
This is easily done by setting the mains
made safe. The moral? Insulate with rubber
voltage selector on the back to 250V, as
sleeves before starting work. Even mains
shown in Fig.11. It would normally be on
plugs in those days had unsleeved pins,
the 230V setting, for the UK. Of course,
as shown in Fig.15. However, I kept the
the real mains voltage in the UK is 240V,
1960s plug because I liked its historical
not 230V as commonly supposed. In
solidity. (Only idiots grab the pins as they
France, it is 220V. In the interests of EU
pull the plug out – please don’t be an idiot!).
conformity, it was decreed mains voltage
One interesting feature is the pre-1971
in EU countries would be ‘harmonised’ at
mains-cord colour-coding, with red for
‘230V’ but with a tolerance of +10% / –6%.
live. None of that brown for live nonsense!
Interestingly, the earth wire was green
Speakers
with a spiral yellow tracer line. A minor
If you can find good 1960s 15Ω speakers,
concession for the colour-blind.
such as the Goodmans Maxim or the
Leak Mini Sandwich (shown in Fig.3)
then they will give good results at low
Circuit history
volumes. I sometimes use two EMI 30Ω
Before we turn on ancient gear, it’s
elliptical speakers in open-back cabinets,
informative to have a look at the
which are very ‘laid-back’ sounding. (See
history behind its circuit design to aid
Audio Out, EPE, June 2016, p.64).
understanding and fault finding. (There
is also a salient tale
here that is a warning
to all aspiring
circuit designers
in PE). Most 1960s
transistor amps
used driver and
sometimes output
transformers. These
were expensive, and
due to their restricted
frequency response
and associated phase
shift, the amount of
negative feedback
that could be
applied was limited.
However, if there is
one thing needed to
Fig.12. Old gear often has lots of exposed mains wiring. The
reduce the distortion
rear terminal on the fuse holder (far rear left) bit me!
Practical Electronics | February | 2021
(Left) Fig.13. The mains on-off switch was often ganged with the volume control on old audio gear. I don’t think it’s a good idea, but
it does mute the turn on/off thumps from the pre-amp. (Right) Fig.14. The fuse holder is now safe (too late in my case) with its (pink)
sleeve, also on the mains switch.
of transistors down to the levels of valves,
it’s lots of negative feedback. A classic
driver-transformer-based Hi-Fi amp is
the 1966 Philips TRA 2506 (www.audio.
nl) which was my parents’ first amp. The
Rogers amps, such as the Ravensbrooke use
the same basic circuit. They are reliable
because they are AC-coupled. The first DCcoupled, transformerless amplifier was the
RCA 1956 HC Lin design (an interesting
man – see: https://en.wikipedia.org/wiki/
Hung-Chang_Lin). Two Plessey engineers
described a 2W version using Mullard
transistors in Wireless World, March 1960.
This led to the Tobey-Dinsdale amplifier
published in Wireless World in November
and December 1961, and which was later
updated in Wireless World, January 1965.
A good quality PDF can also be found at:
https://bit.ly/pe-ps21-02
This design set the template for the
modern transistor amplifier. It was
described by Paul Kemble in his website
as the ‘Grand-daddy of them all’ (see:
https://bit.ly/pe-ps21-03).
Every audio engineer of a certain age
must have built one. I was too young,
though I have bought a few from radio
rallies. They didn’t sound as good as the
valve amplifiers they replaced, but they
Fig.15. Unsleeved pins on a 1960s MK
mains plug; it’s built like a tank and good
for another 50 years.
Practical Electronics | February | 2021
did sound reasonably Hi-Fi, unlike most
of the other early transformer-coupled
amps. However, just like all ‘modern’
DC-coupled amps they suffered from the
‘domino effect’, where one transistor going
short-circuit killed the ones connected to
it. Hi-Fi News covered a Dinsdale amplifier
in the June to September 1964 issues
(updated in March 1966). PE also did one
in its second issue in December 1964!
Now to the story, Harold Leak invited
Dick Toby and Jack Dinsdale over after
seeing their article. After giving them the
grand factory tour and a gourmet meal, he
asked them to leave their sealed amplifier
with him to take some measurements. It
was later returned having been opened
up, with the excuse ‘his staff hadn’t
followed his instructions’. Later, when
the Stereo 30 came out to great acclaim,
it transpired Leak had copied most of
their design. When challenged at its
launch at the 1964 Audio Fair, he said to
the 26-year-old Dinsdale, ‘I’ve got more
money than you, rock my boat and I will
destroy you’. Jack Dinsdale tells the whole
story in a letter published in the October
1996 Hi-Fi News magazine available on
the Hi-Fi History Society website (https://
bit.ly/pe-ps21-04). (I source back issues
of Hi-Fi News from eBay, via companies
such as Zip Audio).
It turned out that the low-pass filter of
Dinsdale’s circuit had been patented by
his employer at the time, Elliott Brothers.
Of course, it is easy to get round patents in
circuit design. Dinsdale’s unpatented idea
of using a heatsink-mounted compensating
diode to control the quiescent current was
circumvented by using a thermistor. The
scratch filter part on the tone control was also
copied. Leak improved the Dinsdale circuit
in conjunction with Mullard application
engineers, mainly P Tharma, (see Wireless
World, July 1963) by using a high-bandwidth
video transistor in the voltage amplifying
stage. They then claimed that 60dB of
negative feedback had been applied. Leak
should be credited with ‘productionising’
the Dinsdale circuit. Harold Leak then sold
his company to the Rank Organisation
for £41m in 1969 (using the PE inflation
index) just before the Japanese moved in.
Only recently was Dinsdale’s contribution
grudgingly acknowledged by Leak’s designer
Ray Whitcombe as an inspiration in Steven
Spicer’s book, Firsts in High Fidelity: The
Products and History of H.J Leak and Co Ltd.
(currently out of print). Dinsdale also did
okay though, becoming the first professor
of mechatronics at Dundee University.
He is regarded as the father of the subject,
the science of controlling mechanics with
electronics. There is a good interview with
him in the June 2010 issue of Hi-Fi News.
Another Dinsdale rip-off design was the
Pye HFS30T. Introduced slightly before
the Leak, but not as successful. It used
Newmarket transistors, except for the NPN,
which was a Texas D451. The circuit is in
the 1964-65 Radio and TV Servicing Book
on p.227. Henry’s Radio of Edgware road in
London used to do lots of Dinsdale poweramp modules, such as the MPA 12/15. He
was properly credited and paid for these
(see ad on back cover PE, Feb 1968). The
Leak in turn was then copied by the Truvox
TSA100. The Vintage-radio.net forum has
some good info on these.
Next month
That’s enough history! Next month, we’ll
move on to the restoration.
WARNING!
When restoring electronic equipment,
do ensure you work with:
• Eye protection
• Fume extraction
• Safe wiring/earthing
These are not nice-to-have optional
extras – you must follow all safety
guidelines to protect yourself and
those around you.
59
