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Old 22nd Aug 2019, 9:47 pm   #1
Nickthedentist
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Default Pye 1004 (Transistor Black Box) amp faults, help wanted please

Hello,

I bought this late-variant (c. 1964, BSR UA15 deck) Black Box cheaply, as a non-worker with the mains lead chopped off.

The manufacturer's data is on Paul's discs. It uses 7 Newmarket transistors, all DC-coupled except for the first one.

The amp is complete and never seems to have had any attention. However, F1 (1A) in the -25V DC supply is open circuit.

Quiescent consumption is stated as 15-30 mA. But powering it from my bench supply set to 25V and limited to about 60mA results in the bench supply's overcurrent protection operating and its ouput voltage dropping down to about 8V. Nothing looks burnt and there are no obvious shorts.

What tests should I do next, please? Transistor amps aren't my forte!

Many thanks in advance,

Nick.
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Old 22nd Aug 2019, 10:34 pm   #2
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Default Re: Pye 1004 (Transistor Black Box) amp faults, help wanted please

Possibly one or more transistors is.are faulty. DC coupled amplifiers can be a bit of a pain because a fault in one transistor or other device produces a 'knock-on' effect bu upsetting the voltage readings on others. I'm not familiar with the transistor Black Box, but I'd be inclined to suspect the output transistors, which almost certainly will be a PNP/NPN pair, no doubt with a common heatsink or cooling clip. If the Newmarket Transistors are as prone to the 'tin whisker' problem as Mullard types this could be the cause of the excess current consumption
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Old 23rd Aug 2019, 6:43 pm   #3
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Default Re: Pye 1004 (Transistor Black Box) amp faults, help wanted please

I would first test the voltage on the negative of the output capacitor (collector of the lower transistor). It should be sensibly about half the rail voltage i.e about 4 volts in your fault case. This should at least give an indication of which transistor is likely to be s/c.
Probably then just an Avo 8 on resistance checking the transistors.
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Old 24th Aug 2019, 7:40 am   #4
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Default Re: Pye 1004 (Transistor Black Box) amp faults, help wanted please

A DMM on its diode setting can be a handy diagnostic tool to check that each transistor has two working junctions inside it. With the amp powered off, if they’re germanium, expect forward voltage of 0.2V to 0.3V; if silicon, 0.6V to 0.8V.

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Old 24th Aug 2019, 11:33 am   #5
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Default Re: Pye 1004 (Transistor Black Box) amp faults, help wanted please

Newmarket transistors must be replaced with the same type. I've found by bitter experience that other types just dont work, especially in Pye products.

In fact you might be better off changing all the transistors, but use NKT453's instead of 452's.
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Old 27th Aug 2019, 1:53 pm   #6
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Default Re: Pye 1004 (Transistor Black Box) amp faults, help wanted please

Thank you everyone for your helpful suggestions.

I've pulled the two NKT452 output transistors, and both measure leaky (though definitely not s/c) in all directions. With them removed, the current consumption drops to 15mA.

All the electrolytic capacitors seem dubious; they all measure very high on my Peak tester (which implies they're leaky), or are open circuit. I plan to replace them all.

In view of Michael's wise words in post 5, can I ask the following, please:

(1) Should the voltages on T1-T5 all be normal, if run in its present state (i.e. with the output pair removed)? If so, and the voltages do indeed measure correctly, should it be safe to fit replacement output transistors with some degree of confidence?

(2) Cricklewood list the NKT453 @ 12 each. Has anybody actually had any success with this model fitting anything cheaper or more readily available? Will these Cricklewood ones be of reasonable quality, or will they be dubious re-labelled modern devices? Or will they be originals, and possibly doomed to early failure because of tin whiskers etc? I don't mind spending that kind of money to get it working reliably, but I can't afford to gamble too much!

Thank you again,

Nick.
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Old 27th Aug 2019, 2:37 pm   #7
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Default Re: Pye 1004 (Transistor Black Box) amp faults, help wanted please

These amps are very component sensitive. If you are lucky enough to have found a good source of NKT453s in 2019, I would get them now. Cricklewood are trustworthy and might even test them for you before sending.
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Old 27th Aug 2019, 2:44 pm   #8
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Default Re: Pye 1004 (Transistor Black Box) amp faults, help wanted please

An alternative would be to simply replace the amp with a modern IC design.
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Old 27th Aug 2019, 2:44 pm   #9
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Default Re: Pye 1004 (Transistor Black Box) amp faults, help wanted please

Edward, in your experience, is it mainly the two TO-3 output transistors which fails in these amps, or will there typically be other faults which have caused them to die?

Nick.
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Old 27th Aug 2019, 2:45 pm   #10
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Default Re: Pye 1004 (Transistor Black Box) amp faults, help wanted please

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Originally Posted by paulsherwin View Post
An alternative would be to simply replace the amp.
Yes, I haven't discounted that, but it seems a shame to "spoil" an entirely unmolested 55 year-old machine.

N.
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Old 27th Aug 2019, 2:51 pm   #11
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Default Re: Pye 1004 (Transistor Black Box) amp faults, help wanted please

AD149 would be a suitable replacement for NKT453, also make sure the base bias resistors are in good condition, especially if there is an adjustable pot.
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Old 27th Aug 2019, 2:57 pm   #12
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Default Re: Pye 1004 (Transistor Black Box) amp faults, help wanted please

That's what I thought until I read post 5
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Old 27th Aug 2019, 4:07 pm   #13
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Default Re: Pye 1004 (Transistor Black Box) amp faults, help wanted please

Yes, I assume Edward and Michael know what they're talking about. You could always redesign the output stage to use different transistors, but then it's cheaper and easier to just replace the whole module if you decide to go down that road.
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Old 28th Aug 2019, 7:59 am   #14
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Default Re: Pye 1004 (Transistor Black Box) amp faults, help wanted please

Quote:
Originally Posted by Nickthedentist View Post
Edward, in your experience, is it mainly the two TO-3 output transistors which fails in these amps, or will there typically be other faults which have caused them to die?

Nick.
The Pye 1004 was the very first UK volume produced record player using a medium powered (c. 6 watts) transistor germanium AF amplifier. Launched in late 1963, surviving examples are now 55+ years old and need treating with some respect. The output pair are the usual suspects and accurate voltage measurement over the circuit is recommended. There were some circuit and loudspeaker changes made over its 15 month production life.
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Old 28th Aug 2019, 9:45 am   #15
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Default Re: Pye 1004 (Transistor Black Box) amp faults, help wanted please

Thanks Edward, that's really helpful.

I ran it with the output pair disconnected last night and did a few quick voltage measurements. I will post the results tonight if I get a minute.

I have also ordered a pair of NKT453 from Littlediode - they cost rather more than I paid for the entire machine, inevitably!

Nick.
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Old 18th Sep 2019, 9:03 am   #16
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Default Re: Pye 1004 (Transistor Black Box) amp faults, help wanted please

Sorry about the slow progress. The new output pair has arrived, but I haven't fitted them yet.

I've removed the preceding 5 transistors and tested them on my Peak analyser. All are identified as Ge transistors with sensible and similar test results, except for the only NPN transistor, VT4, which in my example is an NKT733. This gives an HFE (?) reading of 1, whereas all the PNP examples are well into double figures.

Should I worry about fitting the output pair because of this? Or Is this normal? Or should I source a NOS NKT733 at some expense?

Snippet of circuit diagram attached.

Cheers,

Nick.
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Old 20th Sep 2019, 2:01 am   #17
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Default Re: Pye 1004 (Transistor Black Box) amp faults, help wanted please

Quote:
Originally Posted by Nickthedentist View Post
Should I worry about fitting the output pair because of this?
Yes! Check the junctions properly with an AVO on the lowest resistance range.

Think of the transistor as two back to back diodes and check base-collector and base-emitter in both directions and expect to read as a diode in each case.

There should be no reading collector to emitter in either direction.

I ALWAYS use an AVO for checking transistors like this and I'd only use an actual tester as a last resort to give me HFE values, that's if I used one at all.
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Old 20th Sep 2019, 6:55 am   #18
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Default Re: Pye 1004 (Transistor Black Box) amp faults, help wanted please

Thanks so much for taking the time to reply. I'll report back with some good old AVO tests.
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Old 20th Sep 2019, 11:17 am   #19
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Default Re: Pye 1004 (Transistor Black Box) amp faults, help wanted please

Will also compare results with an AC176 if I can find one as this is sometimes quoted as an equivalent.
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Old 20th Sep 2019, 11:36 am   #20
Edward Huggins
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Default Re: Pye 1004 (Transistor Black Box) amp faults, help wanted please

It might help to know which variant of the Pye 1004 yours is.
If it has 6.5" round speakers (possibly R&A units) is an earlier model.
If it has 6" x 4" twin-cone elliptical Philips units, it is a later version.
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