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Old 18th May 2022, 8:32 pm   #1
samjmann
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Default Leak Stereo 30. Output transistor replacement

I'm looking at a long stored example of this amp. Totally original, but needing a lot of work. The most obvious problem is the failure of one output transistor. This uses a AD140 transistor in a quasi-complimentary set up. These are of course germainium transistors, and I'm almost embarrassed to say I can't remember the last time I took any readings from one with a modern DMM!!

If you look at the attached readings that one transistor has a vastly different reverse C-E reading to the 3 others. What really surprised me is that forward C-E all of them test effectively leaky, but surely this can't be the case. The + and - are the polarity of the DMM test leads in the table. My old RS meter reads 2.895 on open circuit diode test.

This unit had not been powered on, all the original fuses are OK.

I know know that germainium reads different to silicon, but is it as much as this on leakage?

Sorry to look stupid, but the last time I looked at a germainium amp was over 40 years back.

Any help or advice welcomed SJM.
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Old 19th May 2022, 12:04 pm   #2
snowman_al
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Default Re: Leak Stereo 30. Output transistor replacement

Germanium transistors are notorious for increased leakage and it gets worse with age, temperature and so on. I believe modern meters assume zero leakage so give false readings.

There are a number of tutorials on the web about measuring the actual leakage, in µA to mA. They focus on effects pedal transistors, but the principal for power ones is the same. Just uses a couple of resistors and a 9 volt battery. (Example https://www.youtube.com/watch?v=bzWB...IYGuitarPedals or http://www.geofex.com/article_folders/ffselect.htm or find one that suits your style.)

There was a table on an American site, I cannot track down at the moment that gave suggested maximum acceptable leakage currents, from memory for a power transistor 2 to 3mA was 'acceptable'. Over 5mA was a fail.
I tried this for some Armstrong AL102 transistors and they were marginal.
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Old 19th May 2022, 3:05 pm   #3
paulsherwin
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Default Re: Leak Stereo 30. Output transistor replacement

Germanium power transistors do have relatively high leakage.
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Old 19th May 2022, 3:43 pm   #4
greg_simons
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Default Re: Leak Stereo 30. Output transistor replacement

Strangely enough I've just removed the ad140's from an old 30 carcass and they measure quite leaky on a dmm yet they're definitely ok, tried an au113 for the perdio project, even worse! but still a good device, i think you forget just how bad they were.
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Old 19th May 2022, 9:58 pm   #5
samjmann
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Default Re: Leak Stereo 30. Output transistor replacement

Thanks for all the replies: The YouTube video was really informative. The leakage figures are really quite high. If I hadn't had the other channel as a reference, with three others to compare to, I wouldn't have made a correct judgement.

Modern silicon just read perfect today. It was sixty years ago so
things do get better (mostly).

Next bit will be to get rid of the tar from cigarette smoke that has made a lot of the insides a tacky brown colour

SJM.
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