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Old 30th Jan 2023, 9:27 am   #41
samjmann
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Default Re: 1970's GEC power supply issue - help required

To prevent damage to the output transistors, I would suggest using a lamp limiter in the mains supply. You can use a 40w lamp in series with the mains supply. If the amp is ok, it will glow bright briefly then dim down at switch on. If there's a heavy load, it will stay bright!

I would take note of the position of the bias pot, then turn it so as it is at minimum resistance. This will limit the forward bias to the output transistors. The neg thermistor across the pot may have gone high.

I've recently had very similar problems with a Sony STR-3800L, there's a post about this in this section of the forum.

Don't wish to labour the point, but don't run it without a limiter. The Sony nearly went into thermal runaway when I was working on it. The limiter alerted me of the high current.

Good luck with it. As I said earlier, output stages can be challenging!!

SJM.
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Old 30th Jan 2023, 9:33 am   #42
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Default Re: 1970's GEC power supply issue - help required

if the output stage is taking too much current, there will be a loud hum.
You will need to test the output transistors and the others in the output stage.
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Old 30th Jan 2023, 9:47 am   #43
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Default Re: 1970's GEC power supply issue - help required

I'm not sure if the OP has both channels gone or just the one. He mentioned that the transistors were getting hot, but all four or just two? I've every sympathy with the OP. If you're just starting out in this, these types of faults are most frustrating.

SJM.
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Old 30th Jan 2023, 11:58 am   #44
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Default Re: 1970's GEC power supply issue - help required

It would be a good idea to measure the total DC current to confirm the suspicion it is too high.
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Old 30th Jan 2023, 12:35 pm   #45
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Default Re: 1970's GEC power supply issue - help required

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Originally Posted by Radio Wrangler View Post
Have a look on the service sheet for any statement about what they measured their voltages with. Being GEC they're likely to have gone with their own 'Selectest' rather than AVO, but a 20,000 Ohms-per-volt instrument is likely. AVO 8 territory. I don't know the military ones myself, but I think they could well be like the earlier models and much lower ohms-per-volt.

There are areas in these circuits where this can produce significant differences in what you measure.

David
Hi David,

Just for clarification sake, the Test Set 1 I have is this one

"Test set No.1 (a military version of Model 8 Mark 2) Essentially the same as Model 8 Mark 2 apart from 3-10 ranges instead of 2.5-10; no dB scale; resistance on bottom of scale plate instead of top; no LR or Ins. positions. At some point added fuse on low ohms ranges. "
The only other change is that they potted to enable useage in harsh conditions.

Last edited by paul1962; 30th Jan 2023 at 1:02 pm.
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Old 30th Jan 2023, 12:42 pm   #46
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Default Re: 1970's GEC power supply issue - help required

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Originally Posted by samjmann View Post
I'm not sure if the OP has both channels gone or just the one. He mentioned that the transistors were getting hot, but all four or just two? I've every sympathy with the OP. If you're just starting out in this, these types of faults are most frustrating.

SJM.
Hi Samjmann

In answer to your points, both channels are gone, which is the frustrating part as I don't have a good channel to reference with. It is indeed all 4 output transistors getting hot.

In regards my experience, I studied Basic Electrotechnology back in the 1970's, but have been out of the electronic field for about 40 years. To be honest, I have forgotten more than I remember
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Old 30th Jan 2023, 12:56 pm   #47
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Default Re: 1970's GEC power supply issue - help required

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Originally Posted by peter_sol View Post
if the output stage is taking too much current, there will be a loud hum.
You will need to test the output transistors and the others in the output stage.
Hi Peter

There is indeed loud hum. That is the fault with this player and I would liken it to an old Dansette with a leaky mains cap.

I have tested the output transistors with a passive test from my scope (Hameg HM205-2) and components are showing proper waveforms (not open or short circuit) but not checked HFE, I have to admit.
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Old 30th Jan 2023, 11:08 pm   #48
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Default Re: 1970's GEC power supply issue - help required

Hi all

Ok, after much deliberation and as I stated in an earlier reply, I intend to change C329 through to C332 Electrolytics to find out whether this issue is down to capacitance, rather than resistance or Transistor issues. I feel at this point it's better to rule out capacitance, resistance and transistors in that order.

This is where my questions come in.

C329 and C330 are 250uF 16v. C331 and C332 are 400uF 16v.

Since I last had dealings with caps back in the day, it seems that a lot of values are not prevalent anymore, including the required values

C329 and 330 not so much of an issue, i assume that 220uF will do based on a 20% +/- tolerance guess ?
C331 and C332 are the main issue. It seems the nearest widely available is either going to be 330uF or 470uF. I would say that 470uF would be favourable . Based on what's available specs wise, it seems the 330uF has a tolerance of 20% +/- taking it to 396 uF ( a bit too close to max ?) whereas the 470uF with a tolerance of 20% +/- ranging down to 376uF should be better matched ?

Do you guys second my thoughts ? or am I just losing the plot again ?

Last edited by paul1962; 30th Jan 2023 at 11:13 pm.
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Old 30th Jan 2023, 11:16 pm   #49
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Default Re: 1970's GEC power supply issue - help required

Use 470 uF, 220 uF will be fine.
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Old 30th Jan 2023, 11:39 pm   #50
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Default Re: 1970's GEC power supply issue - help required

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Use 470 uF, 220 uF will be fine.
Many thanks for the support Cruisin. It's nice to know that people in this day and age of "sod you" actually still take the time and care to support others
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Old 30th Jan 2023, 11:42 pm   #51
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Default Re: 1970's GEC power supply issue - help required

You are most welcome.
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Old 31st Jan 2023, 8:11 pm   #52
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Default Re: 1970's GEC power supply issue - help required

While changing all those electrolytic capacitors isn't a bad idea, I doubt that it will solve the fundamental problem that you have. The voltages that you have measured show that the voltage at the junction of the output stage emitter resistors isn't too far off half the supply voltage, suggesting that the transistors aren't the cause. The supply voltage is low because of the very high current draw.

The voltage drop across the emitter resistors is very high, confirming the high current, so you need to reduce the quiescent current in the amplifier above all else. There have been a number of suggestions earlier on how that might be achieved.

My suggestion is that you temporarily wire a 15 ohm resistor between the bases of the two output transistors on each channel. That should drop the quiescent current to a safer value. If it does, the fault will either be P309/P310 has a poor contact/high resistance or TH301/TH302 has gone high resistance.

As an alternative to the above, turn P309/P310 fully clockwise (as suggested earlier) and check the voltage drop across R337/339 and R338/R340 with it set like that. If the trim pots are faulty, this may not make any difference. If the voltage has dropped to near zero, carefully turn the controls slightly clockwise until you measure 25mV across the each of the resistors. The method described in the service data will be better to use if you have access to a signal generator and oscilloscope.

Paula
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Old 1st Feb 2023, 8:11 am   #53
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Default Re: 1970's GEC power supply issue - help required

iI think you need to check all 4 transistors in the output stage of both channels.
Do it with an Avo, not some fancy component tester. you can google how to do that.
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Old 1st Feb 2023, 10:13 pm   #54
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Default Re: 1970's GEC power supply issue - help required

Well Everyone, Firstly I would like to thank each and everyone of you for your thoughts, helpful advice and input into this issue

As I put in my last posting on this issue, I intended to change out caps C329 to C332.
I have now done this and voltages at the speaker plugs are now 0v and the loud mains hum is now down to just the token expected for units of this age (1970-1972)
Basically, this appears to have solved the issue, although, reading the caps out of circuit both with a capacitance meter and scope weirdly enough shows no faults I guess that after 53 years, they decided early retirement haha.

I guess it's a case of "the end justifies the means"

Once again, I can't thank you all enough for your great support on this great forum. Be well all and maybe I can pay back for someone further down the line

Kindest Regards
Paul

Last edited by paul1962; 1st Feb 2023 at 10:20 pm.
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Old 1st Feb 2023, 10:28 pm   #55
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Default Re: 1970's GEC power supply issue - help required

Well done Paul, glad for you, nice to get things sorted, you deserve a
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Old 1st Feb 2023, 10:52 pm   #56
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Default Re: 1970's GEC power supply issue - help required

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Well done Paul, glad for you, nice to get things sorted, you deserve a
Hi Cruisin .. you know how you think you have got there, but then realise you shouldn't have counted your chickens ?

I thought as a last check, that I would read voltages across R337 and R338. What a big mistake that was!
Readings are still 0.5v through a 1 ohm load = 500mA. Something still wrong I guess although mains hum is now gone.

Time to look into what is causing this issue now, before something goes awry again

Back to the drawing board (or circuit diagram in this case)
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Old 1st Feb 2023, 11:39 pm   #57
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Default Re: 1970's GEC power supply issue - help required

Ok THIS time I'm convinced that it's cured !!

I followed the advice of quite a few of you, up to and including frsimen's reply.
Turned trim pots anti clockwise, voltage dropped to zero. Adjusted the trims and got to 25mV. Transistors heaving a sigh of relief and are staying cool currently. I have left it on test to see what happens, but are more convinced we are there now

NOW LET ME LET YOU IN ON THE BACKSTORY ....

This player had been repaired previously by a late Electrician. His Daughter inherited the player and plugged it in to loud mains hum.
What wasn't known at the time, was her Father was suffering with Alzheimers at the time he repaired it and she wasn't sure it could be repaired. No one knew what he had done to it or whether he had blown it completely.

THANKS TO YOU ALL ... YOU HAVE MADE HER DREAM A REALITY !!

You will never know just what a special job you have done here ... THANK YOU !!!!

Last edited by paul1962; 1st Feb 2023 at 11:48 pm.
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