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Technics SU-V8 Amp - ICQ problem - dead trim pot?

I swapped diodes D307 and D308 - little round baubles. I've packed the amp up as the missus will be back soon! I'll dig it back out again tomorrow. Really appreciate the help.
 
Hi all,

I've just been back over the thread to see if there are any unanswered queries:

Voltage between base of Q614 & Q616 should be 4V
Click to expand...

I'm not sure how to measure this. I put the black probe at Q614B and put the red probe at Q616E and it read -576mV

SVI2004A said:
what is voltage like on Emitter on Q316
Click to expand...

Voltage between ground and Q316E is -65V (and at Q316B is -64V)

With amp off, check for Collector-Emitter shorts on Q606 and Q610
Click to expand...

I've set the multimeter to OHMs and tried to read between B & E for each of these but my multimeter is not picking up any reading, even with the MM set at very high and very low. Possibly my cheap multimeter is not up to the job here.

SVI2004A said:
MA26-I is like a stabistor diode
Click to expand...

I'm not sure what this is.

So unless anyone has any further ideas on how to test this (remotely!), or unless anyone can spot a definite problem thus far, I think I may have to admit defeat and take the unit to a proper tech. From a low base I've learned a lot in just a couple of days. I'm all for trying to fix things yourself, but I'm also aware that I could start doing more harm than good if I end up swapping things out blindly. Thanks again to anyone that provided guidance - really appreciated.

I'll hang on for a bit and then call the tech later unless anyone has a eureka moment.

Sam
 
Flapper said:
I'm not sure how to measure this. I put the black probe at Q614B and put the red probe at Q616E and it read -576mV
Click to expand...
The request was for voltage between Q614B and Q616B
Your -576mV suggests something not ok.
Measure Q614B to chassis/GND. Repeat for Q616B to GND
 
mbz said:
The request was for voltage between Q614B and Q616B
Your -576mV suggests something not ok.
Measure Q614B to chassis/GND. Repeat for Q616B to GND
Click to expand...

Thanks for walking me through it

Q614B to Q616B is reading : -230mV
Black ground to Red Q614B is reading: 1.84V
Black ground to Red Q616B is reading: 1.71V

mbz said:
Better to use the diode test function on your MM, see diode symbol.
Click to expand...

Q606 MM in diode setting: Red probe B to black probe E: 738
Q606 MM in diode setting: Black probe B to red probe E: no reading

then Q610 works in reverse:

Q610 MM in diode setting: Red probe B to black probe E: no reading
Q610 MM in diode setting: Black probe B to red probe E: 760

mbz said:
Maybe give it another day, people in different time zones need their sleep...
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WHAT! Wake up you buggers! Heh - my impatience is with myself for not knowing what to do, and with the fact that I'd love to get this working again. I'll probably ring the tech after the weekend if we can't solve it. Thanks.
 
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Blimey. Just opened up the mains plug (which I guess I haven't done since buying the amp a few years ago) and there's no earth connection! Surely that's super dangerous and time for it to go to the techie? Live and Neutral are both wired correctly.

Emitter of Q316 and ground is -65V

R348 shows nothing on diode setting. It shows 47.5 on the 200k OHMS setting.

Reluctant to turn it back on now with no earthing.
 
Ok, progress! SVI2004A you're a genius! Swapped Q316 and Q315 and now I am able to successfully set the bias the right channel - which had been bad - but unable to set the bias on the left channel - which had been good.

Audio tested and the distortion is now on the left channel as you'd expect.

This indicates that the transistor that had been in Q316 is bad (now in Q315).

Is it possible to buy a replacement and would I have to replace a pair? [Found some - see below]

https://photos.app.goo.gl/8ruZ5mW6GK3KAdRc7

(by the way I called my local tech and he wasn't interested! But he did suggest that the amp didn't need earthing as it was (most likely) double insulated)

Found some direct replacements on ebay. I'm going to buy two in case the other goes bad:
https://www.ebay.co.uk/itm/2SC2632-...567398?hash=item4b12990ce6:g:kBgAAOxy-W9SPKny
 
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Suggest avoid ebay and so called "new old stock", go with a modern day equivalent from a reliable source. I'm hopeless
on transistor matching thinking maybe KSC3503 as a replacement for 2SC2632, others will chime in with better subs.
 
I’d do a pair... Q314/Q316 (A1124/C2632) if unobtanium replace as set w/ new equivalents- I’m sure there are ppl in the know here on best choices
 
Flapper said:
Blimey. Just opened up the mains plug (which I guess I haven't done since buying the amp a few years ago) and there's no earth connection! Surely that's super dangerous and time for it to go to the techie? Live and Neutral are both wired correctly.

Emitter of Q316 and ground is -65V

R348 shows nothing on diode setting. It shows 47.5 on the 200k OHMS setting.

Reluctant to turn it back on now with no earthing.
Click to expand...

Was thinking of plug from drive board to main board (J5 J6)
Glad the fault has been found and the V8 is good
 
Thanks all. I've ordered some transistors from a couple of different places so hopefully one at least will be good.

I mostly use the amp in straight DC mode bypassing the pre-amp so I'm not interested in recapping that section, but would there be any benefit in recapping the electrolytics caps on the power amp board? I think there's only seven including the 4 large ones in the middle of the board (filter caps?) I wouldn't try this until the other issue is definitely resolved, and not at all if it's not worth it.
 
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