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Sansui Eight help - hum on one channel

Thanks for the clarification on the DBT voltages, that makes sense.

Yes R837 was replaced with 150 ohm, and I did measure the resistance before putting it in. Without pulling a leg, I just measured it, and it is 150.6 ohm.

Here are the TR807 readings: E: +38.8V, C: +39.0V, B: +39.5V
 
KeithD said:
Here are the TR807 readings: E: +38.8V, C: +39.0V, B: +39.5V
Click to expand...
That's a symptom of the problem, or the problem itself, TR807 is turned hard on, which is turning TR853 & TR857 on - hence the huge DC offset.

Try turning VR803 (Bias) trimmer's slider to the R817 end of the track... see if those readings change... (leave the trimmer's slider set there).

If you can - please check the resistance of:-
R828 (680Ω)
R829 (150Ω)
R831 (150Ω)
R832 (680Ω)
 
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Hyperion said:
That's a symptom of the problem, or the problem itself, TR807 is turned hard on, which is turning TR853 & TR857 on - hence the huge DC offset.

Try turning VR803 (Bias) trimmer's slider to the R817 end of the track... see if those readings change...

If you can - please check the resistance of:-
R828 (680Ω)
R829 (150Ω)
R831 (150Ω)
R832 (680Ω)
Click to expand...
I know I got no change in the current reading at the fuses when I tried adjusting that with VR803 following SM procedure (see post #34), but I can try the voltage reading you suggest.
 
Yes, do move the trimmer slider, as requested - and leave it there - that is the position for minimum bias ;)

Those resistance measurements - we're looking for an 'open' or high resistance (out of spec') resistor. ;) - lift one end of each and measure that way - to be sure. (re-solder in place if ok).
 
OK, moved VR803 to R817 side, which is all the way counter clockwise. No change in voltages on TR807.

Here are the resistance measurements without lifting the resistors:

R828 (680Ω): 402 ohm
R829 (150Ω): 137 ohm
R831 (150Ω): 137 ohm
R832 (680Ω): 402 ohm

Will take a little time to test each of those out of circuit. None high in circuit.
 
Resistor values with one leg lifted:

R828 (680Ω): 682 ohm
R829 (150Ω): 151 ohm
R831 (150Ω): 151 ohm
R832 (680Ω): 686 ohm
 
Hyperion said:
Both sides?

Also please give,
Resistance value for R835 & R851 (emitter resistors).

Base Emitter & Collector voltage for TR808 and TR856.
Click to expand...
Yes, both sides are -39.6V when I measured just now.

TR808: E:+37.6, C: -39.6, B:+39.6

Since TR856 is on the R side, I assume you meant TR855, which is on the L.

TR855: E:+37.3, C: -39.2, B: +37.4

Will get the R values a little later when I have time to solder.
 
KeithD said:
Since TR856 is on the R side, I assume you meant TR855, which is on the L.
Click to expand...
Yep - sorry I did it again.
KeithD said:
TR808: E:+37.6, C: -39.6, B:+39.6
Click to expand...
Well, from the looks of that TR808 is faulty (2SA537A), do you have a replacement?

Something like this:-
https://www.mouser.co.uk/ProductDetail/ON-Semiconductor-Fairchild/KSA916YTA?qs=/ha2pyFaduhJa4fTi8HteUt1SNWLw3QaAzOMeavfEcw=

You could steal the one off the other driver board (as a temporary measure) - IF you know it is good. I just wonder why your transistor tester didn't find it? - assuming I am correct of course.
 
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Hyperion said:
Yep - sorry I did it again.
Well, from the looks of that TR808 is faulty (2SA537A), do you have a replacement?

Something like this:-
https://www.mouser.co.uk/ProductDetail/ON-Semiconductor-Fairchild/KSA916YTA?qs=/ha2pyFaduhJa4fTi8HteUt1SNWLw3QaAzOMeavfEcw=

You could steal the one off the other driver board (as a temporary measure) - IF you know it is good. I just wonder why your transistor tester didn't find it? - assuming I am correct of course.
Click to expand...
I don't have that transistor on hand. This is what I have for KSA transistors:

KSA940TU
KSA992FBU
KSA1013YTA
KSA1220AYS

I agree, TR808 did test good. Is there any other explanation for the voltages on that transistor? I keep coming back to the fact that both channels are measuring the same very high voltage at the speaker outputs with the fuses in. Doesn't that point to something being wrong that affects both channels? I know I have not tested any of the transistors on the R channel at this point.
 
Checked pin outs on KSA1220: ECB.

Replaced TR808 with that. On DBT measured voltage at 8H. Started out around 25V, but then moved up to 37V

Checked R835 and 851 in circuit: R835: 10.8 ohm, R851: 0.36 ohm.

Looks like TR808 was not the problem? I did test the 2SA537A after pulling it out: hfe=85, Vf=633. Tested the new KSA1220 before putting it in: hfe=207, Vf=612.
 
KeithD said:
Replaced TR808 with that. On DBT measured voltage at 8H. Started out around 25V, but then moved up to 37V
Click to expand...
That's a nuisance. :(

However
KeithD said:
Checked R835 and 851 in circuit: R835: 10.8 ohm, R851: 0.36 ohm.
Click to expand...
This isn't right - and needs replacing, R835 should be the same as R851. ;)

Correction - I see we have 2 R835's - One on the base of TR855, which should be 10Ω, and the other on the emitter of TR859 which should be the same as R851, - can you check the 'other' R835? (- which I think should be marked R853).
 
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KeithD said:
I'm getting 0.44 ohm for R853.
Click to expand...
Obviously OK.

OK, so none of the semiconductors test bad, (little need to test TR805 & TR806 as these are for current limiting protection). So we are down to resistors, and capacitors, plus the possibility of dry joints or cracked traces, or a driver board connector fault perhaps.

Something just occurred to me, what is the voltage at TR804 collector? - this last record of that voltage confuses me.
KeithD said:
TR804 Left: (?) 46.4, (-1.32) -39.6, (?) 46.3 Right: (?) 46.6, (-1.32) -39.9, (?) 46.7
Click to expand...
So, what is the voltage? is it -39.6?

If it is, check the resistance of R820 (4.7Ω) and tell me if it is OK. - if it isn't Ok - replace it and see what happens. :)
 
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