Sansui Eight with Way too much Gain on left channel of F-1235

[hirscwi]

I'm in the process of rebuilding a Sansui Eight. The right channel driver board was very fried. I've been able to get that working, with much help from the threads here in AK.

But now I've discovered a problem with the preamp board - F1235. I'm getting a sever gain imbalance between the channels. The left channel is delivering about 30 times more gain than the right channel. For example, at about 1/10th rotation from the "off" position, I'm getting .15 VAC on the right channel and about 4.5 VAC on the left channel. This is with a single sine wave signal of perhaps 1 V fed to the receiver. It appears to me that the left channel is the problem, but I suppose the right channel could be too weak. My best guess at this point is perhaps the negative feedback loop on the left channel is not functioning. However I've hit my limit of being able to determine how the circuit is supposed to function. Before I start to pull and test every component on F-1235 I'd appreciate any suggestions.

The schematic for one channel of the F-1235 board is attached.

Many thanks.

 

[Leestereo]

The symptoms described do point to a negative feedback issue; if capacitor C7007 has "dried out", the feedback loop from TR7003 to TR7001 would be "open".

 

[hirscwi]

Thanks, Ben. I think I already replaced C7007 but I'll test it. I'll check to see if it's reversed?

 

[ivandezande]

Check for accidental solder bridges, caps installed backwards, etc.

 

[stereofun]

You said you had 4.5 VAC on the left and 0.15 VAC on the right, resulting from a 1V input - is that at the base of input Transistor TR7002 ? - It could certainly also be the right channel that's too low, depending on where in the chain the readings are from.

Also, how does your DC readings compare left to right ? Looks like a 20VDC rail. Beginning with 0.75VDC at the base of T7002 and ending with a 12.3VDC base at TR7010, before the signal couples to the driver boards. I once replaced all the signals in an Eight - those 693's were noisier than any 458's I've come across - I don't trust them

 

[hirscwi]

Well, I think I have it fixed but am not entirely sure what did it. I re-flowed some old solder joints that looked a bit dicey. However they were likely the originals so that probably wasn't the problem. There seemed to be a short in the low filter switch (between 7ML and 7NL) that I was able to clear up with contact cleaner. I don't know why that would have caused the problem, however. It's working beautifully now but I do worry that it might revert to it's old problem.

I'm not sure it could be the 2SC693 transistors but it won't hurt to replace them to be on the safe side. I've got a bunch of KSC1845s and even the pinout is the same, so I think I'll swap them.

I'll check the DC voltages again and report back.

 

[hirscwi]

OK, I've compared the DC voltages to the ones one the schematic. There are only a few significant differences. Unless noted, the left and right channels are the same.

TR7003/7004 - B = 2.0 - should be 2.3 (not sure this is significant)
C = 8.35 - should be 5.7
E = 1.4 - should be 1.7 (not sure this is significant)

TR7005/7006 - C = 11.6 (left) & 12.4 (right) - should be 4.5
E = .83 (left) & .69 (right) - should be .75 - the variance between the two channels seems to be indicative of something

TR7007/7008 - B - same problem as TR7005 because they are directly connected
E = 10.5 (left) & 11.75 (right) - should be 10.5 (left channel is OK)

All other voltages appear to be normal according to the schematic.

I'd appreciate any reactions. I have not tested it on my QA401 yet, but it sounds fine (quite good in fact) so I'm wondering if these variations are within tolerances.

Thanks.

 

[hirscwi]

Fur future reference, I've found two different schematics. The only difference that I see is R7011 and R7012. One version shows 82K ohm and the other shows 120Kohm. My board has 82K. I didn't compare any other boards except the F-1235

 

[Leestereo]

Since the DC voltages now agree between the channels and it works well, I'd say you solved the problem. The KSC1845 is a good replacement for the 2SC693 wrt to Ic, fT, hFE and noise NF spec.

 

[hirscwi]

Ok. Time to declare success and enjoy the music. Thanks for the assistance!