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Sansui Eight help - hum on one channel

Yes, time to go back to the beginning, especially rectifier and then move downstream. It's quite the mystery with both channels giving rail as DC-Ofset and an unresponsive bias - both sides. And prior to that clear sound both channels, then all hell broke loose when you got your 2nd 2sc1079. :crazy:
 
Thanks for the help and encouragement. I think I should get some micro-grabbers before I do any more testing. Will see what we have at work, otherwise I will need to order some.
 
Found some mini-grabbers and so was able to make some measurements, all on DBT.

PS voltages to driver boards. Schematic voltage is in parenthesis, then L and R channel readings. Note all of the voltage signs are opposite of what the Schematic has.

B3 (46.5V), L: -44.8V, R: -45.4V
B4 (-46.5V), L: 45.9V, R: 46.5V
B8 (?), L: -39.1V, R: -39.3V
B9 (-41V), L: 39.8V, R: 39.8V

Driver board transistor measurements, schematic values in parenthesis, order is ECB, all values in Volts. Channels listed Left then Right. Clear outliers in bold.

TR801 Left: (?) -0.138, (-41) 46.3, (0.14) 0 Right: (?) 0.52, (-41) 46.6, (0.14) -0.007
TR802 Left: (?) -0.17. (-41), 46.5, (0.56) -7.519 Right: (?) -0.55, (-41) 46.6, (0.56) -7.673
TR803 Left: (?) 46.4, (0) 0, (?) 46.5 Right: (?) 46.6, (0) 0, (?) 46.6
TR804 Left: (?) 46.4, (-1.32) -39.6, (?) 46.3 Right: (?) 46.6, (-1.32) -39.9, (?) 46.7

So, if we discount many of the voltage signs being opposite, clearly there is an issue with base of TR802 and collector of TR804.
 
We need to sort out the B3 & B4 voltage anomaly - they are exactly the opposite of what they should be. :yikes: The voltages you have recorded suggest that you have a wiring error (or connector plugged on the wrong way round?). I have never seen the inside of an 'Eight' so I don't know if this could be possible - but that's what it looks like.

Thoughts that occur are bridge rectifier D009 (5B2) replaced and put in back to front? - which would account for why you also have B8 & B9 opposite voltages to what they should be. But I would expect C23 & C24 to complain LOUDLY if you had done this, so....The wiring to the main capacitors C23 & C24 - wiring also reversed perhaps?

OR - more likely (now I have thought about it a bit more :))
What might have confused you is the polarities of C012 (1000µF, 6.3V) and C017 (1000µF, 6.3V) at 'OJ' & 'ON' on F-1201 - which may have prompted a wiring mistake here? - as these capacitors are showing opposite polarities to what you might expect at these connections - but are correct.

Remember, I can't see what you have done, whatever you do don't swap it round without thoroughly checking first. Hopefully you took record pictures? which you can refer to, to unscramble this confusion.
 
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Thanks for you thoughts, John. A couple of things to note.

1. I have not changed any wiring. I have only replaced resistors and the OTs.
2. The Eight was working before I put in the second set of 2SC1079s, with one being shorted.

So, at this point I am not about to change any wiring. That just doesn't make sense to me. What does make sense is that something must have failed that burned up R837 on the L channel after putting in the two 2SC1079 on that channel. So, by my way of thinking either the shorted 2SC1079 caused the problem, or something on the PS board failed that is now causing identical issues on both channels.

I actually did not do any testing on the PS board, but rather tested all voltages on the driver boards at resistors connected to the entry points for those PS lines. I could check the voltages at the source points on the PS board. I assume they should be the same??? I guess the other question is, is the schematic correct for the sign of those voltages? When everything was working, and I did the SM procedure for setting bias, etc., I don't recall any readings being the wrong sign. Of course, balance adjustment has you set VR801 to zero, and bias adjustment is a current reading, so it's only setting TR802 that should have a positive voltage.
 
If you haven't disconnected and reconnected any wiring - then I must be misreading the information you have provided, but.....
KeithD said:
B8 (?), L: -39.1V, R: -39.3V
B9 (-41V), L: 39.8V, R: 39.8V
Click to expand...
It seems like there are some typo's here, B8 should definitely be a positive voltage and yet you record it as negative?
And B9 you note should be negative - but you record it as positive?

Similarly with B3 & B4, a minus sign means the voltage is negative with respect to chassis, no polarity sign implies a positive voltage, with respect to chassis ground.

Can you clarify this?
 
Well this is embarrassing :oops:, but good news! :thumbsup: I went to check the measurements again this morning and discovered I had the probes backwards (red to com and black to positive!) :crazy: Put them in correctly with com connected to chassis, red to test voltages and polarities are opposite of what I wrote above - so all negatives are actually positive and all voltages with no sign are actually negative. So, here are the corrected measurements:

PS voltages measured on driver boards (added + to be clear what is positive):

B3 (+46.5V), L: +44.8V, R: +45.4V
B4 (-46.5V), L: -45.9V, R: -46.5V
B8 (?), L: +39.1V, R: +39.3V
B9 (-41V), L: -39.8V, R: -39.8V

Driver board transistor measurements, did not re-measure these, but measured values should be opposite sign, so here are my previous measurements with flipped signs. Order is ECB, all values in Volts. Clear outliers in bold.

TR801 Left: (?) +0.138, (-41) -46.3, (+0.14) 0 Right: (?) +0.52, (-41) -46.6, (+0.14) +0.007
TR802 Left: (?) +0.17. (-41), -46.5, (+0.56) +7.519 Right: (?) +0.55, (-41) -46.6, (+0.56) +7.673
TR803 Left: (?) -46.4, (0) 0, (?) -46.5 Right: (?) -46.6, (0) 0, (?) -46.6
TR804 Left: (?) -46.4, (-1.32) +39.6, (?) -46.3 Right: (?) -46.6, (-1.32) +39.9, (?) -46.7

Can check these again tonight, sorry for that confusion, but thanks for questioning my readings John!
 
OK, looking much better ;)

Concentrating on the Left channel only.
TR803 and TR804 - could do with being removed and tested - do you have a multimeter (or a transistor checker?) with a 'diode test' range? - and if no transistor tester, do you know how to perform a '6-way test' on a transistor using your multimeter?

Don't worry too much about the voltage on TR802 base(s) this is a symptom of the DC offset on the output.
 
I'm getting this meter that Tom suggested. Should be here tomorrow and I can test those transistors then. I've seen videos on the 6-way test, but have not done one for a couple of years, so I'd have to refresh my memory.
 
It makes life so much easier, so not only can you see if it's outright shorted but also if its misbehaving by say abnormal hfe or switch on voltage. Also one of the most common failures is when a perfectly working replacement is pinned out wrong, which just compounds the diagnostic. I think this meter will also do ESR , uF and leakage on caps.

The right tools are half the job. Glad you got those tiny grappers too. You will get this sorted out and we might all learn along the way.
 
Meter arrived, so was able to pull TR803 and 804 for testing. Results:

TR803: hfe=95, Vf=623mV

TR804: hfe=96, Vf=614mV
 
Hyperion said:
TR852 next, then TR807 & TR808. :)
Click to expand...
OK, just to be clear, SM says TR852 is on the R channel. As you said in post 50, we are working on the Left channel. Just want to make sure you didn't mean TR851, which is the same transistor, but on the L channel.
 
Tested TR851 and 852. Pulled both driver boards so that I did not have to de-solder the transistors.

TR851: hfe=202, Vf=726mV

TR852: hfe=156, Vf=745mV
 
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