I think you need to review what's actually happening and what the voltages are 
Make sure the black lead from the meter is firmly clamped to ground (chassis) and then re-confirm the output offset is still high at around 47 volts. Then check the voltage on the base of TR01 and the voltage on the ends of those diodes, all while leaving the black meter lead in the same place.
Make sure the black lead from the meter is firmly clamped to ground (chassis) and then re-confirm the output offset is still high at around 47 volts. Then check the voltage on the base of TR01 and the voltage on the ends of those diodes, all while leaving the black meter lead in the same place.
You know. it is certainly possible your problem is not a bad part. A broken connection or cracked circuit board trace can just as easily cause this symptom. Avoid the temptation to throw parts at the circuit.
The amp is one big feedback loop. So changes most anywhere will affect elsewhere.
Whatever voltage is sitting on the bias trimmer is more or less what is going to be on the output. Everything from TR09 to the right is essentially a current amplifier for the speakers, not voltage amplifier. If TR05 does not turn on to drag it down towards V-, then it will rise up close to V+. That lack of turn-on at the base of TR05 looks suspicious to me, is TR01 working.
I doubt your trim pot is the issue here. The reason it lights your bulb at some settings is when the amp is over-biased, then it draws a lot of current. WHich lights the bulb. The bias setting is not how to change the DC offset.
The amp is one big feedback loop. So changes most anywhere will affect elsewhere.
Whatever voltage is sitting on the bias trimmer is more or less what is going to be on the output. Everything from TR09 to the right is essentially a current amplifier for the speakers, not voltage amplifier. If TR05 does not turn on to drag it down towards V-, then it will rise up close to V+. That lack of turn-on at the base of TR05 looks suspicious to me, is TR01 working.
I doubt your trim pot is the issue here. The reason it lights your bulb at some settings is when the amp is over-biased, then it draws a lot of current. WHich lights the bulb. The bias setting is not how to change the DC offset.
I took the driver board out to check it and found one of the earthing wires had come loose so I resoldered it and found that I was back to square one where the DBT would light up as soon as I turned the amp on. With a bit of experimenting I found that if I unplugged the NPN outputs (pins 14,15.16) on the left channel,the DBT didnt come on.Then the new 1K ohm trimmer came in the mail today so I installed it. Now the amp starts up normally on the DBT with everything plugged in. I still have 48v,was 47v on the right channel output resistors. Here are the voltages measured with the earth clamped to the chassis. Could the problem come from the driver power board? Im asking because I didnt tighten the bracket to hold the regulator board in properly and burnt the earth track when it started shorting? Although Ive fixed it and checked it over,I may have missed something?
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Tr01 should be working now? I just have to find this broken track or whatever it is,Ive done continuity checks I dont know how many times and looked for cracks and breaks with a magnifying glass and jewellers loup. Now I cant check voltages because the DBT lights up at switch on.Ive been plaqued with broken wires from constantly taking boards in and out testing them.
Flying spaghetti meatball monsters! I gave a bit of a tug on all the grounding wires and one fell off so I resoldered it and shorted TR09. Not only did it not light the DBT. The green light clicked on! Then I snipped the TR09 C to E link and it came on again when switched on. Is there anything I should check before doing the DC offset and bias Mooly? Ive learnt a lot about this driver board from you blokes that I wouldnt have got from anywhere else so thanks fellas especialy you Mooly for hanging in there for me,its really,really appreciated. Oh Oh its late again goodnight fellas!
Ive learnt a lot about this driver board from you blokes that I wouldnt have got from anywhere else so thanks fellas especialy you Mooly for hanging in there for me,its really,really appreciated. Oh Oh its late again goodnight fellas!
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I set the right DC offset no problem but the left side wasnt very responsive,if I let it sit for a minute or two the DC would go up and down 10 to 20 mv by itself? I got it down to an unstable -12mv to +40mv when it started rising quickly untill it got to 450mv and went into protection with the DBT glowing bright? If I unplug the NPN outputs from the left driver board it doesnt go into protection.Ive checked everything,reflowed the solder joints,etc. I took some voltage readings with the left channel NPN outputs unplugged from pins 14,15,16. on the driver board. Ive tested all the outputs again and have only recently taken the output module apart and checked it.
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It sounds like you have something other than an offset problem going on there. No offset error should light the bulb, even if the output is sat at one or other rail voltage.
Two things come to mind.
1: Thermal runaway in the output stage. You can prove that easily by simply shorting the vbe multiplier TR09 out (as we did before).
2: The amplifier may be unstable. This is more likely if you have used alternative transistors to the originals. You really need a scope to check that though.
Two things come to mind.
1: Thermal runaway in the output stage. You can prove that easily by simply shorting the vbe multiplier TR09 out (as we did before).
2: The amplifier may be unstable. This is more likely if you have used alternative transistors to the originals. You really need a scope to check that though.
I think youre talking about the other F2582 P.S. and F-2583 driver board combination that Sansui used on this model? I have the F-2567 P.S. and F-2580 driver board combination. Ive set the DC offset and bias by the manual for years.so thats right.I dont know about the other combination though?
The DBT lights up right away on power up,could it still be thermal runaway because it hasnt had time to heat up? I put a link across TR09 C to E but the DBT lights up with both output connections unplugged? I have all origonal transistors from 2SA847s up to the complementary power drive,where I have MJE15030/31s.(TR15,19,17,21.) Then MJ21193/94s on the outputs. Ive found that I can plug in the left NPN connector if I unplug the positive 55v power supply from the big cap P.S.the DBT doesnt light up so its got to be in there somewhere with the NPN side of the drive? Ive spent today re-doing the wiring but I think Ive narrowed it down to that area between the red lines,I hope? It seems strange that everything reads OK on the DMM and the continuity. Theres a little bit of difference between forward voltage readings when comparing the transistors on each channel but thats about it? Ill get this amp going if it kills me Mooly. lol
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If you have TR09 shorted then there is no chance of thermal runaway.
Quick ways to pin the area of the fault down are to remove drivers TR19 and TR21. The bulb should be off in that condition and the output voltage at the emitters of the output transistors should be zero.
Try that first because the result determines what we do next.
Quick ways to pin the area of the fault down are to remove drivers TR19 and TR21. The bulb should be off in that condition and the output voltage at the emitters of the output transistors should be zero.
Try that first because the result determines what we do next.
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