Mono amp 33V at speaker terminal!

[QSilver]

I have this little kit amplifier that I blew up some years ago and I've been trying to repair it. So far so good - I got the schematic out and found a lot of damamged or blown components which I have replaced. One of the 0.47R resistors had blown so I have replaced that, along with the outputs, drivers and capacitors and the two diodes - not the zener diode.

I used a DBT and got a bright bulb. Powered off and found a short in T6. I repalced this with a KSC1845 - same as T4 and powered back on. Now the bulb goes dim but I have about 33VDC at the speaker terminals and a lot of heat coming from R13 to the point it heats the surround components up after being on for 10 seconds or more. It doesn't burn up but it is close. Would the most likely cause be T3? I have not replaced that yet...

When the amp blew, it took out T8, T7, R19, melted R16 in half, and T4. I repalced T4 and T5 with KSA992 and KSC1845 to get a matching pair and replaced D1 and D2, D1 had shorted.

I then replaced R13, R14 and R15 and all the electrolytics except for the main filter caps as a precaution.

 

[QSilver]

Can I use a different transistor I might have in my stocks to replace a BC639?

 

[QSilver]

I changed T3 for a KSA1845 which dropped the voltage to -40mV. R13 is now just warm but the bias adjust does nothing. Should I replace the gain matched TR1 and TR2? the circuit seems to be reading correctly now but I cant adjust the bias. Is this because I'm using different transistors now? Will I need to make some adjustments to the resistor values in R13-R15?

 

[sregor]

How about some voltage (like around T6 since now it is a bias issue). Also you might want something heftier for T3 than an KSC1845, though for temporary work may be okay. It looks like you don't have pin out issues. Is T6 mounted on Heat sink?

 

[QSilver]

Yes, T6 is recessed into a hole in the heatsink for the output transistors. I was beginning to think that T3 might need to be uprated - it does getting quite warm. I think I have managed to stabilise the Bias - I was measuring from the wrong point and I have that at a stable 10mV now. I was looking into regulating the DC offset as that is still around 33mV and I was considering replacing R3 with a 10k trimmer to adjust it. Would that seem sensible?

What would be a good choice for an uprated T3?

 

[sregor]

Closest match for T3 (but BCwould be MPSA06 or 2n5550 or 2n5551. Slightly larger KSC2383 or ksc2316 or ksc2310. Older (no longer in production) 2sc1509 2sd667 (and a lot of others but I don't remember all the numbers) . Or go to a TO126 (small flat package) BD139, ksc2696 or ksc3503.
Putting a trimmer in for R3 may adjust it some, but I don't like it (just my opinion FWIW) I would try increasing R9 (If you calculate the currents, it seems to be running unbalanced, and increasing R9 would balance it better - 4.3K or 4.7K should be about right.) 33mV is not a big deal to begin with. GL.

 

[QSilver]

Thanks for your opinion on R3 and R9. I had made a simulation with the amplifier and had found that adjusting R9 was having good results however, before I tried it, I went to look at other amplifier schematics to see how they adjusted DC offset and the one I opened, for a Pioneer SA-7500, adjusted the supply to the base of T1 which is why I had thought of making that adjustable.

I replaced T1 and T2 with a matched pair of KSA992's which dropped the DC offset to 15mV. I then tried what you suggested, increasing R9 to 4.7k which dropped it to 0mV on power up, drifting to -7mV and then it began to creep back towards 0mV again. I was thinking perhaps of adding a multi-turn trimmer in place of R9 instead of R3 to fine tune the offset. But for now I will look into replacing T3 as when I'm adjusting the idle current, its running very hot. Rather than tempt a failure. I'll replace that, and then look further into balancing the amp - it's close enough now but I like to experiment.

I'm thinking of going for the KSC3503 as I use these a lot. I might need to thin the leads down to fit it into the PCB - we'll have to see...