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Coconuts 500 22nd February 2012 06:53 PM

Powered up amplifier without one of the psu rails connected - now distorted channel
Hi! I have a simple question:

I (acidentally) used an amplifier without one of the power supply rails connected and now one channel is not working right. Is this a "usual" problem? Will doing what I did typically destroy the amplifier?

More in-depth:

I was cleaning up a very dirty vintage amplifier with a typical power supply. The amplifier had a fuse for each rail. I polished up these fuses, and destroyed one of them in the process without noticing.

After I was done, I tried out the amp and one channel was heavily distorted, practically nothing but noise coming out of it. DC offset was extremely unreliable, varying from around 4 mV to a few volts! Moving the volume control was causing the fluctuations. I I measured the voltages on the power supply - 40 VDC on one rail, and 0 on the other. Checked the fuses and noticed the one I had destroyed. Replaced it and now the voltages are normal. 40 VDC on both sides.

Now that channel is not as distorted. It works but there is still a lot of noise. Like someone is blowing into a microphone - that kind of noise. The DC offset problem persists, especially when using the volume control.

The amp was fine before I touched it. DC offset was about 10 mV on both channels.

There is no service manual to be had, this is a very uncommon amplifier and nobody knows who designed it. It appears to be quite simple, however. It uses STK 0050 modules, but only six pins (out of ten) are connected to the pcb.

Sorry for the long post.

Coconuts 500 22nd February 2012 07:39 PM

Addition: Here is the pinout for STK0050

Pin 1: Input pos(+)
Pin 2: Vcc neg(-)
Pin 3: Output
Pin 4: NC
Pin 5: NC
Pin 6: NC
Pin 7: NC
Pin 8: Output
Pin 9: Vcc pos(+)
Pin 10: Input neg(-)

Could this mean that both STKs are damaged?

EDIT: I measured Pin 3 and Pin 8 towards ground, on both STKs. I got 1550 ohms on them all.

merlin2069er 22nd February 2012 07:45 PM

those ic's aren't too expensive, try replacing them.

Sanyo STK0050 Audio IC power amplifer | eBay

merlin2069er 22nd February 2012 07:55 PM

oh, you should try swapping your speaker outputs around, you might have damaged a speaker.

Coconuts 500 22nd February 2012 09:37 PM

I tried replacing the differential pair on the defective channel. The originals were poorly matched (400 and 470 hFE). Now it got better, but the noise was still there... until I fiddled with the selector switch a bit. Now it works. Damn, I should have tested that more thoroughly right away. I always panic when I get problems... and forget the obvious stuff. Sorry about making this thread.

gmphadte 23rd February 2012 03:34 AM

That is also an experience to learn from, for others. No need to be sorry.

Gajanan Phadte

riotpack 23rd February 2012 07:05 AM

EDIT : Basically said the same thing as poster above...

Coconuts 500 23rd February 2012 03:06 PM

Thanks guys.

I cleaned the selector switch very thoroughly and now everything is fine.

Just one thing.

When I turn it off, there is massive dc offset on both channels with no load connected. With speakers I get some crackle in them, and then that's it. The amp has no protection circuit. This I suppose should be normal, but I still haven't encountered an amp this bad before (usually there will just be a short snap when turning off). Also, the bad dc offset remains when no load is connected. I plugged it in again today (unused since last night) and it still had about 200mV on both speaker outputs.

Bigun 23rd February 2012 03:58 PM

'noises' at turn off are pretty common - they usually measure as large dc offset on the output but generally they are of no concern unless you find it irritating. You should probably check it with a load connected (a resistor rather than a valuable speaker) as it's all too easy to get large voltages when no current is drawn whereas with a load you may see a much smaller offset which may put your concerns to rest.

A steady dc offset of 200mV is a bit high (20mV would be good) and it should be something you could adjust out - would have to see the schematic to advise the best way. But I don't think it's high enough to damage a speaker or have much impact on the sound.

p.s. tracing out the schematic by looking at the wiring and the pcb could be a good learning opportunity.

Coconuts 500 25th February 2012 02:10 PM

Sorry, I was a bit unclear. I meant that the dc offset is measured at 200mA after turn off, when no speakers have been connected.

During normal use, dc-offset is around 1-10mA. Quite good.

With speakers connected, the juice runs out right away and there is no dc offset after turn off. What I was curious about is what could be causing the high dc offset to remain "forever" if there were no speakers connected. I am going to replace all the capacitors in this amp, as I always do with vintage equipment. Perhaps that will change things. Anyway, this is a non-issue, really. I'm just thinking out loud, sorry. :)

The tracing idea is an interesting one, and it didn't cross my mind before. That sounds quite interesting (and certainly educational)! Thanks!

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