Amp repair failure

[Frisbee78]

Hi everyone!

I am trying to repair a friend’s amp but with no success so far.
I have replaced all the components that had blown but when I plugged it in they blew again. I think I’ll leave it as that unless you have an idea?

The pics below show the damages when I first got the amp:
2592 x 1944 - 1600 x 1200 - 1024 x 768 - 640 x 480

1600 x 1200 - 1024 x 768 - 640 x 480

Below, we can see the old and the new components:
1600 x 1200 - 1024 x 768 - 640 x 480

Below are the pics of the damage after I had replaced the component:
2592 x 1944 - 1600 x 1200 - 1024 x 768 - 640 x 480

2592 x 1944 - 1600 x 1200 - 1024 x 768 - 640 x 480

2592 x 1944 - 1600 x 1200 - 1024 x 768 - 640 x 480

Note that it blew again as I plugged it to the mains. As it were, the amp was then supposedly in ‘Standby’ mode.

[subwo1]

Hello, I cannot see much resolution in the pictures, but would suggest checking all semiconductor junctions in the circuit with an ohm meter. You may not have gotten them all, especially the original culprit, and that could easily cause a repeat of the original failure. Also, look for all low value "fuse" resistors in the circuit and check them.

[Frisbee78]

Yes, I probably missed the original culprit, probably causing the thing to go up in flames again

I checked all the resistors of that channel and all seemed OK, including the low value 0E22 wire resistors (even though they browned a little as we can see on this picture).

[Frisbee78]

Originally posted by subwo1
Hello, I cannot see much resolution in the pictures

Did you try to click on the links on the right? (2592 x 1944, 1600 x 1200, etc...)

[subwo1]

If their values are still 22ohms as appears to be printed on them, they should be fine. Also, place a "short light" (I usually used a 60w one myself), between the mains and the amplifier power input when you power it up again.

[subwo1]

Oh, sorry, those alternate resolution links escaped my attention.

[Frisbee78]

I just realised that the NPN driver transistor (2SD667A) was blown from the start and I didn't realise until now (always good to take pics as you go along, as you can look back on previous states). So I guess that the burn marks on the NPN power transistor (2SC3855) mean it's now dead too.

If their values are still 22ohms as appears to be printed on them

I thought that R22 means 0.22 ohms? If that's the case, then I can only assume they are OK as the resolution of my ohmeter is 1 ohm. But if they were dead, I think they would have an infinite resistivity... Am I wrong?

Also, what does connecting a lamp do? Does the 60W value or the V rating matter? (we are in 230V in UK).

Thanks for your help

[subwo1]

If they measure less than 1 ohm, they seem fine as fuse resistors usually go all the way when they do go. I was out of the repair field by the time the European designation method became more widespread so am not experienced with it. The 60 watt bulb rating is independent of using 110 or 220 mains.

[dmfraser]

You plugged in a "repaired" amp without a light bulb in series! I usually use a 150W lamp but then I build much bigger amplifiers. The light bulb in series trick for testing repaired amps has saved me thousands of dollars in parts because you can never be sure to have caught all the bad parts when you do a repair.

[Centauri]

To clarify the 'light bulb' thing - the light bulb is wired in series with the mains to the amplifier under test. If there is still some sort of problem in the amp, the bulb will light up and limit the current flowing, thereby hopefully saving more expensive failures while the problem is sorted out. If all is ok, bulb will briefly flash then just dull glow.

I usually power up repaired amps using dual tracking current limit power supply.

Cheers