Amplifier repair: I think I found it! (need advice)

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#21
Ok, I put in an order for a new set of electrolytics, as well as a pair of trim pots, and a few polypropylenes. We'll see what happens when I replace the electrolytics and trim pots. The polypropylenes are there just in case I need them.

Thanks for all the help. I'll post back here after I do the repairs.
:wave:
 
#22
Curious, though, the DC-offset of the amp measured 2mv in one channel and 60mv in the other before I swapped all the devices, immediately after the rectifier bridge replacement. After all of the transistors were swapped, I got the same readings. I would presume that DC-offset is caused by mismatching of transistors, but it's a real head-scratcher why I would get the exact same results with a whole new transistor set. I'm not all that worried about it since its a tolerable value, but it still bothers me how this could be so. Coincidence? Any ideas?
Click to expand...

if i were you i wouldn't be too cencerned with offsets less than +/-100mv.....
 
#23
Hi,
I know that offsets <100mv are considered "safe," and I think that if it hadn't been as high as 60, I wouldn't be so concerned. Anyhow, I have some new information about this little puzzle.

I found some appropriate sized electrolytics in the spare parts drawer. I soldered them in, and no change in offset voltage.

On a whim, I desoldered the potted transistors and dug them out just to see what they are. Here's what I got:

1 pair each of 2SA1016 and 2SC2362 per channel. I think they were potted to thermally couple them. Anyways, to make a long story short, I resoldered the transistors back into the unit after I dug them out of the potting compound. The DC offset is now 45mv. Does this make any sense? Does it help localize the problem?
 
#24
I think they were potted to thermally couple them.
Click to expand...
Probably. Leave it as it was.
Now that you excluded the caps try this: take your soldering iron and keep it close to the diff-pair while looking at the offset. If the value changes a lot, your diff-pair is not properly matched.
Be careful not to touch anything.
Try switching the transistors from the other channel and look what happens.
Give the devices enough time to settle after you replaced or switched them. Half an hour at least. Then measure the offset.

/Hugo 🙂
 
#25
Ok, finally got around to replacing the differential pairs, as well as replacing the trim pot for setting the bias. I let the transistors settle in for a couple hours. DC offset is now stable at less than 10mv! Thanks so much to everyone who helped with troubleshooting this amp and a special thanks to Netlist for all of his patient help.

🙂 :drink: :drink: 🙂
 
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