Should I buy: Mcintosh 754 with one bad channel

[tdwpgtp]

Alright, all of the components have been ordered. The bigger, incorrect transistors will be replaced with the original models, as will some of the crushed transistors. The mps4126 transistors, one of which was burnt and some of which were crushed, could not be sourced. They are being replaced with N4126 transistors that are as close to the original specs as I could find, and I am told they should work just fine. We will find out after they all come in (from China, so many weeks from now) whether or not these are the only damaged parts, and whether or not it will work. I will post back when the time comes.

Thanks, everyone, so far for your help and input.

 

[Blue Shadow]

I'm concerned with that From China part. We don't order parts from China. A few different sources for regular production parts and some more for the NLA parts until the stockpiles are depleted of known good quality part.

Hope you get the good stuff.

 

[tdwpgtp]

I only ordered from China what I could not find elsewhere. Hopefully they are legitimate..

 

[Blue Shadow]

Just read that UTSource usually has some decent parts. And you didn't say you got them from the bay.

 

[tdwpgtp]

Alright, so I replaced all of the obviously damaged components with their correct replacements, assuming the replacements are legitimate, and the power guard light for the right channel lit up, with no sound from that channel. The left channel works fine, and just for shits and giggles, I swapped driver boards to make sure the right channel didn't have issues elsewhere. The power guard action followed the driver board to the left channel.

Anyway, if anyone has any clue what to attempt next, input is appreciated. I may try to replace all of the transistors again, in case one was DOA.

 

[Blue Shadow]

follow a signal through the amp, left then right to find where the signal gets lost or broken on the right channel.

 

[tdwpgtp]

So I have not yet put a signal through, BUT after some further inspection, I have discovered that along with the transistors mentioned before, a previous owner has also changed the output transistors (the big silver guys on the heat sinks) on the bad channel. The replacement parts say "MJ15003G BM1611" and "MJ15004G BM1645" and the originals say "132188 GE/RCA C8823" and "132189 GE/RCA C8819". I have not yet had a chance to investigate any of these parts' specs yet, but will do so tomorrow. I doubt the replacements were properly sourced, like the BJTs

Edit: on second thought, the right channel will work with the left channel's driver board, so those output transistors aren't the culprit, or at least not entirely...

The search continues

 

[tdwpgtp]

An update for this project at long last. I posted the following reply in a thread I had started dedicated to the repair in a different sub-forum, but I figured it would be nice to update this thread, as well.

Okay, so almost a year after I actually bought this thing, I think we have finally found all of the bad components. Transistor q16 melted a leg, as seen in the first picture in this post, several transistors in the power guard circuit were physically broken, damaged, and/or leaky, resistor R34 was open, and the transistor on the heat sink (q19) that controlled the output transistors had a crack in it. With all of these components temporarily replaced, power guard no longer trips, however I have done no further testing. I will be ordering new components for each of these, adjusting bias where needed, and reporting back from there, once done. I, for the first time, am optimistic.

The first set of n4126 and c2240 replacement transistors that I had ordered from eBay turned out to be leaky and likely counterfeit, as Blue shadow had warned me here. I will be replacing them with suitable replacements, as genuine direct replacements seem not to exist. Only genuine, original transistors will be used in the amplifier circuit, and the replacements will be used only in the power guard circuit.

As for the output transistors mentioned in the above reply, I am still not certain if those are newer Mac replacements or not. I emailed Mac to see if they had replacements available, which they do, but they did not give me manufacturer part numbers. Upon testing this amplifier out, if I notice ANY difference in the channel with the replacement output transistors, I will contact Mac to buy their newer replacements, as they are only $20 plus shipping, and good peace of mind.