A Crest Pro 8200 Amplifier

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I'm not sure if I like this amp very much. It is fairly well made and good quality, but some aspects of the design are not good.

The first thing that made me think badly of it was a visit to Crest's website to get the service manual. I didn't find it. I did find a schematic for the 9200, which will have to do. I wish people would refuse to buy stuff where there is any obstacle to third-party service and repairs.

This one has strange problems. The fans don't work, and it doesn't switch on fully. The power supply generates good voltages and the amps work fine, but they never get connected to the output terminals. The ACL lamps stay on indefinitely.
The underside of the board looks OK. I don't much like these huge PCBs. With the heavy heatsinks they are difficult to handle safely and any mishandling could wreck it very easily.
The problem with the fans was soon solved, they are cheapo fans and have died. There is little dust inside the heatsinks, so I don't think they can have lasted long.
Someone thought it would be a good idea to put components right in front of the fans. All the smoke and moisture and other crap in the air reacts well with metals as you can see. This is far from being the only amplifier with this problem, there are many other manufacturers equally clueless. I don't think it is deliberate planned obsolescence, you could never be sure it would survive long enough.
The nearby gain switch and input terminals are getting corroded. I got here just in time to do something about it, so maybe it's a good thing it failed early. Although it is an old amp, it has not done much work as far as I can tell.
The tracks to this opto are beginning to lose their green, and corrosion is starting.
In the gap between the two large sinks are a couple of trimmers. Probably the Iq adjust. They are in the blast of crap from the fans and are beginning to get crusty. That could end in an interesting fault. This is probably the most stupid part of the design, but it has close competition. The whole thing is just too complicated, and judging from comments on the interweb, this complication does not provide any benefits in terms of reliability.
View from the top. You can clearly see that there are components in the forced air stream at both ends of the sinks.
Here I have replaced the caps and cleaned up and put some conformal coating over it. This was a big mistake. I checked the tracks and all were OK including the fine tracks to the opto. I forgot to check if the opto was actually working. It is the way the power supply tells the amp protect circuits that all is well, and if the amps are also happy they will get connected to the outide world. But in fact it doesn't work, and that accounts for most of the symptoms.
The coating leaves a bit of a mess after I blast the opto off with the hot air.
I think the corrosion has actually gone through the legholes and wrecked the internal clockwork.
The opto pads.
Here I am part way through blobbing a replacement in. I didn't take the board back out of the case and working through the tiny gap between the sink and the case was interesting. The replacement is a MOC217.
After a bit of a clean up and some more coating it doesn't look too bad. I won't be making that mistake again.
This is the 16V regulator area. The board is marked 16V but I calculate more like 15V should come out of it. And sure enough it measures about 15V. I have replaced the 317 because the output looked strangely noisy and a bit bouncy. Not exactly obvious why there are so many supply voltages all over the place. I did start to try and figure it out, but decided my time would be better spent playing tetris than bothering with this museum of diseased imaginings.
New fans in place, and I have also rammed a lump of foam between the sinks to try and stop more crap getting to the Iq trimmers. I will put some more on the input side, but I can't put any on the opto side because I think it needs some cooling on the rectifier sinks.
Assembled for testing. Seems to be OK. I think we will get a few more years work out of it.