I cringe every time I see a reference to this article. I'll give the guy credit in that he put a lot of effort into it. I don't agree with everything he says. It's the internet and everyone has an opinion.
When it comes to silpads I never reuse them, and I've never had one fail. If you install the output screws with a screw gun and the clutch backed off you will have trouble. Last year I bought about a 100,000 silpads at a surplus shop. I'm set for the long haul.
I am a big supporter of changing/ replacement of everything on the output board...
In your experience, did the technician who checked out the amps do it per the procedure in the manual? You can't check bias and offset and say it's good. Were the outputs a mixed bag of generic replacements and date codes?
As previously stated I do support changing all of the outputs because I've had my own 400 go down while idling after I repaired it and reused the "good" outputs. I don't like changing the transistors and diodes on the board unless they have shown that they have failed. If you start off by replacing everything and still have trouble you don't know if it is an original problem or induced with the replacement parts. If I'm charging to repair an amp I want to be able to show the customer what failed.
I start by removing any shorted outputs and leave the rest in the amp for the time being. You can do this with careful resistance readings before removal of any of them. Then I bring up the amp with the 100W light bulb and no load. The bulb has never failed me. If the amp comes up then I see if it will pass a signal. If it does then I know the board is probably ok. If it doesn't then you troubleshoot from there. Then I do the upgrades.
I've recently began replacing the bias transistor with a (larger) TO220 package just because I've seen the leads break on 2N3403's.
When I'm finished with an amp and I'm sure it's in good shape I take a hair dryer and blow a
little bit of heat over the bias transistor while watching the bias voltage. the bias should drop and return to normal when the heat is removed. This checks the thermal stability of the amp. I also take temps of the case of each output when it's very warm to make sure there is a good thermal transfer to the heatsink.
...If you don't get the relay kits at the very least fuse your speaker with some low amp quick blow fuses, I had one fail and it started to make a terrible sound and then silence, blew both speakers fuses, but didn't do any damage to any of the drivers, just had to replace the fuses.
It's very hard to find a fuse that is fast enough and doesn't produce nuisance blown fuses. If you look at the time/current curves for an AGC fuse you see they take a relatively long time to open. To me 200 milliseconds of 100VDC on a speaker is way too long. The relay boards are a lot faster. Another possible issue is the effect the fuse can have due to it's changing resistance with signal level. And then you have to figure out a way to safely mount the fuses.
Finally, many thanks to all of you who for the kind comments on the relay boards I designed. It's much appreciated.
