[Amps] Solid state amp advice needed

[Steve Thompson]

On Monday 25 November 2002 21:54, Jeffrey Madore wrote:
> Steve,
>
> You bring out a couple of good points, so let me pick your brain if you
> will:
>
> In your experience, where does a supply decoupling fault usually show up in
> a low Z solid state final. Do turn to turn shorts occur in the L, or are
> bypass C's more often likely to fail?

It's usually capacitor 'failure'. Unless the set is so old that electrolytics 
might have dried out, the most likely cause in my experience is a broken chip 
(probably mechanical cause) or dud solder joint. 
>
> Also, I have little experience working with chip caps, and only found bad
> ones by probing at it while looking through a magnifying glass. I knew that
> something was wrong and knew that it was highly likely the cap, but I
> didn't know what a bad one looked like. When it flaked apart it was pretty
> apparent that it was bad. But I imagine that one could damage a good cap
> that way too. So could you explain in a little more detail what you look
> for as you reflow the joints?

The common faults I see are either a chip that's cracked in two or the end  
metallisation parting company with the body. Both can be caused by mechanical 
stress from bending the pcb, or dropping it, or from thermal stress - 
excessive heating and cooling. In either case, if you reflow the solder joint 
you will usually see something move - half the chip cap coming off on your 
iron tip is fairly obvious :-)  There was a period when it was normal for 
chip components to be gleud to the pcb before soldering, and this can hide a 
cracked component. Where the metallisation piece from the end has detached, 
it will often come away on the tip as well, but it can get lost in the solder 
blob. Either way, you can usually see the dry joint that's left behind.

I'd also endorse your original comments - good lighting and a 8x magnifier 
are quicker and easier as a first step.

Steve