Amateur electronics questions, but almost on-topic

[Tony Duell]

> 
> I have a couple of questions that I haven't been able to find in my books.
> 
> (1) there has been much discussion in this list about capacitors failing. I 
> recently had a cap on a QIC-24 drive fail with a bit of sound and fury (axial 
> tantalum). Is there any way to test for this other than desoldering every cap 

Yes, tantalum caps are good at that. The good news is that there's 
usually no other damage.

> (at least every electrolytic) and testing it on a cap meter? I don't want to 
> fry my IMSAI, but there are a lot of caps in there and it would take forever...

Cehck for dead shorts ebtween each power rail (at the output of each 
3-terminal regulator on each card) and groud. These caps fail shorted, 
which is why they generally explode.

I would be more wary of the big aluminium electrolytics in the PSU. If 
one of those goes up you'll know it. I would reform them (bench PSU and 
series resistor, there must be a FAQ on this somewhere) before powering 
up the machine.

> 
> (2) (flame risk) My Tek 555 popped and blew a fuse when I had it plugged in 
> last. I disassembled the PS and cleaned off the dust, and I want to test it by 
> itself, but the power-on relay won't close. Any hints, or is this like 
> amplifiers where the tubes need a load?

It's getting late (too late for me to hunt for the service manual), but 
from what I remember you need to have the 'scope connected. The heater 
transformer, for example, is in the 'scope, fed with controlled AC from 
the PSU box.

-tony