[sdiy] sources of hum and hiss in a solid state guitar amp (LM1875) (slightly OT)

[anthony]

holy crap! I /do/ have a scope and I always seem to be forgetting to use it 
when I build effects or fix amps. I use it all the time when building synth 
mods though. I should have used it when I built my tube amp because I'm sure 
I would have arrived at my ideal parts a lot more quickly. (It didn't really 
take me that long anyway.

I did use my scope heavilly when I was modding my Peavey Hotfoot and my 
"Realistic Electronic Reverb".

It's a Beckman Industrial 9060 (actually made by a different company but I 
forget which) 60 MHz scope. It seems to have just the right amount of bells 
and whistles, but I always get the feeling that it doesn't really excel in 
the audio domain. My RCA and Heathkit scopes got trashed in a basement flood 
and my Tektronix got stolen. I never actually got to use the Tektronix but I 
still get wistful when I think about it. I did have a chance to crack it 
open and look inside. I mean... wow...

I think the thing that keeps me from using my scope as much as I should is 
the fact that I really don't have a good probe. I do have a BNC plug cabled 
to 2 mini clips with a low cap coax cable that works pretty well, but it's 
been on my frequency counter lately because that thing has a tendency to 
load circuits under test even using the high impedence input. Another thing 
is the fact that it saw heavy use in the military before it was retired. And 
I'm pretty sure it was retired because it needed to be calibrated more 
extensively than it was worth to bother even though the calibration record 
on the cabinet does not reflect this. It does work and is probably perfectly 
suited for audio at this stage in its life. A good spray in the time-base 
switch with a nice-n-nasty chlorofluorocarbon cleaner spray would probably 
bring it 99% up to snuff. I don't have a backup scope now so I haven't had 
the chance to do the overhaul that I'd wanted to do. When I still had 2 
other scopes my intention was to make a sofisticated high-res audio spectrum 
analyzer front-end for it. I still want to do this, but I wanted to have it 
be dedicated to this use only. I suppose I should put out my feelers at the 
University for another freebie scope that would fit the bill. I might even 
be able to score a Tektronix if a specify a smaller bandwidth. (My original 
intention called for a wide-ish bandwidth because I wanted to upconvert the 
audio and use the full bandwidth of the scope. Actually those DSP capable 
PIC u-controllers might work well for a small compact spectrum analyzer.

I t also occured to me just now that when I first fixed this amp I was 
house-sitting so I only had my mobile elctronics kit with me to use.

Now it's cracked open on my bench and the scope is all warmed up.

> Hey Anthony....
>
> (do you have a scope or are you working by ear ?   you keep mentioning 
> that you are