Hi everybody! I'm new on this board but I could use some help...My friend has a Randall, RB-120, 1x15 combo, bass amp, 1976 vintage. It's been a great practice amp but it's recently developed a hiss/static (all the time) and fizzy/farty distortion (when playing notes). The hiss sort of goes away for a second when a note is played and it fizzes and farts as the note decays...then the hiss comes back. It's definitely more of a hiss/static that gets worse the longer the amp is on...it's not really what I would call a hum.
What do you think is the source of this noisy, static, hiss, fart??? Based on the sound, if it was a tube amp, I would suspect a bad tube or socket. Does this sound like a bad cap, power transistor, etc. The speaker is ok!
It has two, can type capacitors, one of them is leaking black goo. They are similar to the type I have seen in my Champ but I don't think they are "multi-section". There are only two wire connections. The specs. printed on the cans are:
4-46 FIL-CAP
4500MFD 50WVDC
46075338
The caps are 3" long x 1 and 5/16"dia. clamp mounted.
I can't find a schematic online. Does anybody have any idea what I can use as cap replacement? Or any other trouble shooting ideas?
Speaker is ok!
Thanks
It's very probably oscillating, because of bad filter caps.
You don't hear the oscillation itself but its byproducts, as hiss.
Anyway, any goo-leaking cap *has* to be replaced.
You need two 4700uFx50V caps.
Mind you, modern ones will be *much* smaller, won't fit those clamps, but you'll certainly find a way.
If trouble remains, *somewhere* I have that schematic or a very similar one, close enough for service.
Good luck.
Thanks JM,
I've been having a hard time finding replacement cap cans that will fit.
It seems most of them are actually longer than the originals (3") or larger dia. (1.375") and I don't have much extra room to work with.
Any recommendations of where to shop?
So far I've checked Tedss, Mouser, AES, etc.
Thanks for your help!