help with hiss problem

[rutabaga bob]

hey y'all...i recently got a mcgohan p.a. at a sale for a buck...it works, but when you turn up the treble, it reveals an annoying hiss, even with all volume knobs turned to zero. i replaced the one carbon comp resistor it had, but no change.  it has a small (.001uf)ceramic cap as the last component before the treble pot...any chance this is the problem?  any suggestions?   thanks!

[joecool85]

More than likely the treble pot is worn out and/or dirty.  Try cleaning it up or replacing it, should solve the issue I'd imagine.  I don't think ceramic caps ever really "wear out" like electrolytic type caps.

[Enzo]

Hmmm, when i read the post, I wasn;t thinking noisy pot.  I don;t think he means turning the pot makes noise, I THINK he means when the treble is up, the hiss of the system is increased.   Whatever hiss is already in there would naturally be made more prominent by turning up the treble.

I generally associate hiss with a gain element - in your case I assume an op amp or maybe transistors?  What active device is just before the tone control?

[rutabaga bob]

yes, you have hit on it...the pot is o.k., but the hiss increases when the treble is boosted, even with no volume on the controls. i don't have a schematic, and i haven't come up with anything definite by scanning over the pc boards...there are a couple of small transistors on the same board as the pots, and you can see that the emitters are grounded along with the pots, but i'm not sure of much else at the moment.  this amp uses 2 power transistors.  i need to delve into it further...this is a mcgohan model ms-203, 20 watt p.a. head.  THANKS FOR THE REPLIES! 

[J M Fahey]

*Maybe* it's just poorly designed.

[Joe]

Does it sound like the hiss you get in-between stations on an AM radio?

[rutabaga bob]

that is a fairly accurate description...

[joecool85]

that is a fairly accurate description...

I suppose it is possible you are having issues with interference in your amp.  If that is the case, the most you can really do is shield all of your wires, re-do your grounding in a star style format and hope for the best.

[Joe]

The problem may be coming from the grounded-emitter transistor stages, which make good AM demodulators (like having a radio tuned to all the stations at once).

Might try lifting the emitter leads on the small transistors and tack in some small resistors (perhaps 470 ohm) and see if that helps. An input resistor can also help, in the range of 1k-10k.

Good Luck!