Hiss in Fender Champ 25GR

[jim]

Here is a throw-away teaching amp my shop asked me to repair.  I am not good with solid-state.  It hummed and hissed.  I replaced the main filter and now it justs hisses.  Funny, with a Strat plugged in,
it is silent in the #2 and #4 position but any humbucker hisses loudly.  Hiss is both channels and gets loud with volume increases.  I think it is a front end transisitor but I don't know .  How do I trouble-shoot it? Thanks, Jim

[PRR]

> How do I trouble-shoot it?

Similar to a tube amp.

Observe which knobs affect hiss. Treble and Reverb are particular clues. If a knob has no effect on hiss, the problem is likely *after* that knob.

Take it out in the sunshine and look for trouble. Burned resistors. Bad solder joints.

The main burned-resistor item here is the 330 ohm 2W R48 R49 jobs in the power supply; fairly notorious on other Fender sand-amps.

Remember that soldering is a minimum-wage occupation; solder inspector gets a dime bonus. Assume there are solder joints that just-barely worked only for so long.

Is the reverb pan working, is the reverb pan hooked-up? If it's disconnected, bad stuff may happen.

Check voltages. Your + and - 12V rails should be 10V-14V each, not zero, not 20V. All opamp pins (except U2B 567 which follow footswitch state) (except power pins which must be near power voltages) should be near-zero.(DC biasing of bipolar opamps is much simpler than tubes with their +2V and +200V cheack-points.)

> silent in the #2 and #4 position

I don't know the amp. What is this "#2 and #4 position"?

> I think it is a front end transisitor

You say *both* channels; but there are separate opamps for each. While it is possible one contaminates the other, that would not be my first guess.

However there is a major oversight in this amp, and if it has been abused it *might* be part of the problem. U2A and U1A connect *direct* to the input jack. If more than 12V input is applied (say, a loudspeaker OUTput), the chips may breakdown. There's some internal protection, but not much. A partial breakdown could hiss madly. Is there *any* way to insert a 33K resistor between jack and opamps? Maybe cut a PCB trace, scrap the lacquer off the trace, and bridge the gap with a 33K resistor? That makes it much more robust against stupidity.

[jim]

OK thanks for the tips, I will get to work.  Jim


"silent in the #2 and #4 position"
The amp is quiet with the Strat pickup selector in the #2 and #4.  This could just be input gain related.

[super&plexi]

I know it's old topic...just a little addition in case it helps someone else.... 2 & 4 position on strat is humbucking on newer models with reverse wound/polarity middle pickup.      also low cost SS models of Fender (and probably most other brands) are notorious for bad input grounding schemes, & jacks.


My thoughts...a lot of these low price cheapo's are quite useful for practice, rehearsal,...all kinds of things, even gigging, and many sound great.


Almost all benefit from removal of cheap plastic jacks. small hassle cutting off board mounted jacks and soldering onto board some wires to a good switchcraft jack/set. maybe Cliff type for certain apps.