Fender Acoustasonic 30 problem - Noise on Reverb only... HELP?!

[PurpleRocks]

Hey Dudes

New to this site, and I have a problem with a friend's Acoustasonic 30 that I'm trying to help him with... There's an audible noise similar to those described on the thread in the 'Guitar Tech' section, but only when engaging the 'Reverb' option - the noise is cavernous/echoey and scratchy/bumpy, and I think it gets louder as you turn up the Reverb pot... When the reverb is off/ down to zero, the amp performs perfectly and without any problems/unwanted noise/interference...

I don't know much about these kinda electronics, but I thought it might be a broken pot, but from the above replies, could it be a grounding or capacitor issue? Please help a confused newb?!

Cheers from the UK

Alex

[Enzo]

If the control turns the noise up and down,l then the control is doing its job.


Do you have this noise when no instrument is plugged into the amp? If teh amp sits there making nopise, it is probably a noisy semiconductor, and specifically most likely a noisy op amp IC.

[PurpleRocks]

Thanks for your reply. The amp does seem to make the noise when nothing is plugged in, but only with the pot turned up. It sometimes takes a minute or so for the noise to appear - not sure if this makes any difference to what it could be. I'm a novice when it comes to the electrical terminology I'm afraid - is the OP Amp IC one of the chips on the circuit board? Is this easy/cheap to replace?

Cheers for the help

[Enzo]

All the little 8-leg ICs in there are op amps, or at least most of them.

I find replacing them simple but this is what I do for a living. To do so will mean identifying the noisy one, remvong the circuit board, unsoldering adn removing the old part, and soldering in the new, then reassembly

[PurpleRocks]

Sweet - thanks dude I'll get my engineer mate at work to look at it next week, with your advice - thanks!

[PurpleRocks]

Big thanks to Enzo on this one - replaced the 'U7' Reverb circuit Op-Amp with a TI equivalent to the Rohm original IC, and the echoey popping noise has gone! Cheers for your help dude! (shame the reverb isn't very impressive after all on this amp, but hey-ho!)

[PurpleRocks]

Hey Guys. So the reverb now doesn't work at all. I've checked the pan, the resistors and capacitors on this circuit and even the phono lead and they all seem fine - can't help thinking that the Op-Amp that I've replaced the Rohm BA4560 one with (Texas Instruments RC4560) may be 'blocking' the feed into the reverb circuit...?

Noticeable differences in the spec include a slew rate of 5.5V/usec (Rohm was 4V/usec) and a GBW of 15MHz (Rohm was 10MHz) - could the different chip somehow be acting as a resistor or some sort?! Going mad with this now!

Cheers chaps!