Guyatone ZIP 401 with PCB EPK-222

[mtn_silviu]

Hello everyone

I have a Guyatone ZIP 401 guitar amp in need of repair. It was previously opened and repaired by someone else, and i believe it was repaired wrong by using different capacitor types, maybe also different values.

I can't seem to find any schematic on the web, or photos of the original PCB and internals.

Maybe someone has one , or can point me in the right directions of repairing this amp

I will reconstruct the schematic bit by bit, but i don't really have the knowledge of recalculating the correct capacitor values of OP-AMP ics in guitar amps.

[Jon Snell]

What is the fault with it?

[mtn_silviu]

What is the fault with it?

When powering the unit, the initial very loud crack, followed by a increasing loud humming noise. The humming increases for 1 second and stays high, like the volume is at max. Moving the potentiometers did not help.

But reading the PCB ,there are many changed components that don't match what I'ts actually written on the PCB. There are electrolytic capacitors soldered in place of maybe ceramic or plastic film ones, and vice-versa. Made a list of every component installed so i can have a backup in case i want to revert the changes. i replaced some electrolytic capacitors with bipolar ones as the PCB was marked. The capacitors were changed i believe only in circuit dealing with the reverb function, but I'm not sure, I'm not so familiarized with amp circuits.


Now when i power the unit on, there is only a slightly noticeable background humming noise ,probably coming the mains AC 60Hz . Plugging in a guitar does not solve the issue. Could it be a ground issue?

Apart the humming, and the two black 1Mohm pots that were not original and the front panel was drilled with M10 bits to fit them.. the amp seems to work just fine.

[Jon Snell]

Looking at various youtube recordings using these old amplifiers they all seem to hum. I suspect a ground loop if there is no bad earth connection.
Good to read you have effectively a working amplifier.