I recently acquired a Guitar combo.
The power amp had been removed. Consequently the power to the preamp
board was also missing.
I have bought a hy2003 to replace the power amp ( with dedicated psu
and transformer.
I have built a 15v power supply + regulator for the preamp.
Unfortunately the only signal that comes through the preamp is
intermitant and seemingly when the input is at its strongest.
Is my 15v transformer too low a current ? (rated at 6VA) or is it
something more serious ? :?:

"Peter F" wrote in
message ...
I recently acquired a Guitar combo.
The power amp had been removed. Consequently the power to the preamp
board was also missing.
I have bought a hy2003 to replace the power amp ( with dedicated psu
and transformer.
I have built a 15v power supply + regulator for the preamp.
Unfortunately the only signal that comes through the preamp is
intermitant and seemingly when the input is at its strongest.
Is my 15v transformer too low a current ? (rated at 6VA) or is it
something more serious ? :?:


Are you sure that the preamp only needs +15v ? In my experience with guitar
amps, and I do repair a lot of them, it is usual for the preamp to require a
negative supply as well, as the circuitry is usually based on 4558 or
similar opamps.

Also, the supplies for the preamps are very often derived on-board, from the
main +/- 50v or whatever rails that feed the power amp. So +/- 50v goes
across to the preamp, where it is zener'd down to +/- 15v, with a couple of
wirewound resistors and 5 watt zeners.

The clues you should be looking for are does the preamp employ opamps ?
Where the power comes onto the board, are there two electrolytic caps for
decoupling ? Where the power comes into the board, are there any signs of
wirewound resistors ?

The sound problems you are describing could well point to only one rail on
an opamp stage.

If it's a ' famous ' make, then the schematics for it are almost certainly
available, probably for free, from one of the many excellent guitar amp
schematic sites on the 'net.

Arfa