Another Hot Rod Deluxe switching problem !!

[mosrite1]

HELP!!

I have a switching problem in a Hot Rod Deluxe that is killing me ! I have searched the forum and have seen nothing quite like it...anywhoo..

The Problem; Both of the overdrive channels do not work properly , the red light goes on..the yellow does not, it just remains red ( so no green LED). the signal comes through just the clean channel when either switch is engaged . So the Master volume and the drive pots do nothing when the switches are engaged.. Here is the odd thing, the reverb only works when the more drive button in engaged, it's kooky !

I have retouched ALL of the solder joints, I have replaced R78, R79, the LED, U2, U3, K1, K2, S1, S2, Q1, Q2, Q3, Q4 .. I think that's it!!. Both U2 and U3 had 4560 chips, but the schematic has the TL072 in U2, not sure which one to believe

Does anyone have a clue whats going on? I am out of idea's

thanks!!

[52 Bill]

The schematic for this amp has a list of test points and voltages for the switching circuit. Have you actually tested the voltages in the switching circuit? If the red led doesn't change to yellow then either the led is bad, or the voltages going to it are wrong. It never turns to green on this amp.

The amp uses both TL072 and 4560 opamps.

If the reverb only works when the more button is switched then, I would guess that the low voltage power supply (+&-16vdc) is being pulled down by the switching circuit, killing the power to the reverb circuit.

Sit down with the schematic and check the voltages at the test points and see what you get.

[Enzo]

The green comes on to add its coloer to the red so yellow results. The green never is on alone.

Agree with Bill, get out the schematic and go through the test point voltages in the switching circuit. They lay out pretty well what needs to be there.

[mosrite1]

It is R79, no voltage on either side of it, I have replaced everything in that circuit..and still nothing.. all of the other voltages seem fine according to the schematic...what next?

[Enzo]

So you are missing your -16v power supply then. This is basic power supply. That resistor drops -48v down to the -16v zener. If there is no voltage at the 48v end, ther cannot be any at the 16v end. Your resistor may be open, but most likely it has come free of its solder under the board. A common problem. Those two resistors next to each other get hot. A lot of guys mount higher wattage resistors (of the same value) off the board and run wires back to where they had been. But if you simply rebuild what is there, it ought to work.

Instead of poking the resistor leads through the holes, soldering them, and clipping off the excess wire, do this. Remove the old part and old solder. ABrade off the green coating from the copper traces the resistor was soldered to. Nice bare copper, maybe as much as half an inch along the copper from the hole. Now install the new resistor, leaving enough wire up top so the part can sit up off the board like before. But instead of soldering and clipping off, the end of the wire on the solder side of the board should be bent down to lay on the copper trace flat. Now solder along the wire, to solder the length of it to the bare copper. This extra solder will help the resistor conduct heat away from itself.

[mosrite1]

Found it!! There was a broken trace before C42, so there that's why there wasnt any power going to R79..

Thanks for all of your help!!