Fender Hot Rod Deluxe - persisting issue after replacing components

[RazorReaver]

Hello all,

I am sure most of you will be used to people saying that have issues with this amp. I am no different, since my amp recently started cutting off at random after playing it for a while and after some digging found the usual suspects were to blame (R78/R79 overheated and were coming off the board). I swapped the resistors with 7W 470R ones, and replaced the diodes CR13/CR14, plus another resistor that had burned out (R97). I plugged the amp back on and it was playing fine then I switched the channel, and now I'm stuck on the drive channel... which, after some digging, I found it to be a symptom of the problem I thought had just fixed. I'm a bit stumped now, the only other information I can give is that both R78 and R79 give me a voltage measure of +33/-33 when it should be +16/-16. Any thoughts on what went wrong?

Thanks

P.S.: if you want to have a look at the schematics, look no further:
hotrod_deluxe.pdf

[TomCarlos]

Razor....

If you swapped out 4 components, then that is where you should look. Before doing this, can you confirm that you had +/- 16volts before the change?

And you are sure you are using IN5353 Zener diodes (put them in the right way) and that R78 and R79 are measuring at 470 Ohms?

And what about the voltages before these resistors? Are you measuring +/- 48 volts?

[RazorReaver]

Hey Tom, thank you for the reply.

Over-confidence-driven mistake, since the resistors were in such obvious bad shape I swapped them out without measuring... I'm sure it's the right diodes, plus they are giving me normal voltage drops (0.7V) and the right value for all resistors. When measuring the diodes (one probe to the anode, one to the cathode), they both give off 16V readings, am I wrong in thinking they should be around half of that each?

I am not presently with the amp but will measure before the resistors as soon as I have the chance

[TomCarlos]

Look at the schematic carefully (the section next to the Notes in the lower right corner). You should be taking your measurements from the junction of CR13 and CR14 (which is the same point as C40 and C41 which is Ground). From Ground to the CR13 Cathode, you should measure +16v. From Ground to the Anode of CR14, you should measure -16V. That is your output for this part of the circuit... From Ground, Plus and Minus 16V.

[Enzo]

We are used to hearing about this amp because it is one of the most popular amps of all time, and so there are huge numbers of them out there. We always hear more about amps with huge populations than about amps that sold a couple hundred.

Amp cuts out randomly. OK, those two resistors are part of the 15v supplies. Those supplies power the solid state parts, which is channel switching and reverb. Look at the schematic, unless you use the FX loop, the signal does not pass through any solid state stages. So unles your cutout is coming from unwanted channel switching, which would evidence itself by the lights changing, those resistors are not likely involved.

it is not informative to say you have 33v on a resistor. A resistor has two ends, and unless zero current flows through them, there is a voltage drop across them. SO we either need to know what is at BOTH ends or we need to know what is across the resistor. SO do you have 33v at the 16v end? ie the 16v supplies are now 33v? Or do we have 33v ACROSS the resistor, which with 48v on one end and 16v on the other per the schematic sounds about right. 48 less 16 leaves 32v across the resistor schematically. If you had 66v across the IC, then chances are it would explode. +33 to -33 is 66v) The IC needs +16 on pin 8 and -16 on pin 4, which is 32v total. 16v zeners should have 16v across them.

R78,79. Pretty much if you have roughly 48v on one end of each, and roughly 16v on the other end of each, then they and the zeners are working.

Now sound cutting out? MY first suspects for that are the tube socket pins and their solder. In fact I always inspect for this as soon as I open any of those amps for service. Look VERY closely at the solder on the pins of the two power tube sockets. See any tiny cracks in the solder around them? I just automatically resolder them anyway, it is faster than laborious inspection. I also look over the 12AX7 sockets, but they usually are not cracked solder. I always check the two plate resistors on the phase inverter tube, R57 and R58. Sometimes one of them goes open.

Stuck channel switching? Assuming you do have good +/-16v supplies to the ICs. Look at your schematic on the left edge, a little below center. See those notes that say "footswitch operation"? That lists the test points in the channel switching circuits and gives voltages to target. In some it even says with footswitch and with panel switches, because the are different voltages. Go through those. Please don't make up a laborious chart, just report what conditions you find.

And what exactly does stuck mean? Stuck on drive channel totally? Or the lights change but not the sound? Or some lights are wrong? Be specific.