Fender Hot Rod Deluxe Type PR 246 Squealing

[devilsindetails]

Hi again everyone. The problem this time is with a Fender Hot Rod Deluxe reissue.

The amp operates perfectly until introducing gain on the Reverb Pot when a squealing commences.

I did the preliminary troubleshooting...cleaned and lubricated all pots with DeOxit, tested the tubes Power and Pre-Amp for microphonics and noise, substituted known good tubes, checked for visual signs of blown caps, burnt resistors, loose connections etc...and no change.

Tank also checks out ok as do reverb plugs

When I patch in the exterior footswitch the condition remains except when Reverb is bypassed.

Attached is the schematic for said Hot Rod.

When I tap the board area aound R78 and R79, C48 C49 and C35 the squealing increases, subsides etc...

The Reverb pot measures 25K as per schematic so any ideas of where my inexperienced eyes and hands should be looking next? Thks

[funkmeblue]

I chased this same problem and replaced a few parts including op amps. It only did it when the reverb knob was past 5. I never fixed this amp but I was convinced it was the reverb tank. Owner did not want to put any more money in the amp at this point. Pull the reverb pan.....I coild stop the high pitch squeal by lightly touching the springs....

[rock_mumbles]

Are the voltages correct on the reverb opamp?

A friend is a Fender dealer/service center one of the first things he checks on a HRD are the two 470 ohm resistors that drop the 48V to 18V for the ss stuff ... they are R78 and R79 (on the bottom of the pcb) ... they develop broken solders or sometimes fail ... this can cause a high pitched squeal.

[Structo]

Wow, that's the kind of stuff you only learn after working on different models a lot.

Thanks for passing that along.

[M Fowler]

The HRD all suffer from broken resistors all over the place.

The one I have been working on failed after the owner engaged the channel switch. The power amp works fine but no preamp. All tubes were good.
Changing the Relay the amp fired right back up but then the amp went out right away after trying the channel switching. So some where in that switching circuit there is a problem Houston. I want to build another amp out of the chassis/cab but NO of course not.

Mark

[rock_mumbles]

I talked to my friend today and he said there's a 100k resistor in that same portion of the circuit to check out ... it's (R77) the 'tail' of the adjustable bias supply.

My friend's shop almost always has at least one HRD or DeVille in for repair ... today one of each were repaired waiting to be picked up.

[renshen1957]

The HRD all suffer from broken resistors all over the place....

I want to build another amp out of the chassis/cab but NO of course note.

Mark

+1 Fender makes the most minimal PCB with cheapest components. Use the amp with any frequency and the traces lift, the resistors fry. Fix the culprit and then something else goes wrong. For what it's worth, build something they really wanted in the first place, but had to settle for the HRD.

Best Regards,

Steve

[ChrisM]

Are the voltages correct on the reverb opamp?

A friend is a Fender dealer/service center one of the first things he checks on a HRD are the two 470 ohm resistors that drop the 48V to 18V for the ss stuff ... they are R78 and R79 (on the bottom of the pcb) ... they develop broken solders or sometimes fail ... this can cause a high pitched squeal.

Seen that one too when I worked on Fenders. These resistors get very very hot and often will "open" up for mechanically fail.