Fender Hotrod Deluxe Circuit Board Buzzing

[SoulFetish]

Okay, you’re in luck. I just had one on my bench and logged some photos of the work. Hang tight, and ill try and upload these tonight with some suggestions.

[SoulFetish]

Thanks for the reply. Yes. I am hearing an audible buzz from the main board. I am also having channel switching issues. I have replaced the C31 (47uf/500v), C33 (22uf/500v), C35 (22uf/500v) and C36 (22uf/500v) with new F&T caps...

Fine enough, these may not have needed changing. But since it's your amp/money/time, no harm in replacing the with new ones.
There are usually clear visual signs when these are faulty. ie. like this:

.. and the resistors in R61 and R62 with identical equivalents (5W 470ohm cement type).

Ah, now we're getting to the cause of your channel switching problems. Again, I don't think I've seen these components themselves fail. But the traces, board, solder joints all around these often do. You probably didn't have to replace these. But, this is the likely area causing your problems. First thing, in standby, check the power supply at the switching op amp and see if you're getting the -/+ voltages at pins 4 and 8. If you are, then the op amp may have failed and needs to be replaced

The traces around the R61/R62 resistors were badly damaged. I ran jumper wires and confirmed continuity, but something is still not right. I have ordered replacement diodes and resistors for around the R61/R62 resisistors. I stumbled onto this forum after I did that work. I want to proactively replace as much as possible to make this amp reliable.

This is how I prepare and reinforce this area of the board around these components. This example had dry solder joints, but the board only had mild heat damage to the traces. (I uses solder helper tools to aid in this job) I clean the board with isopropyl alcohol and scrape off the discolored soldermask, exposing the bare copper traces around any discolored area, and compromised solder pads like this:



Then I'll often remove the original solder, and using some liquid flux, resolder the joint and flow the solder along the exposed copper trace. This will add some mechanical strength to the connection and help prevent this from happening again going forward.
(If the traces are lifted or broken, I will form some buss wire and lay it along the path of the trace. I'll run it from the through hole, connecting the component lead, to a suitable distance using my discretion.)



This is my preferred flux, and applicator. It makes for very efficient use and goes a long way for lasting.

[SoulFetish]

Since i will have access i think it may be a good idea to replace C62, C63, C64, C65, D6, D7, D8 and D9 . I can find the Diodes easy enough but I am not sure which capacitors to get with the spec "8200pf @ 1.6KV". I know how to solder and follow a schematic but i get a bit lost when sourcing parts. Do you have recommendations for a suitable capacitor replacement? Any help is greatly appreciated.

I'm not looking at a schematic, but if these are the diodes and caps in your LV supply, I almost never see these fail. I can't remember needing to replace them (they're not a common mode of failure in these amps). Occasionally, a zener will fail, but it's rarer than you'd imagine. Usually, it's a faulty solder connection under the board. As a technician, I would really encourage you that you don't need to wholesale re-part you're entire amp. The issues you're experiencing can be tracked down to one or two causes most of the time. Check your voltages, the do some of the things I showed you and report back.

One last thing I do when I pull the boards is clean the faceplate on these amps using this conditioner my tech buddy developed called "Cranky's Amp Spray". They almost always come in rusty and filthy and looking like shit.- like this:



He developed this primarily to clean and protect tolex and cabinet materials and I keep telling him to sell it to stu mack or something, because it's like magic for this kind of work. This is a minute and a half spray and wipe down with standard paper towels:

[nothing2fear]

Thank you Soulfetish! All of this was extremely helpful. I changed the R61 + R62 resistors with 7w replacements and raised them off of the board using spacers. I then replaced Diodes in D13 + D14 because the area looked like it had seen a good amount of heat. I also replaced the resistor in R97 which I believed to be be the main issue. I had to repair some traces but they don't look as nice as yours! Everything is now working well. Thanks again for all of the suggestions!