Bargain of the day: Fender Hot Rod Deluxe 5-minute fix

[xtian]

After reading the intertubes ("channel switching problem"), including some good posts here on Amp Garage, I drove 30 miles to pick up an "as is" Fender HRD. Only channel 1 was working. The Reverb and Channel 2 would not engage at all. I paid $200 cash and drove home with my fingers crossed.

Yes, indeed, in five minutes I isolated an open connection leading to those two 5W resistors that provide voltage for the channel switching and also for the reverb op-amp. I installed an alligator clip jumper to test, and INSTANT FIX for both problems. Nice.

Only other annoying problem is some major buzzing in the baffle in sympathy with certain notes.

[Drumslinger]

nice little score xtian. I believe there are some nice mods floating around on the net for the HRD. You might want to check those out.

[Structo]

Why do some amp companies mount the sand box type power resistors upside down on the PCB?

Does it dissipate heat better that way?

[Zippy]

So the creamy filling doesn't drip out???

Mmmmmmmmmmm, reminds me of a Twinkie...

[vibratoking]

Why do some amp companies mount the sand box type power resistors upside down on the PCB?

Does it dissipate heat better that way?

Yes, that's my theory. The top of the resistor has a more uniform surface and so the thermal resistance is less. This allows more heat to be transferred out of the resistor and into the board. I don't know if that's where you really want the heat without doing a calculation, but that is where it's going in that orientation. The glass transition temperature of FR4 is about 120 degrees C, so I would think it is the correct way.

[xtian]

This HRD is a 50th Anniversary model, built August 1996. Has Sovtek 5881wxt power tubes. The plastic, central index pins were broken off both power tubes. I have reglued with cyanoacrylate.

I read about the mods, but decided against making any tonal changes before I learn what it sounds like in practice.

I'm going to replace the 5w resistors with blue ones from Mouser. Best to keep them off the board and separated so they dissipate heat? I guess I can't lock them down with hot glue like with caps, because they'd melt the glue. Suggestions?

[DonMoose]

I'm going to replace the 5w resistors with blue ones from Mouser. Best to keep them off the board and separated so they dissipate heat? I guess I can't lock them down with hot glue like with caps, because they'd melt the glue. Suggestions?

My suggestion is to do a 'J' bend in the leads to leave as much lead as possible (the leads are a significant part of a resistor's ability to sink heat), tensioned to press the bodies against the board.

Then, maybe, some kind of clip-on heat sink.

Another idea is resistors in a TO-220 package with added heatsinks - tougher to figure out the mechanics and how to secure these 'wrong footprint' parts.

Hope this helps!

[xtian]

Bottom of circuit board exposed. Check it out! Classic.

[Structo]

Yes in that instance I can see where having an air gap would be preferrable.
I was just wondering about the orientation of the resistor body itself.

Is that PCB normally placed in the amp so that the resistors are on the top side of the board or amp? Heat rises..........so that makes sense to allow more air around them.

Is that the amp the de-solders the resistors with the heat from them?

[xtian]

Tom: This is a vertical board, so the resistors are on one side of the board.

Finished resistor replacement photo below. Suggestions for shock-proofing them against vibration?

Speaking of vibration, I discovered the speaker was barely hanging on to the baffle. I tightened the four screws, and hey-presto, no more ugly buzzing.

[vibratoking]

Thanks for showing those pics. They indicate that the FR4 is indeed being heated beyond the glass transition temperature. That's the point that the glass, in the fiberglass, melts and the material separates and delaminates. I guess the designer didn't do his math or test it.

[ampdoc1]

To shock-proof the mounting of your resistors, go to an auto parts store and get a tube of Blue RTV silicone gel. When it "dries" it'll support/cushion the resistors and is impervious to the amount of heat these will dissipate.

a'doc1

[Drumslinger]

good to know, thanks ampdoc.

[FUCHSAUDIO]

Consider moving them to a terminal strip on the chassis, and wire back to the board, or use long standoffs (440 style nylon) to mount them WAY above the board a good long distance. Cold joints from these parts thermal cycling the connections is common too.

BTW: There was a tech release from Fender tech support that raised those resistor values (to run cooler) and also to replace the Zeners and mount them above the board as well. Common issue in some production runs. Later fixed.