HR Deluxe with bizarre hum issue

[Wes]

Wondering if anyone has come across this issue and had an idea of what this is. I've repaired so many of these HR Deluxes and Devilles over the years that I can usually tell what's wrong without even opening one. It's always the typical LV power supply, stuff around the PI, those pesky Illinois capacitors that fail so quickly, or any number of bad solder connections.

This particular unit came in with the bad LV supply. Popped the back off and, yep- the 5W 470's had cooked themselves loose. Along with this, the main caps were spewing, and the power tubes were completely mismatched. Seemed to be the typical scenario.

New HT caps
New power tubes
New 5W 470s and their traces touched up

All controls set to zero-

The amp fires up and has a loud hum.
Click the drive channel on and the hum disappears.
Turn up the master control and the hum signal comes through and is controllable.

Pull V1- no effect.
Pull V2, V3, or insert into the return and you bypass the hum.

Guitar sounds great direct into the return.

At this point the amp has been running long enough that the hum starts to fade away. Go ahead and plug into the inputs and everything works and sounds great.

Channel switching works great, reverb is lush- no issue. LED indicates as it should.

I figured there was something more in the LV supply so I replace the two filters for the +/-16 and the two zeners. Same problem.

Replace the 100uF/100v caps upstream of the diodes, same problem.

At this point I'm recognizing the thermal relation to the issue. Let er run for a bit and it's fine.

After she's ran for about 5 minutes and everything sounds and works great I blow air over the main board and it induces the hum again- fading in and fading back out.

I try to precisely assess where to apply cool air and localized it to the right side of the second relay where R27, 42, and 18 are.

I've tried new relays and all new/different tubes. None of that had an effect.

Could this be another bad electrolytic Like C14?

When the hum is present I get about 30mA of AC at the preamp HT node. That dies down as the hum dies down.

Beating my head against the wall on this one. Have you guys encountered this or have any thoughts? It sounds like 60hz but I'm not 100% as I didn't check that yet.

Thank you guys for any input or advice here.

[The Dude]

Try temporarily clipping in a known good cap across each of the filter caps one at a time and see if it solves the problem. Start with C14 if you suspect it's bad.

[nevetslab]

Makes me wish I was back in the shop where there is another cone of these waiting for me to heal back up as I was before.

I also haven’t run into this particular problem. I would remove all the panel hardware and drop the main board out and put my surgical loupes on and headlight (as I look in my avatar) and make sure all is fully isolated electrically and power it back up, but not until I had a real close magnified look at all the solder points on the back of the PCB. I think you’ve already done that on the component side.

Like the idea of using the blower. I’d be using the heat gun alternately with/without heat to try and hone in on the culprit.

definitely a challenge this one!

[pdf64]

I suggest to reflow every joint on the pcb.

[Wes]

Thanks for all the replies and suggestions.

I checked and found no bad solders on the boards. Blowing cool air, I managed to localize it down to the area just above V2. Using a small tube to precisely blow air, I pinned it down to the two wires that jump the filament supply across the preamp board.

I'm not clear on why the hum would come and go based on thermal conditions and the wires were soldered enough that it worked, but I applied some fresh solder and that seemed to make it go away.

What a weird one....

[Tom Phillips]

Thanks for all the replies and suggestions.

I checked and found no bad solders on the boards. Blowing cool air, I managed to localize it down to the area just above V2. Using a small tube to precisely blow air, I pinned it down to the two wires that jump the filament supply across the preamp board.

I'm not clear on why the hum would come and go based on thermal conditions and the wires were soldered enough that it worked, but I applied some fresh solder and that seemed to make it go away.

What a weird one....

Good work on your part to find that.
My thought is that a faulty solder joint was adding resistance to one of the heater supply jumpers. If so, that would cause a voltage drop to unbalance the heater supply. That could cause a slight hum. Once that starts in a high current path such as a heater supply joint on a PCB it will only get worse with time. The excess heat would eventually melt the solder in the joint and possibly start charring the PCB material.

[Wes]

My thought is that a faulty solder joint was adding resistance to one of the heater supply jumpers. If so, that would cause a voltage drop to unbalance the heater supply. That could cause a slight hum. Once that starts in a high current path such as a heater supply joint on a PCB it will only get worse with time. The excess heat would eventually melt the solder in the joint and possibly start charring the PCB material.

That makes perfect sense! Thanks for that insight. I've spent so much time on this amp that once it was fixed- I stopped thinking about it. lol.