HRD IV Crackle at high volumes

[tdlunsfo]

This thing is driving me nuts. Caps replaced, input jacks replaced, 5W resistors replaced, zeners replaced. Amp isn't that old and the heat damage had only started under the board. I have resoldered every tube socket, and touched up any other joint that looked remotely suspicious including all the ribbon cables. I can't make this thing mess up but once every blue moon and it only does it for a split second. It will not mess up on my dummy load, tapping the board or tubes also make no difference. Customer has brought it back to me twice now. Anyone ever experience this? I've read everything from speaker to output tubes to plate resistors. I would prefer to catch it in the act and actually TROUBLESHOOT but that feels like catching lightning in a bottle at this point.

[GoldtoneWorkshop]

Does it only do it when the volume is cranked? Dirt or clean channel (or both?)

[tdlunsfo]

Only when cranked. Customer said it didn't matter the channel but I've personally only heard it on the clean channel with volume above 4.

[g1]

You mentioned you read about plate resistors. There was a time when this series had issues with them, particularly in the phase inverter position.
If you have freeze spray it might help locate the culprit.

[Delta362]

Plate resistors would be my first thought.

[tdlunsfo]

I changed them out. While putting the tube socket back in place I found a broken joint on the ribbon cable for V1 tucked under the board. Tough to see the way these are connected. I'm still unsure if this is the cause of the crackle but it's a strong possibility.

[The Dude]

If the symptoms are there only when cranked and also do not exist when a dummy load is used, I suspect it's vibration causing the problem. So, I'd be looking for a connection problem or cold solder joint. It could also be a bad tube. I just had a Deluxe reverb in the shop that blew the fuse at high volumes. I found that if I smacked the rectifier tube while it was operating, I could see it arc. I realize this is a different amp, but my point is that sometimes tubes can be vibration sensitive.

[tdlunsfo]

Oh I certainly use a smack on the top, tube tapping, and my chopstick as troubleshooting methods. Unfortunately it didn't yield any results this time.. even when pushing on the ribbon connectors.

[tdlunsfo]

Well the crackle is back and this time with a vengeance. I scope a clean signal coming out of R9 going in to the volume pot and then I get nastiness on R41 on V1B grid. There isn't much in between other than a Volume pot and a drive relay. There is also a small value cap C53 across R4 that is on this version and not on the old schematic. Seems most sensitive around the relay so I think I'm going to start there. I touched up the relay joints previous but I am going to remove it and start over and might even just replace it.

[Helmholtz]

Schematic?

What is a HRD IV?

[GoldtoneWorkshop]

Schematic?

What is a HRD IV?

it's a Fender Hot Rod Deluxe IV

[tdlunsfo]

Here is a link to the original schematic. There have been a few minor changes but this version is still really close. As of right now I have removed the relay and jumpered pins 4 and 6 and the amp is quiet. I'm going to let it heat up a bit and see how it does.
Microsoft Word - HOTRODDLX.doc (thetubestore.com)

[g1]

attached is HRD III (rev.B) schematic in case it is any closer to the IV than the original HRD schem.

[tdlunsfo]

My theory is a weak K1-a relay and the normally closed contacts are coming open due the rattle of the amp at high volumes.

[g1]

If so, I would think a good smack right on the relay with a screwdriver handle should be able to make it flinch?