Fender Hotrod deluxe noise like popcorn

[Gerhard Grass]

Hi to all of you,

I have to repair a Fender Hotrod Deluxe. When fired up, the amp makes popcorn-like noises
when switched from stand-by to on. The noise stays for about a minute and then disapears.
The amp now sounds o.k. without any disturbing noise.
To reproduce the phenomenon you have to switch the amp off, let it cool down for
some hours, the fire it up again. I pulled all preamp-tubes and the PI, but the noise persisted.
So the noise seems to come from the powertubes. I already changed them, but the noise stayed.
Looked at the supply voltage with a scope, looked o.k.

Next thing for me would be a change of the caps in the power supply.

Any hints a very appreciated,

thx inadvance

Gerhard

[nickb]

My first guess would one or all of C26, C27 and C30 going bad.


If you have freezer spray, that can often pinpoint which one.

[Mick Bailey]

Check the joints around the power tube sockets. In fact, check them all for good measure. If they're on the way out you may be getting mechanical disturbance of the joints as the tubes, sockets and PCB heat up. When they fail they have a circular crack around them. Discharge the caps and re-solder them if they look suspect.

You should do this check before changing any components. I always re-solder the joints on this series of amps as a matter of course. Nearly every one fails in this area.

Check the screen resistor values and their solder joints too. A failing resistor or bad joint will produce noises.

[nickb]

^^^^^

What Mick said. I re-solder the sockets on every one of these I see. They are very troublesome.

Check the joints around the power tube sockets. In fact, check them all for good measure. If they're on the way out you may be getting mechanical disturbance of the joints as the tubes, sockets and PCB heat up. When they fail they have a circular crack around them. Discharge the caps and re-solder them if they look suspect.

You should do this check before changing any components. I always re-solder the joints on this series of amps as a matter of course. Nearly every one fails in this area.

Check the screen resistor values and their solder joints too. A failing resistor or bad joint will produce noises.

[Enzo]

I too vote resolder all the power tube socket pins. Don't resolder them just for looking bad, resolder them all ANYWAY.

That really doesn't sound like caps from the description.

[T9c]

My first guess would one or all of C26, C27 and C30 going bad.
[ATTACH=CONFIG]23664[/ATTACH]

If you have freezer spray, that can often pinpoint which one.

Also check values of all resistors in the PI section, particularly R57-82k.

[km6xz]

Sounds like short duration "snits" and continuously burbling sound until it heats up? Sounds like a carbon composition plate resistor with particles separating when cold. Use a well insulated pair of pliers to squeeze the insulated shell of the plate resistors, probably PI, to see if the sound changes.

[Mick Bailey]

He says "I pulled all preamp-tubes and the PI, but the noise persisted"

The PI plate resistors are often troublesome, though. I replace them with 1W metal film even if they look ok - they're listed as CFs in the Hotrod Deluxe. By the time the board is pulled out enough to get to easy access to the component side they may as well be replaced.

[Jazz P Bass]

All tubes removed except the power tubes.
Hmmm.
It may be the output transformer cooking itself.
I have replaced quite a few HRD OPT's.
They seem awfully 'wimpy' for 50 watts.

[Enzo]

So pull the power tubes and see if it still does it. If the transformer is breaking down, it ought not need the tubes to show it. In this area do not forget the flyback diodes, one of those could be trying to fail.

[frus]

What Mick said. I re-solder the sockets on every one of these I see. They are very troublesome.

+1
usually fixes this particular problem

[Gerhard Grass]

Yep!

Resoldered the power tube socket pins. They didn't look bad at all, but it fixed the problem.
The bias pot had a bit of a contact problem. Just turned it several times and all was fine.


THX to all of you!

Gerhard