Fender Hot Rod Deluxe iii - intermittent loss of sound - help needed to diagnose

Philtre

This is one of those very intermittent problems that is hard to reproduce but occurs maybe once every few days. Here's what I do:

1. Guitar lead is already plugged into input 2. Volume set between 1 and 2.
2. Power on the HRD into standby mode.
3. Wait a minute or so and put it out of standby mode.

No sound. (On one occasion there was sound for a few seconds, and then no sound)

1. Adjust volume control on amp. Still no sound.
2. Plug into input 1, and then into the return jack. Still no sound.
3. Plug back into input 2. Still no sound.

But after about a minute or so you can hear the sound coming back, as if it's warming up. After this, the amp is fine. Can play it for hours, no problems.

On a couple of occasions the dropout, and return, of sound has occurred after the amp has warmed up and functioned normally. But this has happened only twice.

I've opened up the amp and I've:

- reseated the power tubes
- checked the bias (fine at 60mv)
- checked fuse is ok
- done a visual check for bad solder joints - looks ok
- checked for heat marks around R78 and R79 - looks ok

Could this be a sign of a bad power tube or something else?

ICBM

If the sound comes back slowly as if the amp is warming up, it’s almost certainly a filament circuit problem - because it is. It’s either a faulty preamp valve or a bad connection at the socket, probably a cracked solder joint on one of the pins. It won’t be anything to do with the power valves since only one being faulty won’t kill the sound completely.

If you look very carefully at the preamp valves when the amp is silent you should be able to spot it - one valve will have only one filament glowing not two.

Philtre

Thanks @ICBM - it had the same problem when plugged directly into the  "Power Amp In" (Return) socket, so doesn't this bypass the pre-amp section?

ICBM

Philtre said:

Thanks @ICBM - it had the same problem when plugged directly into the  "Power Amp In" (Return) socket, so doesn't this bypass the pre-amp section?

No, it means that the problem is almost certainly in the phase inverter. The only other possibility is that it's the whole filament supply, in which case you would see all the valves go out.

A slow fade-out/fade-in is almost always a filament problem.

Philtre

Yep, looking at the schematic, I see that V3 is still in the game after the "Power Amp In" socket. I'll make some more observations on the filaments.

LooseMoose

Chopstick test? I tend to press gently on the solder joints as this often reveals an otherwise ok-looking cracked/dry joint.

Philtre

Hmmm... v3 has a plastic sleeve around its base so I can't observe the filament there. A chopstick test doesn't indicate any loose connections, and solder joints seem OK visually, at least nothing obvious.

This is such an intermittent problem that I might not be able to reproduce it for some time.

jpfamps

If it's not the filament supply I've seen this problem caused by the muting circuit.

The HRD has a mute circuit that engages when the channels are switched to minimize popping. I've seen intermittent issues with this circuit causing the signal to drop out too.

Philtre

@jpfamps Interesting. I'm looking at the schematic but can't see what components might cause a problem. Could it be to do with R78 and R79 flaking out the 16v supply?

jpfamps

Philtre said:

@jpfamps Interesting. I'm looking at the schematic but can't see what components might cause a problem. Could it be to do withR78 and R79 flaking out the 16v supply?

Actually I've just looked at the HRDIII schematic and that feature has been removed, so ignore my previous post!

Philtre

@jpfamps No worries. There are two relays involved when switching channels, but don't think that would be the cause.

Danny1969

The low voltage supply created by the 2 resistors is used to power the opamps that run the effects loop and the verb. Does your amp pass no signal then a small amount of distorted signal that gets better as the amps warms up until it works fine ? if so then I think your problem is what ICBM suggested as I have another amp that does exactly the same thing although once mine has warmed up it does tend to stay on until the next time. I've been meaning to fix it for 2 years now but still happily giging it 

Philtre

@Danny1969 said:

Does your amp pass no signal then a small amount of distorted signal that gets better as the amps warms up until it works fine?

Yes, that's it. The signal drops out to nothing, and then becomes a very quiet crunchy signal (like just as after you put it standby) and then the signal recovers. And then it's OK after that.

I've been unable to reproduce the problem today, so don't really know where to go from here.

Danny1969

I would change the v3 valve, and then resolder the joints on the valve base and check there's good contact between valve pin and socket ... that should sort it

Philtre

@Danny1969 Thanks for that.

I removed v3 and reseated it. Amp is making a rather worrying rattling sound now...under investigation...

Edit - power tube has developed rattle. Maybe time for a re-tube. The amp is 3 years old, bough s/h.

Philtre

Reseated all valves. So far no dropout. Now, one power tube is rattling like crazy on startup but settles down when it's warmed up. Bought the amp s/h a couple of weeks ago, previous owner says they bought it 2-3 years ago, not gigged it but played at home regularly. Is it worth doing a total replacement of all valves?