Fender Hot Rod Deluxe: Sound comes and goes intermittently

[davidkurutz]

I own a Fender Hot Rod deluxe that recently started behaving strangely. Upon power on/warm up/take it off stand by, everything works fine. it plays great on both channels. Then, after about an hour, volume decreases to nothing in about 6 seconds, making crackling sound as if it was put on standby while still playing. Amp power is still on, tubes still glowing. Sometimes, it will stay down for a few minutes and return on its own, but most of the time the amp needs to be turned off for a few minutes and restarted.

I generally keep volume for both channels at 3, so i am not pushing too hard.

The problem is independent of instrument, instrument cable, amp input, and channel selected. I have plugged the amp into several power sources and the behavior is the same.

The problem is that it cannot reliably be recreated. Sometimes it doesn't happen at all. Wednesday it happened 4 or 5 times within 2 hours, last night it happened once in 3 hours.

I have taken the amp to a repair shop, and the technician was unable to recreate the problem, thus unable to troubleshoot. We decided to go ahead and change all tubes, for the amp needed it anyways. The problem persists.

I work in IT, and understand the troubleshooting process, but not electronics/amps. What problem/component could potentially cause this behavior?

Thanks for your time.

[52 Bill]

I work in IT, and understand the troubleshooting process, but not electronics/amps. What problem/component could potentially cause this behavior?

Any number of things could cause this, but the first thing that comes to mind is the FX loop jacks. Have you tried plugging a cord connecting the preamp-out/power amp in jacks?

[davidkurutz]

Any number of things could cause this, but the first thing that comes to mind is the FX loop jacks. Have you tried plugging a cord connecting the preamp-out/power amp in jacks?

Thanks for your reply. I do not use the effects loop, but imagine that your solution is designed to bypass a failing component.

I will try this out tonight and report back. Thanks once again.

[52 Bill]

I do not use the effects loop, but imagine that your solution is designed to bypass a failing component.

The fact that you don't use the loop is usually what causes the problem. There is a normally closed contact on the return jack that becomes dirty or oxidized, and this will cause the signal to drop out.

[Robert M. Martinelli]

Hmm....
since you state that the problem starts after warming up, I'd say that what we have here is just another case of HRDDRS.

( The name I gave to this disease - Hot Rod Deluxe Dropper Resistors Syndrome, and it seems more widespread than the swine flu, I have another HRD with the same problem on the bench right now )

This subject has been dealt with already on the forum, so doing a little search could help, anyway.....

Briefly explained:

There are a couple of 470 Ohm 5W resistors ( R78 on the positive rail and R79 on the negative one ) used, together with a couple of 2N5353 zener diodes, to supply +/- 16VDC to the opamps and to the channel switching circuitry; these resistors get so hot they usually melt the solder joints. After many heating/cooling cycles, the solder joints' material crystalline structure changes, and the solder joints end up cracking, generating false contacts that cause the amp to behave erratically.

Hope this helps

Best regards

Bob

[CharlieP]

Hmm....
since you state that the problem starts after warming up, I'd say that what we have here is just another case of HRDDRS.

( The name I gave to this disease - Hot Rod Deluxe Dropper Resistors Syndrome, and it seems more widespread than the swine flu, I have another HRD with the same problem on the bench right now )

This subject has been dealt with already on the forum, so doing a little search could help, anyway.....

Briefly explained:

There are a couple of 470 Ohm 5W resistors ( R78 on the positive rail and R79 on the negative one ) used, together with a couple of 2N5353 zener diodes, to supply +/- 16VDC to the opamps and to the channel switching circuitry; these resistors get so hot they usually melt the solder joints. After many heating/cooling cycles, the solder joints' material crystalline structure changes, and the solder joints end up cracking, generating false contacts that cause the amp to behave erratically.

Hope this helps

Best regards

Bob

+ 1 there. If you open it up and check those areas, I would bet you would find excessive heat evidence such as scorching. This is a common problem. When replacing these resistors it has been recommended to leave about ¼ inch of space between the board and any components near them to allow them to cool.

[octal]

Those power supply resistors do sound like likely culprits, but these amps were also known for having the plate load resistors open and or go intermittent. The one I fixed turned out to be the plate load resistors for the phase inverter tube. If memory serves, they're 1/2 watt carbon film resistors and are a bit cheezy for the application. 1watt metal films will do nicely. It's suggested to replace all the plate load resistors (they're mounted on the pcb with the tube sockets if I recall correctly.

Nathan

[Euthymia]

+1 on jumpering the preamp-power amp loop.

Also, just for laughs, pound on the top of the amp cabinet (not control panel!) with your fist while it's acting up and see if it affects the problem.

Tube amps with intermittent problems actually do often respond to a bit of banging on the top, and it's good information to tell the tech if you have to take it to one.

[Mars Amp Repair]

Those 16volt + & - zener supply dropping resistors are originally 470ohm 5watt variety. I have had complete no rework success in that past year or so replacing them with 750ohm 7watt pc mount resistors.

Search the forum as there has been much discussion about his exact subject. There are a few different solutions. Select the one you feel most comfortable with. I've only mentioned my solution as it only requires merely substituting the resistors with 750ohm pc mount stand-off variety parts. What could be simpler than that.

You can also use 750ohm 5watt standard resistors & be certain to mount them up off the board as mentioned above to help dissipate the heat. glen

[Robert M. Martinelli]

+1 on jumpering the preamp-power amp loop.

Also, just for laughs, pound on the top of the amp cabinet (not control panel!) with your fist while it's acting up and see if it affects the problem.

Tube amps with intermittent problems actually do often respond to a bit of banging on the top, and it's good information to tell the tech if you have to take it to one.

Hi Erik,
he stated that the problem starts after about 1 hour of playing, before that the amp behaves normally, so I'd say the FX loop is not the culprit ( unless the FX loop's sockets are heat-sensitive ).

I would tend to confirm my previous diagnosis of a HRDDRS case.

+1 for ( lightly ) banging the amp's top when the problem occurs as a basic fault indicator that can point a tech in the right direction.

Cheers

Bob

[dynafreak]

Hot rod dlx problemo

When amp volume cuts out slowly "in about 6 seconds" it is often due to interruption of heater voltage. HRD amps often suffer from bad solder joints where the tube sockets are soldered to the PCB. I repair many of these. I always touch up the solder joints on the pwr tubes, even on new amps. I recommend you unplug the amp, drain any residual voltage, then carefully check all the tube socket connections using good lighting and a magnifying glass.

The problem with the dropping resistors stated in other posts is also good advice. that problem tends to cause erratic switching - are you having that problem?

[daveryder]

About the zener diodes part#

@ Robert - when you said

There are a couple of 470 Ohm 5W resistors ( R78 on the positive rail and R79 on the negative one ) used, together with a couple of 2N5353 zener diodes, to supply +/- 16VDC to the opamps and to the channel switching circuitry; these resistors get so hot they usually melt the solder joints.

did you mean 1N5353 zener diodes? My HRD shows the part# 1N5353 on the diode, a search on mouser for "zener diode 1n5353" gave me these results (zener diode 2n5353 returns no matches):

zener diode 1n5353

so my next question is: since the Mfr. Part# 1N5353B is not available, will 1N5353BG be ok to use? Sorry if this seems stupid, but I'm kind of bouncing around in the dark here...

Thanks for any help

[Jazz P Bass]

The G is not marked on the diode.
It stands for PB free.
Are you having 16 Volt issues?

[daveryder]

yeah, I believe I got the dreaded HRDDRS:

since you state that the problem starts after warming up, I'd say that what we have here is just another case of HRDDRS.

( The name I gave to this disease - Hot Rod Deluxe Dropper Resistors Syndrome, and it seems more widespread than the swine flu, I have another HRD with the same problem on the bench right now )

classic symptoms, opened it up and saw scorching on the pcb under the r78 and r79 5w 470 ohm resistors - I'll do the usual - mount the new ones off the board, dab of silicon between them, etc - I just thought I'd replace the zener diodes too, I mean they're only 39 cents. I just wanted to make sure I got the right ones, so thanks about the "G" info

anyway plan on doing what Mark Black recommended in another post while I've got the board out:

Also, after doing the work to get the boards out, this would be a perfect time to retouch (and that means adding solder - not just re-melting) the solder joints at:

- ALL of the jack solder points.
- ALL of the pot solder points (including the mounting bracket pins).
- ALL of the switch solder points.
- ALL of the LED solder points.
- ALL of the large capacitor solder points.
- ALL of the wire plug-on quick-disconnect terminals.
- ALL of the tube socket pins.
The large zener diode solder points (these may need to have their leads cleaned to take solder well if they have been running hot, and they almost certainly have).
And any other joint you see along the way that looks like it might be cracked or lacking sufficient solder.

Try not to bend the wiring between the two PC boards more than an absolute minimum - the conductors tend to break where they enter the PC boards and every time they are flexed they get a little closer to being broken. This is another reason to get all the solder work done while the boards are out for the resistor fix.

Actually, the HRD is my backup amp - I play mostly through my mesa boogie mark III le - but sometimes I use the HRD for practice, it's so much lighter to carry around. The mesa is a back breaker!

[Euthymia]

Hi Erik,
he stated that the problem starts after about 1 hour of playing, before that the amp behaves normally, so I'd say the FX loop is not the culprit ( unless the FX loop's sockets are heat-sensitive )

I have had many cases of dirty FX loop that didn't manifest until the amp had been on for some time. My theory about what happens is that the metal in the jack warms up enough to flex and move a little, which breaks the connection. Mesas seem most prone to this, but the HRD, with the heat rising up to that top control panel, is a good candidate as well.

In any case, jumpering the FX loop is cheap, quick and easy, which is why I tell people to try it before they attack the problem in more invasive ways. If you jumper it and it still cuts out, there is something going on other than the FX loop. Of course, I have also seen plenty of amps that were cutting out, and I had to repair TWO or more issues that were causing that same symptom.

Low voltage dropping resistors AND dirty FX loop, with a side order of cracked joints on the tube sockets!