Fender Hot Rod Deluxe hum

[soundguruman]

Maybe, First, the input jacks, the internal switches have to close. Replace input jacks if switches are not tight.
Second the soldering of the input jacks, quite often the connections crack. Re-solder the input jacks.
Replace the 68K resistors for the input jacks...replace the 1 meg resistor for the input jacks. R1, R2,R3 on my schematic.
Replace the first stage plate resistor R4.

Basically, all the soldering should be carefully re-soldered wherever it's cracked. Input jacks, controls, more often.
The resistors, grid and plate especially on the first tube, can go bad.
The resistor will read good, but it's defective. The resistor fails partially when it's powered up. Causes a buzzzzzzzzzzzzzz.

Just finished one with the exact same symptoms.
Replaced input jack, replaced 68K resistors, replaced plate resistor R4, re-soldered a few cracked connections on the controls.
now it don't hum at all.

The thing about these resistors, they are cheap junk. They read good on ohm meter, then fail, or are very noisy when powered up.
That's RIGHT. The resistors read GOOD on an ohm meter. Then fail.
I used metal film to replace them.
Now no more noise...

Hmmmm, should have used better parts at the factory.
I am kind of disappointed in the quality of parts that they built these amp with. Too cheap.

But what you said, it HUMS with nothing plugged in....
Reminds me that the input jack internal switches are not closing. Replace worn out jacks.

[guitician]

But it hums with the volume down to 0, how could it be an input problem? Those control connections that make the power ground connection where probably to blame there.

[DGstrat]

Get on R9 (V1B) & measure the Vac ripple.
If that decoupling capacitor is bad (C46) it can cause all sorts of grief.
http://www.blueguitar.org/new/schem/...ues_deluxe.gif

Jazz P is correct. I have also run into one of these amps with a defective C46 filter cap, where the hum was affected by the tone controls. While I have run into plenty of problems with these amps related to bad connections at the input jack, the fact that it is noisy with the volume control turned down would lead me to the cap(s).

I'd also recommend examining the solder connections between the tube sockets and the circuit board in the bottom of the chassis. Plenty of problems there in the past as well.

Good luck.

Dave

[fredcapo]

Thanks for all of the input. The first thing I did was check the grounding strap and burnish the metal. Thought that was the problem...but caps sound like a possibility though dc voltages were good where I checked but did not check all. Probably should have just replaced all the caps in the front end... Will try to get back to the shop before the snow...

[Jazz P Bass]

but caps sound like a possibility though dc voltages were good where I checked but did not check all. Probably should have just replaced all the caps in the front end...

But then you would never know what was wrong.

Check the caps for Vac.

[fredcapo]

OK. Looking at the schematic and thinking about it does seem to finger C46... But it is 60 Hz that s being amplified... 15 ms period...
Would not ripple be 120hz?

Anyway I'll try it tomorrow if it doesnt snow again... Thanks

[fredcapo]

Worked on this again...replaced the decoupling cap c46 as well as the bypass cap on cathode of v1b...no improvement as I kind of expected. But I think I know what is happening...sort of... After a few minutes powered on and for a second when first powered up, I see 700 microvolts of 60 hz on the ground around v1b, as well on grounds on the same pcb trace. This correlates directly with the hum that appears. When I put the amp in standby, the noise still appears on the scope. I have not yet traced the land completely. Beyond that, i don't know what is breaking down (other than myself)... I plan to add several direct ground wires to see if that gets things to an acceptable level... To recap, if I pull v1 the hum is gone and the hum is there even when the volume pot (located after v1a) s fully ccw.

[fredcapo]

OK it's fair to say this problem stumped everyone... To recap, after a brief warmup and for an instance on power up, a fairly loud 60 hz hum was heard with volume down, louder on the hi gain channel with drive down and master up. Raising volume seemed to diminish the hum.

The hum gone when V1pulled and gone when guitar plugged into power amp in.

Last thing I tried before quitting the repair business was replacing the 6L6 pair...which fixed it. I tested them and one showed a short after heating up for 10 min or so in tester.

Thanks wonderful Chinese tubes.

[guitician]

The heater shorted to the cathode and then the AC showed up on V1, which it amplified. Hummm, strange that the power amp by itself was hum free. I'm glad you found the culprit, and I'm not surprised about the cheap tube failing.

[headnamps]

Having almost the problem with a 2009 Hot Rod Deluxe III. Hum appears after cord is plugged in so both input jacks are grounding. Did the resold thing to bottom board for all connections. All tubes check good as well. Grounds are good thru-out the chassis, ground wires, shielded cable grounds, have checked everything already discussed. How for the clincher... plug cord in hums starts. Hum increases with normal channel volume and overdrive channel both drive & master. No reverb issues, grounds & coils good. Then after it gets nice and warm, the volume controlled hum, for lack of better words, stays then it starts producing a low level crackling sound randomly, but the volume controlled hum never stops. I have had my head in this amp for hours, checked everything on the board, BUT haven't pulled board out yet. I hate doing that to these types of "engineered to cause problems by designed and manufacture!" There is no place for rising heat to go inside the closed chassis except up thru and across the circuit board components and right to the top! Switches can get so hot they can & will burn your fingers to switch the amp off. The heat can & will destroy all the pots at the top over time and use. Funny they do seem to last ok for the warranty period, then they can become pain in the you know where, always in the shop, always something failing... but it is not only the Hot Rods, there are numerous brands of same type of design that will also experience the same long range issues. Just have a large wallet! Too bad Fender can't tell you the "fix", surely they have had these problems presented to them numerous times.

[The Dude]

Well first off, this is an 8 year old thread. You should start a new one for your specific problem. That said, I've worked on hundreds of the Hot Rods and similar. Yep, they break. But, when you consider how many of them are out there, I don't think they fail any more than most by percentage. They have some "usual" problems- tube socket solder, bad Illinois caps, but other than that, they are fairly reliable. Rising heat destroying pots? I have yet to see that one. If you're amp is getting that hot, I'll submit it's not a design problem, but a malfunction of some sort.

[Enzo]

I spent 30+ years as an Authorized Fender Warranty tech at my shop. They have made SOOO many of these Hot Rod amps. I agree there are some small details we look for, but cooking themselves to death? I never saw it. Darn few amps have vents to inside the chassis.

Over near the jacks is a green wire from the board to a screw terminal on the chassis. Make sure that is intact. And the rear cover panel is shielding also.

[g1]

Bias spec is 30mA per tube, which works out to approx. 50%. Many insist on biasing them hot at 60 or 70%. That is something that would make the amp run a lot hotter than it is designed to run.