Hum problems from a Hot Rod deluxe

[Enzo]

Hi ogee, welcome to the forum.

Just a thought, it is better form to just start a new thread for your amp instead of tacking onto the end of a thread that has been dead for four years. You will get better response.

[ogeecheeman]

sure.if you pull V1 or V2 hum is gone. I have a laundry list of options to try so after I get back on it tomorrow I'll let you guys know what I have found.

[bsco]

I'll just mention this because it's happened to me, but it may not be your problem.
Fixing a non-hum issue in this style of amp, problem solved but now there is a hum.
a) Sometimes just putting the rear panel back on solves it
b) sometimes the rear panel foil is no longer properly connecting to chassis when re-installed
c) sometimes the ribbon cable has been moved in a way that makes it hum. The "dress" and bend of the ribbon cables can be critical.

I have had that problem with the back panel off...and I also had the problem of that foil coating on the back panel as well.....

[Yelly]

Well here we go...had one of these hot rod dogs on the bench last year.same problem.went through all of the usual fixes,re-cap power supply,rectifier diodes,tubes,reflow joints,check all grounding points. in the first amp the re-cap and rectifier fix solved it.this one not so lucky.I noticed in another post someone said they could kill it by switching on drive channel then turning the drive pot off. on mine, the only way to kill it is to switch to drive channel then turn master off.also if I turn up drive channel and turn on more drive the hum is unbearable.on the scope it looks like AC riding on the DC.checked cap c49 (22uf / 25v) checked good. replaced B+ caps.bias is stable.If I insert a plug into power amp input jack the noise is gone-completely.if I run signal from pre amp out to another amp noise is there.even if I remove all tubes except V1 & V2. heater AC reads normal although I haven't checked 100ohm resistors.interesting to note from the customer, when the problem first showed up it was intermittent.comes and goes. customer re-tubed.ok for a couple of days then he said it got to sounding terribly distorted. he brought it to me and one output tube was red plating. I subbed a good pair and no more red plating but still distorted and one tube sounded microphonic. swapped tubes and trouble followed the tube.after replacing all tubes,no longer microphonic but still distorted.suspected output tranny. customer insisted on trying to use it.a few weeks later he brings it in,very low power very distorted. open 100k plate resistor-pin 6 phase inverter tube. after replacing,clean channel now loud and clean,usable but with annoying hum. turn on drive channel not usable-unbearable hum. shorting pin 2 or pin 7 of V1 no change. shorting pin 2 of V2 kills it which is the same as turning down master volume.it's in the background in clean mode but in drive mode it's just wacked.any insight appreciated.

I´m a little confused of what you´ve tried and not, so far, but have you ruled out preamptube failure or maybe leakage through the psu 22uF cap?
Instead of shorting pin2/7 on V1, try pulling the tube. Is the hum gone?

[g1]

I'll just mention this because it's happened to me, but it may not be your problem.
Fixing a non-hum issue in this style of amp, problem solved but now there is a hum.
a) Sometimes just putting the rear panel back on solves it
b) sometimes the rear panel foil is no longer properly connecting to chassis when re-installed
c) sometimes the ribbon cable has been moved in a way that makes it hum. The "dress" and bend of the ribbon cables can be critical.

[ogeecheeman]

Well here we go...had one of these hot rod dogs on the bench last year.same problem.went through all of the usual fixes,re-cap power supply,rectifier diodes,tubes,reflow joints,check all grounding points. in the first amp the re-cap and rectifier fix solved it.this one not so lucky.I noticed in another post someone said they could kill it by switching on drive channel then turning the drive pot off. on mine, the only way to kill it is to switch to drive channel then turn master off.also if I turn up drive channel and turn on more drive the hum is unbearable.on the scope it looks like AC riding on the DC.checked cap c49 (22uf / 25v) checked good. replaced B+ caps.bias is stable.If I insert a plug into power amp input jack the noise is gone-completely.if I run signal from pre amp out to another amp noise is there.even if I remove all tubes except V1 & V2. heater AC reads normal although I haven't checked 100ohm resistors.interesting to note from the customer, when the problem first showed up it was intermittent.comes and goes. customer re-tubed.ok for a couple of days then he said it got to sounding terribly distorted. he brought it to me and one output tube was red plating. I subbed a good pair and no more red plating but still distorted and one tube sounded microphonic. swapped tubes and trouble followed the tube.after replacing all tubes,no longer microphonic but still distorted.suspected output tranny. customer insisted on trying to use it.a few weeks later he brings it in,very low power very distorted. open 100k plate resistor-pin 6 phase inverter tube. after replacing,clean channel now loud and clean,usable but with annoying hum. turn on drive channel not usable-unbearable hum. shorting pin 2 or pin 7 of V1 no change. shorting pin 2 of V2 kills it which is the same as turning down master volume.it's in the background in clean mode but in drive mode it's just wacked.any insight appreciated.

[bsco]

Thanks for your advice. I checked all the associated components and they were fine. Checked trace continunity as well. Then I started to check anything associated with the word drive...found a capacitor...C48 I believe...it is tied to the drive switch. anyway replaced that and the hum dissapeared. Also found out what was causing the irritating buzz in the drive channel as well. Apparently, the amp was picking up interference from the lighting over my bench. I am running three 40W incandesent light bulbs overhead....When I reassembled the amp the clean channel was super quiet but the nasty buzzing remained. When I took the amp off my bench and laid it on the floor, the buzz went away...I thought that dimmers and floresent lights caused this sort of thing...guess I was fooled. Anyway, I am a very happy camper right about now. I have left the amp on for awhile to give it a good burn in just to make sure there are no other issues. I want to thank everybody for their input and advice...I don't think I could have repaired this amp without it. One thing is for certain..I won't forget this problem any time soon...
Cheers.

[g1]

Grounding V1 pin7 kills the hum, so that doesn't leave much except the relay, R41, or R44. Make sure the one end of R44 is grounded. Check R41 & R44, resolder them and the relay, check for any DC on V1 pin7.

My apologies, had a brain fade and that statement is incorrect. Grounding V1 pin2 did not solve the hum, but shows that the jacks etc. should be ok. The problem should be between V1A pin2 and the volume control. I'm not concerned with the drive setting right now because you will always get more hum/buzz with the drive setting.

[bsco]

The hum is still there in both clean and drive. but it is much more noticeable when in drive mode. Also, when the drive mode is activated, and the drive control turned all the way down, if you turn up the master volume you get a buzzing sound in the speaker...if you then turn up the drive control, the buzzing is louder and then you also get the hum that is coming from the clean channel as well. When in clean mode, you can kill the hum by turning down the clean volume control. When in drive mode you can do the same thing by turning down the drive and master. Note that the hum in the drive mode is more buzzy than in clean mode. I haven't tried removing the relay yet, especially when I found that bad solder pad. Grounding pin7 of V1 will kill the hum in the clean channel but not the hum in the drive channel. You have to ground pin2 of V2 in order to kill the nasty buzzing hum in the drive channel. I am going to check the connections to all the associated components between these two points. If that don't solve the problem I'll pull the relay and try what Yelly suggested. Any other suggestions greatly appreciated. Thankyou very much for your time and advice....

[g1]

The hum is still there whether in clean or drive mode? When in drive you can kill the hum with the drive pot and likewise with the volume pot in clean mode?
Grounding V1 pin7 kills the hum, so that doesn't leave much except the relay, R41, or R44. Make sure the one end of R44 is grounded. Check R41 & R44, resolder them and the relay, check for any DC on V1 pin7.
Did you try what Yelly suggested as far as removing the relay and using a jumper?

[bsco]

Just put the chassis back in and guess what...the hum is still there. I believe that this is the Hod Rod Deluxe from Hell...
Oh well...back to the drawing board.

[bsco]

I did as you suggested and when Pin2 of V1 is grounded, no change...however, when Pin7 of V1 is grounded, the amp is quiet. So I have a problem in that area. Upon further very close inspection, I discovered that a solder pad was broken away from the circuit trace but you would never know it by looking and when a meter lead was placed there to check for trace continunity, the pressure would make the pad contact the trace. I am going to give all pads and traces in that area a very close inspection and temporarily install the board back into the chassis to see what the end result is like. I'll let you know how it turns out. Thank you, thank you very much for pushing me in the right direction.

[bsco]

JUst grounded Pin2 if V1 through a 1 ohm 2 watt resistor and no change. Grounded the other grid, Pin7 of V1 and the hum stopped. So I have a problem between theese two points. Note that when I grounded Pin 7 of V1 and activated the drive channel I got a buzz inthe speaker...I might have two problems so I will tackle he first one..adjustable hum with the clean volume control. Maybe once I find the cause of that, this other problem will dissapear also. I'll keep you posted.

[bsco]

OK. Thanks. I'll try that shortly and let you know what I find. I am beginning to wonder if some of the low level hum I am also hearing might be due to the fact tht the chassis is removed from the amp...never the less the problem I am having at the moment is definately not right....I will get back to you as soon as I try this.....Thank you very much...

[Jazz P Bass]

When you mentioned to ground pin2 of V1 did you mean to ground it directly or through a low value resistor?

You can use a resistor, a capacitor or a straight shot to ground.
Neither one will "kill" the circuit.
But each one can help to shed light, in a specific fashion, on the problem.