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Thread: Hum problems from a Hot Rod deluxe

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    Supporting Member bsco's Avatar
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    Hum problems from a Hot Rod deluxe

    Hi. New to the form....however I have been viewing it for some time now and have decided to join up...Hopefully I can be of some help to members and they can be of some help to me as well...I am trying to repair a Hot Rod deluxe for a friend of mine which has a very bad hum problem which can be adjusted with the volume control....turn down the control and the hum goes away, turn it up and the hum gets louder the higher you turn the volume control. If you kick in the drive and more drive functions, then this hum becomes unbearable. New tubes, one new power supply cap,lots of bad connections, etc...replaced the plate load resistors because the amp was cracking.The amp has lots of volume but can't get rid of this nasty hum. It has dual 6l6's in it. The hum is there even without an instrument plugged in. Looks like there are new input jacks installed......Checked out the 47 uf/16V capacitor for leakage in the first stage......tested ok. Anybody got any ideas????I am open to any and all suggestions no matter how simple or bazzare they may seem.
    Thanks.

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    Quote Originally Posted by bsco View Post
    ...
    . Looks like there are new input jacks installed.....
    Missing ground connection somewhere? Did you check the replaced jacks to verify they were installed correctly? Are they direct replacements, or did someone wire a non pcb mount type (like a Switchcraft) and run wires to the PCB?

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    Last edited by JoeM; 01-16-2012 at 09:02 PM. Reason: spelling

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    Supporting Member Jazz P Bass's Avatar
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    Quote Originally Posted by JoeM View Post
    Missing ground connection somewhere? Did you check the replaced jacks to verify they were installed correctly? Are they direct replacements, or did someone wire a non pcb mount type (like a Switchcraft) and run wires to the PCB?
    Good call on that
    Seeing that the volume control changes the hum, that says that the problem is before the volume control.
    You may want to verify that the IC's are getting there proper voltage.
    Pin 4 (-Vdc) & pin 8 (+Vdc).
    There should be very little (ie: none) Vac ripple on those two rails.

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    Supporting Member bsco's Avatar
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    Thanks very much for the ideas. It looks as if the jacks are direct replacements...they are not "hand wired" to the circuit board and the solder connections are good..I checked that out. I also had thought of a bad ground in the first tube stage but couldn't find anything wrong....and there is no broken wires in that grey interconnecting cable between the mainboard and the small seperate circuit board that has the 1st tube. I had checked the high voltage supply for ripple and found it to be clean. I didn't think of checking the low voltage rails. I will most definately check it out and let you know what I find.....or don't find. I did put a jack into the pre-amp out and you can see the hum on a scope and can turn it up and down with the volume control for the clean channel and if you engage the drive channel, then that volume control will also turn the hum up and down. So is it correct to assume the problem is isolated to the first tube stage?? Then again, assumptions usually get you in trouble. I will modt definately check out the low volatge rails shortly.Thanks very much.
    Bernie

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    If you plug in a guitar, does the guitar's volume control have any effect on the hum? (in either jack)

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    There is a 10ohm resistor (R63) from the input ground to the pot bracket ground, check that it is ok. Any chance of a burnt ground trace on the input jack/pot board? Also make sure the pot nuts (if it has them) are tight.

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    Supporting Member bsco's Avatar
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    Plugged in a Strat and the hum does not change with the guitar's volume control. It is still there but if you turn up the guitar loud enough it will drown it out...but when you stop playing the hum is present. I checked the originating supply for the positive and negative 16 volts....seems ok. I did scope it and set the scope to ac and scoped both rails.....I had to crank up the volts per div to about 20 millivolts and I got an ac signal on the + and - 16 volt lines but was very blurry due to the fact that it was so low I guess. I then scoped the 48 volt test points across C39 and C42.....Set the scope up for 10 volts per division of deflection and I could clearly see approx 1 volt of ac ripple there riding on top of the dc level of plus and minus 48 volts. These two caps are connected to one side of R78 and R79...which are 5W 470 resistors....on the other side of these resistors you have your plus and minus 16 volts with the help or CR13 and 14 and C40 and 41....So it looks as if I have found my problem. I'll let you know how I make out......Thanks for the help......

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    Thanks for all the help everybody...however I still have the problem. I replaced C39 and 41 and tested them before I installed them...They checked fine. Also tested the ones that came out and they tested fine as well but I went ahead and replaced them anyway. Measured the anode side of CR15 and it has -52 volts....but that would be alright anyway as I can adjust the tube bias no problem. Checked for a bad ground from the main board to chassis, and also grounded the pot brackets to chassis ground as well with a temporary connection...still the same...the 10 ohm resistor checks fine. This is obiviously a grounding issue (or at least it appears to be judging by the hum)..... but I can't explain the ac ripple on the positive side of C39 and the negative side of C42.....and both sides have exactly the same amount. Wish I had another amp to check this against. I think I am going to give my aching head a rest for awhile and tackle this again tomorrow....electronics can be so cut and dry......

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    Supporting Member Jazz P Bass's Avatar
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    [QUOTE=bsco;244854]but I can't explain the ac ripple on the positive side of C39 and the negative side of C42.....and both sides have exactly the same amount. QUOTE]
    Move on from this issue.
    Actually, it is a nonissue.
    This point is the raw supply for the + - 16 volts.
    If the + - 16 volt rails are ripple free, that is good.
    So far I have not seen any voltage checks on the preamp tubes.
    The plate, cathode & grid voltage dc measurements can speak alot.
    For instance. There are coupling caps between each tube stage.
    They will have the plate voltage on one side & , theoretically, zero volts on the other side.
    If a cap is leaking (caps do not pass Vdc) it could be upsetting the next tube circuit & causing a hum.

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    OK. Thanks. This I shall check into tomorrow morning. Just a quick question...Is there very much of a difference between a Fender Hod Rod Deluxe and a Blues Deluxe Reissue..as they both have a drive function. The Blues deluxe does not have a more drive function. thanks very much for your time...

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  11. #11
    Supporting Member Jazz P Bass's Avatar
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    I don't think the Blues Deluxe has a bias pot.
    It's fixed.

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    It would be good to trace the signal path and find the point at which this noise is getting in with your scope. That way you can isolate the problem to a specific part of the circuit. Also is the ripple you're seeing 60 or 120 Hz?

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    Checked out the plate voltages on the tubes..VI..has 220VDC on pin1, 0V on pin2, 1.60V on pin3. pin6 has 217VDC, pin7 has 0V and pin8 has 1.62VDC....... V2 has 206VDC on pin1, 0V on pin2, 1.83VDC on pin3, 209VDC on pin6, 0V on pin7, and 1.77VDC on pin8..........V3 has 246VDC on pin1, 21.5VDC on pin2, 33.4VDC on pin4, 231VDC on pin6, 22.1VDC on pin7, 33.4VDC on pin8........V4 has 404VDC on pin3 and pin4, -45.7VDC on pin5,and 60mVDC on pin8.......V5 has 406VDC on both pins 3 and 4, -45.6VDC on pin5,and 59.9mVDC on pin8...... as far as scoping the circuits to see where the hum originates, can't seem to find it with the scope I have...it may be too low for the scope or the scope is loading it down in the pre-amp ckt...

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    Just got my hands on another Hod Rod Deluxe but this one was made in 1996. All the jacks on the input side of the amp are different. And the amp is super quiet...no hum in this one even when the volume is maxed out with no signal connected. Compared the voltage readings on the tubes in both amps and they are pretty much the same...The amp with the hum problem was made in 2002 at least that is the date code on the main board. I wonder if those input jacks are the right ones...but I have no way of knowing if they are..unless I get another one with the same date code.....I am going to contact Fender with this particular serial number to see what they have to say about it. Thanks everybody for the help so far...if anybody has any more ideas, I am all ears......

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    Does your newer amp use the 4 pin plastic input jacks? Apparently they changed brands of jack to neutrik around 1999. The newer style jack would have different holes in the board and not fit an older style board. So likely you have the correct jacks.
    You said the jacks look like they have been replaced, has the amp worked properly since then? I'm wondering if there is a cracked trace on the circuit board which was overlooked when the jacks were replaced. Also the legs on the jack can break but you might not notice because the good legs are still holding the jack in place.

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    Last edited by g1; 01-18-2012 at 02:30 AM.
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    Supporting Member bsco's Avatar
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    the older amp uses a jack type that has staggared pins. The newer unit has a jack that has the solder pins located at the four corners of the bottom of the jack. (mono)... so these must be the newer style you are referring to...they are soldered in place and flat to the board so they must be correct. Actually I was beginning to think I should take a look at the traces, but because the guitar played ok, I shrugged it off so to speak. And if I turn down the clean volume control, the hum turns down...so the problem has got to be before the volume control...... the tube has been replaced and voltages measured,components checked,.so the only thing left really are those jacks.........and it also happens to the drive channel. That volume control will also turn down the hum... Good idea.....I will have another peek tomorrow and post what happens....
    Thanks..........

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    Hot-Rod DeVille | Input Jack
    Excellent web page.

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    Thanks for the link. The jacks that are in this Hod Rod Deluxe are the newer type...I'll have to take a look today and post my findings....

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    As Jazz P Bass mentioned, you should measure DC voltages after the coupling caps too.
    But, I do not think this is the problem. First, I would narrow down the problem:
    You say that the hum disappears when you turn the Volume down. So the problem likely is just around this pot or anything before that.
    Since the problem also appears at the Drive channel, problem is unlikely the coupling caps C18 and C23, for they should both be leaking DC then.
    Are you positive, that V1 is ok? Try grounding directly at pin 2, what happens? Try pulling V1 out, what happens?
    Since both Drive and Clean signalpath goes through K1 Relay, have you checked this relay? If you don't know what to do, pull it, turn drive to 0, jumper wire wiper on Volume to joint R41/R44 and try the amp on clean, is the hum still there?

    You have to isolate more instead of changing components everywhere.

    all the best

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    Thanks Yelly for the advice. I have had the flu for the last week but I am feeling much better now. I'll check out you're suggestions and post what I find. Once again...thanks for the help.

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    I pulled V1 out as you suggested and the hum that was originally there went away but another non adjustable hum appeared which was like a buzzing sound that cycled on and off. It also occured with the drive channel activated as well. When I put V1 back this buzzing hum was still there. I then changed the 3 12ax7a tubes around and this hum went away...don't know what caused it...now I am back to the original porblem..These tubes are all new....When you mentioned to ground pin2 of V1 did you mean to ground it directly or through a low value resistor?

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    Supporting Member Jazz P Bass's Avatar
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    Quote Originally Posted by bsco View Post
    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.

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    Supporting Member bsco's Avatar
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    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...

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    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.

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    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.

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    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.

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    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?

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    Supporting Member bsco's Avatar
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    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....

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    Quote Originally Posted by g-one View Post
    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.

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    Supporting Member bsco's Avatar
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    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.

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    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.

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    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.

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    Quote Originally Posted by ogeecheeman View Post
    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.
    Im a little confused of what youve 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?

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  34. #34
    Supporting Member bsco's Avatar
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    Quote Originally Posted by g1 View Post
    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.....

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    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.

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