Hum problems from a Hot Rod deluxe

[bsco]

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.

[JoeM]

...
. 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?

[Jazz P Bass]

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.

[bsco]

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

[JoeM]

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

[g1]

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.

[bsco]

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......

[bsco]

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......

[Jazz P Bass]

[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.

[bsco]

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...

[Jazz P Bass]

I don't think the Blues Deluxe has a bias pot.
It's fixed.

[GregS]

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?

[bsco]

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...

[bsco]

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......

[g1]

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.