Hum problems from a Hot Rod deluxe

[bsco]

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?

[bsco]

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.

[Yelly]

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

[bsco]

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....

[Jazz P Bass]

Hot-Rod DeVille | Input Jack
Excellent web page.

[bsco]

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..........

[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.

[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......

[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...

[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?

[Jazz P Bass]

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

[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]

[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]

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......

[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......