Fender Hot Rod Deville problem

[ArnC.]

I have a Hot Rod Deville which the clean channel sound fine but when I use the 2nd channel via pushhbutton, (this has the moredrive button) with more drive not turned on, I have super high volume at lowest pot setting and if I push in the moredrive, I just get major high pitched squeal. It almost seems like maybe both channels are on at once or something. Like I said the volume on the 2nd channel is super loud. Is maybe one of the relays not working or maybe I lost a resistor or something. I would appreciate some help on this as I am going insane trying to isolate the problem.

Thanks!
ArnC.

[Jazz P Bass]

The "Drive " channel uses the drive pot & the master pot.
Where is the master set at?
Do you have the schematic?
There are a bevy of test voltages available.
K1 & K2 are the relays.
Not engaged is the clean channel.
Did you try changing out V2 12AX7?

[ArnC.]

HRD

This would happen with master pot all the way down.
I do think that at the time I had just tried the clean channel and that stayed where it was set.
I have the schematics and yes I did change out all 12ax7's.
I changed a few caps(electrolytics, checked all solder joints and touched some up.
Also replaced the ribbon cables with good wire.
Just don't know where to look next.

Thanks!
ArnC.

[52 Bill]

Here are two things to check.

The Master Volume pot itself should be checked as well as its soldering to the PC board.

If you look at the schematic relay contacts K2A controls the Master Volume. If the contacts don't switch, then the MV is not in circuit and the preamp output volume will remain full on.

[ArnC.]

I checked the pot, it is okay, I also checked the pots solder connections they are also okay. I had a feeling about the relays, in fact I have an order for two of them ready to go. I will pull them out first and check that they are switching. Thanks!

ArnC.

[ArnC.]

Checked out the two relays

I removed the relays from the board and tried them with my power supply and meter.
Neither one of them is switching, both are stuck in normally closed postition.

On order: two relays!

Thanks!
ArnC.

[ArnC.]

I drummed up some relays that are suitable subs and installed them. It appears now that I have a bit more of control but can't put the gain above two, it is so loud! And again if I push in the more gain button, it squeals, like it is being overdriven. Man, this thing is killin me! Any more suggestions where to look for a problem?

Thanks again!
Arn C.

[Jazz P Bass]

What are the chances of two relays being bad?
If you really want to repair this amp, you are going to have to troubleshoot the circuit.
The main issue that I see is the drive channel.
Start at K1.
Clean Channel: With your meter leads on K1 pin 4 (or where R41 & R44 meet) & ground, rotate the clean channel volume pot. You should be reading the pot value as you rotate it. 0 ohms to 250K.
Now engage the Dirty Channel. Rotate the Drive pot. Now you should read the value of the pot. Same thing. 0 to 250K.
If these tests are good, you just confirmed that (1) the relay works & (2) the pots are good.
Test the points that Fender has in circles. These are Vdc test points. No signal applied. Write them down on the schematic.
If they agree, start testing the points that are in squares. These are Vac signal voltages. This is with a signal applied.
Troubleshoot from left to right. The faulty circuit should show itself as a voltage (DC or AC) that is incorrect.
Then & only then, when you pinpoint the troubled circuit, can more assistance be offered.( at least on my end)

[ArnC.]

HRD

Yeah, I figured so much. Time to pull out the meter and get to work. I did change both relays, It has to be something in the drive channel(ex: a failed resistor) that is letting too much signal through to make it overdrive prematurely.

I will start checking the test points and also your recommendations.

Thanks again!
Arn C.

[52 Bill]

Test the points that Fender has in circles. These are Vdc test points. No signal applied. Write them down on the schematic.
If they agree, start testing the points that are in squares. These are Vac signal voltages. This is with a signal applied.

On my schematic the ovals contain the AC signal voltages and the rectangles are DC test points.

[ArnC.]

testing

"Clean Channel: With your meter leads on K1 pin 4 (or where R41 & R44 meet) & ground, rotate the clean channel volume pot. You should be reading the pot value as you rotate it. 0 ohms to 250K.
Now engage the Dirty Channel. Rotate the Drive pot. Now you should read the value of the pot. Same thing. 0 to 250K.
If these tests are good, you just confirmed that (1) the relay works & (2) the pots are good".

I tried both pots and they switch properly but on each one I am only reading from
0- 49K This makes no sense? Could both pots be bad? Doesn't seem like they would?

Stumped


Arn C.

[ArnC.]

I removed the pots and checked them and they are okay. 0-234k

Arn C.

[52 Bill]

You were reading the parallel resistance of the pot and R44 which is 56K. Removing the pots did conclusively answer the question though.

[ArnC.]

Okay, I got rid of the high pitched squeal, wire orientation was the problem at v2. It still seems as tho it has way too much volume. I will attack it again tomorrow. To me it seems like it must be a resistor not doing its job. Just thinkin out loud.

Arn C.