Hot Rod Deluxe Oscillation.

[brilliantblue]

150hz is right.

Treble knob slightly reduces the frequency - very slight.

Drive knob does not affect volume - Master knob controls volume.

I haven't had a chance to check test points yet or pot frame continuity.

I did put my finger on the relay to feel for buzzing, but due to the slight volume emitting from the speaker, I'm not clear on if I felt the relay buzzing, or speaker vibration through the relay. I have a dummy load that I'll try it with next, hopefully tonight, to clarify this. What I did feel was slight enough to make me wonder if I was imagining it in the first place.

[52 Bill]

150hz is right.

I did put my finger on the relay to feel for buzzing, but due to the slight volume emitting from the speaker, I'm not clear on if I felt the relay buzzing, or speaker vibration through the relay. I have a dummy load that I'll try it with next, hopefully tonight, to clarify this. What I did feel was slight enough to make me wonder if I was imagining it in the first place.

I would think that the relays would buzz whenever the drive was turned on, not just when the more drive was turned on. But that isn't a reason to not test the relays.

Do you have a scope? If you do try looking at the control voltage that is at TP36 and at the gates of Q1 and Q2. Speaking of the fets, what did you finally use to replace the dead ones?

[Enzo]

I am not married to the relay idea, I just have seen it in the past.

Remember, the way it works. In drive, both sides of the LED are on to make yellow. The current for the relay coil flows THROUGH the LEDs. When both sides are lit, there are two more or less parallel paths, with a 270 ohm resistor each. When you turn on More Drive, your mind want to think in terms of turning on more something, but in reality you are turning OFF one of the LEDs, so now the relays must get their hold current through just the one LED. SO that condition is more likely to fall short than when both sides are on. In other words, the more drive position is more likely to have relay issues.

[52 Bill]

When you turn on More Drive, your mind want to think in terms of turning on more something, but in reality you are turning OFF one of the LEDs, so now the relays must get their hold current through just the one LED. SO that condition is more likely to fall short than when both sides are on. In other words, the more drive position is more likely to have relay issues.

Good point about the LED. I forgot that the LED is part of the circuit that supplies the relay voltage.

I've had bad relays before, and early on there was a resistor change in one of these models because the relays would sometimes float between positions.

[brilliantblue]

So, I hooked up a dummy load and powered up the amp. There is a high pitched though faint buzz that seems to emanate from around the OT area. It only kicks on when B+ is on. I tried pulling the output tubes and applied B+ and the hum was near existent. Tubes back in, switching between Normal/Drive/More Drive made no change in the sound.

I could not feel or hear buzzing from either relay in any mode.

Regarding my question about the circuit-board mechanically grounding to the chassis: There is continuity between control frames. I determined that a junction of R87 was contacting a support when the circuit board was backed a bit from "home" position. Sorry for the alarm. Operator Error.

Still there is oscillation in More Drive

I replaced the dead jfets with J111's. The run around with the R87 grounding out may have caused me to believe that the originals were bad when they may have actually still functioned properly. Its frustrating to think that, considering the oscillation didn't exist back then, or at least it wasn't presenting itself then. Anyway, I'm going through the TP 31-36 with a multi-meter now. The schematic I have does not give any expected voltages for these TPs. I will look around on the 'net, but if any of you know what they should be and see a discrepancy, please let me know:

All using Panel Switches - no footswitch installed.
DRIVE
TP-31 = 1.06v
TP-32 = 9.75v
TP-33 = -12.66v
TP-34 = -5.13v
TP-35 = -0.5v
TP-36 = -14.57v

MORE DRIVE
TP-31 = 1.06
TP-32 = 9.75
TP-33 = -13.13
TP-34 = -4.83
TP-35 = -9.65
TP-36 = 15.23

Using the meter on the gates of the jfets I see
Q1 & Q2 = 4.65v in MORE DRIVE Mode, & -14.26 when in Drive

Thanks for your attention and knowledge.

EDIT - I did indeed find a mistake in my record of voltages, and corrected above. TP-33 was not changing polarity. Thanks for the eyes, Jazz P-Bass!

[g1]

The schematic in your first post shows the voltages. Area B8.

[Jazz P Bass]

TP 33 is changing polarity, when it shouldn't.

This statement makes no sense. "Using the meter on the gates of the jfets I see
Q1 & Q2 = 4.65v in MORE DRIVE Mode, & -14.26 when in Drive"

In your list of TP voltages you stated that TP 36 is -14.57 in Drive & +15.23 in More Drive.

These voltages (TP 36) are good.

BUT, they are one & the same as the J111 gates.

Something is not right.

[brilliantblue]

"
This statement makes no sense. "Using the meter on the gates of the jfets I see
Q1 & Q2 = 4.65v in MORE DRIVE Mode, & -14.26 when in Drive"

In your list of TP voltages you stated that TP 36 is -14.57 in Drive & +15.23 in More Drive.

These voltages (TP 36) are good.

BUT, they are one & the same as the J111 gates."


I take it all this goes together - but wouldn't that R87 load of 100k cause a voltage drop between TP36 and the gates?
If that isn't accountable for the amount of drop, would this indicate C44 or CR16 as possibly faulty?

[g1]

CR16 will prevent positive voltage from getting to the gates as it will be reverse biased. So I think you are ok in terms of switching voltages.
Looking back at the original post, am I correct that this problem occurred when addressing another issue and with the chassis out of the amp? My question would be whether the amp has ever been completely re-assembled and found to still have the issue?
I've seen a few weird problems in this style of Fender amps that only exist when they are opened up. I'd hate for that to end up being the problem.

[brilliantblue]

Thanks - g-one

The circuit board is currently in the "home" position, installed in the amp. I've kept it essentially hooked up the entire time, lifting it up and out to solder when replacing components. And, I've put the back on to see if the oscillation went away, but it did not.

I just discovered that I put the wrong voltage for TP-33 - it is not reversing polarity. I will re-check all test point voltages and edit that post as needed or post a new chart of results if its too late to edit.

Thanks again

[brilliantblue]

Happy holidays bump

[52 Bill]

Happy holidays bump

Happy Holidays to you too.
The only thing that I can think of trying is to remove Q1 and Q2 one at a time and see if the noise happens with one or the other removed. That way you can see if the problem relates to one or both of the tube stages.

Did you ever check c44?

[brilliantblue]

Happy Holidays to you too.
The only thing that I can think of trying is to remove Q1 and Q2 one at a time and see if the noise happens with one or the other removed. That way you can see if the problem relates to one or both of the tube stages.

Did you ever check c44?

I swapped out c44 since I had another in stock. No change to the oscillation. I'm beginning to wonder if the relay is buzzing, and its just not strong enough to be heard as a mechanical buzz.

I've yet to try pulling q1 and q2. I wish i could say there was no "trace" of me working in that area, but the traces are somewhat damaged there and I've hesitated to do more in that area. I have the continuity and discontinuity (no apparent shorting) that one would want to see in that area. However, tonight after work I'll try pulling either q1 or q2 and see if there is a change.

I've synced the oscilloscope to the oscillation and can test different points, but I'm not clear on where I should look first. If anyone has a point-by-point plan for what they would do with the oscilloscope and care to offer it, I'd love to try it.

Well, thanks again for everything!

[52 Bill]

I swapped out c44 since I had another in stock. No change to the oscillation. I'm beginning to wonder if the relay is buzzing, and its just not strong enough to be heard as a mechanical buzz.

Before you spend too much time looking at the relays, I'd look back at the fets. Try reading the resistance across the two 100K resistors that are across the drain and source legs of the fets with the more drive switched on and off. You should be able to do this with the amp still in standby mode. Do you get both high and low readings?

If you have the scope set up, I'd look to see if there is any ac component (at around 150hz) to the control voltages going to the fets gates.

[brilliantblue]

I tried responding to this earlier today. Seeing 100k at the resistors in Drive, through the roof in More Drive. Something's still funny in the jfets.