Hot Rod DeVille switching Weirdness

[Jro81]

I know, right? Another HRD with switching issues. I've read through most everything I can find and I haven't found anyone with this issue yet. Hopefully it is something obvious to someone more experienced.

I've changed the typical low voltage supply parts, caps, resistors, zeners. But that was prior to getting the amp in a state that I could test the switching. So, unfortunately I don't know if I caused the issue or not. I'm generally tidy, and neat and careful with my work, so I don't think that what I did is the issue.

Anyhow, getting to the point. Clean to drive works fine, but drive to "more drive" doesn't work. The LED goes off entirely, and actually switches back to the clean channel. All the test point voltages seem correct or within reasonable tolerance based on the schematic notes for the TP's. -15.86v instead of -16 for instance. Only thing left to test that come to mind are the J111 FETs, the relay, and the LED itself.

Just odd behavior and was looking for any thoughts while I continue poking around to try and understand why it would behave that way as I'm somewhat of a novice.

[xtian]

The circuit will not function if the LED is burned out. Test the two LED elements with diode tester to make sure they're functional.

[Jro81]

LED seems to test ok. On to something else as the cause. I guess it may just be a really wonky relay.

[mikeydee77]

What is the behaviour when tapping the PCB with a chopstick all over the board? I often find HotRod problems originate from the flimsy PCB tracks and cracked pads, dry joints and earlier repair work related. Especially around the large resistor on the top left, near the jewel, Droppers and Zeners and the LED.

Wondering if the LED checking out OK could be a red herring as it is a bit of a dry joint weak spot - I believe that some users press the LED thinking it is a button.

[Jro81]

What is the behaviour when tapping the PCB with a chopstick all over the board? I often find HotRod problems originate from the flimsy PCB tracks and cracked pads, dry joints and earlier repair work related. Especially around the large resistor on the top left, near the jewel, Droppers and Zeners and the LED.

Wondering if the LED checking out OK could be a red herring as it is a bit of a dry joint weak spot - I believe that some users press the LED thinking it is a button.

Tapped around earlier in the process thinking maybe there was some wonky trace or bad solder joints and didn't get anything to happen. It's not jumping back and forth or anything.

Clean works, drive works. Then I hit the more drive button and the LED cuts off and it goes back to the clean channel. Kinda like the relay is bugging out

[Jro81]

As an experiment I bypass R23 and R24, which is essentially what the J111 pair are doing with the switch I believe, and it didn't seem to do anything. No extra gain or anything. It's like nothing switched. I guess maybe I'm missing something.

[mikeydee77]

Can you check out the push button switch itself, the FS jack socket and the associated components CR25, C49, R100 etc. dry joints cracked traces etc. Is the result the same using an external FS? Is the behaviour the same in standby?

I have a HR open here if you need some comparison measurements

[Jro81]

Can you check out the push button switch itself, the FS jack socket and the associated components CR25, C49, R100 etc. dry joints cracked traces etc. Is the result the same using an external FS? Is the behaviour the same in standby?

I have a HR open here if you need some comparison measurements

Those components seem to be fine.

Update: my alligator clips that I previously used to jump R23 and R24 we both broken. So after using new jumpers the gain is "more gaining" when those resistors are bypassed.

[Jro81]

Can you check out the push button switch itself, the FS jack socket and the associated components CR25, C49, R100 etc. dry joints cracked traces etc. Is the result the same using an external FS? Is the behaviour the same in standby?

I have a HR open here if you need some comparison measurements

At this point I'm suspecting the j111's or the driver circuit and potentially the relay.

What are the voltages you're seeing on the FET's? The schematic doesn't really list any test points for those. All voltages for test points for the switching circuitry prior to that are within spec.

[Jro81]

The FET's are functional. All voltages from the switching circuitry seem correct. I guess all that would be left would be the relay. As soon as the "more gain" is engaged (via panel, I don't have a foot switch) it just switches to the clean channel. There don't seem to be wonky or cracked solder joints. And I guess it would explain why the led doesn't changed

[mikeydee77]

I will take some measurements today... let me know if anything specific you need.

[Jro81]

I will take some measurements today... let me know if anything specific you need.

I can't think of anything specific honestly. All the test points seem to read fine. It's as if the relay K2, when you engage "more gain" just bugs out and flips back to the clean channel. No wonky solder or anything or components in the circuit seem to be out of spec. I guess I'm a bit out of my depth if I change the relay and it's still having an issue.

[g1]

The LED is a critical part. If it does not do it's job, nothing else in the 'more drive' circuit will. Solve that and the rest will follow.
You said you tested the LED. It is a 2 colour LED, were you able to make it shine red when testing? If not, it is not proven good.

[Jro81]

The LED is a critical part. If it does not do it's job, nothing else in the 'more drive' circuit will. Solve that and the rest will follow.
You said you tested the LED. It is a 2 colour LED, were you able to make it shine red when testing? If not, it is not proven good.

Yes. When I test it with the diode test on my multimeter, it lights up red just fine. When I engage the switch for more gain, all test points read as they should, and have even verified that the FET's are switching correctly so the more gain would happen if the relay didn't switch back over to the clean channel.

[xtian]

Yes. When I test it with the diode test on my multimeter, it lights up red just fine. When I engage the switch for more gain, all test points read as they should, and have even verified that the FET's are switching correctly so the more gain would happen if the relay didn't switch back over to the clean channel.

Sorry to repeat this...again... but the LED has TWO elements inside, with three leads exiting the LED package. You need to make sure both the red and amber elements are functional, NOT JUST the red one.