Hot Rod DeVille switching Weirdness

[g1]

It's a bit confusing, the HRDIII schematic (attached) makes it more clear.
The 2 sides of the LED are Red and Green. Yellow is when both sides are on.
So you get no LED in clean mode, and yellow in drive mode? But no Led at all in 'more drive' ?

[Jro81]

It's a bit confusing, the HRDIII schematic (attached) makes it more clear.
The 2 sides of the LED are Red and Green. Yellow is when both sides are on.
So you get no LED in clean mode, and yellow in drive mode? But no Led at all in 'more drive' ?

Yes. No LED in clean. Then Green-ish when drive is engaged and nothing in "more drive" and it pops back to the clean channel.

Diode testing both sides the LED lights up green and red just fine. But when the "more gain" is engaged LED goes off like in the clean channel and flops back to clean.

[Jro81]

I suppose Green-ish and yellow is kind of subjective.

[Jro81]

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.

Yes, both sides like when I test each side with a multimeter. Everything in the "more gain" circuit works. Voltages test ok from all I can see with all of the test points. The FET's both drop to about 30 ohms resistance essentially bypassing the 100k resistors, so the more gain circuit is functional, but the relay trips back to the clean channel and doesn't feed that stage or come through the master volume any longer because that relay trips back to the clean channel.

[Jro81]

Maybe something in D18/D19 is causing some weird voltage issue that is causing the relay to pop back?

[g1]

Possibly, or Q3. First double check the reading at TP36 that it is switching polarity from minus 15V to +16V when going from 'drive' to 'more drive'.
Then measure E,B, and C voltages for Q3 in 'drive', then EBC voltages in 'more drive'.

[Jro81]

Possibly, or Q3. First double check the reading at TP36 that it is switching polarity from minus 15V to +16V when going from 'drive' to 'more drive'.
Then measure E,B, and C voltages for Q3 in 'drive', then EBC voltages in 'more drive'.

TP36 tests fairly close to spec. Clean: -14.88, Drive: -14.7, More Drive: +15.73. it's bit lower than spec, but close, and crucially the negative to positive flip is happening.

Where I'm not 100% certain is on what to expect for the transistor behavior.

Q3 Drive: E = -5.3, B = -6.03, C = -8.16

Q3 More Drive: E = +16.28, B = +15.78, C = -13.39

[mikeydee77]

The HotRod I was working on ended up having a problem with switchin as well hence the delqay getting back to you. In my case it was a dry joint on the relay. It was visible but not easily and the nature of the fault seemed be point to a cracked track. So frustrating yet good to get it isorted.

Here are some measurements I took. Let me know if I can check anything else for you.

INCORRECT VOLTAGES TABLE REMOVED

[g1]

Green-ish when drive is engaged and nothing in "more drive" and it pops back to the clean channel.

Is R104 (and it's solder connections) good?

[Jro81]

Is R104 (and it's solder connections) good?

Yes. R104 seems ok. I think I'm going to step away from this one for a bit. Thanks for everyone thus far that's chimed in, it is much appreciated.

[mikeydee77]

Apologies, my table of voltages was missing minus signs on the LED.

[mikeydee77]

Thanks g1, no excuses should have double checked. It was a heavy weekend is all I can say.