Hot Rod DeVille- gain switching problem- no yellow mode only red

[nickb]

TP34 is stuck at -16V yet CR22 has the correct -5.1V on it's anode . This tells as C22 is OK and so is R93. That leaves just one resistor R94 between the correct voltage and the incorrect at TP34. Also U3 has been replaced so that should not be it (it's prudent never to assume - I've been caught out my bad new parts before). Turn off and allow the +/-16V rails to go to close to zero then measure the resistance between pins 3 and 4 of U3. I'm expecting a short. If there is, look for a solder bridge between those pins.

[BStringThumper]

Hi, Everyone-

Thanks nevetslab & nickb for the additional ideas. I checked U3 pin 3 to pin 4 and there is no short. I tried another new BA4560 & NJM4560, I got the same -16 at TP 34. I tested R93, R94, both are good at 10k ohms.

CR17 was removed, it tested good, but I put in a new 5.0V zener just to make sure. CR22 is testing good, R101, R96, R93, R94, R23, R24, are all good. Q4 has been tested.

I'm going to replace Q1,2, with the correct J111. Any other thoughts?

Thank you for taking the time to read this stuff and your very appreciated advice.

[nickb]

Hi, Everyone-

Thanks nevetslab & nickb for the additional ideas. I checked U3 pin 3 to pin 4 and there is no short.

Darn!

I tried another new BA4560 & NJM4560, I got the same -16 at TP 34. I tested R93, R94, both are good at 10k ohms.

CR17 was removed, it tested good, but I put in a new 5.0V zener just to make sure. CR22 is testing good, R101, R96, R93, R94, R23, R24, are all good. Q4 has been tested.

I'm going to replace Q1,2, with the correct J111. Any other thoughts?

Thank you for taking the time to read this stuff and your very appreciated advice.

I think replacing Q2 and Q2 is pointless as until you find out why U3 pin 3 is at -16V it will never work. What resistance did you measure between pin 3 and 4?

[nevetslab]

I looked up the FET you had used, and it's the same pin-out, and Fairchild even stated 'See J111 for characteristics' on its' data sheet. So, it can't be the FET's.
The voltages at TP32 & TP35 are ok. You are NOT getting the 0.5V difference between Drive and More Drive...it remains the same. So, that voltage controling the base of Q5 is puzzling. That's about the time I curse at it. I can't see how C45 would be involved.

And, you're NOT getting the YELLOW LED to light. If ou have another Bi-color 3-leaded LED, I'd change that out. It has been stated before (maybe not in this thrad) that if it's bad, the circuit won't fully work. I've had the Drive NOT work, while the RED, YELLOW and No LED light status, with U3 or the JFET"s being the culptrits.

[Enzo]

If you get red but no yellow, that means the green side is not lighting. You should be able to test the green side by shorting over to the red drive.

[BStringThumper]

Hi,

Thank you all for the additional suggestions. Nickb, your suggestion to check for a short between pins 3 and 4 on U3 was excellent advise, there was in fact a
solder bridge between those pins. When I responded saying there was no short it would have been nice if I tested the correct pins?? DOH! I was looking at the solder side when I tested what I thought were pins 3 and 4. I wish I would have seen the tiny little solder string/bridge on the 3 and 4 pins. Enzo, before removing the board I did short Q3 and the green light does illuminate to make the yellow light. Nevetslab, thanks for taking the time to compare/confirm those JFETS. I wonder what data sheet I was looking at then?? Thanks so much for the help, sorry I missed something so simple as that solder bridge. I hope to have time to reassemble the amp by Monday and report back. Thanks!!

[BStringThumper]

SOLVED FIXED DONE.

The amp works perfectly now. My sloppy solder job on the those two little pins sure was a "make work program" for myself.

I learned a lot from my mistake. Another thing I learned is that I couldn't have done it without the skilled advice of everyone on this thread.
Thank you! I hope someday I can help someone else out with a problem.