Another Hot Rod Deluxe switching issue

[Randall]

Hot Rod Deluxe won't go into More Drive mode. Based on my readings at test points 33 - 36, input voltages look good, outputs do not, I changed out U3. While I was in there I changed out the two big 470 5 watt cement resistors, cleaned up some heat damage, and raised up the new ones off the board. No change. I know the red LED is functioning, because if the amp is off for a bit, and it is switched on in More Drive, mode the LED goes red for a while. I can watch voltage at U3 pin 1 go from 15V with the LED red, and after maybe 20 seconds it drops down to just about 5v and then jumps to -15v with the LED yellow. I do not hear the relay click when it goes yellow.

Here is what I'm seeing compared to the TP notes on the left:

TP33
15.8v normal
-10.1 v drive in yellow
-10.8v more drive in yellow

TP34
-11.8v normal
-11.8v drive in yellow
-11.8v more drive in yellow

TP35
0.54v more drive out in yellow
-9.9v more drive in in yellow

TP36
-15.5v normal
-15.5v drive in yellow
-15.5v more drive in yellow

https://www.thetubestore.com/lib/the...-Schematic.pdf

edit: I just read another thread where lowell had the same problem, and he replaced U3 with a 4558, which as nickb pointed out, will not work. Well, guess what? I did the same thing, because a 4558P is what I pulled out if there. So it seems someone has already tried to fix this. I will get the proper 4560 op amp and see what gives.

[nevetslab]

edit: I just read another thread where lowell had the same problem, and he replaced U3 with a 4558, which as nickb pointed out, will not work. Well, guess what? I did the same thing, because a 4558P is what I pulled out if there. So it seems someone has already tried to fix this. I will get the proper 4560 op amp and see what gives.

^^^^^^ What Nickb said. Should do the trick.

[Randall]

Indeed! Same exact problem and fix. I put in a proper BA4560 op amp for U3, and a fresh 2N4401 for Q4 and all is well..... except for the bias being set to 122mA!

Thank you nickb!