Not your average Hot Rod Deluxe problem

[8wattjack]

 Hey guys. I have a HRD George benson model on the bench. This is the 3rd time this amp has been in and has been a major pita. I am usually patient and work through these fine but this one seems to not want to be fixed. Initially it had no channel switching, and had an unusual distortion at a certain frequency.  I did the usual: lifting low voltage supply off the board, recapped main filter caps, and low voltage supply caps. Someone else had worked on it prior, but didn't seem to have done any harm, but I found U3 and both relays to also be faulty. I relaced U3 and put Omron relays in. The second time in, the channel switching was still working well, but the lower freq. problem was back. It was very hard to reproduce and not consistent. All voltages seemed in check, and otherwise sounded great. After hours of searching with a magnifying glass i found the ground pin of the volume pot to be cold soldered. I desoldered and resoldered that and any others that looked the least bit wonky. This seemed to have fixed the problem until I started to reassemble everything and realized NO channel switching at all! I have went through the voltage checklist and cant seem to find the fault. I have checked all the diodes(in circuit that is) and they seem fine. I checked the switches and they seem fine.  No light, no click at all. Q3 has 16v on all 3 legs. Thats not right is it? I know the Benson schematic is slightly different but I don't have access and the switching seems pretty much the same unless I am missing something. Also NO mods were done of any kind beyond raising components off the board. Any help is appreciated.
 
19 -0.15vdc
20 -15.5vdc
33 16.25vdc
33 16.25vdc
34 -12vdc
36-15.58vdc

[8wattjack]

Pinout of u3 voltages.
 1=  16vdc
 2=  0vdc
 3=  11.8vdc
 4= -16vdc
 5=  1.35vdc
 6=  0.13vdc
 7=  16vdc
 8=  16vdc

I replaced and socketed all 3 4560's so I didnt have to keep pulling the board. I also verified that there is no change anywhere when I use the switch(that I found so far) and I have checked the connections at the footswitch jack (all good it seems). I verified 39vac at the input of r97, but this is where I get lost on these. I have a difficult time diagnosing something I don't understand, and I cetainly don't understand the "hall effect" function of these things. I cannot find any ac voltage present in the switching circuit beyond r97. D21 and D25 both test shorted IN CIRCUIT but with one leg lifted they test fine.?? All other diodes tested okay in the circuit without lifting. If the relays have gone bad again can they clamp the entire circuit? When they were bad before the switch still worked. I bought them from a retailer and they are omrons. Not sure why they would fail again. I alway discharge everything evey time before I pull the board on these to avoid spikes. ?? 90% of what I work on is these and peaveys. Never had a major problem till this one. There is still something underlying popping low voltage stuff. The only lytics left that werent replaced is the .47uf's because I didn't have them in axial and have yet to see one fail.

[BrianS]

-Why not pull and test Q3?
-Why would there be any AC voltage after R97?
-Why socket the chips?  Seems unnecessary.  Also, this would be considered a modification of the circuit.
-How did you determine that U3 and the relays were bad the first time around? Just curious.
-Judging by what I read on this page, it's very common for people to post "I did X to fix Y problem, now Z does not work".  What is the common denominator? Work was done.  Check your work.  It's highly unlikely that the relays and chip would fail again, unless a mistake was made that would cause them to fail.
I have found that the Peak DCA75 is a cool little tool that is helpful for testing transistors and some other solid state components.  A couple months ago I used to find a bad transistor in an old Sunn amp.  The transistor was not completely non-functional, but the measured hFE number was much lower than the same transistor used in other parts of the circuit.  Replacing that transistor with a properly functioning one was part of the repair solution.  My point is that the DCA75 can be a useful tool when working on solid state circuits.
Good luck.

[8wattjack]

 Thanks for the response. The first time around all the pins on u3 tested dead short and relay coils tested open so it was a no brainer. I am going to pull q3, but I have already redone the ribbons, so I am trying my best not to pull it anymore until I have a more clear path. I see that I don't have voltage on pins 2 and 6 which if I am correct comes from the pedal switch side?? I know these are common questions to you, but honestly I have done dozens of these and never had an issue. Usually the occasional op-amp, relay or plate resistor is all that is needed beyond fixing the obvious problems. I have never had to troubleshoot the side of the power supply that comes from the pedal switch, so I am frustrated with it. I will figure it out. I always do. Thanks for the response anyway. 

[BrianS]

Did you ohm out R97?  That one heats up a lot too.

[8wattjack]

Yes I did. I replaced it the first time and raised it off the board. I just tested d23 d24 and d28 out of circuit and d28 tested open. 600mv opens d23 and d24 so i think they are okay but I will replace them anyway since I clipped them in testing. Wouldn't you know I have plenty of 1n4448 but no 1n5240 10 v zeners. Gonna scrounge junk boards and see if I can find one just for testing.