Fender Hot Rod Deluxe Op Amp question

[DrGonz78]

I am currently troubleshooting a stuck in drive channel problem with this amp. Pretty sure I have deducted that the U3 BA4560 chip is bad and I am about to replace it. However, looking at the opamps U1 = TL072 and U2 = TL072 U3 = BA4560 according to the schematic. This one has U2 chip as BA4560 for the drive and return on the reverb. The reverb actually works just fine but I was curious if anyone else has seen TL072 chips switched to BA4560 before in this amp? When I replace U3 should I change U2 back to TL072 or does it even matter?

[Enzo]

They are just dual op amps. If I needed a TL072 and had none, I would stick a 4560 in its place in a heartbeat. Someone probably replaced the original IC during a repair.

Follow the notes on the left edge of the schematic, they will lead you through most switching repairs.

[DrGonz78]

Thanks Enzo! Yup I have been using the notes on the schematic to troubleshoot. All the voltages at the test points are way off and the -/+16v power supplies are intact. I tested the LED and it lights both green and red. I will definitely be back here for help though if a new BA4560 chip doesn't cure it.

[Pedro Vecino]

A week ago I saw one with two 4560 (reverb and switching) and one TL072 (loop). The problem with the amplifier (no lead, no more drive) came from a defective 10V zener (D28 in H.R.D. version III). 4560 on that circuit was fine.

[J M Fahey]

Define "way off"

[cjenrick]

measure voltage across relay coil when engaged, one lead of DMM on the plus 16 rail and the other on coil side or R104 - 270 ohm and read DC volts...

tap on relay with small screw driver and see if it flips on,

sometimes amp builders use too low of voltage to engage relay,

[DrGonz78]

Well the new chip fixed everything and all is working as it should. The relays were fine it was U3 that was dead in the water and I am guessing that U3 is a type of comparator circuit for switching logic using Q3 and Q4 as well.
Here is example of how "Way Off" the voltages were... LOL. Good for documentation purposes as well. All the resistors and zeners were testing fine as well as Q3 and Q4. So it just all seemed to just point right at U3.

TP31 (weird part was that R91 had 16v on one side and 0v on the other)
-0.26v normal = N
-0.26v Drive = D
-0.26v More Drive = MD

TP32
0.46v out (Panel)
9.5v In (Panel)
0.49v off (Foot switch)
2.61v on (Foot switch)

TP33
-8v N
-8v D
-8v MD

TP34
-9.6v N
-9.6v D
-9.8v MD

TP35
-9v N (Panel)
-9v D (Panel)
-9v MD (Panel)
-9.3v N (Foot switch)
-9.4v D (Foot switch)
-9.4v MD (Foot switch)

TP36
-7.7v N
-7.8v D
-8.1 MD

[Familyortiz]

Excellent troubleshooting! Very methodical.

[Enzo]

Are CR20,27 shorted? R91 should have +16 on top and about 1v on the bottom - the sum of the two diode drops. And that would explain the low voltage you have at TP31.

Tp32 is right.

TP33 looks stuck, bad IC?

TP36 stuck at -8 and TP34, 35 stuck at -9, again all pointing to bad IC.

[DrGonz78]

Are CR20,27 shorted? R91 should have +16 on top and about 1v on the bottom - the sum of the two diode drops. And that would explain the low voltage you have at TP31.

Yup that is why, when I started getting my voltage tables to troubleshoot, TP31 stopped me for a few minutes to investigate further. I tested CR20 and CR27 right away and they tested good.

[Enzo]

And did you have zero volts across the pair? That is what zero volts on R91 implies.

[DrGonz78]

And did you have zero volts across the pair? That is what zero volts on R91 implies.

Well yes I did have zero volts across CR20/CR27 since my reference was chassis ground. Now I have 1.2v on that side of R91 and .6v across each diode since replacing U3. The diodes are not shorted and I know my measurements were accurate as I had just put fresh batteries in the Fluke meter. Now I am trying to rack my brain on how there could have been zero volts on R91. Strange indeed. Just to mention though it wasn't zero on the dot... It measured -0.26 when the normal or drive mode button was toggled and -0.38 with more drive pushed in on the panel.

[Dave H]

Well yes I did have zero volts across CR20/CR27 since my reference was chassis ground. Now I have 1.2v on that side of R91 and .6v across each diode since replacing U3. The diodes are not shorted and I know my measurements were accurate as I had just put fresh batteries in the Fluke meter. Now I am trying to rack my brain on how there could have been zero volts on R91.

Perhaps the dead U3 you replaced was shorted from its pin 5 input to the -16V rail? That would cause the potential divider formed by R90,91 to put 0V on the anode of CR20