Fender Hot Rod Delxue channel switching issue

[jazztastic]

Yep, yet another thread about channel switching on an HRD.

The first time around, it was an easy fix as the over-heated R78 & R79 were quickly identified. I replaced them with 330ohm 5watt resistors as well as replacing the correspond zener diodes.

Amp then worked great and channels switched as expected.

After troubleshooting other issues, I decided to switch back to 470ohm 5watt resistors and float them off the circuit board with Hi-temp epoxy.
I then noticed that once again the channel switching quit working. Stuck in Clean with yellow light on.

Here's my voltages:
either side of of R78: +48.7 and 17.3V (off top of my head but close)
either side of R79: -48.7 and -17.3V (off top of my head but close)

Drive switch On OFF
TP31 .84vdc 1.02vdc

They should be 1.02vdc & 1.37vdc

Question: If my voltages are off already at the first Test Point TP31, what would that point to as being the culiprit?

[Tone Meister]

Check LD1 and make sure it doesn't have any broken legs and that all 3 legs are soldered to the board. Sometimes the legs of the LED can get bent and touch each other or ground out, or even break when putting the board back into the chassis. I just saw one recently where the legs had somehow become de-soldered and that caused switching problems.

[jazztastic]

Checked LD1 to make sure it was not shorting out and touched up solder. All 3 colors seem to work as I accidentally discovered by shorting the legs with a small screwdriver.

[52 Bill]

What are the voltages at TP32? Are you using a ft switch or just the panel switch?

[jazztastic]

TP32 is good at .63vdc W/ footswitch; Select OFF.

That was as far as I got this morning. Footswitch LED's light up exactly like they're suppose to, just the relays never kick in.

[52 Bill]

Well, that's an entirely different thing then. What voltages do you have across the relays?

[jazztastic]

Will check when I get home tonite. Am I suppose to 16vdc on one side and 1vdc on the other?

[52 Bill]

There may actually be a negative voltage on one side, but in any case there should be enough voltage to turn the relays on. I would read the voltage across diode CR18 which is in parallel with the two relay coil windings.

[jazztastic]

CR18 reads 17.15vdc & 8.42vdc

[jazztastic]

Also, why would I be getting crazy high voltage readings on these test points:

TP2 225.6vdc (Nominal .1vdc)
TP5 229.7vdc (Nominal 0.252vdc)
TP8 235.5vdc (Nominal .85vdc)
TP11 224.7vdc (Nominal 13.3vdc)

[Enzo]

Because you are reading DC volts on test points with signal AC voltages specified.

Test point in a square are DC voltages, and test points in a circle are AC voltages.

[52 Bill]

CR18 reads 17.15vdc & 8.42vdc

I assume that these are dc voltages read to ground? If that is the case, then there is somewhere around 8 volts dc across the relay coil. This is with the distortion channel turned on?

[jazztastic]

Doh! Thanks Enzo, rookie mistake. I was going off of the 'hot rod deluxe test points spreadsheet' I downloaded. It only had TP16-18 as being (VAC).

[jazztastic]

Yes dc read to ground. Yellow LED does get brighter when channel switch is turned on.

Switch off: CR18 reads 17.15vdc & 8.57vdc
Switch on: CR18 reads 17.15vdc & 6.5vdc

[MarkusBass]

If would be nice if you posted the schematic of the amp. It's because this would allow you to understand that the 17.15vdc is not important (because this is just a power supply). Important it the other voltage which does not change as expected. The schematic contains also a detailed explanation of voltage changes when using the footswitch (or change the channels manually). It means that you don't need any spreadsheet because everything is provided on the schematic. I think that much more important are voltages in T33 and T36 test points (outputs of U3B and U3A).
It's funny because yesterday I was reading a Technical Note from Fender regarding simmilar problems with channel switching. And the explanation was that the problem was caused by 4560 IC from incorrect (not approved) supplier (I think it was Samsung). But I would check first voltages on outputs of U3A and U3B. If the voltages are incorrect, I would check voltages on the inputs.

Mark