Fender Hot Rod Deluxe Switching Problem.

[lovetone]

I have a hot rod Deluxe in for repair with a switching problem.

This is not the usual one where it switches channels it self.

With this the relays buzz when switched to OD, the output of the relay driver U3 is square wave hence the buzzing to the circuit is acting like a multivibrator.

The power supply is working as far as I can see perfectly, all components are good no dry joints, I have replaced U3 no difference, it does not respond to the more drive switch.

R97 (1.2K 2W) is good, Tested all diodes and transistors all good.

Anyone else ever had this problem?

[pompeiisneaks]

Not that specific problem, no, but the power resistors on that part of the circuit tend to over heat a lot, and can cause damage to the traces so they either break or become more like resistors as they get beat up. Make sure you're getting good continuity and the right voltages across that part of the circuit.

I recently did a cap job and the traces were wrecked. I had to solder jumper wires across the leads of the caps I replaced etc. to get it back into working order.

They're quite poorly designed as usual for newer fenders.

~Phil

[xtian]

+1. But if you confirm voltages are good in the switching system, and no heat damage to the PCB, I'd suspect the filter caps next.

Hmm...another test--pull out all the tubes and see if the relay(s) still chatter.

[Stevem]

I would call out the filters also for that low voltage power supply as the coils are chattering on AC ripple, not running on good dc.

[DonMoose]

If the +/-16V rails have almost no AC voltage on them (bad filters, resistors or zeners in the LV supply), then:

I'd be inclined to look at R89 and R92 - they provide hysteresis to the 4560 being used as a comparator. Or pins 5 and 3 on U3.

Then I might think about C48, C45 or C51

Is it JUST the OD or MoreDrive as well?

[lovetone]

Thanks for all the advice with this.

The fault was due to two faulty zener diodes and one regular diode.

These all tested good in circuit with the component tester, even the zener's still had the knee in the trace.

All good now.