Thursday, January 19, 2017

Debugging The Channel Switching On A Hot Rod Deville

(Note: This probably applies to any other amp in this series that uses a similar switching arrangement-Ed.)
(Note: The numbers refer to the Hot Rod Deville schematic, Fender dwg no. 050384 rev, B)

I've had a Hot Rod Deville here for a while as I was trying to sort out a number of problems. The customer complaint was that it had a sort of a crackle as certain notes trailed away. I concluded that it was one of the usual problems that these amps are prone to-bad solder joints on the tube board where the phase inverter resistors are located, a bad R97 resistor (these get hot), or a problem in the +/-16v supply for the solid state devices  that this amp has some of. Also on the list of possibilities were the usual bad solder joints, dodgy filter capacitors and so on.

After some initial testing I discovered that although the bias supply was appropriate for the pair of power tubes this amp had on board, the bias voltage could not be adjusted. That led to  having to remove the main board again, installing a new 25k bias potentiometer (R82) and ultimately replacing the tube board with another.

Great. That problem was cured and it was only then that I found that the channel switching had  disappeared down a rat hole. Since the owner does not use the dirty channel he'd probably never noticed it, all of which led to me dreaming up an amp checkin and evaluation form  so as to evaluate any and all systems on board before disassembly.

The problem manifested itself as the relays started chattering and a lot of noise was being injected into the signal. The system was unusable. Also the dual LED would not light red as it's supposed to when "more drive" is engaged.

After I'd replaced a bunch of small zeners and diodes with no luck I got suspicious of U3, which is a BA4560 dual op amp.  I installed a socket (good thing when removing ICs) and plugged in a 4558 op amp which is said to be equivalent. It quieted the relays but I still did not have the 'more drive' function (which is a couple of FETs).

So back to the drawing board and I ordered all the small parts I didn't have including some real BA4560s. They arrived today and plugging in a genuine BA4560 cured the problem. I think I'm home free on this one.

Points of information. I'd been informed by someone else that had this same problem that installing  a pair of new diodes D26 and D28 cured the problem for him, but the entire episode leads me to believe that if any of the small solid state components in the switching system fails you're likely to run into the same problem.

Best guess? They're cheap, so take the schematic and buy a few of everything you will need to 'cut and paste' your way through this problem.

And beware of "equivalent part numbers."

In the schematic I've marked out the different areas of the switching circuit. 1 is the +16/-16v power supply, 2 is the the switching circuitry including U3, 3 is the 820 ohm 2w resistor R97 and 4 is the more drive FETs.

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