1996 Fender Hot Rod Deville Switch Channel Problem

[tomcurrie]

I have a Fender Hot Rod Deville on the bench currently....

When the amp is turned on, it automatically turns to the "drive" channel. Red LED lights up. I press the "channel selector" switch to try and get it to the "clean" channel but nothing happens. I can only get it to change channels with the external footswitch. I used contact cleaner on the switches but to no avail.

I took the board out and touched up the solder joints on the switches. I also checked the 330 Ohm 5W resistors that are a main cause for problems in these amps. They both test to spec. I also touched up the solder joints on these resistors to make sure.

Any ideas what else I could check out? I would really appreciate it.

 

[Jerry Glass]

Check the voltage on the low voltage rails that supply the channel switching, reverb and effects loop. If you dropped one rail (+ or -) it will do what you describe.

 

[tomcurrie]

Jerry, I have absolutely no idea where the low voltage rails that supply the channel switching, reverb and effects loop are. Could you please point me in a direction? Components to check, etc.

 

[stratified]

Google " hot rod deville schematic"

Click on the first hit

 

[WaltC]

occasionally one of the relays actually fails, but that's relatively rare. What are the voltage readings on either side of the 330ohm resistors relative to ground?

(you know to be careful in there because of the potential fatal voltages present, right?)

 

[Moddjobbs]

I don't have the schematic in front of me, but I recall there are 2 5 watt Zener diodes connected to the 330 ohm resistors you have checked out already. One side of each Diode should be grounded, and the end that is not you should have your 16VDC +/- (positive VDC on one, Negative VDC on the other) The power supply section of the schematic will help you see how everything is laid out.

Check for lifted traces and bad solder connections in this area. Walt C is right about the relays failing, but as he alluded to, it's much more commonly in the associated low voltage PS rail.

 

[tomcurrie]

The voltages on both 330 Ohm 5W resistors (R78 and R79) are as follows:

R79= -15.8v on "top". -33.3v on "bottom"
R78= 16.4v on "top". 33v on "bottom"

Voltages look good to me. I'm a bit stuck at this point.

Anywhere else I should be looking?

 

[WaltC]

in the back of my mind I seem to remember a problem I found on a Hot Rod Deluxe that had the same symptoms and I noticed that if I moved the channel LED the channels would switch. I desoldered the LED and the amp was back to operating normally. But that was a long time ago and I've slept a lot since then so....