Fender Blues Deville Reissue switching problem

[nonost]

Hi! I have a problem with a Fender Blues Deville Reissue. It's not switching channels properly at all. Drive channel works fine with both Drive & Master knobs operating. But when switching to the "clean" channel the amp stays in the drive channel but with the Drive and(or) Master at max. Any of this knobs work there: Master, Drive and Volume are all off.

Relays are clicking and the ceramic power resistors are perfect with the right voltages. Actually, the whole PCB is in really great condition: clean and without any signs of damage. I thought about changing the relays (even though they "click"), because obviously the problem is there or pretty close. What should I check as well?

I don't like the scenario where I replace the relays and it doesn't solve the problem. And I think it's likely to happen...There are a lot of circuitry for the damn switching.

I cleaned all jacks, tried both inputs, used a patch cable from Preamp Out to Amp In and tried an external footswitch...The problem it's still there. Any ideas?

Thanks!

[Greg Robinson]

It's quite probably a fault with the low voltage power rails - check for ripple on the +/-16V supplies. IME C43/44 are likely candidates for failure, C38/39 can also be suspect, and of course R85/86 and CR22/23 (usually marked D22/23 on the board silkscreen).

[nonost]

Ey Greg thanks. What schematic are you using as reference? I'm attaching the one I got.

[nonost]

I've checked both relays without voltage. Coils read 1.4k, looks fine. I tested for continuity and one of the relays is missing it in one of the poles. The K2 relay passed the continuity test, this is the relay that turns on/off the Drive & Master pots. However, the K1 relay doesn't pass the continuity test in one of the two poles. This K1 relay, turs on/off the Volume pot. So it kind of makes sense: K2 works fine and disengages the Drive & Master pots, whereas K1 should engage the Volume pot but it doesn't. The result is all pots disengaged and full volume.

I would also like to check the voltages at the components indicated by Greg.

[Greg Robinson]

Well if one of the relays has a pole not switching despite proper voltages, that'll be your problem.
I was referencing this schematic.
Component designators are slightly different between the two, but you should be able to identify what I suspected as problem components by comparing the two.
Should have been more explicit in my previous post - R85/86 and CR22/23 (my schematic) R78/79 and D13/14 (your schematic) tend to overheat and desolder themselves.
C43/44 and C38/39 (my schem) C40/41 and C39/42 (your schem) often go bad and lose capacitance.

[nonost]

Thanks Greg.

I tested all 4 caps, but in circuit I don't get capacitance values. I think I have to remove them from the PCB. Do you think it's worth it? Caps look fine, and PCB around them it's also ok.

I tested for continuity between pins C and NC in both relays. And in one of them, the C pin is not connected to neither NC or NO. Would this replacement relay work?

https://www.te.com/commerce/Document...%7F6-1393243-8

One thing that I find weird is the bias. It's set at around 30mv, and it should be 60mv.

Thanks a lot Greg! I really appreciate it.

[nonost]

I forgot to add that the two diodes are ok.

[g1]

One thing that I find weird is the bias. It's set at around 30mv, and it should be 60mv.

Check the power tube sockets' solder, the screen resistors, even try one power tube at a time. If one of them is not running, you will get 30mV at the test point.

[nonost]

Hi g1, thanks for your help. I get 490v at both screen resistors, and around -53v at both grids. I have to check plate voltages. Maybe the bias is just off? It looks a huge difference for it though...

I removed the relay and off the board it suddenly passed the continuity test. Put again on board and it's working fine. I guess I should replace it anyways. All the capacitors referenced by Greg are ok and with the right capacitance.

[g1]

Hi g1, thanks for your help. I get 490v at both screen resistors, and around -53v at both grids. I have to check plate voltages. Maybe the bias is just off? It looks a huge difference for it though...

Try them one at a time. It won't damage anything. I think you will find one of them gives around 30mV and the other gives nothing. That would be a bad tube. If they both give 0mV (or something very low), try the other socket.

[nonost]

I'm back. I try them as g1 suggested. One of them gives around 18mv and the other one around 12mv. Socket placement didn't make a difference. Should I try to bias them? They are really off... I mean, is it safe?

[g1]

You said you found 490V at both the screen resistors, but you need to measure both sides of each resistor. It should be close to 490V at both sides of each screen resistor. If not, measure their resistance and replace as necessary.
If the screen resistors are good, check the resistor for the test point, it should measure 1 ohm. If so, go ahead and adjust the trimpot til you see 60mV at the test point.

[nonost]

Screen resistors and bias resistor are fine. I got around 2v difference across the screen resistors. With the bias pot maxed I get around 45mv, a bit more after it warms up. At this setting I'm getting around 470v at screen resistor. Anything else I should check before trying a new set of power tubes?

Thanks a lot g1.

[g1]

Check the DC voltage at the wiper of the bias trimpot, all the way clockwise, then all the way counter-clockwise.

[nonost]

I get -47v and -63v at the wiper's trimpot.