Blues Deluxe switching problem

[Alex R]

...tearing my hair out about this. The amp won't switch properly into drive. With drive selected the relays chatter, the led comes on but often not brightly, and the amp falls silent, presumably because the signal remains grounded at a relay. Or sometimes there is no chattering but no switching either apart from the drive channel remaining silent.

I eventually replaced the relays and all the caps and all the opamps in the switching circuit, with no effect on the problem. R71 is 100K, and all the other resistors check out ok, and all the diodes beep nicely. The first thing I did was to fit two +-15v supply dropping resistors, wired off the board.

At TP30, where the switching is initiated, when the selector switch is in drive position, the voltage remains too high to initiate the rest of the process. However, with the opamp removed (I installed a socket), the TP30 voltages are as they should be. The opamp is ok, I swapped in plenty of good ones.

Here are the TP30 voltages:

Opamp U3 in:
clean 2.9vDC (fulfilling the schem requirement of >0.6vDC easily)
drive 0.6vDC or slightly more (not quite fulfilling the <0.6vDC required on the schem)

Opamp U3 out:
clean 2.9vDC to 3.3vDC (varies, I think according to how long the amp has been on for)
drive -1.7vDC (again slightly variable)

The "Opamp in" condition, ie hovering around the 0.6vDC level at TP30, means that none of the other drive conditions as marked on the schem in the test point boxes are fulfilled. TP31 either stays at about -15vDC or sometimes produces a small positive voltage, which makes the relays chatter, but not switch. With the opamp out the TP30 voltage goes healthily negative.

If I ground TP30 with drive engaged the relays switch fully and drive channel comes on loud and clear.

One more thing. A slight low-frequency whine is just audible on the clean channel, slightly and slowly rising in pitch. It is there with v1 and v2 removed, but is silenced when the PI tube is out.

What am I missing here? It's one of those jobs that sits by the bench and keeps getting looked at, nor solved, put back... it's been here months! The amp has had a lot of use and also been stored; previous repairers have failed to solve it so the owner is happy to be patient... I'd love to solve it!

Fender Blues Deluxe Service Manual.pdf

[g1]

You have healthy 38VAC going to R59?

[Enzo]

OK. is there good +/-15v rails right at each IC?

TP30 stays too high at +0.6v? Ground it with a clip lead. Switch now? Oh, you did that already.

The diodes beep nicely? Fine, but this is a voltage sensitive circuit, so what voltage drops are across each diodes, especially D5,6,7.

Look at how the circuit works. There is 38vAC coming in through R59. Assume the drive switch is open. CR6 shunts most of the negative to ground. R60/61 form a 1/11 voltage divider, so 3.45vAC (positive side) enter CR7. Drop half a volt or so through CR7 and we get pretty close to your 2.9v. Now close the drive switch. Now all but the lowest half a volt of those positive waveforms are also shunted to ground, leaving a chopped off AC form of a half a volt peak. Run that through the 1/11 and we ought to have a very low voltage at CR7, too low to turn it on. R63 ought to pull that pin close enough to ground to get under the 0.6v. But don't forget that CR9 is the reference this comparator uses, so if its half a volt is low...


A whine? Grasp each relay body with your fingers. Is any buzzing/vibrating?


You replaced a million other things, why not those super cheap 1N4448 diodes? DO use that type and not the 1N4148 so common elsewhere.


Do we have the footswitch, and if so does it act the same way? Power off and measure the contact resistance across the cutout contact in the FS jack. If the contact has resistance, that raises the voltage going into the 1/11.

[52 Bill]

Do we have the footswitch, and if so does it act the same way? Power off and measure the contact resistance across the cutout contact in the FS jack. If the contact has resistance, that raises the voltage going into the 1/11.

You beat me to it. It could be a dirty foot switch jack NC switch.

[Alex R]

Thanks so much guys. Yes the 38vAC holds up. I will go check the jack. I don't stock 1N4448 but I have changed this policy and ordered some! I'll let you know what works.

[Alex R]

It's nt the jack - same problem using footswitch, no resistance across the cutout, no change to problem with the cutout contacts shorted out from below. I will swap out all the diodes and let you know!

The +-15v rails are good at the ICs.