Mesa DC-3 LDR switching problem

[UnluckyFett]

Hi, I come here when I'm stumped because you guys are the best!

Ps: I'll post the schematic when I get to my desktop.

The amp I'm working with has a channel switching problem. Rhythm works fine but lead sound like an LDR isnt working, the channel switches but the sound is super thin and low gain. I've swapped tubes to no avail but as I was sitting there looking at the schematic the channel faded into what it's supposed to sound like worked for 5 min and faded back out. So I believe the LDRs are working fine, it's a voltage problem.
The blue wire off the pt is only 50vac vs the print which says 70vac. I know mesa prints are less than accurate so that probably not a big deal. Off the tip on the footswitch jack I'm only getting 11v the print calls for 22v which is explained by only 50vac off the pt. On the leg of ldr2 there's only 1.5vdc

I also just realized I have 1.5vac on the chassis.

Is the 50vac off the pt a problem?

Could it be a bad cap? That would explain leaking the 1.5vac to the chassis and the fade in fade out of it working.

If a bad cap it has to be one of the 2 in the low voltage power supply right?

Sorry for the scatter brain post. I've been up all night trying to figure this out.

Thanks for all the help, past present and future.

mesa-boogie-dual-caliber-dc-3-schematic.pdf



Edit: I changed the batteries in my meter and my voltages look better. Did some more testing and I think the 4N33 might be at fault

[Mick Bailey]

What points did you measure to get the 1.5vac reading? You could be picking up on the ripple from ground currents in the chassis.

[UnluckyFett]

I was checking 2 ground points if i remember right. my meter was acting up so who knows, but thats fixed now. I was confused when my b+ was 30vdc lol. Any insight? Im still leaning on bad ldr. I havent pulled them out to test but based on the assumed part number the ldr side is something like 150k when on everything i found said it should be around 100ohm, also 1 ldr has 26vdc on it while the other has 3.3v. Im lost on this thing.

PS is it normal for ldrs to be microphonic?

[Mick Bailey]

Measuring between two different ground points can show up a low voltage with any amp and is not in itself a fault.

See here for testing Vactrols (the name for the LED/LDR combined device);
http://music-electronics-forum.com/t33158/

Any component can be microphonic, but how much this shows up depends where it is in the circuit. You commonly get microphony with plate resistors and coupling caps especially in high-gain situations.

One vactrol is always on and the other off, depending on the channel selection, so the readings will be different. Your voltages don't make sense to me; you're getting 26v on the LDR but 11v on the footswitch. The LDR voltage is derived from the footswitch voltage at point H. I suggest measuring the voltage on the back of the footswitch socket to verify the point H voltage is 22v. It should toggle between 22v and 0v depending on your channel selection.

Are you 100% certain that your meter is good and the battery is fresh? Whenever I get whacky voltage readings I always suspect my meter battery.

[UnluckyFett]

OK, I got the issue with my meter resolved.

With the switch on lead I have 26vdc at the foot switch jack and ldr2. I have 3.3v on ldr2 and ldr1. The other side of ldr1 reads .5vdc.

When testing the vactrols, ldr2 reads .77vdc one way and shorted the other. Ldr1 reads .26vdc in both directions.

Is it safe to say ldr1 is the culprit?

[The Dude]

I'm guessing you mean ldr2 reads .77vdc one way and OPEN the other direction. If it read shorted one way it would almost certainly read shorted the other way.
As to ldr1, you are also reading across the series resistance of a 100 ohm and a 47 ohm resistor. Testing it in circuit this way is not going to yield proper results. You'll have to unsoler one lead and test it out of circuit.

[Mick Bailey]

As the LDR side is in the signal path you could measure each LDR resistance to see if it drops to close to zero when 'on' (it should toggle on/off with the channel switch). These devices nearly always fail to a high-resistance state. I usually do this with the amp in standby as it's safer, plus sometimes there's a marginal grid voltage with some 12AX7s that can affect resistance readings.