Mesa Boogie Studio Preamp channel switching problems

[Mikestorm]

Hi to all
I got a Mesa Studio Preamp in need of help.
The preamp needed a cap change, most of the signal path capacitors were open or shorted, so I proceed to replace them.
After all, I powered the unit, and the clean is working fine, but the odd cames when the lead channel is engaged.
The lead works, but the lead master volume and the fat option doesnt work.
So in order to see if the switching system works(this one uses ldrs) I unsoldered the vactrols and soldered a led in place of the led side of the vactrol.

here comes the strange thing:
the lead master and the fat option leds lights up in clean channel mode, and not in the lead channel mode as they should.

I've tryed to manually short the cell side of the ldr when the lead channel is engaged, and everythin works as it should.

The vactrols are fine, I've tested them, and just to be sure, I've made two by using leds and photocells, and they works just fine.

I've attached a copy of the schematics, just dont consider the mods in blue.

Any suggestions or idea?

Thanks in advance
Mike

[g1]

It's hard to follow what you are saying Mike. Not sure what was happening wired up normal, and what was happening with vactrols unsoldered.

[Mikestorm]

Thanks G1 for the reply.

Sorry, my bad, I know its a bit confusing.

the problem seems to be in the switching system.
In order to keep out a possible vactrol problem, I just unsoldered the led side of the vactrol(vtcl1 and vtcl4), and solder in place a common 5mm red led, just to see, if, while changing channel, the leds lights up as they should.

In clean mode, no led should lights up, acept the front panel clean led indicator.
As I mentioned in the post, I was seeing the fat option and lead master led light up, but this was my bad, I just soldered the led backwards...

So now with the leds soldered right, when I am in clean channel, only the clean status front panel led is on.

When I switch to Lead channel, the lead channel front panel status led is on, the two vactrols vtcl2 and vtcl3 are on(i've got drive sound nice and clear) but as the schematics shows, also vtcl1(fat switch) and vtcl4(lead master volume) should be on(leds should lights up) but they dont.

Its a couple of days i'm on the piece, but I cant see the problem. Seriouselly I'm stuck...

I took voltages reading, i noticed that across zd2, in clean ch i see 5.59v, and thats right because its a 5,6v zener diode, but when i switch to lead channel(the circuit puts to ground all 4 katode of the vactrol, I read 1,49v. Its a bit low for a led to lights up, so I decided to replace zd2, but the issue keep going on.

Any clue?
If there is something not clear, i will explain better...

[g1]

Ok, so you are just subbing in LED's to observe the switching commands. Good idea. Have you verified that your LED's have a similar forward voltage drop to the LDR's? I seem to recall the vactrols being fairly high forward drop. It may give incorrect results if they are not similar.

Unfortunately, the boogie schematics are notorious for being incorrect, so you will need to verify a lot of things just to not be lead astray.
The switching schematic shows LDR's 2,3, and 4 all in parallel. That strikes me as odd though I haven't looked at the logic states in context yet. Can you confirm that portion of the drawing is correct?
edit: the circuit looks like that is correct and 2,3,&4 LDR leds should all be in parallel.

[g1]

Check that LDR4 anode is connected to LDR 1,2,&3 anodes.
Also check that LDR1 cathode is connected to fat switch and fat switch is working. It appears that they have mis-labelled the switch attached to LDR1 on the switching schem., you need to verify. (they show it connected to rhythm brite instead of fat)

[Mikestorm]

Hi G1

Thanks for the reply, and sorry for my late reply.
Yes you were right! Electronically speaking, I was so arrogant, thinking that the old but working vactrol and the ones I made myself would work together... but not, as you suggest me, they have different forward voltages.
I was measuring 1.45v when lead channel was on, because, the two vactrol, were imposing their forward voltage, that is exactly 1.45v.
So just to see if, the switching circuit was working as it should, as mentioned for the 2 broken vactrols, I went and replaced all 4 vactrols led side, with simple red led, and everything worked as it should.
I confirm that all the led part/side of the vactrols are in parallel, except for the "fat lead" (yes, in the schematics I've posted is misslabeled as bright rythm) that is in parallel with the same orientation, but in order to be switchable, pass through a front panel switch(like drawn in the schematics).

If you have any questions, that may help some users in the future, let me know, I can take reading as I plan to end the mods on the unit for the next couple of days.

Thanks again
Mike

[g1]

So is there still any fault with the amp, or is it ok now?

For anyone else looking for info. later, I will mention that 2 other similar models whose schematics I referred to are MKII C+, and Studio .22+

[Mikestorm]

G1 I am making all 4 optocouplers, with all leds with the forward voltages as close as I can get.

I will keep posting updates.