MKIII PCB underside.

[msi]

Has anyone ever taken a photo of the underside of the mkiii pcb board? I'm trying to trace an issue with the RY2 switching, but the amp is really croweded in that area. I'm hoping a picture of the underside will help me trace the possible issue.

 

[Boogiebabies]

RY2 works in conjunction with LDR 3. If LDR 3 does not open the signal stops instead of shunting to
it's ground reference resistor that is used for controlling the volume change from Ch. 1 to Ch. 2.

First, check the Mid pot switch for continuity. No switch, no voltage supply. See if the foot switch works.
If the switches work but the issues is still present, check the voltage at the rectifier bridge above the 39?/50?-50?/39? resistors
on the bottom right of the board in front of the first and second 30uf/500V filter caps.
If you don't have around 6V DC, the 39? resistors or the diodes may have been cooked from
a power tube spike.

If you have voltage at the rectifier bridge, check if you are getting 6V at the 2 Purple wires on the Mid pot switch. If you are getting 6V, check for
a voltage drop when the switch is pulled. If there is a 2+V or so drop, the relay is working.

Lastly, if everything checks out, replace LDR 3.

 

[msi]

Yea, LDR3 is working. Initially I was thinking the relay was not switching, but that seemed to be working when I checked it with the multimeter. I have an idea what components are being switched in and out and when. I've identified what parts are where even with the errors on the official schematic. Such as there is an extra LDR on the MKIII that is not in the schematics. So what I'm trying to figure out is where a few components are connected under the board rather than just probing things wildly. I got it switching, I'm just trying to figure the discriptency out. I'm trying to avoid the hassle of removing the board myself as I'm sure someone has done it already.

 

[Boogiebabies]

Well slim, your on your own. I would rather put a bullet in my foot than to pull a MK III PCB.
I'm sure most of use have the same sentiment.

Are you cloning the circuit of the amp or fixing it ?

 

[msi]

No, not trying to clone it, just trying to figure out how it works after replacing a few parts and noticing the schematic was wrong.. It's my MKIII so there would be no reason to clone it. Specifically whats going on with the ldr that is missing in the schematic. It is off the cathode of V2A connected to a 15uf capacitor. It is switching the cap in and out of the circuit. When measured with the multi-meter it seems to be going to ground, but the trace goes to the underside of the board even though there is a ground trace next to it for R12 I believe. I'm asking because I want to know where the trace goes, or if I decide to mod it. With all that being said, I agree with pulling the board, which is why I was hoping someone already had.

 

[Mongo1]

Maybe some of the pics in this post might help you see past the clutter.

http://forum.grailtone.com/viewtopic.php?f=3&t=19477&start=14

This Simul-Satellite is built on a MKIII circuit board.

 

[msi]

Thanks, I can see whats going on under the relay in those pictures. But the two ldrs I was speaking about would be are obscured by wires. The trace of one of the ldrs goes under the board, and thats really what I'm looking at.

 

[msi]

RY2 works in conjunction with LDR 3. If LDR 3 does not open the signal stops instead of shunting to
it's ground reference resistor that is used for controlling the volume change from Ch. 1 to Ch. 2.

First, check the Mid pot switch for continuity. No switch, no voltage supply. See if the foot switch works.
If the switches work but the issues is still present, check the voltage at the rectifier bridge above the 39?/50?-50?/39? resistors
on the bottom right of the board in front of the first and second 30uf/500V filter caps.
If you don't have around 6V DC, the 39? resistors or the diodes may have been cooked from
a power tube spike.

If you have voltage at the rectifier bridge, check if you are getting 6V at the 2 Purple wires on the Mid pot switch. If you are getting 6V, check for
a voltage drop when the switch is pulled. If there is a 2+V or so drop, the relay is working.

Lastly, if everything checks out, replace LDR 3.

I do appreaciate your help though, don't misunderstand me.

 

[Boogiebabies]

No, not trying to clone it, just trying to figure out how it works after replacing a few parts and noticing the schematic was wrong.. It's my MKIII so there would be no reason to clone it. Specifically whats going on with the ldr that is missing in the schematic. It is off the cathode of V2A connected to a 15uf capacitor. It is switching the cap in and out of the circuit. When measured with the multi-meter it seems to be going to ground, but the trace goes to the underside of the board even though there is a ground trace next to it for R12 I believe. I'm asking because I want to know where the trace goes, or if I decide to mod it. With all that being said, I agree with pulling the board, which is why I was hoping someone already had.

A cathode circuit has to go to ground. It also has to have a cathode resistor, so the 15uf is being added in by the LDR as a cathode bypass
for more gain and frequency tuning. That 15uf cap on V2A has to terminate at the cathode resistor for that triode.