C13 Clipped on Mark IVB?

[stevehollx]

I had to pull my chassis out last night to troubleshoot a no sound issue due to a power supply problem on a Mark IVB.

I found the culprit of the issue- a fried power resistor (R124). Poked it with a chopstick and the sound returned. That baby will get replaced with a fresh one.

Two things I noticed, though:

-The resistor that fried appears to possibly not be stock. I bought the amp second hand. I notice almost all of the resistors on the board are tan metal film vishay/dale resistors, and any other higher watt resistors are carbon resistors. This resistor (R124) is the only one I notice that is a Xicon resistor. Does anyone know what type of resistor is normally here? Or does anyone have some decent shots of a known-stock IVB PCB? I'm just curious if I should be suspecting that this resistor has blown in the past.

-While scoping around for the culprit, I noticed that C13 has been clipped (or broken). This is a 250pf cap bypassing R323 (220K) off of v4a's plate. I'm not sure if this is clipped by accident or intentional. Is anyone aware of a factory/non-factory mod that clips this cap? I have a feeling this cap is responsible for letting some highs bypass the R, and would brighten the lead channel up slightly. I'm going to connect it back, I was just wondering if anyone had any insight as to (if it was intentional) why it would be clipped.

 

[stevehollx]

Oh another question. The MV always seems to be at 1.8 or 2 on these amps. As a result, it's super touchy. I know that this is common in practically all Mark series amps, but while I have her open, I'm willing to fix it.

Has anyone ever tried throwing a 220k resistor across the wiper and leg of the pot to allow for a less touchy MV?

 

[phyrexia]

Talkin' Output or channel MVs?

 

[stevehollx]

Talkin' Output or channel MVs?

Oh yeah, Im talkin' output master.

I have to say I hate the way all of the volumes are designed on this amp. I have the clean volume nearly at 10, and the lead volume at under two to balance the two channels volume out, and then the master volume stays below 2 most of the time.

 

[JimAnsell]

Talkin' Output or channel MVs?

I have the clean volume nearly at 10, and the lead volume at under two to balance the two channels volume out, and then the master volume stays below 2 most of the time.

wha???

i've never had that problem on any of the 7 mark IV's i've owned.