Mark IV problem

[highroller]

Seeking some help !

My mMark IV (A) has very low power when Level cranked to the max on all channels but still have a nice lead preamp sound. I also noticed that the "Presence Pull-Shift" has no effect on LD channel.

I installed a fresh set of power and preamp tubes and connected another speaker cab and tested all modes but still have the same issues. I also connected a PODxt preamp directly in the effect return (power amp in) and I still have very little power.

Complete re-cap job done a year ago and amp always functioning well.

Seem to be something related to the power section, maybe LDR20 but not sure. Hope this is not the output transformer.

Any clues ?

Thanks

Dom

 

[highroller]

I measured voltage of power tubes at pin 4 (all 6L6):

V8 (A) Tweed : 385 v
Full power : 459 v
V6 (B) Tweed : 387 v
Full power : 461 v
V7 (C) Tweed: 387 v
Full power : 463 v
V9 (D) Tweed : 385 v
Full power : 457 v

 

[highroller]

hey Boogie experts...no solution yet ?

I will replace some Vactrols and see if it will resolve the power and presence pull-shift problems. I will also change the power tube sockets since the contacts are in a very bad shape.

Prefer to go the cheaper way first, then If I don't find the problem I will have no choice but to send the amp for repair $$$.

 

[zool]

The plate voltages are perfect. Like most boogie designs, you need to run a cord through the effects loop to see if it has a bad or bent contact.
Optocouplers have nothing to do with the presence functions. They are ceramic caps wired right on the back of the R@ and LD presence pots. When you pull them, they are in circuit. The only other device that would mute your circuit is the pull silent on the Master Volume. It cuts off the signal from the phase inverter and directs it to the record out via the graphic EQ.

Give it a whirl.

 

[highroller]

I remember a couple of time that I experienced sound cuts after connecting/disconnecting devices from my effect loop. I will check the loop like you told me and hope I will get bakc my Mark IV power and also resolve the presence pull-shift problem.

Thanks for your help !

 

[stokes]

Pin 4 is the screen not the plate.Plate is pin 3

 

[zool]

A screen grid resistor will barely lower the voltage. Those readings are typical for a MK IV even at the plates.

 

[stokes]

Yes,I am aware of that.Just didnt want the guy to think he had the plate when he didnt.