mark IV power loss

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mlaverty

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hi guys

i ve got a Mark IV when i play it looses power!if i put the stand by switch on and of the power comes back!and goes again

what could it be?

any clues.?

thanks
 
A bad phase inverter or possible the master volume pot.
Most likely, the PI. When you turn the amp off it kills the voltage to the tubes and it may come back to life after the recycling of the power.
A good pot cleaning would not be a bad idea anyway. They get dirty and scratchy after only a few years of use.
 
thanks
i'll go for a master pot cleaning!

little precision,it only happens when i use the mark4 with it's preamp
usually i plug my triaxis in the loop and have no problems
 
I forgot to mention running a patch cable through the loop. Since Rabies is a Boogie diagnostic genius, he should have told you.

If the loop returns shorting jack gets left open, it won't pass any signal or may cause intermittent volume drops.
 
the tubes are old!a few years maybe!but they still sound ok

how can i test the preamp and amp tubes?
 
I got a major power loss problem with my Mark IV (1991) some time ago and it was caused by a bad resistor and a bad capacitor in the power stage circuit. I experienced some intermittent sound cuts long before I lost the power completely. If you tried all of the above mentioned tips and the problem is still present, bring your amp to a tech !
 
I had the same thing, solved it by switching a 12ax7, but NOT teh phase inverter, the v2 was the faulty one.
 
Boogiebabies said:
I forgot to mention running a patch cable through the loop. Since Rabies is a Boogie diagnostic genius, he should have told you.

If the loop returns shorting jack gets left open, it won't pass any signal or may cause intermittent volume drops.

I think BB is on the right track here! I've had this happen myself when using the amp without anything plugged into the fx loop. I have replaced these jacks with heavier switchcraft metal body types and every couple years or so the return jack seems to get "sprung" and won't close the circuit. If your triaxis works fine in the loop return, this rules out problems with your phase inverter or master (global) volume pot as the fx loop is prior to the V2b and the MV pot.
 
I would say the first thing that you need to do is what BB suggested: plug a cable from the Effects send to the return or simply turn the effects loop off from the footswitch. After you determine the loop is working then you should start trouble shooting. I don't think that V5 is a problem either since the amp works fine with the triaxis in the loop which says: the return jack, PI tube, and output tubes are all working.

Replacing the jacks every couple of years is unnecessary maintenance as a Q-tip with some alcohol with clean them right up. It's just a bit of dust that gets in there and breaks the connection to make the amp thing there is a cable plugged into the jack. Every amp with a serial loop potentially faces this problem but it is an easy fix.


Greg
 
Boogiebabies said:
A bad phase inverter or possible the master volume pot.

Most likely, the PI. When you turn the amp off it kills the voltage to the tubes and it may come back to life after the recycling of the power.

A good pot cleaning would not be a bad idea anyway. They get dirty and scratchy after only a few years of use.

I have been having the same problems with my Mark IV-A as the one reported by the original post here. I was also finding it was solved by flipping the stand by switch.
Thank you Boogiebabies for your note. I will replace the PI tube and clean the master volume pot and the effects send and return jacks.
 

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