1994 Mark IV B Switching Matrix Problem.

[ArrudaCamara]

Hi there people... First post here... (Although I always come here for help).

Well, My MKIV had no output sound at all... I found the R124 was fried and open. Replaced with another Resistor (6.8 ohm) and it also fried...
Just to trace the main cause of this problem, I replaced it with a 5w and then a 10w resistor... It gets very very hot (around 70c, 158f). Has anyone had this issue before?

While working, there is no heat when I select External switching and keep it in a "nulll state". The V1 Is okay and filament is as red as the other tubes. I can measure 14V before R124 and 12.4V after R124.
When I select any of the channels the resistor starts heating until I just can't touch it....
(current goes around 0.4 A) in the multimeter... Any idea of what it could be?

 

[BlackBoxy]

hi, it seems that the heater on MarkIVb V1 is dc, if this resistor heat, its because somethink behing leak it to the ground!

i would check vr5 and the resistor r332, and how 5v power output from it , first!

F 15v seems to work as you are able to use the footswitch, but maybe the C441 isn't working, would check it out, to find the culprit

 

[ArrudaCamara]

Vr5 works fine, output is 4.95v
R330 measures 100ohm and resistance to ground is 4kohm.
I'm afraid the problem is in the switching matrix... Just got no idea where....
I already replaced the two 1000kuf 16v caps and still have the same problem.... :?

 

[BlackBoxy]

Hi,

How could the switching matrix could get troubles on your power supply filter circuit?

what you got around is heater for V1 and 6v6, and 12v supply to switching matrix!

this resistor heat is because its bleeding to ground !

removing the footswitch does the same?

 

[ArrudaCamara]

I believe somewhere is bleeding to ground i just dont know where... with or without footswitch the problem still occurs. Could somebody point me the C431? I could not find it in the board... Also, resistor only heats when switching matrix is active (all positions but external switching).