Yamaha EMX640 blew out 7amp fuse

[DrGonz78]

Cool Roly thanks man!!

Especially for letting me know the recovery time concept as equal or lower for replacements. Now I see the light!  :tu:

[g1]

  You must sort out the transistors insulation from the heatsink.  The transistors meant to be insulated should have some kind of mica spacer or silpad, along with some kind of plastic bushing for the screws.
  If all the transistors are insulated, then you should not have any collectors shorted to the heatsink when they are all tightened (as the heatsink itself is floating, correct?).  If they all have insulators, one of them must be cracked or damaged.  Find out which one.

[DrGonz78]

Yeah I will keep checking to see why *if* the insulation is failing, but it did not seem to be the connection. I will even pull out some mica insulators and just go that route in the end too. The power board has the four screw mounts connecting to the heat sink that have trace connections to the board. Still I am gonna have to figure it out before I feel comfortable plugging it in to the main voltage.

[g1]

 Well, if the heatsink screws connect to traces, then you are probably ok.  Remove those particular screws and if your transistors are all insulated from the heatsink you should be good.
  Sounds like you have already done this but doesn't hurt to double check.
Edit:  Looking again, it appears the heatsink is grounded (it screws to chassis), so you shouldn't have any collectors or parts of the bridge rectifier shorted to the heatsink.
  You appear to have shorts from both + and - rails to ground.   Find the source of your shorts.

[Roly]

 :tu:

If you can mount the transistors and check they are fully isolated before you connect their legs to anything, do so.  Keep an eye out for damaged insulating washers on the screws; sometimes the spacer sleeve gets damaged at lets the side of the screw shaft touch the device tab.

Generally, with a mica insulator in particular you need to be sure that it and both surfaces are clean and free from any grit before you apply thermal compound.  They are very thin and brittle and grit can punch through.  Those rubbery insulators are normally much less picky.

[DrGonz78]

Yeah I ordered TO-3P silicon type insulators last night, so yeah mica would have been nightmare in this case. New insulators are probably needed here in this instance. This type of transistor does not require insulating washers on the screw so that is not a worry. Definitely will figure this out once new insulators arrive and will not proceed powering up unit till I figure this out. Thanks for the input guys!! :tu:

[DrGonz78]

Yup got it working now! It was the bad insulation and it was just strange that each side(screwed down) would be fine until I started screwing in the the other side. So the insulation was failing when both sides were tightened down and that just had me confused. Thanks to all that contributed to this success...
Enzo, Roly & G1 you guys Rock!  :tu: