Peavey 300 combo blowing main power fuse

[Lownotebb]

Please forgive my "noobness"...trying to diagnosis recent aquisition of peavey 300 combo, 400bh power board, blows main power fuse immediately on power up...checked transformer output, checked rectifier diodes, replaced 35v filter caps, even though old ones did not prove to be shorted.
If I unplug red wire secondary tap, amp will light up and stay on, so seems to be short somewhere on higher voltage tap side....
Now I'm stuck....can someone give me a push?....Thanks.....Enzo?..

[nosaj]

Please forgive my "noobness"...trying to diagnosis recent aquisition of peavey 300 combo, 400bh power board, blows main power fuse immediately on power up...checked transformer output, checked rectifier diodes, replaced 35v filter caps, even though old ones did not prove to be shorted.
If I unplug red wire secondary tap, amp will light up and stay on, so seems to be short somewhere on higher voltage tap side....
Now I'm stuck....can someone give me a push?....Thanks.....Enzo?..

Red wire supplys the -+52v rails for the outputs
Check your output transistors for shorts.PV 400BH.pdf

nosaj

[Lownotebb]

I will do that and report back...Thanks for the push!...

[Enzo]

Agree, almost certainly shorted output transistors. If so, maybe also bad driver transistors. Very little else ever goes wrong with 400BH.

[Lownotebb]

At the suggestion of nosaj, found 1 output transistor shorted in all directions, of course it was the last one!
Going to replace all...can you guys direct me to the driver components that I should check also...be gentle...not well versed in finding my way around....Thanks

[Lownotebb]

Do I need to look at Q2 and Q12?

[Enzo]

I see no reason not to.

Look at the schematic. The drivers are Q3 and Q13. Q2, Q12 are pre-drivers.

Check all those 10w 0.33 ohm power resistors for opens.

Also check resistors R26, R54, R55, R58 for opens.

[Lownotebb]

I'm on it! Thanks again I will let all know what I find....do I need to remove all thise components for testing?...sorry

[Enzo]

The resistors ought to test where they sit. The transistors if they read shorted, then pull them and test off the board, after all if they are shorted they had to come off anyway, right? There is nothing in a circuit or on a board that can make a shorted component appear not to be.

[Lownotebb]

Makes sense....

[Lownotebb]

Okay.....here is where I'm at..R26 blown, R58 blown, Q12 shorted E-C, Q2 has voltage check readings of.24v B-E
.58v B-C
.24v E-B
.93v C-E or E-C
What is the significance of the 47ohm resistors being f.p.? What are good numbers for output transistors to replace SJ6357?.....Thanks again for keeping the new guy on track!
Have I about covered everything? What do you think may have happened to this thing??......

[Enzo]

What happened? Something failed, nothing more cosmic than that.

FP means flame proof.

2SJ6357 are MJ15003 type. You can but these at Mouser, or you can order direct from Peavey. I always bought my predrivers from PV, the 5332 and 5331, or whatever they were using.

[Lownotebb]

Just wanted to thank u guys again, got all said parts replaced, built a DBT and fired her up. All seems good.....like to close out the thread with a good ending, so many you never hear the turn out....

[Enzo]

Great, we hope you continue as a member here, and maybe take on other projects.

[Lownotebb]

Hunting for another now......