Amp Head blowing fuses

[Jazz P Bass]

Where, oh where are the test voltages?

You must have:
Proper & stable (minimum Vac ripple) +VE & -VE
+ 15Vdc & -15Vdc.
No more than 1.2 Vdc on the output transistor bases.
Less than 1 Vdc at IC8 pin 7.

These are no load, no signal test conditions.

[craig4739]

The +15 and -15 VDC are good. Out of town for a few days but will check the other voltages when I return

Thanks for your help

[craig4739]

Well, an out of town business trip and emergency oral surgery kept me away from this project for a short time but got back to it today.

the rails check out good at +- 15.7 volts and +- 41.5 volts. There is .01 volt on pin 7 of IC8. Unable to read any voltage on the output bases as I screw up the bias transistor TR11. got to get a new one first.

Stay tuned

[Tone Meister]

...something else to check would be that the output transistors and TR9 are all insulated from the heatsink. The collectors of the output transistors are connected to the case so they use insulting bushes and washer to prevent shorting. Check they are OK with meter...

Had this same thing happen with a similar model Marshall (MG100HDFX) a couple years ago. The problem turned out to be missing insulating washers from the screws holding the output transistors to the heatsink/fan assembly. In my case, someone had replaced the fan previously and lost a couple of those tiny insulators, which are designed to center the screw in the hole and keep them from touching. On the MG100, when those screws touch it creates a dead short when the output transistors are mounted.

A short piece of heatshrink tubing on the screws with missing grommets solved my problem. I wonder if your problem could be as simple?

[craig4739]

Good Thought. I know I saw insulators but will try powering it up with the transistors not attached to the heat sink. Just long enough to test a signal through.

Thanks for the post

[g1]

will try powering it up with the transistors not attached to the heat sink. Just long enough to test a signal through.

Don't know if I would recommend this, they heat up pretty quick. If you do it, keep a finger on them to monitor the heat. A better way would be to measure resistance from each power transistor collector to the heatsink.

[J M Fahey]

Good Thought. I know I saw insulators but will try powering it up with the transistors not attached to the heat sink. Just long enough to test a signal through.

Thanks for the post

Don't.
There's less destructive methods to test insulation, a.k.a multimeter on the ohms scale

[Audiotexan]

Don't know if I would recommend this, they heat up pretty quick. If you do it, keep a finger on them to monitor the heat.

Also, as a side note, years and years ago, my boss saw me doing that, and 'got onto me'. He then provided me with a nice little tip that I've made a habit of ever since, as it could potentially save a life.

If for *any reason* you decide to 'test thermals' using the 'finger method', then use the back of your finger. For a couple of reasons:

1. the back of your finger is more sensitive to heat changes, than the tip is!

2. God forbid, you ever slip (or get bumped into) and latch onto something that's 'live' (think HV), if your hand DOES constrict, your palm is facing you, and you at least have a chance at pulling away. If you do it finger-tip down, then there's a chance you could 'permanently latch' onto that HV you accidentally grazed!

[craig4739]

Great tip.....thank you.....One can NEVER be too careful when it come to electricity.

[Enzo]

A friend of mine was working on his Ampeg V4b chassis, and reached for the panel, and his finger went too far down, and he applied B+ to the finger, which clenched his hand. he couldn;t let go. he wound up throwing the whole chassis across the room to get free. Made a chassis size hole in his drywall. And a renewed respect for a live circuit.