Ampeg PF-350 repair

[xtian]

Arrived with blown 6.3A mains fuse and obvious smoke residue at FETs. I'm just starting to probe components to decide what to order. Schematics attached here.

First, component TH3 is measuring shorted, and I'm wondering if this is a fault. I THINK this device should normally be open. Part number is 67L060, and datasheet is linked here.




PF350 AMP and PSU ASSY.pdf

PF350 AMP and PSU SCHEMATIC.pdf

[g1]

According to datasheet, L indicates 'open on rise' so should be normally shorted til reaching spec temperature.

[Jon Snell]

Which FETs failed; Power supply Q1 & Q2 or amplifier output FETs Q10 & Q13?
If power supply check the clamp circuits on each side, both high and low.
If amplifier FETs, check the drivers after removing the output FETs.

[xtian]

This project has ended; owner declined to repair.

One observation: Ampeg made this unit so you cannot remove a single FET by itself--you have to unsolder all the components that are screwed into the heat sink first, then remove the entire heat sink...THEN you can remove individual components from the heat sink.

[nevetslab]

Well, easier to repair than a Roland KC 800 Keyboard amp.

[g1]

One observation: Ampeg made this unit so you cannot remove a single FET by itself--you have to unsolder all the components that are screwed into the heat sink first, then remove the entire heat sink...THEN you can remove individual components from the heat sink.

Could you not unscrew them (without de-soldering) and remove the heatsink? Or I guess the screws are blocked by other stuff?

[xtian]

Could you not unscrew them (without de-soldering) and remove the heatsink? Or I guess the screws are blocked by other stuff?

Yes, the screws securing the FETs to the heat sink are blocked by the filter caps.

[g1]

Yes, the screws securing the FETs to the heat sink are blocked by the filter caps.

Since they are owned by Yamaha, this reminds me of my Yamaha bike engine. It's almost perfect except you can't do a simple split of the bottom cases to work on the transmission. Of all the bolts holding the bottom together, there are 2 that are hidden away, requiring total removal of the heads and cylinders to split the bottom end.

[glebert]

Pretty sure this amp was designed before Yamaha bought Ampeg. The tight layout is pretty consistent with other class D designs. It seems they basically assumed board replacement, not repair.

[Enzo]

So let me understand, you have to remove one cap or two to access the hardware?

[Axtman]

I have repaired two Ampeg PF500 amps by just buying new main boards and swapping them out. I admire anyone that can do a component lever repair on those complex and compact PCBs.

[The Dude]

So let me understand, you have to remove one cap or two to access the hardware?

^^^^^^^^That. Wouldn't it be easier to just unsolder a couple cap leads?

[glebert]

In my limited and very frustrating experience with Ampeg class D amps, it is never just one component. There is usually glue covering half the board, sometimes multiple components in the way.

Like this PF500

[Enzo]

Yeah, looks like a pain.

[xtian]

Give it the "horse cure." I.e., shoot.