Ampeg PF-500 replace main board with generic Chinese EBay board

[chrisdski]

I have an Ampeg PF 500 bass head with the fault light on. I’ve read several posts on here about repairs of this board, which is pretty complicated due to it design and construction. Easiest would be to buy the replacement board from Ampeg, but due to supply issues they are out of stock.

I have a mad idea to Frankenstein the main board (which is a power supply and power amp combo) with one of these:

https://www.ebay.com/itm/354298126108

here are the schematics that I have for the PF 500

Ampeg PF500 Preamp.pdf

Ampeg Pflx 500 pwr amp.pdf

I do not have the output board schematic.

The eBay board can supply the correct voltage to the preamp board. I think I can figure out the other connections. The one big question is the 52 volt mentioned from the main board to the preamp. I’m not sure I can get that to happen. Any thoughts on what the 52 volt does thru the pre amp? Overall, is this viable? I only have $25 into the PF 500 as I bought it DOA as a project. Worst case is to flip it to EBay for someone else to play with it.

[g1]

Look at the fets near the input jack, Q1, Q2. They are running off the +52V.

[chrisdski]

I don’t think I can get the input AC voltage to that level DC. Bridge rectifier, Filter caps- but I won’t be able to lower the voltage easily, correct?

[GoldtoneWorkshop]

Doesn't look like the board you listed in the eBay link has something than can do the 52v, I just see +/-15v and 24v rail for a fan. May need to run a separate PSU for that rail?

Have you reached out to Full Compass? Not sure where you're located, but I've had success ordering through them or AudioLab in GA.

Edit: looks like AudioLab is currently out of stock. It says they have one that is on order and due 1-20-22 Their site is not the best though, so I'd reach out to them (or full compass for that matter) and see what they say.

[chrisdski]

Full compass has none in stock, maybe this summer. The board is $200. I’ll probably pass on the project as the board repair is beyond my knowledge.

[The Dude]

Those amps can be had used around $300. The factory replacement board is about $200 + shipping. Then there's labor. I certainly wouldn't go that route. If you do, be sure you are getting the most current revision board. Early runs of that board were plagued with problems. Have you tried troubleshooting the existing board? I would at least do some simple checks first before scrapping the original board.

Edit: Sorry, I was typing and missed the previous post.

[g1]

The current needed to run the preamp Fet's is small. I think a simple boost converter running off the 15 or 24V would do it.
Something like this: https://store.qkits.com/dc-dc-step-u...0-50-v-dc.html

[chrisdski]

The current needed to run the preamp Fet's is small. I think a simple boost converter running off the 15 or 24V would do it.
Something like this: https://store.qkits.com/dc-dc-step-u...0-50-v-dc.html

hmmmm. Well , let me do some more thinking on this. That certainly solves the 52V issue. At the lower right there is a 5V noted - is that an internal component of the power amp? If so, not an issue.

[g1]

At the lower right there is a 5V noted - is that an internal component of the power amp? If so, not an issue.

You mean on the power amp drawing? Yes, that would be coming from the main supply. I don't see any 5V on the preamp drawing so I think it would only be needed by the original power amp module.

[sugeda]

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[chrisdski]

Thanks for the full power amp schematic. Do you have the output board schematic?

[sugeda]

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