Ampeg PF-500 Fault Condition

[bobloblaws]

Hey everybody, today I'm taking a stab at a SS class D amp repair, an Ampeg PF-500. Fault light is on, preamp output is good, but no signal from power amp output. I've done some troubleshooting and suspect either T2-A or L15. Unfortunately I don't even know what these components are or what they do exactly so I could use some guidance. Maybe it's something else entirely?

I'm measuring DC voltages from HGND. Output of BR-1 rectifier is 330VDC. On the anode side of D1 I measure 165V but on the L15 side of C34 things get dicey.After powering on if I check voltage it starts at about 130V and then steadily drops to about 0.4V but never completely settles. But this only seems to happen when I am measuring voltage. If I remove my probe and wait for a spell then apply the probe again it basically picks up where it left off. On the other side of T2-A where I am expecting something in the neighborhood of 82V in fact I have only 0.2V, but this reading fluctuates somewhat as well, never settles. Any help is much appreciated. (schematic attached)

- Bob

ampeg_pf500_main_power_supply_2034544-01-c02_sch.pdf

[Enzo]

OK, you realize this is an SMPS? And that they switch at very high frequencies? T2 is working at those frequencies, and your meter will never remotely respond to them. You either have the 82v rails or you don't. It could be anything, even those two parts, but they would be the absolute last things on my list of likely failures.

Are D8/10/24/25 shorted?

Actually if the preamp works, it is likely your SMPS works. Check the power amp.

[g1]

There are a couple versions of the power amp schematic that have been posted here before. Let us know which of the attached matches your unit.

[bobloblaws]

There are a couple versions of the power amp schematic that have been posted here before. Let us know which of the attached matches your unit.

Thanks G, this one is rev J03 so that corresponds to "Ampeg Pflx 500 pwr amp.pdf"

[bobloblaws]

OK, you realize this is an SMPS? And that they switch at very high frequencies? T2 is working at those frequencies, and your meter will never remotely respond to them.

Actually, no, I had no inkling.

Are D8/10/24/25 shorted?

OK as far as I can tell.

Actually if the preamp works, it is likely your SMPS works. Check the power amp.

I'll give it a shot, thanks!

[J M Fahey]

SS class D amp repair, an Ampeg PF-500...... Fault light is on, preamp output is good, but no signal from power amp output. I've done some troubleshooting and suspect either T2-A or L15. Unfortunately I don't even know what these components are or what they do exactly so I could use some guidance. Maybe it's something else entirely?

I'm measuring DC voltages from HGND. Output of BR-1 rectifier is 330VDC. On the anode side of D1 I measure 165V

Besides it being an SMPS not sure you are fully aware that HGND is NOT "Ground", it is actually HOT and DEADLY, it IS sitting 165V from real Ground and , besides killing you if you touch it, it will also kill your scope and any bench instrument you "ground" to it.

Meter survives only because itīs "floating", powered by its own battery,so no reference to Ground.

Leave the hot side of that SMPS untouched, itīs apparently working so not your problem.

You can only work on the secondary side of that supply, including preamp and power amp (which I suspect bad).

but on the L15 side of C34 things get dicey.After powering on if I check voltage it starts at about 130V and then steadily drops to about 0.4V but never completely settles. But this only seems to happen when I am measuring voltage. If I remove my probe and wait for a spell then apply the probe again it basically picks up where it left off

HGND and Audio ground are NOT connected except by a safety capacitor or maybe just parasitic capacitance, I bet that wonky capacitor charges up and simply measuring discharges it, I bet thatīs what you are measuring, so not really significant.

On the other side of T2-A where I am expecting something in the neighborhood of 82V in fact I have only 0.2V, but this reading fluctuates somewhat as well, never settles.

You have there , as Enzo mentioned, a 200-500kHZ 330Vpp squarewave signal, easily 700W to 1000W power capable, since it feeds a 500W amp and accessories, which your meter will not read it but will happily die trying.

Even worse, there you have TWO power supplies (why make things easy when you can make difficult?)

PS1: the main one: U5 , Q1 , Q2 , T2 , it generates +/- 82V fr main amp.
"Safe" only to the right of D8-10-24-25 , I write"safe" only because itīs no longer MAINS referenced (notice it connects to GND, not HGND) but it has its own deadly 165V between rails.
Even 82V single rail to Ground will be quite a shocker.

PS2: the auxiliary one (Chinese SMPS must be a dime a dozen, not kidding): control and switcher integrated inside U2 (probably a laptop brick PSU type) + T3 (notice primary connects to HGND) , anything to its right is "safe".

Personal opinion and in good faith: you took one of the most difficult and complex: dual SMPS + high power Class D amps out there, to boot itīs unnecessarily complex because itīs quite dated, modern ones are simpler, I would leave Ampeg techs deal with it ... which will most probably replace a full board and call it a day..

Or at most try the gross, obvious suspects: shorted supply diodes, bulged caps and blown MosFets but not deeper.

If wasting beyond, say, 1 or 2 hours, Customer wonīt pay: "hey, I can get a used one on EBAY/Craigslist/Guitar Center for that"

The curse of all techs.

[bobloblaws]

Besides it being an SMPS not sure you are fully aware that HGND is NOT "Ground", it is actually HOT and DEADLY, it IS sitting 165V from real Ground and , besides killing you if you touch it, it will also kill your scope and any bench instrument you "ground" to it.

Meter survives only because itīs "floating", powered by its own battery,so no reference to Ground.

Leave the hot side of that SMPS untouched, itīs apparently working so not your problem.

You can only work on the secondary side of that supply, including preamp and power amp (which I suspect bad).


HGND and Audio ground are NOT connected except by a safety capacitor or maybe just parasitic capacitance, I bet that wonky capacitor charges up and simply measuring discharges it, I bet thatīs what you are measuring, so not really significant.


You have there , as Enzo mentioned, a 200-500kHZ 330Vpp squarewave signal, easily 700W to 1000W power capable, since it feeds a 500W amp and accessories, which your meter will not read it but will happily die trying.

Even worse, there you have TWO power supplies (why make things easy when you can make difficult?)

PS1: the main one: U5 , Q1 , Q2 , T2 , it generates +/- 82V fr main amp.
"Safe" only to the right of D8-10-24-25 , I write"safe" only because itīs no longer MAINS referenced (notice it connects to GND, not HGND) but it has its own deadly 165V between rails.
Even 82V single rail to Ground will be quite a shocker.

PS2: the auxiliary one (Chinese SMPS must be a dime a dozen, not kidding): control and switcher integrated inside U2 (probably a laptop brick PSU type) + T3 (notice primary connects to HGND) , anything to its right is "safe".

Personal opinion and in good faith: you took one of the most difficult and complex: dual SMPS + high power Class D amps out there, to boot itīs unnecessarily complex because itīs quite dated, modern ones are simpler, I would leave Ampeg techs deal with it ... which will most probably replace a full board and call it a day..

Or at most try the gross, obvious suspects: shorted supply diodes, bulged caps and blown MosFets but not deeper.

If wasting beyond, say, 1 or 2 hours, Customer wonīt pay: "hey, I can get a used one on EBAY/Craigslist/Guitar Center for that"

The curse of all techs.

Thanks JM, I appreciate the valuable insight. So I went and checked Q10 and Q11, the latter seems bad, there is continuity between all 3 pins. Digikey does not have the IRFB4227 in stock but they do have IRFB4228, voltage spec is lower at 150V/83A compared to the IRFB4227 which is rated at 200V/65A. They are both N channel mosfets. Would the IRFB4228 work as a replacement?

[bobloblaws]

Thanks JM, I appreciate the valuable insight. So I went and checked Q10 and Q11, the latter seems bad, there is continuity between all 3 pins. Digikey does not have the IRFB4227 in stock but they do have IRFB4228, voltage spec is lower at 150V/83A compared to the IRFB4227 which is rated at 200V/65A. They are both N channel mosfets. Would the IRFB4228 work as a replacement?

I just noticed Digikey had IXTP86N20T listed as a substitute so I ordered a few of those.