Ampeg PF500

[pnut5150]

I have an Ampeg PF500 here that the fault light comes on as soon as amp is powered on. Of course I haven't had any luck with schematics or any repair/troubleshooting tips on the web. Any ideas? I have a feeling that it could be a transistor, but would like some info before I start pulling amps. Thanks in advance.

[DrGonz78]

Just so you are aware and anyone else, they have all these schematics online now. The portal link is...

https://service.loudtechinc.com/

Just sign up real quick and look up Ampeg Schematics > Portaflex folder. They are all available now.

[pnut5150]

Thanks

[Jazz P Bass]

I was on the Loudtech site.
Can't seem to locate the PF500 schematic.

So here is the PF 350.

Ampeg PF 350.zip

Having a 'feeling' that the problem is related to 'a transistor' is not a good sign of things to come.

These amps use a switch mode mode power supply & a Class D output configuration.(also switching mode)

Both can be a real bugger to troubleshoot.

[nashvillebill]

On the Talkbass forum, there have been many user complaints of similar failure with that particular amp, particularly the first generation. That does not mean that your amp failed in the exact same manner as all the others, or even that all the others shared a common problem, or even that that amp has any more problems (%) than any other amp! Nevertheless, the "consensus" over there isn't kind to that particular model.

[DrGonz78]

The PF500 is right there on the site Jazz. Not sure if the link here will work but it might if you are signed in to the site already. The thing is that it shows different revisions.

https://service.loudtechinc.com/loud/#category/1845

[Jazz P Bass]

Ah, it's a PortaFlex.

Thanks.

[Enzo]

Always be aware that problems are amplified on the internet. people have a problem, they go online looking for a solution. People that buy ne and have no problem don't do that. So you find a bunch of people with a problem in some model and no one without the problem. What that ignores is all the thousands of amps that never had the problem.

[pnut5150]

I've found a lot of posts online with the fault light being on, but I have not found any solutions or any causes of the symptoms..but with it coming on as soon as it is powered up, I'm sure there is a mosfet blown or a SM resistor open...somewhere...was hoping one of u guys may have worked on one of these before and could shed some light.

[Enzo]

The problem with that question is that so many things can turn on that light. What we d is we track down what is actually wrong.

The fault light is driven by the mute circuit. The mute circuit is triggered from two places, a temperature sensor and the power supply. They trigger a timer IC, which has its own little 5v supply off the +15 rail. it is itself a circuit, of course, so the timer itself could be bad as could the trigger circuits. The temperature circuit has its own little 5v zener supply as well. So that and the sensor and the comparator could be bad. The power mute is held off by the SMPS, so anything that causes the SMPS to shut down will trigger that. What does that? SHorted power amp, shorted rectifiers in the SMPS, and a list of other things. Overvoltage on the output can trigger. The SMPS has a couple layers. There is a first section that should fire up to power the low level circuits, then if that is all OK, the high voltage supplies come up. ANy failure anywhere in that system causes fault.

So there really isn't "a solution" to the fault light. We have to determine its cause in each case.

[pnut5150]

Thanks for the good info Enzo..inwas hoping that with all of the fault light issues reported on the PF series amps, that there may have been a bad run of one component..maybe a cap or transistor..don't appear to be that simple..did I mention I hate troubleshooting solid state amps?

[Enzo]

That's the problem, almost anything that goes wrong will turn on the fault light.

[pnut5150]

I havent had time to mess with this in awhile, but what I have discovered is that R34&R2 get so hot they glow when powered on..but looking at the schematic there's not much in front of them..any ideas?

http://www.elektroda.pl/rtvforum/download.php?id=575453

[nickb]

I havent had time to mess with this in awhile, but what I have discovered is that R34&R2 get so hot they glow when powered on..but looking at the schematic there's not much in front of them..any ideas?

http://www.elektroda.pl/rtvforum/download.php?id=575453

Could someone have changed them for the wrong value? That's the only way I can see that happening. That needs to be fixed but it's not your main problem either.

[pnut5150]

The schematics call for 470k there, i was lifting the leg of R34 and I broke the the leg, it had 470M in both spots so I put the 470k's in both as the schematic calls for..