Flaming MPS-2500, need help to repair

[interestingfellow]

Oh my!

First, thank you for the forum, I've been browsing around a bit.
Also, I have already discussed this at another phorums website dedicated to PG amps, and they have been quite receptive and helpful (I use the same username everywhere, btw).

On to my issue.
I got my hands on an amp, and identified it as an mps-2500. In the process of identifying said amp, I was also advised to recap it as the power caps were leaking.
Did that, no problem. I was excited to have the oportunity, enjoyed it, and always like to DIY (er, DIM) and see things working afterwords.
This was not the case (although I don't believe it was my recapping of the amp that failed).
I hooked up the amp with no speakers, 6ga supply wire, and (like an idiot) a 200a fuse. Fired up fine, the green led was flashing, and about 4 minutes later (while getting my laptop hooked up to an rca cable to test it out) the power fets quickly started blowing up or flaming out.
I believe my issue stemmed from my having brute forced all the fet's and bjt's off the heat sink (like an idiot, I used a long & thin fillet knife, and basically beat it in behind those components)(I have since learned a hard lesson, AND the correct way to get them out).
Anyway, I need/want to fix this thing, and I have an electronics repair guy who has been doing component diagnosis and repair (I believe) since Franklin did that thing with the key and the kite. Kind of reminds me of a non insestual/pedophilic Morgan Freeman. Back on topic; he said he would hold my hand and walk me through the repair process, for free, if I could get the schematic for him, which I have since discovered, is pretty much not gonna happen.

Anyway, I have lots of mid resolution pics here.

My concern is if i go ahead and replace the power fets, that the other fet's and bjt's will fail. I wanna get this right, the 1st (well, 2nd) time and think it wise to go ahead and replace all those TO's I forced off the sink (also checking/replacing their respective resistors).

I'm also having a hell of a time trying to find everthing but the rfp044's (which I'll probably replace with 54's). Anyone know where to get'm?

well, that's what I've got... I feel confident with my abilities, but I'm no EEE, so please, speak slow and use small words, and thank you soo much!

[1moreamp]

Well the flashing green LED was a big clue. You should have seen the RED, and Yellow LED's light up and then fade out and then the green LED comes on and stay on solidly with no flashing.
At least that is the sequence that is supposed to happen in a perfect world
Please test amps with a small fuse like 5 or 10 amps only inline on initial fire up and test out. fuses are cheap and plentiful and easily replaced or use a current limited power supply like a Astron, etc.. with variable current limiting built into them.

To your problems, Your caps were leaking badly so their replacement was a good thing. I am doubtful that their replacement caused any issues. I am suspect of the reassembly of the amp into its chassis...First timers at reassembly almost always short the bottom of the amp PC Board to the case somehow, Me included. This is the only Nit-pick I have about these amps as they do require extreme care on re-assembly or there will be trouble as you have found out. All of these amps are somewhat of a pain to take apart and reassemble requiring more time and attention to both of these actions then a typical repairs takes to perform.

Yes you should have replaced all of the sil-pad insulators especially since you were most likely the first person to ever take the amp apart, the Sil-pads were most likely acting like glue between the power devices and the sink.
They have to be forced apart, as gently as possible by applying pressure ever so softly to the top edge or by gingerly prying them off the sink with some sort of wedge device like a large flat blade screwdriver while using a soft rubber cushion against the sink so as to not mark or gall up the surface's in any way.
Once all devices are freed up I usually use acetone to remove any residual Sil-pad material and alcohol after to final clean the surfaces for reassembly.


The Fet's in the power supply are directly driven off the control IC so you might want to buy replacements for those also while your replacing the power Fet's.
IRFP054 and 064 are suitable replacements and the IRFP064N is also usable. Be sure to replace the 100 ohm FUSIBLE resistors located on each gate lead, I am sure they do not survive the ordeal your amp went through. These are FUSE type resistors which mean they are there to prevent fires so please look well to replace these with the same type resistor. They are made to blow open like a fuse, and if they are open and you place new Fet's in the amp and apply 12 volts the Fet gates not being tied to anything will self enhance to destruction again. < Turn-on full bore and explode again>

SIL-Pads can be found at most electronics part dealers, and on auction sites also sometimes. Any source will do as long as they fit properly. Also clean out the threads of all the clamp screws so there is not binding and reassembly. Aluminum and steel do this easily so being clean and tidy help reassembly.

While the amp is out of its sink I would ohm out all the power transistors, and if they all pass muster then ohm out each channel and all of the fusible resistors used in each channel. They are Green and Grey bodied in color and look just like 1/4 watt resistors except for their unique body color...
If there are any blown transistors in either of the channels I am absolutely sure there will be blown Fuse resistors. The amp was designed this way to prevent catastrophic destruction upon failure. It was the only amp of this era to be so well protected by design, and one of the reasons I became such a fan of them myself. They were design very well by careful and well educated engineers that cared about weather the amp would be repairable after a failure. Not many other amp makers have ever impressed me is such a manner back then or even now.

Also check the main turn on transistor located just underneath the noise filter dead center of the board. It is a MJE-371 these can and do blow open like fuses, here again to prevent certain failures. The collector current rating of this device is the key to why it acts like a safety fuse also, so replacement with exact parts assures protection by design.


This is about all the info any tech might need to repair the power supplies in these amps. paying attention to safety parts like fuse type resistors and using some exact parts like the MJE-371 and you will be just fine getting this repair completed.

If the amp had no other issues before the failure except leaking caps, then you will most likely be looking pretty good at this point.
If on the other hand you accidentally shorted something out in the channels on re-assembly then you might have some open fuse resistors in the channels, and or some shorted transistors also. Any tech can ohm out semiconductors with meter. And your tech friend should be able to show you how to do this.
If not then may I please suggest you look at Perry Babin's tech tutorials on his web-site. His training is the best, and has 100% of my support and approval. Yes even I read his training program, that is why I support and promote it the way I do. Just look any of his posts here on the DIY and you will see his links at the bottom of his posts just click on them and you will be sent to his web-site. Perry has done all of the excellent hard work of assembling a training guide to help anyone at all levels.

PG never released schematics openly at any point of their business venture, and if I was privy to their documents I would be bound by promises not to distribute such things openly on the net or otherwise....I have been inside PG amps for some 20 years now ever since they were first marketed. I am always here and will do my best to keep you informed to only the correct information so just post any question you may have and I will answer as soon as i see the post. I can also be reached by PM if you like...

[interestingfellow]

Cool,

I had a feelng (and hope) you'd be the next poster....


Not to be short, just to the point:

Fuse, yes 5a is what I should have used. I knew better.

I realize that I am here, with this problem, but I was soo carefull about shorting the board. All the insulator spacers were in place, as well as the sheet, which I inspected after the melt down dissasembly. I'll hold it up to the light 1 more time, as I need to find out what went wrong, so I don't toast $50 in new components.

The pads were actually in great shape, no tears etc, but I'll be replacing them this time around. I had been told the proper way to dislodge them was to use a well fitted rod in the screw hole, and pry back gently. Your thoughts?
Also, I did noticed that most of the toshiba's corners were chipped as a result of my sledge hammer like tactics.

I thought the ifrp054's were the fets in the power supply (if not, where are they/to which ones are you reffereing)?
Fusable resistors were already on the shopping list, I'm not chancing another melt down or total melt down.

None of the fusable resistors are visably blown (I don't know if you litterally meant that they would blow open, you can see the board after it's failure at the bottom of this reply). I had planned on hitting them with a DMM before replacement.

I agree about the quality and care. the primary reason for my desire to restore this bad boy. Well, that and 12" IFX-12DVC subs. I intend on running them 1ch @ 1ohm and was looking forward to the amp being protected, JSYK.


Well, I guess I should start with a DMM, and get some parts in my hands, although I'm still looking for in stock parts??? The more I read, the less I believe I cracked the fets or BJT's, of which I have examined carefully for visable cracks. I gotta figure out what caused it,too.

I'll check out Perry's link(s) and get goin.

Thank you, agian!

[1moreamp]

Any prying action on the screw holes on the power devices will most likely cause them to shatter there plastic insulator coverings. < past experience speaking here >,
And any device that would work in the mounting holes will be of such close tolerance that it will most likely only be good for a couple uses before its too deformed to fit properly, plus it might deform the metal base of the device also, and this is something we never want to do as it will most definitely prevent the device from seating properly and cause premature failure.
I prefer a flat blade screwdriver as a wedge at the top of the power device with a plastic or rubber sleeve slid over the driver shaft to prevent marking of the case, and a very slow and gently prying action. Also you might want to apply some warm up heat from a hot air gun to soften the Sil-pads so they release somewhat easier when cold and not expanded from heat.

Fets: according to my info source proper device was IRFP044's But IRFP054's work nicely and were used in other models. The IRFP064's and N version are also acceptable replacements. The 064N devices being super duty rated at 110 amperes each, or twice what the original fets were rated. it won't make any more power but it will make the supply side a bit more rugged.

Fuse type resistors will look perfectly new and will show absolutely no signs of being open. They open up inside like a fuse instead of burning up and smoking all over the place, and leaving burnt brown residue all over the pretty board that PG used. So they must all be ohm'ed out individually in or out of circuit, a open one will read anything but near what its rating is as read by the resistor code strips on the body. they are either Green or Grey body color ceramic looking style 1/4 watt sized resistors. I can not stress hard enough to use only these type resistors when they need replacing. they can be had at most any parts dealer that sells repair parts for home and commercial gear as these were used to prevent fires in most home audio gear for years all the way back to the late 70's. PG was the one and only car amp back in the 90's that used these. < again why I liked them so much >

As we get further along I will tell you how we can test the whole board out side of the sink for a very short duration just to check ourselves before we re-assemble the amp into its sink, and I will tell you how to use a simple ohm meter to check for case shorts before applying power to the amp.
None of the board hold down screws need to be used until you are sure the assembly is good to go, only the power devices need to be attached firmly to the case on initial test and start up.

[interestingfellow]

And I feel like an ***....
I just realized that I never torqued the ******* screws on the fets/bjts/etc to the heat sink. I set them, but didn't give them that final dog down.

This would allow those IRFPs to overheat quickly, and pop. But 6 of the 8 did explode or flame out. Would the first one failing contribute to the rest of them failing sequentially? If I recall correctly, all the power caps on one side are tied in parallel, as well as the IFRP's for each side. The first one popped on the left ch side, and about 6 or 8 seconds later the first on the right ch, and then every 4 seconds or so, another one after the other.

Idiot

[1moreamp]

The Fets operate is pairs, and if one burns up and folds over on itself the one beside it will follow just behind.
This amp does not draw much current at idle as most car amps do. So the 15 watts or so it should hsve had to dissipate to idle each channel should not have killed it unless we are missing something.

I always run bench testing after any invasive work on a amp to monitor its idle current draw and output biasing and to verify thermal tracking of the entire assembly. Yes its a rather bothersome time sink but it has proven to be a valued and trusted test standard over the many years.

For test purposes you will only need to install one fet of each set. 2 fets total one for each primary coil per channel will do for simple diagnostic testing. You can leave the other thru holes cleared and ready to receive their device. This will lessen the cost if we have missed something. I still recommend a variable voltage and current bench supply rated no more then 15 amps or so to do all testing with. You will be able to limit the current draw to a very safe 4 or 5 amperes max and it it does pull more the bench supply will shutdown and prevent more serious damage. A simple headlight from a car can also be used, but I would like to see you use your bench meter setup inline as a current meter to monitor the current draw this amp has as it turns on and stabilizes itself. Anything over 2.5 amps total current draw and we have problems to resolve before completing the supply repairs.

I'll be out this morning for a while but will be back in the afternoon


PS the very first MS-2125 I repaired ever back in 1990 had been installed and was being power up for the first time brand new and the installer had failed to cover the amps internals while cutting wires near the amp. the huge gauge very fine stranded wire actually had trimmings fall inside the amp and due to the beautiful gold PC Board they shorted out the power fets in one channel and the amp burst into flames on initial power up with 12 volts. you should have seen the face of that installer . I was his hero that day as I was able to repair the supply in short order so that brand new amp could go out the door that day...lol, lol, lol... I still have that exact same amp sitting here in my collection it found its way back to be some 12 years later needing only as set of internal rail caps as one had a broken lead. I keep it as a memento now, and for the fond memory of my first PG repair ever...And with a set of new caps in it it plays like new still to this day, with just a minor blemish where the copper wire trimming shorted out all of the fets in one supply side..lol...memories...lol

[interestingfellow]

Thanks for the info. I work, ALOT, and haven't had time to dedicate to this repair. For now, I've just been doing recon and trying be as prepared as possible (after all, I can read a few pages with nothing but my Toughbook and 10 min).

I'll be checking out perry's guides tonight (hopefully) and calling my guy this week. I also plan on putting together a culmination of all the test procedures and important notes I've received from both of these forums.

That's awesome about your first repair, I always enjoy incidentally revisiting past accomplishments. Then again, I don't believe in chance.

[interestingfellow]

Awesome,
I had already gone through the bcae website once .

It makes me feel good knowing that I was already on the right track!

[freshkryp69]

I could and have read "1moreamp's" posts for days......he know's probably everything there is to know about car amp repair! I really liked it when he was on the PG forum....!!!!!!!!!!!!!

[1moreamp]

Originally Posted by freshkryp69

I could and have read "1moreamp's" posts for days......he know's probably everything there is to know about car amp repair! I really liked it when he was on the PG forum....!!!!!!!!!!!!!

Thank you!, but I am just one of many committed members that provide support to anyone here on the DIY. Although I appreciate your comments greatly, please do not leave out all of my friends here on the DIY that help. They are some of the finest people you will ever meet, and some are so dynamic they handle this site by themselves most of the time. My contributions are minimal in comparison.
I must admit I was unaware of any of my previous work on any other site still being available. I thought most of that earlier work would be gone by now. I can only guess someone forgot to delete it after I moved on to the DIY. Hopefully some of that work is still helpful albeit somewhat dated...