Phoenix Gold MPS-2500 Repair Help, Please

chriscl14020 · Post by **chriscl14020** » Sun Aug 30, 2015 7:52 pm

I have (had, in a way, now) a Phoenix Gold MPS-2500. It is a beautiful amp, and I ****** it up. Now I want to cheat death for this poor little guy. Here's the story.

I bought this maybe 6 months back on eBay. It came in pretty goofy condition -- someone had clearly already done something to it (I will be posting pictures, in hopes that you can tell me what exactly was done; I took some when I took this apart for the first time to do the obligatory capacitor replacement). While I was in there, I had to replace the wiring to the RCA jacks. This is I believe revision B, which used the old smaller terminals). I have an MPS-2125 bus bar/terminals/end cover set, that I was using for a while, but I switched back because I could not access the gain control like that; otherwise everything was fine. (this is irrelevant though, the only difference it made was a difference in physical connector sizes). Anyhow, a few weeks ago, I was simply sitting in a parking lot, not listening to anything but radio on, when I smelt The Bad Smell -- the smell that means GTFO your car and flip the breaker, don't wait for the 150A fuse to give out. Sure enough, my amp had turned my custom plexiglass cover a nice nasty soot color, and melted 2 of one channel's transistors nearest the caps. Anyways, I figured that is weird, and simply replaced the transistors (obviously cautiously fuse testing it before powering it up, it worked fine, so I put it back). However, I purchased instead of the original (can't remember 50 or 55v), 57A transistors, some (matching 50 or 55v, can't remember), 70A transistors, at the recommendation for a good substitute from the sales rep (B&D Enterprises, they're close and I get a lot of my parts there).

I replaced the 4 transistors on the damaged side (the channel that is on your right-hand side, if you were looking at the visible side of the circuit board with the bottom off and the capacitors nearest you), and it worked fine after that... For about two days. I noticed it started to sound 'distorted' about 5 seconds before it died completely. When I went to the trunk to see where my bass went, and I found the red (protect) light on. I took the amp out, and inspected, to find no burnt looking anything, but a blown off (literally, off the board) 50v 10uf capacitor (capacitor C221, the one labelled 4.7uf 100v on a MS-2125 schematics diagram, which is not the same as this amp I know -- the capacitor im a matching spot on the other channel is also a 50v 10uf cap) and a blown fuse (the one closer to the edge of the board of the two on that channel). I replaced this capacitor, and fuse, and now get nothing -- no power light, no protect light, nothing.

So, how bad could I have ****** it up, I want to ask (any ideas)? Did my sale's rep's recommendation and my dumb decision to not look further into that, cause this issue? Any clue where to look from here? Also, bonus question -- what are these modifications to this amp? Are these repairs, or some sort of modification? Note: pictures all from before amp blew up.

Last question. Anyone have the schematics diagram of an MPS-2500, preferably early revision? I have the schematics of a MS-2125 if anyone needs those. Can I attach that file here, or is that not allowed? Can email upon request, if that's not okay. Repair materials should be accessible to all, in my opinion.

I couldn't find the original pictures I had taken, so I took some of the board in current state. I have included 'orientation numbers' to help you get an idea of where the zoomed in pictures are. None of these are components I have added, these were all in place when I got it. Any idea what these components added on are for? Any way I can revert this back to 'stock'? Thanks!

Pictures:
http://i.imgur.com/ROFzGBN.jpg
http://i.imgur.com/6bsO8s0.jpg
http://i.imgur.com/vDLazgH.jpg
http://i.imgur.com/PBxmM4V.jpg
http://i.imgur.com/3CQEyDo.jpg
http://i.imgur.com/zXKLJcj.jpg
http://i.imgur.com/1BmZnpW.jpg
http://i.imgur.com/ZBHCHva.jpg
http://i.imgur.com/usDWqEf.jpg

Mr. Wild · Post by **Mr. Wild** » Sun Aug 30, 2015 9:13 pm

I'm not going to guess what's gone wrong with your amp, but I can give you some general knowledge about these amps.
Hope this helps.

Power supply fets ("57A transistors") don't blow without a reason. There has probably been a short circuit in the speaker wires or inside the amplifier. Could be a loose strand of wire or some other loose stuff.

When you replace PS fets, you must also replace their gate resistors, which are of the fusible type. Different type fets need different ohm value gate resistors.
When you fail to do this it is likely that the fets blow again.
When fets blow, the gate resistors and the PWM control circuit (SG2525) can be damaged also. At this point the amp won't power up at all.

Phoenix Gold amps often have mods which don't seem original from factory. Many of them are done by phoenix gold because they did a lot of improvements on the fly. They didn't change the PCB layout for every mod.

chriscl14020 · Post by **chriscl14020** » Sun Aug 30, 2015 11:09 pm

Okay, so I looked into it a bit further. After stressing my brain over why I could have signs of life (voltages present in various places), but no actual life, I realized one point of failure I hadn't checked... The remote wire. It seems when I had moved the terminals to gain deeper access, I had broken the few strands inside the actual terminal block itself. So, I replaced the remote wire, and it appears to be working (got 'green' light, only powered it up for about two seconds though -- I try hard not to run things outside of a sink for over 5-10 seconds tops). I'm going to replace all 8 PS transistors with the original spec transistors just to be sure. Where are the gate resistors? I'm assuming we're talking the three attached to the PS resistors? They all appear fine, according to my multimeter. Should I still replace them even though none appear shorted/blown?

Also, one last issue to tend to. Would I be able to make custom aluminum 'rails' to run on top of the transistors, sort of like how the MS-2125TA design is? Would that hold heat, or trap it? The reason I ask, is one of my transistor bolts snapped upon removal. Would love to be able to create a more even, non-localized, and less stressful pressure, I do so hate the direct 'through-hole' bolt design. I already have new sil-pads coming, I know that is a must.

Thanks for your advice! I will give further info once I either change transistors and put it back together, or clamp it to the heatsink for testing. I honestly think I might get it fixed, and once it's up to par to be put back, run it for a few weeks to be sure it's good (I don't want to deal with the headache of something with undiscovered gremlins), and then sell it. I've loved it a lot, but I have a feeling someone else may be able to treat it better (I am only running it at 2 ohms stereo, though, with a custom fan endplate I got from a previous original owner of an MS-2125 -- I'm not that hard on it

) and it might be time for me to be on my way to a downgrade to some other old-school amp.

trickyricky · Post by **trickyricky** » Mon Aug 31, 2015 3:12 am

Quick question did you replace the silpads on the transistors mounted on the heatsink? If you reused them then there's a likely chance one or more are damaged enough to cause a short between the transistors case and the amplifiers chassis (which is tied to the signal ground), so keeping that in mind you have to make sure you trim the pins on the new transistors after you solder them (otherwise they might touch the metal chassis and cause failure).

I hate when a break one or two of those transistor mounting screws after am assembling the amplifier...lol. I've had luck drilling it out with small drill bits then re-thread it with a tap drill (4-40, might not even have to if you are careful and lucky enough the screw will come out in a few pieces). If you think you can do the same as the TA designs then go for it, it will not cause any damage or more heat.

All bare wiring connections that go to the amplifier MUST BE TINNED to prevent loose strands from going through the block terminals and ending up inside the amplifier's board (causing a short/damage).

After verifying all components are good (as MR WILD stated) then I would make sure you have 12v on the terminals, after the filter toroid and again after and before the fuses. You'd be surprised on how the amplifier will not power up if you forgot to tighten the screws on the terminal blocks that hold the buss bars

Hope you already replaced that remote wire

chriscl14020 · Post by **chriscl14020** » Mon Aug 31, 2015 10:05 am

-I did use the old sil-pads, but upon removal I was sure not to damage any. It is not easy ( this is maybe the 5th one of these I've taken apart, but mainly for recapping ), but doable. I'm replacing them this time for peace of mind, and to not tempt fate.

After I replaced the transistors (when they went the first time), it worked fine. A capacitor had exploded off of it. I replaced the capacitor, and it works fine (the reason I thought it was not working was that my remote wire was broken).

I did replace the remote wire. I am going to replace the new transistors with originals. I made sure to bend transistors to shape and trim before installing. That is a good idea with tinning ground/power/remote/speakers. I never thought of that, stupidly enough.

Anyhow, I clamped it down and tested it for a minute, and it seems to be fine. Anyone know how to set the DC bias and offset (if this is doable) on this amp? I always try to re-do that any time I make any change to the circuitry. Thanks again for all your help