Blown Amp-Help Please

FriendOfTheDevil · Post by **FriendOfTheDevil** » Mon Jun 15, 2009 2:44 pm

Hey, new member here. Referred by a friend, I heard you PG guys know what you're talking about.

Anyway, I had an amp blow a couple of weeks ago, a Memphis PR70.2. It was out of warranty, so I decided to just take it apart and try to find out myself. After all, it's already broken...

I found out that the spark/smoke came from a mosfet. Can anyone help me out? How do I go about finding out if this is the only culprit? How should I replace it? I'm pretty quick to pick up on technical/audio/electrical/computer type stuff, I just need some guidance. Any help you guys can give me would be appreciated.[/img]

knightrider358 · Post by **knightrider358** » Mon Jun 15, 2009 2:53 pm

First get rid of that pic and repost mine I edited the size for u. No one will be able to really look at it and also post some picsof the whole board so we can really look at it. Also welcome to the forum u'll love it here and check out every thread you will learn allot from this forum compared to most others. and wait ull get some great advice from people (they know everything). Here's my biggest tip for you chuck the amp

(No offense) and get a PG

. Youll never go back LOL.

FriendOfTheDevil · Post by **FriendOfTheDevil** » Mon Jun 15, 2009 3:22 pm

Thanks, didn't think about that. The only reasons I'm fixing the amp are 1) the learning experience 2) So I can sell a working amp. I am in the market for a replacement however. What is the model for either a reasonably priced 4 channel or 5 channel amp? I would be interested in hearing someone's PG setup, I have never heard any of their equipment.

waynehead · Post by **waynehead** » Mon Jun 15, 2009 3:29 pm

I got a question for you sir, are you really a friend of the devil or just a grateful dead fan?

knightrider358 · Post by **knightrider358** » Mon Jun 15, 2009 3:30 pm

If you want something half way descent actually good but cant spend allot look at the PG RSD series amps. You cant go wrong there plus they are dirty cheap right now online. Then anything before say Ti, ZXTi, ZX, MS, MPS, and the M series they are freaking awsome jus do this if you ever can Look at all those series amps boards and compare them to any other brands and itll SHOCK you what the quality is in PG.

FriendOfTheDevil · Post by **FriendOfTheDevil** » Mon Jun 15, 2009 3:44 pm

I guess he's not really a "friend" of mine, more like a "business associate," but a I'm Dead Head nonetheless.

waynehead · Post by **waynehead** » Mon Jun 15, 2009 3:48 pm

Right on. I like listening to my DEAD at high volumes too. Welcome to the phorum bro.

wash with gasoline · Post by **wash with gasoline** » Mon Jun 15, 2009 4:17 pm

welcome. this phorum moves a little slow sometimes, but the knoledge floating around here is huge. id try an help you but my amp repair skills end at changing a blown fuse

FriendOfTheDevil · Post by **FriendOfTheDevil** » Tue Jun 16, 2009 3:29 pm

bumpski

Post by **Jacampb2** » Wed Jun 17, 2009 5:01 am

I'll take a stab at some advice, but keep in mind that I just screw around with repairs/troubleshooting as a hobby and have no formal education...

Anyhow, it is very unlikely that there is just one blown mosfet. Something caused it. Also, when power supply mosfets go, they typically take out several other items as well.

Here is what you need to check-- off the top of my head.

1st) Even if only one power supply FET is blown, and the rest test A okay, still replace them all. Chances are excellent that none of them will even test good, and you just have one that showed the physical damage.

2nd) Replace the gate resistors on the FETS. Look up the data sheet for your blown part, find the lead marked "gate" and follow it back until you find a resistor. It should be a low value part, somewhere in the neighborhood of 20 ohms. Replace them, it is cheap insurance. If you replace the fets, and neglect the resistors and one of the reistors is open, or goes open on powering up, and you will end up replacing the fets again. Their gates CANNOT float when power is applied or they will destroy themselves.

3rd) find the pull down resistor from gate to ground, should be one pull down for each bank of fets. It will be in the trace that goes to the gate resistors and then on to the fet's gates. The pulldown will be before the gate resistors, but there will be only one which pulls all the fets on that bank down.

4th)find the bjts which drive the mosfet gates. They are typically blown. PG used mostly the common A06/A56 pair, I couldn't tell you what memphis did. A lot of times one or both of these drivers are shot. There will be a bipolar pair for each bank of fets.

5th) it is really, really rare for a PS to just blow up for no reason. In my experience there is typically a high current draw after the PS that causes the issues. I have only had one amp through that had a blown PS and no attributable cause. It was a Ti1200.1 and it had about a 1.5" square of PCB totally burned through by one of the supplies. I rebuilt the board and supply, and it works perfect, I never did find any other issue with it...

Good Luck,
Jason

FriendOfTheDevil · Post by **FriendOfTheDevil** » Wed Jun 17, 2009 1:03 pm

Wow... ok. So I guess this repair is probably out range for a novice? I was thinking this would be a pretty straightforward issue of "this is broken, I will replace it" and thought this would be a good one to try for me to learn what I am doing. Sounds like it's a bit more complicated than that.

FriendOfTheDevil · Post by **FriendOfTheDevil** » Sun Jun 21, 2009 3:49 pm

Anyone else with anything to add?

Post by **stipud** » Mon Jun 22, 2009 6:51 am

FriendOfTheDevil wrote:Wow... ok. So I guess this repair is probably out range for a novice? I was thinking this would be a pretty straightforward issue of "this is broken, I will replace it" and thought this would be a good one to try for me to learn what I am doing. Sounds like it's a bit more complicated than that.

Well that's pretty much it... you just have to replace the broken parts.

The issue is that when one part breaks, it often cascades to others, so you have to figure out what else might be broken. Otherwise you replace the one visibly broken part, put it all back together, and it fries again!

Post by **Jacampb2** » Mon Jun 22, 2009 8:10 am

I say go for it. You have to start somewhere. If you have questions as you go along, ask, we will try to help you. I pretty much described where to start. Most checks can be preformed with a common DVM although there are far more specialized instruments for checking detailed specifications of parts.

Do some research on line. www.allaboutcircuits.com is an excellent primer on the basics toward mid level, including details on how to check parts with a DVM. Best of all it is free. Mosfets can be checked to see if they are shorted with a DVM, they typically fail shorted. It takes a more elaborate procedure to check if they will actually switch, but if you look up "meter check a mosfet" you will likely find some easy ways to do it.

A cheap ass harbor freight DVM actually has a beta (HFe) check function for BJTs, I have no idea how acurate it is, but it could help in a pinch and they are only $5.

Good Luck,
Jason

FriendOfTheDevil · Post by **FriendOfTheDevil** » Tue Jun 23, 2009 3:34 am

Thanks guys, I think I might give it a shot. Now I just have to find a couple hours to do it. THAT'S going to be the hard part...