Amp Repair Gurus-Got one for ya!!

csperl1 · Post by **csperl1** » Sun Jul 06, 2008 5:04 am

Hi Everyone,

I replaced the 12 caps in my 2125 because a few were leaking (16V 2200 uf). I reinstalled the pc board and powered it up and it drew over ten amps of current and I immediately turned it off and searched this forum for a solution. I found an old thread that indicated I may have overtightened the screws that hold the transistors to the sink causing it to short some of the components. I removed the screws and gently lifted the parts off the board and it powered up fine. I then removed the board, chamfered the aluminum screw holes and gently sanded the high spots on the chassis, and cleaned out the chassis of any loose aluminum shavings. I then inspected the back side of the components and wiped them clean to make soure no aluminum slag was left on them and screwed the board only back into the chasis. I did not screw down the components. Now when I powered it up it plays music, all lights are on but I now notice a humming sound from the toroid area and the amp slowly increases its current intake from 1.5 A intital and climbs slowly up and I shut it off when it gets to 2.5 amps.

Any ideas what could be wrong with my baby?

Thanks in advance for any help you could give me.
Chris

Post by **stipud** » Sun Jul 06, 2008 9:03 am

Sorry, no idea...

You should try giving rolandk a private message. He used to be the lead repair tech at PG

Post by **Jacampb2** » Sun Jul 06, 2008 1:14 pm

Did you put the insulators back in place under the transistors? If it doesn't have clamp bars for the transistors, make sure there are insulators on each screw as well. Another thought is your heat sink compound. Some heat sink compounds are not dielectric. If you have far to much it can bridge across conductors causing all sort of problems.

Good luck,
Jason

soth · Post by **soth** » Sun Jul 06, 2008 6:00 pm

Here is what I eventually had to do in order to get mine working again.

Blow out the board/heatskink with an air gun really good.
Remove all thermal compound and clean all pads and heatsink with alcohol.
Apply artic silver compound from newegg.com to the sil pads again. Not too much, just enough give a thin layer.
Attach the board to the chassis and make sure not to over tighten the screws.

Cecil use to travel these boards, but doesn't do that anymore. He was a amp guru technician

naughty · Post by **naughty** » Sun Jul 06, 2008 6:59 pm

Apply artic silver compound from newegg.com to the sil pads again. Not too much, just enough give a thin layer

very little of it is right - and be careful not to get it anywhere else on the board cos that stuff can be conductive - so dropping it on the printed traces or anywhere where stuff can short is a definite no no

csperl1 · Post by **csperl1** » Sun Jul 06, 2008 7:21 pm

Thanks for all the replies.

I removed each pad from the component and wiped them down in order to make sure there were no chips of aluminum still stuck in them which could cause a short to the chassis. Currently, the board is mounted in the chassis but the components are pulled slightly away from the sink and are unattached.

Could it be that the components not being attached are causing the slow increasing draw of current. I am afraid I may have damaged a part the first time it was powered up (with overtightened components) and this is causing it to do this. There was no popping or burned components smell or anything like that. It plays music fine but it hums (the amp itself not any noise through the speakers) and slowly draws an increasing amount of current. All board lights are on now (one side was out when it was overtightened).

I was EXTREMELY anal when I cleaned out the chassis and fully inspected everything but if you guys still think it is a short I will disassemble this thing and do it again.

Maybe I will pull the board out of the chassis and try it that way...

Post by **Jacampb2** » Sun Jul 06, 2008 7:37 pm

You really should not apply power, even for a short time, with the heat sink removed. The switching power supply generates a fair amount of heat, even at idle. The output devices always have bias current applied, and will heat as well. Very generically, as semiconductor junction temperatures rise, so does current draw. It is quite possible that the fact that the HS is not attached, is what is causing the slowly increasing current draw.

BTW, I am no amplifier repair guru, but I like to tinker, so take what I have to say w/ a grain of salt. That said, I would still recommend reattaching it to the HS and testing again. It is much more likely that you are doing damage to the output devices with no heat sink, than that you would damage them with power applied during a *brief* short.

Am I correct in my assumption that the amplifier was working fine prior to your taking it apart to replace the caps?

Good Luck,
Jason

csperl1 · Post by **csperl1** » Sun Jul 06, 2008 7:57 pm

Thanks jason,
Yeah it was great when I took it out of my car a year ago. I am replacing the caps and installing it in my friends car for him.

I think you may be onto something with the non-attached heat sink. I put my fingers on the output deivces while i powered it up they got warm pretty quickly. I will attach them in the morning and put my small p/s on it and monitor current draw.

BTW is that your buggy? I have an 83 CJ fully restored/built. Locked F/R, SM465, 98 grand 4.0, etc. I wheel at Rausch Creek in PA.

Post by **Jacampb2** » Sun Jul 06, 2008 8:08 pm

Yes, it is. I own a 4wd fab business that I operate in my spare time. I haven't been doing much in the way of sales lately, but I used to run CJ's and before eBay went to hell, I sold a lot of Jeep gear on there. The buggy was 100% built in my shop, it runs a F.I. Ford 460, C6, Atlas 5.0 comp case, locked front and rear D60s and 42" TSLs.

Good luck again on your amp. I think you need to at the very least continue testing on a HS. I know it is a royal PITA, but it is the wise course of action.

Later,
Jason

csperl1 · Post by **csperl1** » Sun Jul 06, 2008 8:18 pm

Nice rig man. 60's and a big block. Damn that makes me nervous i the nether region. Seems we have similar ways to spend our cash and time huh? I kicked the jeep addiction for now and have moved back to my car audio dependency. Lather Rinse Repeat.

One more question before I dig back into this thing-Should my amp "hum"?

Post by **Jacampb2** » Sun Jul 06, 2008 8:35 pm

Radiated hum, like from the toroid you mentioned, or hum at the outputs?

I have heard the switching power supply start to whine, it is normally very high frequency, and should be well over the audio bandwidth, but if the amp is starving for current, or some other anomaly, then it can force the switchers PWM IC down into the audio realm. It normally takes something really out of whack in the supply voltage, or a serious current draw after the amps power supply from what I have seen. If you are talking about transformer hum, then yes, all transformers hum. It is due to the interaction of the magnetic fields and the iron core. In a switch mode supply, like the amps, the frequency of the applied AC to the toroid should be light years above audible. If it shifts down into a audible range, this sometimes signifies a problem. Possible shorted power supply FETs, or perhaps one of your caps you just replaced are bad.

Typically what I have heard out of switchers in torment is more whine than hum. I realize that this is subjective description, but most switchers I have heard in torment are at the upper audible frequency ranges. Hum is the noise I would tell you is coming from a 60Hz AC line transformer.

Later,
Jason

csperl1 · Post by **csperl1** » Sun Jul 06, 2008 8:42 pm

It is humming when no inputs or speakers connected. It hums if the amp is off and a bit more when I connect the remote turn on and power it up. I guess buzzing could also describe but not whining.

csperl1 · Post by **csperl1** » Sun Jul 06, 2008 8:43 pm

Yes from the toroids.

Post by **Jacampb2** » Sun Jul 06, 2008 9:27 pm

That is really odd. The power supply should not even be on with no signal on remote. Do you measure any kind of current draw with the remote disconnected? Did you do anything with the 12V stiffening capacitors? It is possible that you have one or more power supply FETs that have failed. They typically fail as a dead short, drain to source, but a lot, if not most times they fail catastrophically w/ visible damage. If you have some shorted PS Fets, they could be passing 12Vdc through the toroid(s) causing the cores to saturate, and yes, that could cause a hum (think electromagnet). However, I would think that if one or more of the PS FETs had failed, that the power supply would not function correctly when powered up. I.E. if there is a shorted path to ground through the FET's which are supposed to switch said path, then the transformer should not be able to work at all, in which case, you would have little or no rail voltage and no output.

I have a MS275 at home that I have never had apart or used. I will power it up over my days off, and see if I get any hum from it. This is really an odd one in my limited experience.

Later,
Jason

csperl1 · Post by **csperl1** » Mon Jul 07, 2008 3:47 pm

I am going to try to put the meter on it in the morning to check current draw without the remote turn on triggered. Thanks again for all your help.

rolandk · Post by **rolandk** » Tue Jul 08, 2008 8:04 am

Hi Chris-

Sorry I was off for a week and just got back today. First, if the insulators have any thermal grease you might clean it all off or better yet install some new ones and don't put any thermal grease on them.

If you are handy with a multimeter set it to ohms and with the amplifier power off, measure from the B+ terminal to the left negative speaker terminal (audio ground). Should be a high number, if its a dead short then you most likely have a short from one of the PS FET's to the heat sink. I'm assuming there isn't a short from an output transistor to the heat sink because the amp will instantly draw a ton of current and be very unhappy.

Not sure about the transformer humming, do they both hum, or just one? If the amp is still working fine disconnected from the heatsink, try attaching one transistor at a time and power the amp up for a few seconds until the problem appears. Take your time and make sure the transistors have plenty of time to cool off before you power it up again.

csperl1 · Post by **csperl1** » Tue Jul 08, 2008 10:33 am

Ok I think we have a combination of things working here.

First when I ohmed it across the L- and B+ it was not shorted-it read really high so there is not a short there.

I think what Jason said about current demand increasing on unsinked components is correct combined with my crappy power supply. I am using an old sears car battery charger to power this thing up. I hooked one of my MS2250TA's up and it also drew a slowly increasing (not as fast as the 2125) amount of current. So I checked the voltage and it is slowly DROPPING! Now I think this is exacerbated with the 2125 not having the components screwed down so the transistors get hotter faster and want more idle juice. Does this make sense? Initial idle current was 1.58 ish for both.

However, both toroids still hum and the TA did not hum.

This time it did not hum when the remote was disconnected and it drew 0 amps through the power source.

I am going to screw down the components one by one and see what happens-gulp!

Post by **Jacampb2** » Tue Jul 08, 2008 11:18 am

Well, IMO, that make more sense. Even good automotive battery chargers are not well regulated or filtered. A lot amount to nothing more than a xformer and rectifier. You really should not test with a battery charger. The un-filtered or inadequately filtered DC will have significant ripple in it still, and may cause all sorts of unexpected test results. I would, at the very least, put a battery in parallel with the charger, just to help filter the dirty power, other than that, get a real PS for testing. Putting a battery in the mix kind of blows the idea of having a current limited supply in case you run into problems, but if you power through a low value fuse, it should be reasonable enough.

Later,
Jason

csperl1 · Post by **csperl1** » Tue Jul 08, 2008 12:00 pm

Success!

All components are screwed to the sink and it draws 1.58 A still at idle and plays music. The current climbed (assuming voltage dropped) to 2.18A and has remained there steady for about 15 mins now. Any other voltages I should check on this thing to make sure all is well and it will live another 15+ years?

It still hums and I have discovered it is the middle toroid. When I push down on it the hum stops. Could this be caused by my shabby power supply sending "dirty" power into it? Does it even matter?

Post by **stipud** » Tue Jul 08, 2008 12:26 pm

csperl1 wrote:It still hums and I have discovered it is the middle toroid. When I push down on it the hum stops. Could this be caused by my shabby power supply sending "dirty" power into it? Does it even matter?

To remedy this in the Xenon series, I believe they used hot glue to stick it down. Not the most elegant fix, but it does the trick.

Post by **Wakeup** » Tue Jul 08, 2008 1:03 pm

Cecil had told me to epoxy mine down in my TI900.7 cuz it hums as well...I never got around to doing it....but thats what he told me to do....Let me see if I can find his email...I may not...it's beena long while...

rolandk · Post by **rolandk** » Tue Jul 08, 2008 4:02 pm

csperl1 wrote:It still hums and I have discovered it is the middle toroid. When I push down on it the hum stops. Could this be caused by my shabby power supply sending "dirty" power into it?

I think that is correct. The middle toroid is filtering out crud on the B+/B- input. If your amp seems to be working fine otherwise then try it with a different power source.

nico boom · Post by **nico boom** » Wed Jul 09, 2008 1:15 pm

Correct; dirty powersource allright!
Had the same problem after adding two fans in my powersupply, which were fed with the output of 14.4 volts.
They made the ms1000 toroids "hum" like hell...
Now the fans are powered by an external small powersupply; problem solved.
Or just add a 1 farad cap; works wonders in these cases!

As for the ms2125; if you got an old one; BE SURE TO CHECK THE SCREW WITH THE INSULATOR RING AROUND IT TO BE CENTERED IN THE METAL PART OF THE DEVICE IT IS HOLDING.
After screwing down, check for possible fault by measuring with your dmm; zero ohms= screw short-cicuits the metal part to the heatsink= not correct.
I have more suggestions, if you still can't find the problem...I've been having these problems already in the past and learned a few tricks

csperl1 · Post by **csperl1** » Wed Jul 09, 2008 4:12 pm

Thanks everyone. I think all of the issues I was having was due initially to shorted devices of some sort because of slag aluminum mashed between them and the sink. I realize I overtightened them and should have cleaned the hell out of the case and deburred the holes when the board was out. The second issue (current draw creeping up) was due to a cheap power supply and the components not tightened to the sink causing them to heat up quickly. The humming I will just deal with. When I hook it in up it will be in the trunk of a car and it was barely noticeable but I had never heard it before. Plus it will be on a battery which is a cleaner more stable energy source than my 70's era sears rig.

One thing I noticed when I was in there (a few years ago and recently) was some discoloration of a resistor and diodes on the board. The opposite side of the amp with the mirrored circuitry looks fine. It almost looks like the clear insulating material on the diodes is discolored but the resistor looks a little dirty as well. It measured ok at 500 ohms. Should I be concerned?

nico boom · Post by **nico boom** » Wed Jul 09, 2008 10:05 pm

Discolouration of that part [the two diodes ] is normal.