At wits end with class D repair! Help! - diyAudio
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Old 5th July 2005, 02:19 PM   #1
NUTTTR is offline NUTTTR  Australia
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Default At wits end with class D repair! Help!

Hi All,

I'm in the process of trying (!) to repair and not having much luck, i replaced the obviously problematic parts (they had definately been overheated, despite the fact that the resistor near Q62 (obviously burnt) is obviously burnt, it still functions perfectly.... After replacing the transistors (Q62 and it's partner right behind it), they stopped getting massively hot on turn on, however Q61 (top blue circle in second pic - also replaced this pair...) (and partner, they are the same transistor powered as a pair...) is still getting massively hot, i mean, in the first ~10 sec of running they will be at 60deg already! The resistor that's giving it "power" is right above the burnt one, it's not dropping the voltage much at all it's about 150ohm from memory and is feeding about 42 or so into the transitors... They get hot quick - one thing - they run as a pair - normally MPSW42 - i replaced them with MPSA42 - because that's what i've got lying around - they have a combined capacity of about 1.2-1.3w - however mpsw about 2w, but to me this doesn't account for the massive heat in such a short short time. The circled diode in the second pic was faulty, short circuit. It's been replaced.
I've checked what i can (there is a missing capacitor, i had a problem desoldering it, so am getting a replacement, i suspected it had shorted out! not so )

So where i'm at is - amp function -
on turn on it has a ~4.5 amp (limit of my p/s) drain for about 5 seconds (always about the same time, regardless of how many times i've off/on'ed it), after about 5 seconds it goes back to about 900ma drain, which is acceptable, but then the "de thump/protection" circuit cuts out (amp's "started" so it lets audio out) and it goes "thump" through the speaker and then protects, then does it again about 5 seconds later again (protection cutting back out again)... and so forth... The outputs (i've measured) appear to be OK - not shorted and no odd readings i can see....

What happend: user had it in the car and turned a corner and it blew 60 amp worth of fuses... Which would indicate something shorted.. but no go, nothing shorted, if i don't connect a speaker it doesn't produce a "thump" in the power drain at all.....

ANY help anyone can offer me? Sorry the images are big, but i can downsize if some prefer but i am much appreciative of ANY help i can get here!!

http://www.behavingbadly.net/diy/ampsmr.jpg
http://www.behavingbadly.net/diy/ampsmq.jpg

Thanks
Aaron
P.S. Amp is an MTX500D (65v rails!)
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Old 5th July 2005, 03:09 PM   #2
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I'm not to familiar with the class-D designs but here is what my guess might be:

1. The constant-current source feeding the transistors that keep burning is probably damaged (again my lack of class-D knowledge rears its head - there might not even be a CC source in this design) and therefore the current flowing is too high through those units and they keep blowing.

2. Resistors are pretty tolerant to heat damage. Once they cool back down they are usually fine.



Not really sure where else to look....




-Matt
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Old 5th July 2005, 08:45 PM   #3
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Hehe, 500d's. The two 150 ohm resistors, transistors around them, and the zener diode you circled are all a part of the preamp power supply. The zener is a 1n4746 (4744 will not work). The transistors come in pairs, and you can replace them with a larger bipolar transistor, just watch the pinout.

Here's the fun part. Sometimes when the preamp supply fails, it connects the main rail (+/-55V) to the op amps, so you might need to replace those as well. Sorry to bring the bad news


Let me know if you need more help, I've done a ton of these.
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Old 5th July 2005, 10:48 PM   #4
NUTTTR is offline NUTTTR  Australia
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Quote:
Originally posted by bkoopman
Hehe, 500d's. The two 150 ohm resistors, transistors around them, and the zener diode you circled are all a part of the preamp power supply. The zener is a 1n4746 (4744 will not work). The transistors come in pairs, and you can replace them with a larger bipolar transistor, just watch the pinout.

Here's the fun part. Sometimes when the preamp supply fails, it connects the main rail (+/-55V) to the op amps, so you might need to replace those as well. Sorry to bring the bad news


Let me know if you need more help, I've done a ton of these.

I suspected that was what it did, but the transistors are still getting very hot....... is there a reason for that? Or is the preamp just draining massive power (short?)
These transistors appear to drive the NE5532N's - is that right? Is that what i'd have to replace? Anything else that's causing the "thump" and heat? Any help would be awesome!!!

Thanks
Aaron
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Old 5th July 2005, 10:58 PM   #5
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Yeah, sounds like the op amps are out. Check the NE5532's and TL072's. If you have spares, you might just want to replace all of them. Just make sure you're seeing +/-18V on the power supply pins, or you'll have to replace the amps again.

The way to check the opamps is to see if the output makes sense with respect to the two inputs. i.e. if the + input is higher than the - input, the output should be high. This is a bit tricky, and sometimes you don't know. If all three pins are reading 0 (or any other value), the op amp is *probably* good. Checking to see if any of them get hot is also a good indicator. They should run warm, but not hot.

So, see if you can fix the power supply first. Once you can get +/-18V, start going through the op amps. If you can't get +/-18V and you think the (preamp) power supply is good, you'll have to guess which op amps are bad. Sorry for not being good at explaining this.
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Old 5th July 2005, 11:11 PM   #6
NUTTTR is offline NUTTTR  Australia
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Thanks for the help!

BTW as i remember, the really burnt resistor is 443k ohm.... the one above it, less burnt is 150 ohm...

I'm thinking one of the opamps (or more!) are shorting, causing big drain through those 2 transistors.... nothing else really gets hot as i can't leave it long enough to find out!

Thanks
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Old 6th July 2005, 07:28 AM   #7
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Yup, I know how this goes. I got a pallet of MTX amps once that had 35 of these. This was the problem on about half of them.

The two resistors right next to each other are each 150 ohms. The regulator is simple: take a zener diode, ground one leg, and connect the other leg through a resistor to a voltage rail. Now connect a bjt as an emitter follower, where the emitter leg is the regulator output, and the collector goes to the power rail through the 150 ohm resistor. The resistor just eats up some voltage so the transistor doesn't get as hot.

When the transistor dies, it usually shorts. This connects the zener diode to the power rail through the 150 ohm resistor. The resistor usually wins, and the zener shorts. After that, the resistor is just connected between the power rail and ground, and quickly goes to a better life.

If you haven't already, you'll have to replace the transistors (two in parallel) associated with the failed regulator. They're MPSA42's and MPSA92's (NPN/PNP), but I usually bend legs on something larger that I have handy (2n6488, tip102, any bipolar or darlington will do). You have to bend them so that they don't smack the case, but it works like a charm.
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Old 6th July 2005, 08:29 AM   #8
NUTTTR is offline NUTTTR  Australia
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Ok So the 443kohm resistor should be 150ohm?! damn, that's a buggered resistor

Thanks, will have to order some parts now

Aaron
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Old 7th July 2005, 11:10 PM   #9
NUTTTR is offline NUTTTR  Australia
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OK..... I'm pretty sure all the preamps are blown - i literally went around and cut the Vcc+ leg off all 5 chips, just in case... Powered up and the regulator works, sort of...... voltage takes ~30 sec to rise to 18v output from the regulator!! I changed the resistor that's reaaaaally burnt to 190 ohm for now (have to go pick up some 150 ones).... Now there's another problem Regulator doesn't get hot at all, because there's no load... but the power supply - of the amp - there's something wrong with that, i've looked into it about as far as i can with this type of p/s, but the control lead for switching the input fet's is very inconsistent.... it's switching from 3.4khz to 2khz, etc contstantly going all over the place... This is obviously running through the system, but the p/s takes about 30 sec to get to full voltage, slowly building up...... and pulsing....... the pulsing is making the whole amp now work right (the regulator takes 30 sec to run up to 18v, as an example!)
Have you ever had this happen? I figured maybe a diode somewhere - went through every diode tested OK (off board) - so this comes down to a cap or a transistor - or maybe even the TL494 or the LM339 *i think that's the one that is in the amp...*....... problem is this amp is full of transistors and there is no indication of which one is causing a problem... I don't have a scope to test it, unfort......
Any more help?

Thanks
Aaron

P.S. I'm using a pretty good fluke meter so i'd guess there's no problem with my meter and that most of the measurements i made are about right
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Old 7th July 2005, 11:33 PM   #10
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Aaron,

I'm not sure if I can help you if you have no scope, but I don't think the power supply is bad. If the preamp regulators were getting hot before, that means you were at least getting power to them. If you can get +/-18V from the regulators, go ahead and swap the TL072's, you might just get lucky.
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