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Old 26th February 2007, 04:31 AM   #1
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Default SAE A502 Keeps smoking protection transistors

After a long search for the schematic, and a longer wait for my friend to get around to bringing me his busted amp, I finally got a chance to look at it.

He told me that he simply powered it up one afternoon, and saw a puff of smoke and it would not come out of protection.

Here's the section I'm having problems with:
Click the image to open in full size.
(nice big pic, so you shouldn't have a problem reading it )

Anyway, I found Q241 (circled) toasted, as well as R243. I could not find any other problem, so I assumed Q241 was a latent failure. Replaced it, and R243, and ran it for a few days. Gave it back to the owner, and he ran it for a week or better.

After he had it for 10 days or so, it did the same thing...on power-up, puff of smoke, and would not come out of protection. He brought it back, and again Q241 and R243 are toasted.

Any idea what might be going on here? I've drawn red lines to easily show where the base of Q241 connects, but I cannot see what the circled 'A' and 'B' are supposed to connect to. I'm not even sure I understand the full function of Q241 and Q242. Neither amp channel is blown, and I can only suspect some intermittent failure here.

Full schematic is still available from TomWaits in this thread (thanks buddy!).

FWIW, this thing is awful to work on.
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Old 26th February 2007, 11:56 PM   #2
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No thoughts, ideas. or opinions?
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Old 27th February 2007, 05:21 AM   #3
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Looks like a subtle problem.... I hate those! I don't know anything about that amp... but here is my stab. If either points A or B (the ones circled with a question mark) or CON13(P8) or CON17(P8) get turned on for an extended time or rather slowly... I wouldn't doubt that those two parts (R243 and Q241) could smoke. The 10 Ohm resistor does not do much for current limiting if there is a substantial voltage on CON8 Pin3. We need some more info about where CON8 Pin3 goes before anything can be deduced. I would just guess that this circuit is intended to be pulsed or turned on for just a brief time. Maybe just long enough for a relay or some other protection scheme to take over. Need to find out where those points (A and B) are going. Also, We don't know the intention of Q311L (where does the input come from and what turns him on). Sorry, but without seeing that amps full schematic... I am just grasping in the dark. Keep us posted on what is going on.
Later,
GregoryD
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Old 27th February 2007, 06:49 AM   #4
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Sorry...I didn't notice that the full schematic was linked. I looked them over a little bit and these are some of my first reactions. It looks like points A and B go to thermistors for over temp. protection. You might check them to see if they both have the same restistance. If one of these go open.. the parts you mention will surely burn-up. Looks to me like Q311L is some sort of output short circuit or low impedance detection ckt. Check with your friend and see if he is trying to drive a really low impedance load or if his output cables are shorted (maybe bridged-mono into a 4 Ohm load?). CON8 Pin3 goes to the +15V power supply so there is a good chance this circuit will fry if the relay circuit does not disconnect the load. I would surely be tempted to raise the value of R243 if it weren't for that 470uF capacitor it was trying to charge in the DC offset detector. Let me know what you think.
Later,
GregoryD
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Old 27th February 2007, 07:49 AM   #5
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I found A & B!!

I swear...I scanned that schematic 100 times and could not figure out where they were supposed to go to. Now that you pointed it out, I can't see how I missed it...

Isn't that the way it always goes? Thanks for tracking those down. Duh!

Yes, the basic function of the circuit is to kill the relay if Q241 detects excess current flow, and now that I know A and B go to thermistors, Q242 protects from overtemp conditions.

Under normal operation, both of these are off. C251 is just part of the AC filter for the DC offset detection. Q241 is turned on if there is either excessive negative DC offset, or if an overcurrent condition exists, which will forward-bias Q252 directly and kill the relays. Excess positive offset will turn on Q251, when then will turn on Q252 and kill the relays. It may be possible to replace R243 with something larger.

I may not have that whole scenerio down pat, but I believe I got the basics...

The failure of the amp has twice been on power-up...not after some time of operation. Amp is powered up, smell of burnt resistors, and stuck in protection.
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Old 27th February 2007, 02:43 PM   #6
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Look at the amp output, before the relay and see if there is a positive low frequency thump at turn-on. That could turn on Q241 and destroy it. I don't see how this should happen ... but since you say its at turn-on, I would check it out ( maybe if there is some DC offset that gets multiplied by the gain). Also, you may want to check for the same thing with the mono/bridge switch in either position. Check Q251 to see if the B-E junction is OK. If this is open or shorted, it could put a lot more stress on Q241. Q209 and Q210 are the normal peak current limiters.
Later,
GregoryD
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Old 27th February 2007, 08:19 PM   #7
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Thinking a little more on the problem... Looks to me like this protection circuit is just waiting to burn up whenever it is called into action. Seems pretty poorly implemented. I don't know if you want to keep it in stock condition, but if it were my amp I would change the value of R243. Depending on the original wattage rating and size of this resistor, I would opt for either a 560 Ohm, 1/2W value or a 680 Ohm, 1/4W. If you can accomodate an even larger wattage, then lower the value of the resistor a bit more. For a 1 watt resistor, I would use 390 Ohms. The response of the relay disconnect will slow a little bit, but it won't destroy the protection circuit. The time constants will remain about the same as the DC protect circuitry. The overtemp (thermistor/heatsink) time constant will be greater, so that part of the circuit will be OK. There may be another subtle problem with the amp that causes this circuit to activate in the first place. I'm pretty sure that you are correct that Q311 is designed to disconnect the relay during overcurrent especially when operating outside the SOA for a period of time. What do you think?
Later,
GregoryD
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Old 27th February 2007, 11:32 PM   #8
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Monitoring the output before the relay on power-up is going to be a real trial...this thing was not designed for the tech. One thing I see is that the output Zobel is disconnected until the relay closes...I wonder if oscillation due to this might be an issue...?

I'm not sure about the overall effects of increasing the value of R243. I have some parts coming in for this amp, and when they do I may consider this.
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Old 28th February 2007, 01:23 AM   #9
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You might be on to something! If the amp broke into oscillation before the relay closed, then the protect circuit could activate. This would surely fry the parts you are having trouble with. It sure would be a good thing if you could rewire the Zobel to the input of the relay. IMHO, that is how it should have been done in the first place. Sounds like this amp is a real challenge to work on. Keep us posted.
Later,
GregoryD
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Old 3rd March 2007, 09:32 AM   #10
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I've got some parts in, but I'm going to bump this in the hopes that I can get one or two more opinions before I put it all back together and cross my fingers.

Anyone else care to comment? Please?
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