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Post by mastertech on Jun 20, 2019 10:39:18 GMT -5
Pretty much nada from what I could find.
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howie
Junior Member
Posts: 88 
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Post by howie on Jul 4, 2019 15:04:01 GMT -5
I'm on vacation next week so I'll try to put some time aside for this. All 5 modules are exactly the same so my thought is to pull one of the working ones out and start comparing in circuit readings from one to the other.
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Post by mastertech on Jul 4, 2019 20:34:21 GMT -5
Sounds like a plan.
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howie
Junior Member
Posts: 88
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Post by howie on Jul 11, 2019 5:04:19 GMT -5
I sat here with my DMM and a pot of coffee until I got the shakes. Had two identical boards side by side and everything I probed matched or damn close to it. Back to the internet (I think I found the end of the internet). I found a post listing parts and showing pictures of a repair. The one thing I did not probe that is right smack in the middle of the board is a "DC Servo OP AMP". The original was a AD705AQ and it was replaced with a LT1097.
Can this be tested/checked in circuit. The other parts replaced were high wattage resistors. The readings I got from those (in circuit) matched the working board.
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howie
Junior Member
Posts: 88
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Post by howie on Jul 11, 2019 8:36:00 GMT -5
If you look at and expand the image posted on the first page, R506 on the bottom left is one of the resistors replaced on the repaired board that I found on line. I believe it's a 1K 87 1% resistor but the printing is mostly burned off I don't know the wattage. Both the working and not working boards resistors at this location are over heated and not readable. It is 1/2" long and about 1/4 inch in diameter. This one is showing a crack in it. I'm going to nip one leg on both the functional and not functional board so I can get a better reading off it. Perhaps a higher wattage here would help if it turns out to be the problem.
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Post by mastertech on Jul 11, 2019 20:41:37 GMT -5
I would also look around for any broken traces. Especially near any over heated parts or areas.
As far as testing an op amp you could do a voltage test at the output of the good and bad one. If going into protect is a problem for testing it then I usually jump the replay while testing. If you do this I would also test for DC on the speaker output of the bad channel to get an ides of what is going on.
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howie
Junior Member
Posts: 88
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Post by howie on Jul 14, 2019 6:46:29 GMT -5
OK, back to this for a little while this morning. I'm going to power up the amp with the bad module out so it will power up normally. I will first check the input voltage on a working modules op amp at pins 2 and 3 and then (correct me if I'm wrong) test the output at pins 4 and 6. Should be 1.1V to 8.0V depending on the resistor size between pins 2 and 6 1.8K to 135K. And then put the bad one in
Wish I had a service manual and schematic for this.
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howie
Junior Member
Posts: 88
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Post by howie on Aug 22, 2020 8:11:49 GMT -5
This extremely heavy paper weight is still on my bench exactly where I left it a year ago. I'm going to clean the shop this fall and will give it one last try at that time including looking for another used amp module to just slide in. This thing is really pissing me off just seeing it there.
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howie
Junior Member
Posts: 88
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Post by howie on Dec 13, 2020 14:14:58 GMT -5
Just thought I'd post a followup on this POS. A year ago I replaced all the caps on this module. Half of them capacitance wise were out of spec and a couple of them pegged my ESR meter. After this, It still was banging off on protect. I was unable to find anything else wrong on this module. So it sat here on my bench for way over a year mostly due to it's weight. I have another project that will require the space this pig is taking up so I thought I'd give it one more look at before high end amp heaven. With the bad module out, I powered it up and took as many readings off the next "good" board in and then reinstalled the bad board. Mind you, I've done nothing to this but let it get dirty on my bench. Now it powers up perfectly and I can't get it to fail. Go figure.
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howie
Junior Member
Posts: 88
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Post by howie on Dec 15, 2020 11:15:09 GMT -5
Well so much for that. It ran on the bench for over 48 hours and then sometime on the third night it went into protection. I don't know what the threshold is for setting DC protection but when it was up and running, all read close to the same neg voltage on the speaker output with nothing connected to it.
#1 was -0.980
#2 was -0.972
#3 was -1.077
#4 was -0.962
#5 was -0.945
After setting protection, they were all the same except #1 which was -2.375. #1 is the one that has been the problem all along. I suspect #3 will be the next problematic one.
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Post by mastertech on Dec 15, 2020 14:44:02 GMT -5
How does that module connect to the board? This almost sounds like a poor connection problem.
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howie
Junior Member
Posts: 88
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Post by howie on Dec 15, 2020 15:20:45 GMT -5
How does that module connect to the board? This almost sounds like a poor connection problem. Its not a connection problem. It's power connections are hard screwed to the main board and if it requires being grounded to the chassis, there is 8 screws doing that. There is a 10 pin ribbon connector that connects all five amp modules to the power supply. There is only 3 of the ten pins being used. That is the protection circuit to the power board. You can unplug one board at a time to see which board is doing it or unplug at the power board disabling protection to all 5 amp modules at the same time. Right now I have the amp powered up unprotected so I can look at it |
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howie
Junior Member
Posts: 88
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Post by howie on Dec 19, 2020 7:02:40 GMT -5
perhaps this will help. Will have to pull it out again to look for additional information  I pulled the module back out and removed the board from the heat sink (27 screws w/spacers). I was unable to test those big power transistors in circuit so I removed them and tested them. They all test good. Damn. On the center transistor, top and bottom, there's another device. Those are thermal switches that are normally closed. Those test fine also. Wish I had a service manual or schematic. |
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howie
Junior Member
Posts: 88
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Post by howie on Dec 23, 2020 16:50:45 GMT -5
All those transistors tested perfect so I reinstalled them and just for giggles tried it again in the amp No Go. So I pull it out again and start looking at other things. So I grab my meter and start poking all the diodes. In circuit, all the little glass diodes test fine.(current flow in one direction only) however, there is 17 of the black with silver bands and only 2 of them appear to be holding. Will I have to lift one leg of each of them to test?
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