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Old 31st October 2008, 05:14 AM   #1
jol50 is offline jol50  United States
diyAudio Member
Join Date: Nov 2007
Default MTX 202 power on shields

Have a MTX Thunder 202 that has been sitting here for a while. It had a smd diode behind the RCA that was cracked in half. I finally remembered to order one and put it in today. It had worked as I recall but no audio in one channel, but everything on the sinks tests good. So I fire it up and I have good offset (had 80mv bridged then went down to <5mv in few seconds), rails 22v think it was (is 2x35rms@4ohm@12v rated). Just about to test it and I check RCA (that shields were loose and I soldered them on) has 15v on the shields. Has few mv or less on the case to ground or speaker common/center tap to ground. Also has 15v on negatives of high level input plug. I've not seen this before, only shorted transformers making the common hot. The RCA center also had some voltage but it would go down if you held the meter on it, the shields would not.
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Old 31st October 2008, 07:32 AM   #2
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Location: Louisiana
This sometimes happens when the 18v regulators fail. Do you have both positive AND negative regulated voltage on the op-amps?
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Old 1st November 2008, 02:41 AM   #3
jol50 is offline jol50  United States
diyAudio Member
Join Date: Nov 2007

1. 15.36v
2. 12v then climbs to 15.26
3. 13v then falls to at least 5 I stopped
4. -20.78
5. 13 and falls
6. 15.33
7. 15.33
8. 20.42

Hope I didn't miss a - sign in there.
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Old 1st November 2008, 04:05 AM   #4
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Join Date: Nov 2003
Location: Louisiana
This op-amp appears to be defective.

If pin 2 > pin 3, pin 1 should be near the negative rail.

If pins 6 and 7 are shorted, they should match pin 5 within a few millivolts.

Was it a Zener diode that failed?

If replacing the op-amp doesn't solve the problem, send me a few decent photos of the inside of the amp so I can see what we're dealing with.
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Old 1st November 2008, 08:21 AM   #5
jol50 is offline jol50  United States
diyAudio Member
Join Date: Nov 2007
There is another op-amp near a larger IC that has similar readings. The diode is mouser# 512-ES2D. IIRC it has a fixed HP/LP filter is all. I double checked and those numbers are correct for the op-amp. I'll pull the board and see what diode is hooked to. Diode and that op-amp are less than inch from RCA input, diode closest.
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