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Author Topic: Fixed: MP-1 with ALL clip leds lit solid and NO sound (long post)  (Read 1135 times)

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Pork_Chop

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Had an interesting fault on my beloved MP-1 a while ago...  Below is an excerpt of the troubleshooting I did, and with the help of MJMP we got it sorted out.

ADA MP-1
V. 1.38
Rev F

History:
Has worked great ever since I have had it except for a microphonic tube years ago (not the ADA’s fault obviously).
Been close to 4 years since I did:
-   Battery holder mod
-   Input jack mod
-   Replaced PS and tube board electrolytics

Has worked flawless, as I said for at least 20+ years that I’ve had it.

Problem:
Last gig I turned it on and all the clip LEDs were lit and no sound.  Panicked and flipped the switch a couple times and it came good…. For about 5 songs. Then it locked up again and has not recovered.
No noise… no pops… no strange smells… All clip LEDs are lit and no sound on any output, not even effects loop (when I got it home I discovered some random static present on the headphone jack)

What IS working:
-   Power supply voltages are at or near spec (+22, -22, +15, -15, +5, +8) all check ok
-   The digital part of the unit seems to be ok. It boots up normal, flashes the front panel LEDs and display shows the 1.38 firmware, flashes “ada”, then goes to the last preset.
-   MIDI in works. Can change presets no problem
-   MIDI thru works. CC messages from ART X-15 pedal are sent to other units in the MIDI chain (MP-1 is my first unit, so it’s the IN)
-   Can set the MIDI channel
-   Can edit and store all parameters in a patch
-   Program retention is ok, both the patch parameters and last preset retain
-   Factory reset works, displays lp and loads the stock patches
-   Error check works. Pulled the battery to reset the ROM. It counts up c1. . .c128 like it should, displays an error code like it should
-   All PS voltages on the digital section chips are measuring what they should (within tolerance I’d say)

What SEEMS to work:
-   PS +/- voltages on the chips in the middle section of the board are measuring good
-   -15v on the op-amps and chips on the left (audio) side of the board are bang on -15v
-   +15v on the op-amps and chips on the left (audio) side of the board are GONE. NOTHING. NO +15v!!!
-   Tube board voltages seem to be good, and tubes heat up ok

How I determined this:
Since the clip LED’s were all lit, I started with U1
-   Clip LED supply voltage is indeed +5
-   No +4.7v supply measured on pin 4 of U1 after the Zener (measures -0.0491v)
-   No +15v on input side of Zener (measures -0.7909v)
-   Verified the loss of +15v on several of the chips (U9, U10, whatever I could see using +15v on the schematic) They all either have the +15v present (middle section of circuit) or -0.7909v on the + input pin, so problem seems to be in audio part section.
-   All chips that are supposed to have -15v do indeed

MJMP said he has seen broken traces and bad vias before, so suggested tracing the +15v line and find where it dies.  Well, not having the eyes or steady hand of a younger me, I did just that but in reverse.  To avoid too much poking and risk shorting anything out, I unplugged and clipped on to the input side of the U1 zener and (moving away from the zener) measured continuity on everything that should have had +15v. I made it all the way up to the trace on the back edge of the pcb that carries the +15v, and about midway on the board I finally lost continuity on the trace vias.

Sure enough, hidden behind some caps there was a tiny bit of crud on the trace and it had corroded a small section of the trace away.  Used a fiberglass pen to rub the solder mask off the trace and soldered a small piece of wire across the gap, regaining continuity. Powered up and measured +15v at the zener. Indeed, the clip lights were off, and I had full audio again. What a relief!!

Thanks to MJMP for the sanity check, and now this is documented here in hopes that it can help someone else.  :metal:
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rnolan

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Great wright up, very appreciated  :thumb-up:
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MarshallJMP

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Thanks for posting this, could help people in the future.
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