A pair of MP-1 Preamps with issues needing assistance.

[kissfan113]

I have a pair of ADA MP-1 preamps that I'm making repairs to. One is a unit that functions well but has the rear input jack and headphone Jack broken and needing replacement. It is a version 1 unit with the top mounted input switch. The second unit is brand new to me, just came in the mail today and I'm having a few problems diagnosing the issues it has. It is a version 2 model without the top input switch. Upon arrival the unit was apparently doa, wouldn't power up at all. I cycled the switch several times and the unit now comes online and goes directly to the clean indicator lit and the numeral 0 on the display, no self check, no ada or model or program information, no leds on the panel light during startup or with buttons. Preset buttons do nothing, I can however press combinations of buttons like the arrow keys, treble, and one or 2 other buttons and it will illuminate several leds along the strip above the buttons but does nothing to change presets or other parameters. I don't seem to be getting audio out of the unit either. The battery is stone dead (original soldered in unit) and I plan to replace it with the battery mod during repair just trying to see where I stand before going in depth on other issues. EPROMM chip is version 2.01. Voltage through my Furman pl-plus dmc reads a solid 125 volts. Any help you can provide me in troubleshooting these issues is greatly appreciated.

[rnolan]

Hey Kevin, ok your eprom is the last MP-1 version made (v2.01) which added some more factory presets and midi/sysex dump & load basically.  The battery just keeps the RAM powered between power cycles, obviously needs changing but won't be causing the other issues, it should still work when powered on just won't remember any program changes after you turn it off.  So since it's DoA, should you not send it back? or did you know that before you bought it?  One thing you could try which is relatively cheap to do is replace the Z80 CPU, I think I paid $1.50 for one on ebay a while ago, cost more in postage...  Now this won't hurt anything and "should" force a full diagnostic (as will removing the current CPU and re inserting so you can also try that).  MJMP can help you out with the jacks for your working unit (http://www.marshalljmpmodshop.net/miscellaneous_parts_and_replacement_tubes.htm) and also hopefully help get your new one sorted.

[kissfan113]

I knew it was broken going in, took it on as a project based on a description stating it was working but something happened during a previous sale shipping or otherwise so I figured it had to be something somewhat simplistic to repair. The seller did provide me with screenshots of his prior sale complaint and has photos of 3 error codes from the prior buyer, shows err1, err2, err3. He said the prior buyer got those codes then the unit refused to start back up, I just got those photos this morning, after my power cycle I got the condition I posted in my first post.

[kissfan113]

Tried a few things today, pulled both tubes and tried to start, no change in condition, tried pulling the cpu, epromm, and memory chips with no change in condition or startup either. The unit almost acts as though the memory has been completely flashed and it holds no original operating data, is that even a possibility?

[MarshallJMP]

So you pulled the RAM too? (6264 next to the eprom)

The err1,2,3 codes followed by the counting of C1 to C128 usually mean you had a dead battery. Since you have another mp-1 you could try to swap the eprom and RAM chip. But this will erase the presets from the other MP-1.

Me personally never saw a busted Z80 but that doesn't mean it can't happen. And BTW you can still buy them new.

First thing I would check is that you have a good 5V. On the left side of the Z80 is a small capacitor just next to it. If you measure over it you should be able to measure the 5V. Voltage should be 5V +- 5% (4.75-5.25V)

On the back near the transformer you have 4 voltage regulators, the one on the far right is the 5V regulator for the logic, is it a 78M05?

[kissfan113]

MarshallJMP, I will have to give that a shot, I'll check the 5v first and try chips next, I don't have any presets in the second unit and it's down for my uses until I get replacement jacks for it anyway. (Rear input and headphones) I did pull the cpu chip and the ram chip one on each side of the epromm chip earlier today in the V2 unit with no change in condition or startup process upon restarting the unit. I just ordered some 2032 battery holders and am awaiting their arrival to replace batteries in both units as they both have the original soldered in batteries. I had seen those were the replacement used in the mod, not sure if it's worth getting the holders for the original battery number or not but if there is a benefit I'll order them and install the original size battery instead. Still shows an immediate clean indicator and the number 0 button presses don't change anything, combinations of buttons give another 0 in a different part of the screen and several top leds light but nothing changes in the program. I will update once I get a chance to test the 5v and try the other chips.
Thanks
Kevin

[kissfan113]

I will also check that regulator for the proper part number when I get in there.

[MarshallJMP]

ADA used different 7805 5V regulators but the 78M05's can fail sometimes at startup (when the current is at it's peak).

It seems it wants to start but can't. Sometimes either U41 or U44 fail, these control the select lines for the RAM and Eprom. Then you can have some strange beheviour too.

Do you have any electronics knowledge? Do you have a oscilloscope?

[kissfan113]

I took 3 years into an electronics engineering program, but that was over a decade ago so my skills are pretty rusty, and I've forgotten probably as much as I've retained. I have a scope but I don't know that it would do much good it's an old Simpson model 466 and isn't in the greatest condition. I could potentially borrow a fluke 196 off my dad long enough to test a few things if it becomes necessary.

[kissfan113]

I read 5.0v even across the capacitor next to the z80, 5.0v on pin 14 of U41, 4.4v on pin 14 of U44, the 7805 is labeled (9006, LM340T5, 7805 P) it also resides on a heatsink

[kissfan113]

Replaced all 3 chips (one at a time ram, epromm, z80) with chips from my working mp-1 again no change in startup or function same display, noticed the clip leds dimly light at initial power up sometimes od1 gets brighter but it's intermittent when it comes on

[kissfan113]

Ok, got to thinking a possible dirty connection may be the cause of my issues on the v2 model, I took the ribbon cable connection on and off several times and low and behold the unit fires up in a normal manner, sound output is normal now, all functions work great, I still get error 1,2,3 but I am attributing that to the dead battery and should be solved as soon as the 2032 battery holders come in. That leaves me with battery mods to do on both units and rear input and headphone jacks on the v1 model. MarshallJMP do you know a source for the jacks I need? Thank you to everyone who has helped me along while I've been poking at this problem.

[MarshallJMP]

Ok good thinking, strange though, usually if you unplug the connector to the midi board nothing happends, didn't think about this since you had some activity.
7805 is good. Maybe you can find those jacks on Mouser. Are you going to use the rear input? If so this is a line in not a guitar input. It's not the same as with V2 MP-1's.
BTW it's normal that the clip leds go on briefly at startup.

[kissfan113]

I'll check Mouser for the jacks, I know they have a huge catalog so it may take a little while to find what I'm looking for, im planning to leave the V1 as originally built where I have the v2 with the guitar input on both ends just to add some flexibility to my system and have a line input if need arises at some point until then it will probably be in my studio rack where things get plugged into the front input anyway. As best I can tell the jacks I need are 4 pin Jalco jacks.

[kissfan113]

Got the batteries replaced today, cleared all errors and working like a new one, all I lack is finding the proper jacks for the V1 unit and all is good in my world.