How did the M9990 survive this?

[thinkchronicity]

Well, so i had it apart for 18 months sitting on the floor after fitting an ebay 'belt set' that was way too tight - it messed up the speed and w/f figure. I got cheesed off with it and had plenty of non boomer contraptions to fix. Anyway, bit the bullet once more and got some proper size belts in today. I was connecting up the blue & orange wires, 16v, from the power supply to test the deck and accidentally connected them on the board to the 1.5v clock circuit instead.
Realising quickly what i'd done i looked at the clock face expecting nothing or chaos to be displayed and it was happily ticking away (in an lcd fashion).
Disconnected those wires and tried the red & black pair from the 1.5 v battery on the clock circuit lugs to check if it still worked, and it did.
I don't get it! Does the clock chip accept that range of voltage?
Please don't try this at home.

 

[Superduper]

It's hard to say but I wouldn't look a gift horse in the mouth. Sometimes, those connections have oxidation after decades and you don't get a good connection at first. If you use a DMM a lot, you'll often hook up probes only to get no or poor or unreliable readings. Then if you brush the solder pad or clean with isopropyl, and maybe polish the probe tip, you may suddenly find you can get good reliable readings. Maybe that is what saved it. Some things in life aren't worth worrying over. Vow never to do that again, be glad that it didn't ruin your day, and forget it ever happened.

 

[thinkchronicity]

Vow never to do that again, be glad that it didn't ruin your day, and forget it ever happened.

Haha, good advice there.
Things have got interesting in other ways today. The new belts haven't cured the w/f which is dire. The 3k waveforms on the scope tell me that. But the torque on the play takeup reel is really weak and easily stopped from turning by my fingers, so i guess it must be the clutch, from reading about it on here. Weird thing is the w/f was perfectly fine 0.11 or something before i changed the floppy belts. Maybe i disturbed something when i unscrewed the bracket that holds the clutch in order to slip the short fat belt on.
Just done a read around of all things m9990 and m9994 and found your great restoration account. I'm also cleaning the micro leaf contacts as i go - you talk of using a preservative on them after sanding. Can i ask what that stuff is exactly? curious
Thanks Superduper

 

[Superduper]

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