MCI JH16: reel motors DC offset problem

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Mailliw

Well-known member
Joined
Jan 13, 2005
Messages
196
Location
Halifax, Canada
Hello,

I've been picking at this machine for a while, but recently I re-soldered all the molex connector pins and re-socketed/re-chipped the analog torque board. It *seems* to have fixed a torque problem with the supply reel, but now I have a problem with the take-up reel spinning fast on idle.

I checked the DC offset for the reel motors (as per the maintenance manual) and I measured +5.5V on the take-up reel (TP1) and +2.5V on the supply reel (TP2). The manual says they should both be +/- 0.1V or less. The 10K trimmers are having no effect on the DC offset. I tried the other adjustments (idle, full reel, empty reel) but none of them have any significant effect on the DC offset.

I did hours upon hours of re-soldering to restore some consistency to this machine, meaning my soldering iron touched a lot of these circuits. The problem could be anywhere.

On the bright side, the problem does affect both reel motors (the take-up reel a little more so).

Schematic:
In order for the schematic to be legible, it is 3000 pixels wide!



Any help is appreciated. :grin:
 
Ah yes another trip down memory lane...

DC offsets in the reel motor circuits can indeed be trouble and the numbers in your manual are correct.

Do some voltage checks with nothing moving. You can unplug the reel motors if a motor is "stuck on".

With no tape moving you should see low voltages (less than .25 volt max) in the tach circuits. If the voltages are not both low, check continuity through the tachs (tach DC resistance, at the analog torque board with tachs standing still) and check the tach preamps on the analog torque board.

You can short the tachs and then check for zero volts through the tach preamps. Look for bad opamps, bad sockets, bad molexes.

Again, if tape is not moving the tach voltages should be quite low. The tachs cannot generate voltages if they are not moving, though something might be screwed up in the wiring or the preamp. Check for open tachs or a tach preamp with a DC output that should not be there.
 

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