Never work on your Power Supply with it connected to AC power!
Always remove the power cord at the wall and at the Power Supply
to insure there is no AC power to the tape machine!
ALL MCI tape Recorder Power Supplies suffer from a couple flaws
in their design: The Molex Pins on the Regulator board WILL slowly heat up
and often soften up the solder just enough to loosen the pin or cause a
'cold solder joint' at the base of the pin. This requires you to open up
your Power Supply every couple years and resolder those pins,
making sure to supply enough temperature to the metal pin itself to make a
good solid full solder connection.
Next you will have to use a 400 grit metal sandpaper and clean
the Molex Pins lightly from time to time, as well as
Spray-cleaning ALL the connectors.
Next If ANY of the IC Sockets are the 'Red' style,
you MUST REPLACE ALL OF THEM asap !!!
Also if you see a brown or black area on the mating Molex
connector, that means there has been arcing between the connector and
the various pins. You MUST either replace the female mating pin (best)
or take it out of the connector and clean it with sandpaper, then re-insert it
and of course, cleaning the mating Pin too.
Another problem point is that the Power Supply fans will stop working after
years of service. And if one of those fans stops drawing air through
the Power Transistor Heatsinks, the Power Transistors will overheat,
becoming weak or going bad, causing all sorts of problems.
Those fans are only about $ 25.00 each, so replacing them every few
years is both easy and cheap insurance to keep your machine running well.
Make it a point to check your fans once a month !!!
There were a bunch of slightly different Power Supply Regulator cards made by
MCI, so make sure that you have the correct schematic for your version.
Finally you MUST check your Power Supply voltages at least every
couple months, and I'd suggest you do so once a month.
Next on the Tension Control Card, check the + and - 15 volt rails.
These MUST be within .2 volts or better !! (i.e.: +15.1 and say -14.9)
If they are not you must replace one of the regulator IC's - these are on the back of
the Power supply and simply unscrew. Match the + / - voltages as close as possible.
Next check your 22 volts at the Pinch Roller or Tape Lifter card
This needs to be a good solid 22 volts (or higher up to 25 volts)
when the machine is running.
If it gets 'weak' - less voltage - when in the play mode, you will have to repair
the 22 volt card in the Transport Power Supply.
If you do this, INCREASE the 1n5252 (24 volt) Zenier diode to a
1n5253 (25 volt) or even a 1n5254 (27 volt).
This will increase the voltage of the 22 volt regulator up to 23 or 25 volts...
to better drive the various solenoids.
NOTE: This works only on the 22 volt regulator card PN: 2500B0190
Some 22 volt regulator cards have a trimpot which you can adjust for 24 volt output.
Note that there may be Cold Solder Joints on the power supply molex pins in the power supplies!
these - if you have them - MUST be repaired ASAP !!!!
Her's a pic of a bad solder joint - the dark area in the middle of the solder is a cracked
'cold-solder' connection - and may cause intermittent connections causing massive failures.
All this needs is a good resoldering, making sure to heat the pin itself and the solder pad
before putting solder on the connections....