Problem with MCI 16-track 2" JH-114 transport

[Michael Tibes]

I have a problem with an MCI 16-track 2" JH-114 transport (build 1977): when I press play, both the play and stop-light come up but the tape won't play. If I either turn the tape a bit or turn the capstan by hand then everything starts as it should. I tried to follow the schematics for the logic circuit, but reading logic is really not at all my world :sad:
To me it seems like either one logic state or -impulse is missing or the capstan should start by itself but doesn't.
Any hint of what to try or look for would be very apreciated!
If schematics are needed I could scan them and work out how to post them here (they're not too small...).

Michael

 

[bovox]

Hey Michael, I´m not a "tape" guy but I have one of those JH114. Mine is from 77 as well. The Electronics seems to be a micture of Ampex and Mci but the logical part is certainly "molexed" MCI stuff. Molex is a prime problem. I had not exactly the same problem as You describe but similar. I took out those locgical cards under the reels-when you lift it up,disconnected them, brushed the molex-contacts gently and put
it together.. and it worked again. Actually I have done it just twice in a 4
year period.
If you have any scheme´s from the analouge part I would preciate if you
can send it over.
Hope you can find the problem and I know for sure that there are great
tape guys in the forum- they just have to wake up in the spring of the
GOOD NEW YEAR-everybody by the way.
Cheers Bo
[email protected]

 

[Michael Tibes]

Thanx for your reply, but unfortunately this doesn't seem to be the problem in my case. I've cleaned up everything and replaced all the red sockets, so the standard causes for trouble should be sorted out...
What are your audio cards, I believe there are different types of each?

Michael

 

[Brian Roth]

Check the "offset null" adjustment pots on the analog torque board. If these are mis-adjusted, then the "logic" will believe that the tape is still in motion and thus will not allow the machine to enter "play" mode.

There are two test points located at the bottom of the Analog Torque board, at least on the machines that I recall, and the idea is to tweak the "offset" pot that corresponds to each test point so that the voltage is essentially 0VDC when the tape is not moving. I prefer to use an oscilloscope set for "DC" input vs. a meter for the adjustment.

Bri

 

[Michael Tibes]

Thank you for that suggestion, I will look into it tonight. Anyway it looks like I might have narrowed things down to the PLL-board which drives the capstan - if I swap it with one from another machine, then my machine starts as it should. at this moment it seems like some 'ignition impulse' on the PLL-board doesn't happen...

Michael

 

[amorris]

there is a fast start circuit that ramps the capstan up to speed and then switches over to the regular drive circuit. the fast start driver transistor or the switch could be faulty.