HELLO, is anybodddy out theeeree?
Lol, just kidding. Not expecting a giant response on here these days but I'll give it a go:
These irritating varispeed wiggles are a reoccurring problem on my 78' JH16 - when speed control is set to EXT. from my AL III I get short bursts of unpredictable speed increases - this has made tracking with vari on (which I do frequently) very tedious and impossible for some things. I discovered a mod on Michael Blackmer's website (http://blackmerdesign.com/resources/michael-blackmers-mci-electronic-mods-and-maintenance-tricks/
) in which you do the following:
On the Phase Lock Loop (PLL) board add a .01uf ceramic disk cap connected between the reference select jumper common and pin 7 of IC19 (which is a ground). This dumps the high dv/dt overshoots in the reference frequency waveform back into the ground of the signal source IC. These overshoots have been seen to make the one shot double trigger, which makes the PLL hiccup.
This modification has seemed to restrain the issue but not rid it completely.
Is this a common issue with JH16 transports?
I notice that when the jitters are happening, there seems to be a lot of tension reiterating in the reels, I can feel the tension quickly and intermittently tense up and loosen rather sharply with the back of my finger. The dancer arm also rocks fairly aggressively during these episodes. The sound it produces are short, pitched up and very unstable. Sometimes the machine goes into a full on freak out for 10 seconds in which very consistent garbling and erratic reel tension go wild with wreckless abandon.
Is it the Analog Torque Board tension potentiometers (R8, R41) that are the culprit? That wouldn't explain it not exhibiting the same issue with the speed set to fixed or internal. Or is this issue likely more related to the PLL, capstan? My initial though was that something must be faulty with the stability of the oscillator signal coming from the AL III, but I couldn't find an oscillator in there (at least like the one that's in the PLL). There's a socketed LM555 (timer) in very close proximity to the pot used to adjust speed from the remote, but I do not know enough about electronics or the design of the JH16 or ALIII to know if a timer IC has anything to do with varispeed control. There's a 6.144 MHz crystal oscillator in there also - again I don't know if this component is related to the machines varispeed control.
THANKS FELLOW MCI ENTHUSIASTS (I know you're still out there - if nobody's going to fix this forum lets fill it up with good information again.)