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+  MCI Console and Tape Machine Forum
|-+  MCI Tape Machines
| |-+  JH-16 and JH-24 (Moderator: Westrek)
| | |-+  Ext. Cntrl Varispeed "Jitters"
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Author Topic: Ext. Cntrl Varispeed "Jitters"  (Read 874 times)
rennywilson
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« on: November 24, 2016, 06:09:28 PM »

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:
Quote
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.)
« Last Edit: November 24, 2016, 06:16:02 PM by rennywilson » Logged
gcjh16
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« Reply #1 on: November 27, 2016, 01:59:46 AM »

is ref freq stable in ext mode?
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Harrison
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« Reply #2 on: November 29, 2016, 04:10:02 PM »

The reason for the ext question is that with the deck in ext but AL3 NOT set to varispeed, the 19K2 reference is just fed back to the external input of the PLL board from the AL3 itself.

Does the deck do local varispeed correctly?
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gcjh16
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« Reply #3 on: November 29, 2016, 09:29:16 PM »

^^^what he said^^^ Grin
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Harrison
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« Reply #4 on: November 30, 2016, 02:41:53 PM »

The 555 timer in the ALIII is the varispeed from that.
When you toggle "mode" on the ALIII you switch between deck 19K2 reference fed to the ALIII (and straight back again) and ALIII local timer varispeed. (IC7 on the display board does the actual switching).

If it is ONLY the ALIII varispeed that is the problem, change the 555 timer and its associated timer capacitor. (C1).
You might have to swap out the 741 and C2 and C3, but I'd start with the timer.

Do not piss with the analog torque board. Nothing wrong there.
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rennywilson
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« Reply #5 on: November 30, 2016, 10:36:49 PM »

The 555 timer in the ALIII is the varispeed from that.
When you toggle "mode" on the ALIII you switch between deck 19K2 reference fed to the ALIII (and straight back again) and ALIII local timer varispeed. (IC7 on the display board does the actual switching).

If it is ONLY the ALIII varispeed that is the problem, change the 555 timer and its associated timer capacitor. (C1).
You might have to swap out the 741 and C2 and C3, but I'd start with the timer.

Do not piss with the analog torque board. Nothing wrong there.


^this is the reason why i'll be sad to see this forum go. Thanks for this answer.

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Harrison
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« Reply #6 on: December 01, 2016, 01:02:13 PM »

As Brian Roth has suggested, we'll probably all move back to analogforums.com.
I personally CBA'd with Facebook etc. but I gather Roginator went that way.
Mr. Grumpy here.
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Harrison
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« Reply #7 on: December 01, 2016, 01:54:29 PM »

Sorry

http://forum.analogconsole.com/

PC
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rennywilson
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« Reply #8 on: December 22, 2016, 04:30:46 PM »

ONE MORE QUESTION (before this forum bites the dust)

I replaced the timer and the op amp, this took away my jitters.

But now it threw off the vari's overall range. It wont go as "low" as it would before, and now when it peaks over about 20.5 ips it goes into what is essentially ffwd speeds (reading 44 ips) and cannot be slowed down by turning the vari pot down (you must engage the stop button)

I know there's a pot in there that broadly adjusts this varispeed, but apparantly not broadly enough, the overall range is a lot higher than it was before i switched the IC's.

Merry Xmas Everyone!
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Harrison
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« Reply #9 on: January 05, 2017, 09:28:04 PM »

As I recall, put the main VS pot into the middle. ie go to one end and count 5 turns.
Put into Al3 VS and adjust the trimpot next to the 555 timer for centre speed.
This sets the "home" frequency. The 10 turn pot adjusts this +/- away from this.

That is certainly how the PLL is set up on the main transport.
You put the deck pot to 5 turns and adjust the local pot on the PLL board to get centre speed.

I would imagine it's the same for the AL3 VSpeed.

Forgive the delay in reply.
And did you change BOTH ICs at the same time!? (As opposed to one by one).
Only ever change one thing at a time!
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Harrison
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« Reply #10 on: June 14, 2017, 09:04:06 AM »

Either I'm missing something.....because you said earlier this was fixed.

Jitter in external
Is this BOTH in normal and varispeed switched via the AL or only in varispeed? This with the deck switch in external.

If it is only in AL varuspeed, try changing the timer cap associated with the 555 timer.
If it is both, then you might have an IC7 problem, but you might need a scope to check this.
PC
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