MCI JH-24 Issues. No REEL spin without pinch roller FF-RW ok

Hi Guys,

I got a MCI JH-24 that wont tension right... I'm pretty new to all this, so forgive me if these are silly questions.

NO red sockets... The MOLEX issues I think are pretty well sorted, but could be an issue...

If I switch of the pinch roller both reels will stop in a bout 2-4 seconds. Is that normal?

The problem description is incomplete.

I do not know that specific machine.

"Usually" if you can enter Play but lift the pinch, what you want is Edit. The tape will play if you rock the reels by hand, but the reels do not take-off and move tape by themselves. This lets you back-forth to find a cough you want to cut-out.

There really must be a Manual for this or a same-chassis machine which tells how to check the tension. Tension in Edit is un-critical, though it maybe should pull tape to the heads (cheap machines didn't). There was a 3-finger tension-meter you would use while in Play (with pinch). There may be a way to trick the machine so you can get tension with a fish-scale on a reel, but I would not guess.

PRR,

This particular machine, as I'm reading, should keep moving when you disengage the pinch roller... it has a pinch defeat switch called PUCK OFF... no joke.

It very much has a tension flowchart/instructions that are super simple and straight forward, however, there is some unusually symptoms that are making the procedure not work as described in the manual.. as I understand it so far,

You press PLAY and it starts moving okay, but if you turn off the pinch roller everything stops...( after a few seconds)

I'm also curios if the chassis ground should be at the same potential as the ANALOG TORQUE BOARD ground, as it is when the machine is off, and then NOT when you turn the machine on....

Here is what I posted on GS... a little more details

"Hi Guys,

MCI JH-24 coming out of storage.

Seems to FF & RW fine. Keep in mind there are NO RED sockets, and I have exercised the MOLEX issues fairly well I believe.

In PLAY mode it runs for a few turns then the tape starts to fall off or wind in a weird way... too loose.

So, I went into doing the tension adjustments from step one.

1. The first major issue I noticed is if you turn the PINCH ROLLER off, the tape stops moving all together. As I understand it, the tape should keep moving without the pinch roller?

Also seems to only move it HIGH TORQUE setting... so best guess is just not enough tension or NO tension command getting through the ANALOG TORQUE BOARD.

2. CHASSIS Ground is NOT and ANALOG TORQUE BOARD ground (or any of the other PCB grounds) when the machine is on. i.e. with the machine off there is 0 ohms between the gold colored chassis with the large transistors mounted to it. With the machine ON, the resistance goes to about 500k... This most certainly is because my Fluke is picking up the .068 VDC between those two points. Is that an issue.

3. The tach voltages seem to be working right, they swing from 0VDC to +-3vdc in play mode, but again, only if you reference the PCB ground, not chassis ground, might be a goose chase, but...

4. If you have the PINCH ROLLER off and then hit play, the reels will spin for about 2-3 seconds and then stop.

5. Intermittent. At one point it started working fine. I was able to adjust the DANCER ARM into the middle... but then it stopped, working.

My assessment. It sure seems like there is just NO TENSION signals getting to/through the ANALOG TORQUE BOARD? And/or they are so weak as to be effective 0?

Theory: PINCH ROLLER pulling makes the SUPPLY REEL look like it's working. PINCH ROLLER "working" makes the TAKE-UP REEL have very little "work" to do and so it can spin under IDLE or whatever weak amount of "TENSION" signal it's getting???

Perhaps you should put this on the MCI forum, lots of bright people over there. Also read the manual, study the schematic, figure out what should be happening and measure measure measure!
As always with MCI's Molex is a possible issue.

The description is -as already mentioned- perilously incomplete, so I may be making far too many wrong assumptions here, but ONE "cause" for a "problem" may be too much friction from the tape... in other words "sticky tape shed".

MCI's are a little agricultural by comparison, but one "rough & ready" check of things being "happy" on a Studer is to put it into play and -once things are settled- pulling the pinch roller away and observing what the transport does.

On a well-set-up machine, with nice tape, the Studer will continue playing at close to 'correct' speed for several seconds (indicating that the pinch roller is not being "strained" to regulate the speed exactly).

On an MCI, there is no tension sensing like the Studer has both sides of the pinch roller, and they rely on calculating the open-loop tension -rather approximately, if I recall.

The CORRECT way to measure tensions is with a Tentelometer. All other tests are poor approximations which CAN give some indication of large problems, but are really only of use to the trained eye.

If you ARE going to try the "pull the pinch roller" trick, the most important things are thing to observe how FAST the loss of pinch-roller-speed-regulation causes things to start going off-speed. And given the rather agricultural nature of the MCI transport, it's not consistent across the tape distribution. Try it at 50:50 (middle of the reel), then 99:1 (start of the reel) and 1:99 (end of the reel). You'll find different behavior under different load conditions, but under IDEAL (frictionless) conditions, late-model Studers could possibly regulate almost perfectly for several seconds with NO pinch roller.
If you have sticky tape, it's going to grind to a halt quickly no matter WHAT machine you're on, and no matter HOW well the tensions are set... So how good is your tape.

An experienced person who looks at lots of the same model of machine can interpret results when he performs a test on a machine which he has performed on others of the same type... an inexperienced person probably cannot.

Is your machine working correctly? are you "looking" for a problem that you have definitively identified as being present?

If you're "just checking that things are okay" and haven't had any real indication that there's something wrong to begin with, may I politely suggest you STOP right now? I've seen more machines messed up by well-meaning amateurs than I've seen 'improved'.

I have taken the tape off the machine and tricked it with a card.

The pickup reel spins for a few seconds (Extra start up torque)and then comes to a stop. i.e the Analog Torque Board doesn't take over(?)

As I understand it, this is a problem.

The PLL board doesn't seem to be sending the tach pulse from IC10.

The tach pulse is on the pin 1 **edit PIN 5) input to the "Tape velocity Pulse" generator IC (74121) but the output seems held at 0.

You should be seeing the tach pulse going INTO IC10 at pin 5, with something coming OUT (can't recall exactly what...) of pin 1.  The 10K (or maybe 20K) resistor plus the 0.47 uF cap in the collector circuit  of the transistor driven by pin 1 act as a filter, so the voltage leaving the PLL card to the AT board is a variable DC voltage which varies depending on the RPMs of the capstan motor.

Bri

Thanks Bri,

Yeah, that's what i found... nothing coming out of PIN 1... swapped out the whole card and all better... sure suspected the chip, but had a spare card.

Glad you tracked it down.  That DC voltage from the PLL board is a critical signal in order for the AT board to function correctly.  Years ago, I chased down an erratic tension problem caused by a failing 0.47 uF output filter cap on the PLL card.

Bri