Otari MTR 90 mk2 troubles

[davemascera]

I've been using this machine for 2 years now, not really any serious problems to speak of besides a rumble in certain channel cards that was easily solved..

At some point, we noticed the lights on the meters dimming and checked it out... the power cable had begun corroding at the contact point, so we cleaned it out, filed the contacts, etc, and the machine performed just fine for a while.

Recently, while recording, we noticed a 'fuzz' on recently recorded tracks, which would resolve itself after a while or after a machine reset sometimes as well. In addition, the lights (a 6.3 vac line straight from the transformer) began dimming and flickering, probably worse than they ever had. We checked the power cable and it seemed pretty clean, and measured no resistance between the wall plug and transformer input.

I started taking measurements on the output of the filter caps and power transistors for each line and didn't notice anything fishy, except for a 30v line with 2.5% ripple and that the -40V rail was operating at 2V less in difference from zero than the positive side when the machine was taped up and at idle (-38V, as opposed to the +40V on the other side, simultaneously), and would dip significantly more than the positive rail when the machine was set to fast-forward and rewind. The dips would coincide with the lights dimming quite perfectly.

So we decided to measure the current on the negative and positive 40V rails drawn by the machine. What we found was that the positive rail drew approximately 120 ma at idle (tape loaded up and reels providing tension), but the negative rail was drawing 4200 milliamps. This result doesn't seem correct to me, but I really have no reason to believe it was incorrect after further tests (outside of my gut thinking that a machine shouldn't need 4.2 amps to idle).

Prior to taping up, the positive and negative rails would sit at appr 120 ma each, and during a fast forward or rewind, the positive would jump to a few amps, and the negative would jolt up to a significant few more than that. I melted an alligator clip testing this.

There are 3 -40V lines going to the motor assembly. The first line goes to the middle, which seems to operate fast forward and rewind operations, and the other two lines go to the left and right reels and seem to provide tension.

I was hoping that i'd find a specific side that was leaking current and dragging down the supply, but when I checked current drawn by the negative line on either side, they were roughly equal, and disconnecting either side prevented the light flicker/dimming.

This leads me to believe that either the current currently being drawn by the negative rail is either perfectly intentional, or there is some sort of crazy incorrect voltage provided to the base of a large transistor behind the reel motors. So I'm stuck.

Anyone have any ideas for me?

 

[radardoug]

The motor drive amps are essentially big power amps driving the motors. Once the machine is in tension, its possible that both spooling motors pull power from the negative supply. To test this theory, monitor the current, and move the tension arm on one side, and see if the current tracks the change. It does seem a bit high to me. Are your tensions set with a tentelometer?

 

[davemascera]

The motor drive amps are essentially big power amps driving the motors. Once the machine is in tension, its possible that both spooling motors pull power from the negative supply. To test this theory, monitor the current, and move the tension arm on one side, and see if the current tracks the change. It does seem a bit high to me. Are your tensions set with a tentelometer?

Yes, when the tensions were set, a tentelometer was used. I'll definitely try the test you mentioned.

Any clue on where I should look if it's looking like this is typical operation?

Also, I can't find MDA schemes anywhere, but it shouldn't be hard to trace out.

Is it at all possible that a rectifier is failing, and simply needs to draw way more AC current than normal to provide what the load demands?

 

[davemascera]

Also, found these schemes.... going over it and seeing if there's much i can find. Pretty sure I blew my ammeter fuse, so either I have to take the risk and short it to do this today, or just buy a new one.

https://drive.google.com/file/d/1xhz21kz6kQxpKsdXp-R1BSCUVeAWMLAj/view?usp=sharing

 

[davemascera]

Not sure if anyone has been following, but i've tried legitimately everything (including the wall power). Even tried operating the rectifier in half wave on either side of the diode pair (still runs within spec in terms of ripple!).

Only thing I haven't really done is crack the transformer out of that box and see if there's something going on there because that just seems highly unlikely, but for the most part I seem to be getting fairly clean power, and a reasonably stable set of voltages.

I tried the test doug mentioned and it seemed to work as he said it might. So, naturally, if I lower the reel control tension gain, the problem goes away for the most part. I think the transport is having to work a bit too hard.

Maybe since I fixed the power cable the first time, the transport and servos need to be recalibrated, as the machine isn't being starved of current.

I'll perform a calibration and hopefully feel pretty dumb when it works and report back.