Otari MTR-90 MK2 Capstan speed control problems

Kid Squid

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Bore Da Chaps,

Was up my mate's studio the other morning, and we got talking about his MTR-90, well, it's not working right.
aparently, a while ago, on switch on, the capstan motor spins off out of control !, so he fired it up, and the capstan spins off, at a rate of knots :shock:

Anyway, It turns out, that he had an ' Otari' guy in, to have a look at it, and he thought the problems were around the drive motor power transistors.
what happens next is a bit of a oopsy :roll:

aparently, he changed one of the drive transistors for a wrong type (npn for pnp), and blew a hole out of the back of the replaced transistor :green:
anyway, there were a few phonecalls, one big red face, and a disappearing tape machine guy..........

So, I thought I'd try to have a look at it. - replaced drive transistors Q9, Q10, as these were blown, switched on, .... and capstan runs off.
checked Q9,Q10 again, these were blown ( not with catastrophic guts bow out , tho !)
shorted from base to collector to emitter etc,etc., all otherpower transistors on heatsink tested ok. didn't have time to have a look at the smaller transistors, on inside of heatsink, on board PB9A449, had to pick the missus up from shopping :wink:

Have any of you come across this before,
Any schematics lying around ?, I've checked the link for the manual here, but it's not letting me see it! needs authorization.

BTW. here are the transistors, they are not cheap !
http://www.precisionmotorworks.com/shop/enter.html?lang=en-us&target=d2.html&lmd=39157.739375

Any Info is much appreciated,

Steve :thumb:
 

SSLtech

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Yep.

VERY common on these machines.

Is yours a MkII or a MkIII (or -heaven help us- a MkI)?

During the production run for the MkII's, they went from four transistors to six. It helped share the load.

Basically you have to replace the WHOLE LOT. Specially now as the whole lot has been needlessly stressed by the NPN/PNP reversal (or was it worse than that..?)

Basically, the capstan MDA (motor drive amp) is a DC amplifier runing from ±40V power rails. The motor that it drives is a 24V DC motor. reversal is handled by the output (feeding one motor pole) swinging positive or negative. The motor's other pole goes to ground.

If any of the PNP or NPN trannys go short, you get the full 40V applied to the 24V motor.

:shock: :shock: :shock:
-It's quite a surprise when that happens!!!
:shock: :shock: :shock:

Anyhow, do yourself a favour. Put a plastic inline fuseholder loaded with a 10Amp fuse inline with the motor's positive leg. This will mean that NEXT time this happens (and it's quite common with the MTR-90's, specially the early MkII's) the motor will just coast limply down to a stop. -If you want to add a visual warning, wire the two AC terminals of a bridge rectifier to either side of the fuseholder, and wire a red LED in series with a 3.6k ½Watt resistor in between the positive and negative terminals of the bridge rectifier. Now the LED will light whenever the fuse blows. You can test this by pulling the fuse and telling the machine to go into fast wind or fast rewind.

Replace ALL the MDA output transistors. Use sufficient heatsink grease. Check that the fan which is RIGHT by the capstan MDA heatsink is turning freely and replace it at the slightest suspicion that it might be noisy, sticky or running rough. Otherwise you'll be baking the next set of transistors that you put in there, too.

Wear a mask and brush out the dust from the fins and the cooling path of the capstan MDA and the two reel motor MDAs which are to the left and right of it.

There was also the "mod" to reverse the fan flow from the factory's original installation: the factory fitted the can to draw air out of the machine right by the capstan MDA heatsink. That means that the heatsink gets warm air pulled past it. Reversing the flow means that the MDA gets fresh air pulled past it, INTO the machine.

That's all for now. To save the printed-circuit motor from potential abuse, fit that fuseholder, and leave the fuse out. Use a scope or a meter to look at the MDA output, and only when it drops from the 40-or-so volts should you connect the motor again. If it takes you a few hours of testing to fix that MDA, you could also burn out part of the motor, and if you think that the capsatn MDA transistors are expensive, lemme tell ya, that's NOTHING compared to the cost of a new motor!!!

There's lovely,

Keith
 

stuartpre

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Aug 14, 2014
Messages
41
That was interesting. I am restoring a machine that has a baked Capstan MDA  and no capstan. My replacement capstan is on the way so i'll be following the above advice regarding in line fuse and possibly the fan flow mod. Cheers.
 

stuartpre

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Hi SSL tech, i now have the replacement capstan...in Australia we have 240V fuses of various Amperes, is you suggested a 10 Amp fuse..is that for the 110v us? So do I want an Aussie 5A 240v?

If i want to fuse the reel motors also, what values of fuse? Is it the same..ie 240v 5amp?
 

gyraf

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...and make sure to check proper functioning of the capstan tacho before powering up fully..

Jakob E.
 

PRR

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> amps ...volts

A fuse should be rated for the voltage it will have to *break*.

If you would use 10A 120V here, you would use 5A 240V there.

At least here, we only have "40V" fuses for cars and "250V" fuses for all normal wall-power work. There is no problem using a 250V-max fuse on 117V. I *think* euro-fuses for wall-power tend to a 250V rating default. (The occasional 260V line is not a problem, they allow for utility follies.)

Using a 40V-max fuse on 117V can set the fuse on fire when it tries to break the high voltage and isn't big enough to do so.

You "have" to use 40V fuses in modern cars because there may be many dozen fuses and not much knee-space to hide them in. They make them small, sacrificing the ability to break the high voltages that cars don't have.

FWIW: there are 600V and higher fuses for street and factory wiring, some a couple feet long, but you don't want to go there. You can fit .22 caliber gun-ammo in a 1/4" fuse-clip, but you better run.
 

stuartpre

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Aug 14, 2014
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Hi Jakob, i'm not sure if you agree or diagree with 240v 5A inline fuse for each motor?

Further, can you explain to me what you mean and the procedure for testing 'capstan tacho'? Is that part of the capstan, like something on the pcb attached below the capstan? Or is it the tacho, the one that has the other puck on in?

PS thanks for those diy projects you put together, I learnt a bit 10 years back making the: 1176, pultec and ssl bus comp, nice one!
 

stuartpre

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Aug 14, 2014
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Hi, Anyone?..
wait i'm ready a thread from the other place:

https://www.gearslutz.com/board/geekslutz-forum/8931-otari-mtr-90-capstan-motor-problem-2.html

and the manual 5-19 to 5-21...

 

gyraf

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Yep, good find!

I was looking for that thread, but forgot which forum it was on (a while ago..)

Jakob E.
 

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