otari MTR90 MKII capstan problems

[pucho812]

out of nowhere, the machine at the studio  took off without warning.  Luckily the tape was at the end of the reel and it was a fresh tape which we were about to stripe with tones and alignment.The machine had been working fine up until that point. So I turned it off. Now when I turn it back on, the capstan starts to spin, and gets real fast, to the point the whole machine is humming/vibrating. 
None of the fuses have blown in the PSU. I tried  pulling the various cards control cards with the power off and then running it to see if that was it but it wasn't.  I disconnected the remote and had the same results.
I would suspect the machine is stuck in rewind or FF but there is no indication of that on the transport. About the only thing the transport indicates is all the controls(play, stop, rewind, etc) are all lit up and the tape unload seems to be the brightest... I am not sure what is going on here other then the machine is probably stuck.
Anything I should be looking for?

 

[gyraf]

!!..first of all, protect the capstan motor: don't turn on when attached to a blown motor driver..!!

Second - a well known MTR90 problem is blown power and driver transistors on the capstan motor driver board, which will max-speed-and-then-some the capstan motor. Small-small chance it's the capstan speed feedback circuit though.

Open back (hinges down). Disconnect output of motor driver for capstan. Check if the recommended series-resistor capstan protection is in place, otherwise mount this as first priority: You won't find a new/replacement motor for the '90 any longer, so it's a matter of life and death for the machine.

The motor drivers are basically crude high-current power amplifiers. check voltage output vs. input

If you need to replace power transistors, it's safest to swap all at once, as the ones that seems to have survived has seen a serious overload too. And check the driver transistors too, they'll instantly kill newly-installed power transistors at power-on if they are flakey...

I'm sure we have had a similar thread either here or on GS..?

/Jakob E.

 

[pucho812]

thanks mate. Will get to that right away. 

And yes,  didn't want to run it knowing full well damage can be happening. 

 

[pucho812]

well,

we didn't have the power transistors in the parts bin, I found someone prior had replaced the 2sb552 and 2sd552 in one position with an MJ15015 and MJ15025. I would assume those failed before and that whoever worked on the machine prior did not replace all the power transistors.  the mj15035 seems to cross reference with the SB552BN, and the MJ15015 cross references with the MJ15015 in a motorola guide I  found online.
I found a few spots dealing in discontinued parts that have stock and are not ebay.  I also found some NTE part numbers that claim to work in their cross reference.  Lastly I found precision motor works which sells a drop in replacement made my on semi conductor that is their own part number. 

decisions, decisions

 

[gyraf]

..for the MJ15xx, I can whole-heartedly recommend not to source them from unknown suppliers on the 'net.. Don't ask how I know  :-[

/Jakob E.

 

[pucho812]

Well good news and bad news.  The good news is I got replacement power transistors and put them in. 4 out of the 6 power transistors for the capstan motor were faulty. 2 measured as a dead short, 2 measured as a diode and two measured as full transistor.  The replacement transistors came from precision motor works and are a custom part for them made my on semiconductor as replacement for the unavailable power transistors by Motorola. 
SoI replaced all of them, put up a shop reel and the machine ran. It would FF rewind, play and stop.  took the reel off and put up another reel. Now the machine will not play or do anything. If I try to lock the swing arms, you know where the swing arms lock and then you hit the stop button so things are in place,  they do not lock. I wonder if there is a stuck solenoid or something in the swing arm path.  With the shop reel when it was FF after the  supply reel was empty it took off like a rocket on the takeup side. 
Hmmmmm time to dig  deeper..

So if I tension the reels I get the following,  on the reel control card, #1 in the lower card cage, I get both yellow led's when there is no tension on the swing arms. This is normal per the manual. When I tension the arms up to where the should lock, the yellow leds go away and on the T.up side of the card it turns green. This is also normal. On the  Supply side of the card it goes of the yellow but does not turn green.  sounds like a mechanical calibration is in order and sounds like I may need to make an adjustment on the reel control card.

Calibration happened and it's back to working as it should.

Thank's for the help Señor Jakob.  You sir are a delight