[mrclunk]

Hi,
I need to repair my mates Otari mtr-90 mkII. (It used to be in our studio)

The supply reel is stuck in Rewind at switch on.
Think it's blown a drive transistor or two..
MDA schematic below is for a MKIII but assuming it's similar to the MKII.


The part's no longer made, so i was wondering if there's an alternate?
The old parts are Toshiba 2SD552 [NPN] and 2SB552 [PNP]
http://www.svntc.com/TPDF/1604.pdf

Possibly this Motorola part may do it, need to study the datasheet in more detail..
http://www.farnell.com/datasheets/313519.pdf

Also where is the MDA board located on the MTR 90, is it round the back by the fans like an Ampex MM1200?
I can't see it in the service manual.
thanks.

[mrclunk]

Ok,
looks like MJ21193 and MJ21194 will do for the 2SD552, 2SB552 drive transistors. Readily available.
Will stick some fuses in there too!

[Matt C]

I believe the MDA board is around back, towards the top of the machine (behind the upper fan)

[mrclunk]

Thanks, going up tonight to have a look.
Looks like BDX54C, BDX53C will do for the 2sd970 and 2sa971

[gyraf]

pretty sure that the back side hinges down, exposes capstan and reel drive assy's.

MJ21193 and MJ21194 are the ones used in later 90-II's IIRC

Even if some of your transistors in that driver circuit still measures OK, you should replace them all anyway. They tend to get weak and upset from being in the crash.

Constantly spinning reel can also be a reel controller card failure (or a badly seated such). If you are on a parallel-type locator (CB115, CB120, SSL-machine_ctrl), failure can come from here too - try running it disconnected from all externals.

Jakob E.

[mrclunk]

Thanks Jakob,
I had it open last night. Yes it all hinges down and the drive transistors are also on sockets so it's a dream to service.
Two were shorted, they were the original Toshiba part so i'm replacing them all with MJ2119x. Thanks for the tip.
Back up there tonight to fit them, he doesn't have a computer in  studio so no multitrack, no recordings.
Brave man!

EDIT:
Replaced last night and machine back up and running.

[digitall2000]

yeah
on the one that i am working we turned the fan around to blow cool air onto
the heat sink for the mda's instead of sucking he warm air from inside the
machine across the mda's.... this helped to stabilize the swing arm jitters
 

[musiklab]

you should make sure the MDA transistors are well cooled,  by replacing the heat compound  and screwed tight
these transistors do some heavy lifting and get loose as they work then overheat and blow.
- at every maintenance I  tighten each one  carefully .
btw I have a bunch of these transistors,   and they should be matched ,  to ensure all share the load .

[Matt C]

btw I have a bunch of these transistors,   and they should be matched ,  to ensure all share the load .

How do you go about matching them? I've been thinking of putting fresh ones in my machine

[Matt C]

Another question about replacing these MDA transistors - Are people generally replacing ALL the power transistors on all three boards? Or just the board serving the capstan motor? Are those the batch most prone to failure?  I foolishly ordered parts thinking there were only ten transistors to replace (in a MKIII), then opened the thing up and there's thirty!  (this is just proactive preventative maintenance, nothing is fried yet)

[gyraf]

No reason for changing transistors, if they havent been overloaded yet. But for preventive maintenance, make sure that you have the (later recommended) current limiting power resistor in series with capstan motor.

Jakob E.

[Matt C]

make sure that you have the (later recommended) current limiting power resistor in series with capstan motor.

I did put a fuse in series with the capstan motor, but I haven't seen anything about a current limiting resistor.  Do you remember the recommended value?  or is there a service bulletin I can find about it?

[gyraf]

There is a service bulletin, but I don' t remember when and where. will try digging..

Jakob E