Otari MTR90 MKII - Replacement Power Transistor Capstan MDA? in Australia?

Hi, thanks for reading.

I am blowing fuses on my capstan motor ( in line 5A 240v fuse mod), i kind of left the fans off by accident following replacement of fans with external psu computer coolermaster fans.,(silly I know)...

So now after a 4-5 hours (fans on) the fuse blows. I suspect stressed power transistors ( the  3 x npn, 3 x pnp 220v 15A to3 style) in MDA.

Originals are:
2SD552 (NPN) and 2SB552 (PNP). My local electronics shop (Jaycar) has something close 140V,  20A , they are MJ15003/MJ15004 ...

The MDA's run on +-40v rails...
Were the originals selected for performance or because that's what was cheaply available in the design phase? ie. is 140V ok?

I suspect the answer has something to do with  peak current draw and resistance...
Thanks.

https://groupdiy.com/index.php?topic=68001.msg864376#msg864376

Update:

I had a closer look at the reel motor mda's and found some mj15003's installed. So it appears the previous owner already found this part as suitable. It has worked fine.

Also upon visual inspection i found the Capstan power transistors appear slightly 'buldged' whereas the reel motor transistors are flat (this is comparing the identical parts), so most likely i guess the capstan transistors are close to failing completely and would already have failed if not for my ol' mate the in line fuse

Hi Ho, Hi Ho it's off to Jaycar Electronics we go!

Transistors are not like capacitors, they dont bulge when broken. In fact they usually just go to short. The bulge will be a different top shape, quite common.

Is there a diode across the motor? I've fixed an A77 that was popping fuses due to a bung diode

Hi, thanks for the replies.

The machine is working fine with new transistors. I will give it a full session workout tomorrow and see if the fuse goes when everything is warm (that is the issue).

I have kept the old transistor set as emergency spares (to get a tape off or make a session saving transfer if neccessary).

As regards the shape of the old transistors, well they look 'buldged' compared to the other sets with identical parts and batch numbers...so yeah, maybe they do sometimes buldge!? Hard to argue with it when it is right there in front of you. Anyhow, perhaps my i sight is not as good as it used to be

The diode suggestion is a good one, given the potential heat stress of all parts due to the 'no-fan' accident. It was bloody hot.
I have ordered modern equivalents for the driver transistors q2, q7. TIP142/147. They are cheap and I will replace all ( i love the machine so why not?).

I recently had cause to contact my old mentor in the industry and opened with;
'Hi Dave, just calling to say Gidday. I was replacing some power transistor on my Otari yesterday and I thought, 'I wonder how Dave and his Otari are goin?'. 

A bit of work to keep 'Tapey' going but worth it for the memories.

..there's no diode protection, the capstan needs to go both ways, and is powered way beyond what goes on in an A77

Jakob E.

Thanks Jacob. I wasn't sure what the other chap meant. I only knew that heat and semi-connectors is sometimes bad for them. I did notice some diodes in the circuit, but to be honest, I do not understand there function in the circuit. I am still learning.
Thanks for chiming in