Re: PS503A/TM504 issue

Kevin Oconnor

I've been following this TM504 thread, and in my experience with 3 of these units, an alarm goes off in my head if any plugins mis-function. There are a lot of different plugin design styles, each using different resources in the mainframe. But those plugins that rely on the mainframe NPN/PNP mounted power transistors are very likely to be damaged by the presence of a shorted pass BJT. When a plugin stops working, I first check that the pass BJTs are ok.
I had one TM504 with a shorted NPN in one slot. When I moved a DM501 over into that slot I released great quantities of smoke. The DM501 uses the NPN with a LM723 to make 5v. When the NPN is shorted, (and they always seem to fail that way!), you get ~33v VCC and barbecued TTL. I gave up trying to fix the DM501 and got another 502.
Moral: the TM50x are not passive boxes. Make sure they are not the source of the problem, or you may propagate faults.

BTW, the original Motorola BJTs in the mainframe are now unobtanium. The MJE3055/2955 NPN/PNP pairs and their variants are an upgrade replacement. 10A, 60V, 90W 2Mhz ft TO126/225 style package.

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