HP 5384A (5385A) Crow-Bar SCR Help

[MitSenf]

Hi,

While installing an OCXO in my HP 5484A, I got a little too handsy and shorted the 5V rail to chassis. This let out the ghosts on SCR CR9 (See attached Schem), the 5V Reg, and possibly the PNP Q10 - no shorts detected, however, hFE measured zero.

The service manual list the SCR Mfg P/N as SCR1153K (no google hits) and the part looks to be some custom Motorola job - Markings are (top) R002, (mid) 8, (Bottom) 4-0221.  No luck tracking any of these numbers down either.

I replaced the SCR with a 2N6507G I had in the parts bucket, as well as the BJT with a MJE2955T (service manual P/N = X45H28I; Modern Equiv. -> D45H5).  This appears to have fixed the problem, or at least it allows me turn the thing on without blowing any fuses.  Nevertheless, I'm not overly familiar with SCRs/Crowbar Circuits, and replacing them.

Will I be okay with this sub on the SCR? Or if not, does anyone have an idea on a proper sub? 

Also, the PNP BJT I replaced had a gain approximately double that of the 2955T @4A, yet the 5V switch-mode seems to be operating fine.  If this is adding more strain than I'm thinking to the switching circuit, then I'd be okay ordering a D45H5.

Thanks very much for your help, and I certainly appreciate any input.

[thinkfat]

The SCR is likely not so tightly spec'd, just make sure it's able to blow the fuse when it fires. The series resistor will take some of the heat. The one you've chosen looks a bit heavy to me, with its 25A current rating. Gate current also seems rather high, you need to make sure something can deliver that current.

CR10 is a zener diode that sets the trip point of the crowbar circuit, plus the gate trigger voltage of the SCR.

If you're worried about the BJT, maybe use a TIP42C-Y as replacement.