[time-nuts] Re: Need help to DATUM Power Supply MPS (Type MFS Modular Frequency Systems)

[ed breya]

Yes, that transformer sure looks burned out. It's hard to tell how big 
it is from the pictures, but my impression is that it looks kind of 
skimpy to run a FRK Rb plus whatever else is going on like a GPS RX and 
uP system, and maybe battery charging too. You can easily estimate the 
VA rating by measuring the dimensions and comparing to standard 
transformer frame sizes. Generally, the VA rating should be at least 
twice the total raw (not the regulated output values) DC power produced, 
with conventional rectification and filtering. This can be exceeded for 
a while, say during warmup of the Rb, as long as it goes back to normal 
in a reasonable amount of time. It's mostly about temperature rise - if 
you have good cooling, you can get more out of it.

Transformers are pretty tough, so having one burn out in normal service 
calls for some investigation of why it happened, before you risk taking 
out a replacement too.

Regardless of the VA rating that should be used, you're probably stuck 
with using the same size and style as the original, just to fit it 
mechanically. If it's plenty big enough VA-wise, then all's well. If 
it's marginal, you can at least add enhanced protection to avoid another 
burnout.

Regarding DC supply voltages, the main one will be something around 24 V 
for the Rb. I would guess that the DC-DC converter on the supply board 
makes +5 V (or 3.3 or whatever) for the brain and GPS RX, and the 78M12 
makes +12 V for the analog, and that there are no negative supplies - 
unless there's more to the supply system that's not shown. Since 
external 24 VDC can supposedly run the whole thing, I don't think you'd 
have to worry about making any of the voltages from the AC transformer 
except for the 24 V, even though it appears to have a multi-tapped 
winding. I didn't see anything in the OP about whether the thing works 
with just external DC, so this should be confirmed.

There's a lot more circuitry on the board than seems necessary just for 
power, so it may be worthwhile to reverse engineer it a bit - especially 
the four big transistors and U3 and U4, which looks like two identical 
functions of some sort. Maybe extra voltage regulation, or maybe 1 PPS 
amplifiers?

Once you do figure everything out and get a fresh transformer, note that 
the original is banded to reduce magnetic emission. It appears to have 
both the copper strip around the bobbin zone, and the steel (or 
sometimes mu-metal) band around the core, but not the third thing 
commonly done, which is insulating the core mounting. It will function 
without these, but may interfere with the Rb unit, especially if it's 
nearby. You won't find these features in run of the mill OEM replacement 
transformers, so you'd have to specify them, or add them yourself. If 
you get a transformer with same dimensions as original, you can 
transplant these pieces from the old one.

Ed