[time-nuts] Efratom EFR frequency controller schematics

[Michael Ulbrich]

Hi list,

I recently got my hands on an Efratom FRT (with FRK Rubidium oscillator 
inside) + EFR frequency controller combo set for VLF disciplining by 
DCF77. The original FRK didn't lock but I had a working replacement at 
hand so after some additional swaps of dead tantalums, an open bipolar 
transistor and a noisy 10-bit D/A converter I had the unit up and 
running. Sort of ...

I'm trying to wrap my head around the phase control loop since then. It 
seems to work correctly at times but then again goes completely nuts by 
pulling away the Rb oscillator from the reference frequency. Time 
constant seems to be in the order of days, so one needs some patience to 
find out what's going on. Began drawing the schematics by tracing the 
PCBs but soon realized that completing this will cause a major headache ...

My question therefore is if any helpful person on this list might have a 
schematic for the Efratom EFR frequency controller?

It's comprised of 4 modules in Eurocard format: synthesizer (77.5 kHz 
from 10 MHz by fractional-n division (?)), converter (current output to 
FRK c-field coil via 10-bit D/A), data (analog output of phase and field 
strength for x/t paper recording) and the receiver module, which - 
according to the current state of investigation - is responsible for 
putting out the up/down counting flag and count pulses to the converter 
module. I guess the fault must be somewhere around the receiver board. 
Build date of this unit is mid seventies.

Thanks in advance for any help - it would be greatly appreciated!

Best regards ... Michael U.