[time-nuts] New Time nut FRK-H repair questions

Rex rexa at sonic.net
Thu Nov 22 14:53:16 EST 2007

Corby gave you good information. Probably you need to monitor the 
resistance to ground on the lock signal for low (mine was about 140 
ohms) when locked. In the manual referenced below, it looks like the 
lock output is pulled up to +17V so you *would* see a voltage change if 
yours had that circuit. Mine had no voltage unless is was pulled up 
externally, just a high resistance unlocked and low when locked.

The 12.2 V tune monitor voltage sounds high and you should probably try 
to adjust the xtal trimmer to bring the voltage down, as Corby described.

In case you do have a problem, here is a link to a manual:

I have a paper copy of  an FRK (H or L) LN manual which matches the one 
I have, and the circuits are a bit different, so see how close things 
look if you need to get inside. The general operation and theory should 
be the same.

The FRK I had did not lock when I got it. The xtal oscillator in mine 
had aged to the point where the trimmer could not achieve lock even at 
extremes of range. I had to change a fixed capacitor in the trimmer 
circuit. In the manual (linked above) that would be C-11 on page A21. On 
the FRK I had the circuit was different and it was C-7. I actually had 
to remove it completely to get the tuning range centered on mine.

If you decide yours is not locking and if you have access to an accurate 
calibrated counter, you should be able to set the tuning voltage to a 
fixed center value with a potentiometer soldered in (as described in the 
manual) and adjust the trimmer for 5 MHz output. If it was far from 5 
MHz to begin with (more than about +-15 Hz) the adjustment should let it 

When I got mine I didn't have a counter I could trust, so I monitored 
the servo signals on TP-2 or TP-3 for the 127/254 Hz indication to see 
when it was locking.

Hope some of that may help.


swingbyte wrote:
> Hi, I've been reading this group for some time now and have recently 
> purchased an Efratom FRK-H unit.  Now I need to make it work!  I have 
> powered it up and after warmup it draws ~400mA at 26.5V.  I have a 10MHz 
> output sinewave and a lamp voltage of 8.14V.  The xtal ctrl voltage is 
> 12.2V and the lock is 0 v.  I haven't been able to find out what the 
> lock voltage should be for correct operation, but having the xtal 
> control voltage so high makes me think that if might be at its limit.
> Can someone suggest my next course of action?
> Thanks for your help
> Tim
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