[Racal] RA1792 Frequency Standard

[Dan Rae]

Peter Worrall, G4GJL wrote:

> I recently acquired a Type 9442 5MHz standard for my 1792.
>
> I am perplexed as to how to get it going correctly, though I have 
> proved it working to spec and have connected it into the RX.
>
> I was under the impression that the presence of the high stability 
> standard signal  (quoted as >800mVpk tp pk, in the maintenance manual) 
> usurped the TXCO  (The RX has been running on this until now).
>
> So, am I missing something?


Yes, in a word!  You can only have one internal standard fitted and 
powered at a time, there is no automatic switching between two internal 
standards, why would there be a need for that?  Both your new oven and 
the existing TCXO are trying to feed the base of Q5 in the reference 
circuit at the same time, which it will not like at all...  You will 
need to remove or disable the TCXO when you fit the oven standard.   The 
int / ext switch set to ext, will, as you have discovered, switch the 
internal TCXO and Q21 off, but the Oven standard will continue to be 
powered when the external input is selected, and selection is done by 
U2.  If you don't want to remove the TCXO completely, which is perhaps 
the best solution, then just remove the 9 Volt power feed to Pin 1 of 
the TCXO, but that will involve minor surgery to the PC board.

Funnily enough, I just went the other way on one of mine and fitted a 
TCXO, I find it's lack of warm up drift preferable to the Oven, even the 
fast warm up 9442!

Cheers
Dan
ac6ao / g3ncr