RFS 1000 Off Air Frequency Standard

[807]

Like I said earlier, when I flooded the unit with a 198kHz signal it had no effect, but a 60kHz signal killed it, even with the generator set to around -30dBm with a piece of wire dangling from the output socket.

I also assumed it was 198kHz. I can't remember what the original Ebay listing said, as it was bought in 2008. I paid £71.99 for it. Can't remember wher it was auction or BIN.

I've just done the same test and mine is 60kHz also. I got it for £10 as I think the previous owner was expecting it to stop working soon 

I was wrong about the accuracy, I misread my Racal frequency counter. It jumps between +400Hz and +500Hz on 1MHz.

Mike

I'm using a Racal Dana too. A 1998. Yeah, it's not very stable. As they're sold as frequency standards I thought they would be better than that.

I've taken the lids off the cans. I'll take some photos & post them up.

[Solder_Junkie]

I also have a Racal 1998. The ovens were excellent at one time, but they are up to 36 years old. I have a manual dated 1987.

The oven in mine is the 04B option, but even though it’s the highest stability one, it doesn’t have the accuracy or stability mentioned in the manual. Maybe if I left it running for several days it might improve, but having 3 GPSDOs and a rubidium, it is not worth bothering with the internal oven.

SJ

[807]

My 1998 has the bog standard (black box) oscillator. I have a few OSC5A2B02 OCXO units that I intend to replace it with (along with another dozen or so projects on my list).

As you say, with a better standard plugged in the back it doesn't really matter, but on the off chance that I might need to use it somewhere else, it would be nice to know it has the best built-in reference possible.

[807]

I've attached 2 photos of the RF & logic boards inside the metal cans. The 3 chips on the right side of the RF board have had their markings removed. All the chips on the logic board are still readable. The yellow & purple wires at the top right of the logic board go to the front panel VCO trim control.

The 10kHz output is taken from the ribbon cable (you can see the gap in the ribbon cable where the black lead under my thumb is soldered to it). The other frequency outputs are taken from the ribbon cable that connects to the front panel.

[Mike99]

Thanks for the photos 807 (BTW the last time I saw an 807 was in the PA of a No. 19 set in the late 1960s).

I suppose the logic board might be useful to provide divided outputs in another more accurate frequency standard. Do you know what the input frequency is?

Also will the outputs of this device drive 50 ohms?

Mike

[807]

Yep. I had a number 19 set many years ago. Managed to get the receiver working, but not the TX. Since then I've built a few transmitters using 807's. I used to be a medium wave radio pirate. Still got my old vave rig in the loft that I built in the late 70's.

I probed a few components on the RF board & could see the 60kHz input. I thought about taking the board out to reverse engineer it, but there are 2 blobs of solder on the edge of the board soldered to the can. I really need to find my big soldering iron before attempting to remove it.

When I used my sig gen, I noticed that the peak output was at 55kHz, so I might try adjusting the green trimmer in the top left of photo.

The outputs are not 50 Ohms. All the outputs measured 5v5 p-p on scope, but when fed through a 50 Ohm terminator, the p-p dropped to 550mv. Strangely, when the 10kHz output was connected to 50 Ohms, the signal went down to zero with a continuous tone on the beeper.

[Mike99]

The outputs are not 50 Ohms. All the outputs measured 5v5 p-p on scope, but when fed through a 50 Ohm terminator, the p-p dropped to 550mv.

Yes I found the same.

What is the correct procedure for adjusting the vco trim control?

Mike

[807]

I assume that the VCO trim should be set for half way reading on the phase meter. The unit goes out of lock if it's adusted too far from this point.

Something I forgot to mention in my last post. Only the 1MHz output is a square wave. All the other outputs are pulses, with a 20% duty cycle.

[Mike99]

OK thanks. My phase swings between 4 and 6 on the meter with occasional larger excursions so that sounds about right. What about the Sync switch, what does that do?

Agreed on the waveforms, here are mine.

Mike

[807]

My phase meter swings about a bit too. Sometimes it's quite stable, other times it bangs against the end stops!

The sync switch is still a bit of a mystery. It doesn't appear to have any effect on the meter or the outputs. I traced it back to the logic board & it connects the output of one of the AND gates on IC17 to the RESET A & RESET B pins of the dual BCD up conters (IC15 & 16).