Hello Ed,
The replacement of the power supply is a success. I decided to install a 9-way power connector in order not to molest the main PCB any more than necessary,
even in the case the replacement supplies would not function satisfactorily, and plan B would have to be activated (whatever plan B is...).
The three small boxes happen to fit precisely (and I mean _precisely_) into the cavity of the removable metal shield of the original supply.
In two of the three perpendicular directions, the margin is about 1 mm !! See the pictures below.
Of course first I performed a dry run, connecting only AC in, not DC out, and measuring the output voltages. Then the DC wires were fixed to the new power connector,
and after triple checking of the connections, the moment came to plug it in, and turn it on. No black smoke, the display showed "NO SIGNAL",
and after a while the unlock light turned off. Ah, great!
The first guinea pig to hook up was a simple and small digital signal generator. The reading on the PM6685R deviated only a few ppm from the setting,
so that was pretty good. Of course I hoped that by far the major part of the deviation as due to the generator, not the counter.
I played around a bit with the frequencies, and found the counter could measure down to 4 Hz on an amplitude of 5 V. So far so good.
Next, I hooked up an OCXO that I had calibrated to my GPSDO roughly three years ago. It is stand-alone, housed a big metal box with a decent toroidal transformer in it,
and linear voltage regulators. When I switched it on, the deviation was significant, around 100 ppm or so too high. But the box started to warm up, the deviation reduced,
and after about two hours the reading had stabilized to less than 100 mHz too low on the 5 MHz target. That is 20 ppb.
Not bad, given both instruments had been last calibrated years ago.
The proof of the pudding was the GPSDO itself. Once I had got it going again (likewise, it was years ago since it last was fired up),
the reading on the counter swung between 10.000,000,00 and 10.000,000,01, roughly evenly, so the deviation is about 0.5 ppb!
That is the best that could be hoped for, I would think. I will not even touch the calibration pot!
You should take a close look at the power for the fan. I think that if the Rb is installed, the fan is supposed to run continuously. There might be another problem or an option set wrong. You could remove the Rb oscillator, power supply, and distribution board and put them into a seperate box. This would allow you to set up a nice passive heatsink with no fan to cool the Rb.
I did check all jumpers, and they all seem to be set correctly. And the counter seems to operate just fine. Do you have any suggestions what else I can check?
The suggestion of putting the oscillator in a separate box was indeed the plan in case the counter could not be resurrected.
But now seeing it working so nicely, I'd like to keep them together. Unless the fan issue should force it?
The software isn't alive when the counter is in standby. What are the power supplies drawing in standby? The power switch turns the +5V on/off as described in the power supply section of the manual. The reason the oscillator is necessary is that it acts as the clock for the CPU. I've also seen that in a counter from Racal.
From memory, the supply current in standby is about half of the current during operation, so roughly 300 mA for the 5V rail.
As far as the clock is concerned, there is a 10 MHz crystal on the main pcb, so I had assumed that that would deliver the main clock for the processor.
If not, then what is it there for? If the system requires the Rb clock to be always there, the crystal ought to be superfluous, no?
The last thing to check is the GPIB interface.... I will get to that once I have a hardware driver assembled!
Cheers, EJ
Pictures will follow in the next post (there seems to be a bug in the system: canceled attachments are not deducted from the total attachment size)
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