I gutted an ancient CB radio PSU yesterday which served as my main 12 V PSU along with a 5 V supply I built from scratch many years when I was in school.? It's a 40 Vrms CT with 22 AWG leads.? It used a bare LM317, so not a beefy transformer.? I considered it 1 A max when I repaired it 50 years ago.
I had been planning to use the existing chassis, but as I contemplate what I want to put in there I think a bigger chassis will save a lot of misery.? So I think the transformer and fuse holder may be the only parts I use.
Stage one will be a brute force full wave rectifier PS putting out about 28 Vdc with an IEC EMI filtered socket for the mains feed and large caps and chokes on the output.? The LM317 datasheet shows -70 dB ripple rejection with a 15 V drop across the LM317.? The LM399 needs 10 Vin so that makes the drop across the 2nd LM317 a bit marginal so I'll have to trim the pair of LM317s to see what produces the least ripple.
For the 10 V feed to the LM399 I plan to use a 5-10 F super cap and similar on the 6.95 V output and also for the PS for the ultra low noise op amp that will actually supply the OXCO Vctl divider.? Not yet sure about the 5 V supply to the OXCO.? At turn on they pull almost 1/2 A which is a lot to ask of an op amp.? Once they are warm they drop to about 1/4 A.
As there has been quite a bit of FUD about the CTIs "jumping" in frequency, I've attached a screen dump from a recent multi-day experiment using a tinyPFA, Bodnar GPSDO, new PCB OXCO,? HP 5386A, Instek MSO2204EA and Instek GPE4323 PSU.? The GPSDO and OXCO were feeding JFW 1:5 splitters so that the DSO, 5386A and tinyPFA all had the GPSDO for the reference signal.? Over the course of 3 days I observed a constant 0.01 Hz (last digit) bobble on the 5386A and a 0.005 Hz bobble on the tinyPFA which was consistent with drift of the signals on the DSO.
I attached the DSO to the PSU using a BNC to dual banana plug cable to capture the screen dump. I don't yet know what the source of the noise burst is. It is periodic and I know that the DSO has major SMPS noise problems and will be replaced with a low time Tek 2465B by the end of the day. The Instek PSU is a linear supply so it should be much quieter than the SMPS in the DSO.
The CTI OSC5A2B02 datasheet specifies +/- 2 ppb for +/- 5% supply variation. That's 8 ppb/mV which translates to over 90 ppb peak shift during the 200 ns noise burst. The voltage regulation on the new PCB OXCO reduces that substantially, but measuring actual supply variation to the OXCO module at that voltage level without external EMI pickup will not be easy and will definitely require significant effort and a custom circuit.
So I plod along in the midst of property management, trying to organize far too much stuff I have bought and sell the mountain of tube era electronics and other stuff Dad left me.
Anyone interested in an NOS Montgomery Ward 3.5 cu ft cement mixer still on the original shipping pallet? FOB Heber Springs, AR.
Have Fun!
Reg
