HP-8640B problem

[Edward Cain]

I just picked up an HP-8640B opt. 323 signal generator. Looking at the output (modulation off) on my scope (sweep rate at 5ms/div), I see the waveform in the attachment.

Any ideas as to where to look first?

Thanks,
Ed

[WA1GFZ]

Check the power supply for ripple. Pull the regulator card and reinsert it.
I had one with messed up power and it worked fine after I cycled the card.
Paid $100 for it and a late model to boot. Also could be an issue with ripple on the leveling. I think in the box the output amp is located in. ECL power to the divider section could also be an issue. I would take a look at the cavity output on a spectrum analyzer if it isn't the power supply. I had a cavity with the screw threads loose on the cavity lid front section that made a weird waveform.

[KM1H]

Thats the USAF model AN/USM-323 and it doesnt have all the features of the commercial model such as phase locking the cavity.

I have the same model and suddenly had a lot of broadband noise on the signal. Reset a bunch of boards and its been fine.

Carl

[WA1GFZ]

Carl,
Many mil 8640Bs lived ther lives sitting on the floor with the face up. I read that in time the grease from the lead screw flowed down into the cavity making them unstable or stop oscillating. I agree on cycling the board connectors. Edge connectors get tarnished. At least HP used good plating on the contacts. Never take an eraser to them.

[W3SLK]

I'd like to find me a nice 8640B 

[Edward Cain]

Thanks for the response GFZ and Carl. I tried reseating all the boards that can be reseated with no luck. What ever is happening seems to be happening at about 60 Hz so I think I'll concentrate on the power supply. I'll look for ripple first.

Forgive my ignorance GFZ, but what is ECL that you refer to?

Thanks again,
Ed

[WA1GFZ]

The 8640B cavity oscillator tunes about 250 to 500 MHz. Emitter coupled logic  is a string of high speed divide by 2 flip flops to divide the operating frequency down to the lower bands. The waveform coming out of the dividers is not a perfect sine wave but close. I think there is some filtering  in the divider section. I hope that isn't your problem because it is a trip to deal with the cam switches. 60 hz I would look at the power supply. May have an open diode, Then I would look at the modulator section.

[Edward Cain]

GFZ,
   I found a bad cap in the power supply. It's a 3900 uF at 50 V which I'll have to order (luckily found an exact replacement). I hope this is the only problem. I'd hate to get involved with the ECL (thanks for the explanation).

Ed

[WA1GFZ]

A larger value is ok as long as the voltage is the same or higher. not critical

[w1vtp]

Any time one  shows a pic of "ripple" it would be nice to know if the period is 8.3 ms (120 hz) or 16.7 ms(60 Hz).

I assume you have a manual.  If not, short of reseating boards solving the problem, you will need one.

Al