The AM Forum

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

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.

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

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.

I'd like to find me a nice 8640B  :o

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

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.

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

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

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