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8640b problem

We have an HP 8640b that will not work in phase lock mode. When you turn on phase lock, the unit tries to phase lock for about two seconds, then the display blinks. It does this on all frequency ranges. Looking at the output, when phase lock turns on, the signal jumps up in frequecy and keeps climing until the display blinks, then the signal returns to the original frequency. I haven't opened this one up yet, figured I'd ask if this is a normal problem before I dig in.

I have checked for the obvious problems, i.e. cracked gears, loose boards, burnt components..

All help appreciated.

Thanks, Steve

Reply to
Steve
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Does the digital frequency readout work correctly? I've seen many of these go intermittent or worse. As you might expect, if the digital readout is flaky, the phase lock is unlikely to work.

Reply to
Ancient_Hacker

The readout seems to work correctly, the frequency is accurate and it isn't intermittent. It seems like the error amplifier is calculating too high an error and overcorrecting, causing the frequency to drift until it is no longer phase locked.

Thanks, Steve Kamego Calibration Specialty Inc.

Reply to
eletlabs

I've seen a high failure rate of the MECL IC's under the right circuit board-- they normally run quite hot, which might have cut their life. I wouldnt be surprised if the problem is in there.

If you have another 8640B you might try narrowing down the problem by swapping those three boards in that area.

Reply to
Ancient_Hacker

Thanks so much, I'll try that when I get it opened up again.

Regards, Steve Kamego

Reply to
eletlabs

open the lid, look in the back, there is a power supply board plugged in, actually boards! , they have LED's that indicates them being on. you mostly will find one not on. flip the unit over, remove the bottom lid, there is a can transistor there that likes to have it's screws in the collector come loose. Tighten.. I have 2 of those units with all the options, they both have that problem at times.

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Reply to
Jamie

I'll give that a shot and let you know, I don't think I'm going to open it up again this week, too much other work going on. Thanks for the reply, come to think of it I didn't check all the supplies status LED's, just made sure the boards were tight.

Regards, Steve

Reply to
Steve

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I checked the supplies, all are present and in tolerance, I checked the transistor and it's good and tight.

Thanks again, Steve

Reply to
eletlabs

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