HP 8640B

First thing is to check the 5V bus for a short. If that is OK then check the regulator card and external pass transistor for problems. A little patience and I'm sure you'll find the problem. Good luck.

Nanu

Thanks for the heads up Nanu. The bus looks good as does the card components. Tomorrow I'll check the pass transistor which is a 3055. I have a sneaking suspicion that is my problem.

Tracy

Found out that emitter to collector was shorted in the pass transistor. They are very easy to replace. I noticed that it had been replaced before at some point. They used a mica insulator between the transistor and ground. The insulator was broken. I think the collector might have been grounded. I had a 3055 to replace it. Now I just have to get some of those hard to find little fuses that solder onto the regulator card. I only have 35 dollars in the generator so if it never works I'm not out too much.

Thanks, Tracy

Dumb question time...how do ypu ID the card that has the 1 ghz option. As of yet, I have not powered up the unit for more than a few seconds.

Thanks for the tips. I also found info on the problems with the delrin gears.

Tracy

It does have the 1024 range on the switch. Maybe that's a good sign eh?

I won't know until I get a fuse for sure.

Tracy

the power transistor on the botton side comes loose! tighten the screws, also check for bad caps. other than that if you got the doubler option for 1 Ghz, you have something there. its an easy thing to repair/

i have 2 of those units with the 1 Ghz option. old but very nice with the hard line feeds in side ect. :)) P.S., make sure you lubricate the range selector universal because the original grease (safe for plastics) tends to harden up. also check for alignment and make sure there isn't any slack between the joints.

I replaced the pass transistor. I installed a 5 amp fuse. It calls for a 3 amp. I did not have one so that will have to wait a few days. The transistor runs mighty hot. Is that normal for this particular generator? It seems to work fine though. I noticed whomever replaced the last transistor did not use any conductive paste. I'm thinking that may be some of the heat issue.

Tracy

Hmm, i not sure about that, i also got these 2 units from ebay, the selector dial has the 1024 position on it, when you snap the switch over to the mode your freq counter should display it. as far as i know its simply a doubler module put in there.

Tracy-

Apparently the doubler can be added to one that doesn't have it. As I recall, the internal counter displays frequencies in the 1024 range, even if the doubler is not there. I have the impression that the counter is not connected to the output, but to an earlier stage, and displays a calculated value.

Look at the HP label that has "OPT" and see if any are listed. I think the doubler is option 3.

If you need a manual, do a search for TM 9-4935-601-14-7&P.pdf. It is about 10 Megabytes.

Fred

mine does not run hot, you mite want to check for leaky caps.

Thanks Fred...

There were no options listed on the decal so I', inclined to believe it is the base model.

Tracy

I did not have any paste handy. I ordered some. I also noticed that one of the screws that holds the transistor down will not tighten. It seems to be stripped out. Once the paste gets here, I'll repair the screw problem and see if that helps. If it does not, I'll move on to other things to see where my problem might lie.

Tracy