HP 8640B opt 323 ref oscillator adjust?

[Edward Cain]

   I've noticed that my opt 323 generator differs significantly in actual freq output for a given set freq. Using two separate frequency counters confirms the 323 to be off.
   I don't have a manual for the 323 so I wonder if anyone here has adjusted their instrument and can tell me where the adjustment is located. I have the 8640B manual. Is the adjustment location the same ( under the bezel)?

Thanks,
Ed

[WB6NVH]

What is "significantly" off frequency?  I haven't set the reference on an 8640B before (despite having a pile of them) but I am thinking that the reference doesn't adjust far enough to make a huge difference.  Maybe so.

I will try to find my service manual and see what the procedure is.  I think someone is making them on CD now.  I wound up paying $ 65 for a photocopy back in 2000 as the thing is the size of the LA phone book.

I sorta think you have some other issue going on if it's really off, especially if someone has been into it already. 

[KM1H]

Opt 323 is the military version that is not phase locked. The manual is available online
http://www.ebay.com/itm/HP-8640M-SIGNAL-GEN-OPS-SERVICE-MANUAL-USM-323-/370105851116

Carl

[Edward Cain]

   Thanks for the reply, Carl. I emailed that seller and found out that the manual is for the 8640M and not the opt. 323, which is a different instrument. As far as I can tell, the 8640M is an 8640b in a military can with all or most of the features of the 8640b, such as phase lock, counter input etc.

Ed

[Edward Cain]

Thanks NVH. By significant I mean approx. 10-20 ppm. This becomes significant when using the 11710B down converter to get 50 kHz as the converter subtracts 50 MHz from, in this case, 50.050000 Mhz.

I appreciate your looking for your manual.

Ed

[KM1H]

The AN/USM-323 operation and maintenance manual is

T.O. 33A1-8-684-4
(TM-07570A-15)

And the Parts List manual is

T.O. 33A1-8-684-4
(SL-4-07570A)

They should be available for d/l on the military manuals site, someone on here likely has the link handy, Id have to do serious digging to find it.
I have both of them on a CD with a total of around 560MB, not exactly emailable I dont think but Im game to try.

Carl

[Edward Cain]

Thank you Carl.
   Is the website you refer to LOGSA? I have had no luck on that site for the TM #'s you gave.

   May be I don't know how to use LOGSA.

Ed

[w3jn]

The opt 323 (AN/URM-323 as I recall) manual isn't on LOGSA, unfortunately.

[KM1H]

Maybe because it is an AN/USM-323?

[w3jn]

That, too    It ain't up there, so far as I could find, which is strange because  there were scads of these things made.

[KM1H]

Well anyway I hope you have a fine Greek Christmas John. I'll be pigging out with my Greek inlaws after I leave the Polish ones

[w3jn]

Xmas isn't a really big thing here.  New Years and Epiphany are bigger, although I went to a couple malls yesterday and they were jammed.

I'm going to a colleague's for dinner and fine company - hope you have a merry one, Carl!

[WB6NVH]

I finally got home and located the manual. 

One correction- the Maintenance section of the 2-part manual is T.O. 33A1-8-684-1 .  Not 684-4, that's the parts book.

The alignment section is hugely complex with stuff you don't want to know, such as testing for residual AM, pulse shaping, and on and on.  It's depressing just looking at it.

At the moment I don't see any section with a quick way to line up the frequency on the nose, but this is what I would do and what seems to be what you want to do too--

There is a counter which measures the output.  Presumably if everything is functioning correctly we just adjust the timebase so that the counter reads what is coming out the spout. 

Now...Board A3A4 Time Base Assy. has what looks like a square or rectangular metal module on it with a 5 MHz crystal oscillator inside, which makes the square wave timebase reference signal.  I am presuming there is a hole in that module somewhere which allows you to access the trimmer capacitor or inductor in order to warp the output enough to put the counter on the nose. The manual just shows it as a square box on the schematic, as presumably it was replaced as a module.

Where is Board A3A4?  I don't remember!  I posted this response in case you know where it is and wanted to start right away.  I don't have any USM-323's open at the moment but I should be able to find the board by plowing through the 2" thick manual, I just don't have time this morning.  Presumably it's under the cover near the front that says "Counter Assembly" but maybe that would make too much sense.

[Edward Cain]

   Thanks for taking the time, NVH. All the manuals I have on the 8640 variants, civilian and military, have a section in chapter 5 regarding "internal reference oscillator adjust". I assume your manual does not.
   I haven't had the time, yet, to pursue the issue without knowing exactly where to go. But, your identifying the board will help a great deal.
   Thanks again,
Ed

[WB6NVH]

I found the pictorial diagram in the parts section.  The time base board is the bottom "mother" board lying flat inside the cast counter housing, i.e. the housing in front that also houses the LED display.  Now...the timebase box shows a hole in it facing forward toward the front panel, as if you could shove a tool through the front panel and adjust it.  Is there a panel access hole?  I need a forklift to reach my sample generators to look, but I am guessing there should be.  

This process is not listed in the calibration instructions in the manual I have, unless New Year's Eve affected me more than I think.  It certainly should be, but doesn't seem to be.

I suspect the civilian 8640B manual shows the same layout.  The only thing is that the military unit has more screening and a different panel and it's just a question of how the tool inserts.  It would be to the left of the display, as far as I can tell.

Geoff

[Edward Cain]

   Thank you again, Geoff. I think you've solved the adjust location problem.

   Just to the left of the frequency readout there is a screw which I took as a panel mount screw. However, it is, in fact, a cover. Underneath, there is a slot head adjustment.

   It will be a couple of days before I set things up for an adjustment attempt but I really appreciate your taking the time to track this down for me.

   I'll post my results.

Ed

[WA1GFZ]

Yes, My 8640Bs have a hole in the front panel.