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Warmsteel 20th Jun 2016 7:49 am

HP8640B questions
Greetings all

I have seen a number of posts about the HP8640B signal generator where a number of very knowledgeable people have provided their advice and I am hoeing that the same people will assist me.

Having recently squired and early model (serial # 1536U00693) and successfully repaired the rectifier board to get it functioning I have now come across the broblem of lose attenuation knobs.
I have managed to remove the grub screws from both the step attenuator and the pot attenuator without damaging the knobs but it appears that the grub screws have been over tightened or the wrong size Allen key used when earlier removal was attempted as I had to use varying sizes of torx drivers to remove the grub screws.

I do not want to use the same grub screws and I am hoping that someone will be able to tell me what the correct size of grub screw is used on both attenuator knobs, if anyone would know I would be grateful too hear from them.

Whilst I have found a service manual it does not cover my serial number and I am also asking if anyone would happen to have a copy of the manual that covers my serial number (having seen the warning on the front pages of the other manual) that they would be will to let me have a copy of or if anyone knows where there is a manual that covers my serial number.

Thank you to everyone


Radio Wrangler 20th Jun 2016 8:05 am

Re: HP8640B questions
They will be imperial threads.

It may be that the original grub screws were not damaged... some of them were built with "Bristol Spline" type, not hex.

The U says your instrument was built at South Queensferry in week 36 of 1975 and it will have the inspector's stamp of an old (sadly late) friend all over the mechanical bits.

At a guess the threads may be 4-40, american style.


Humber888 20th Jun 2016 8:14 am

Re: HP8640B questions
My HP8640B is a bit later than yours and my manual does not cover serial numbers as low as your one. You could try contacting, say, Telford Electronics and quote your S/N. They may very well have one that is correct for your vintage of circa 1975. I assume you have already checked the usual online sources such as Keysight themselves, BAMA, KO4BB, etc? Another possible source is Artek Media in the USA for excellent manuals on CD.

My 8640B is currently in the loft, so I can't check the size of grub screw used on the knobs directly. However, HP tended to use two main sizes for most of their knobs, so you stand a good chance of sourcing replacements e.g. from scrap knobs used on a range of HP kit. The grub screws will be American standard ones.


Nymrod121 20th Jun 2016 9:16 am

Re: HP8640B questions
Hello John

I've done a bit of work on this type of generator and my records refer to the following:

4-40 UNC x 0.125" small cup point (hex key = 0.05" across flats)
6-32 UNC x 0.125" small cup point
2-56 UNC x 0.094" small cup point (hex key = 0.035" across flats) grubs

They do a range of suitable hex tools as well.

I assume from the early serial number that your instrument has the earlier system of 'range vs deviation interlock' - otherwise known as 'A9' assembly - that uses wafer switches rather than the later PCB-tracked switches? - if so, I've done a lot of work regarding replacement of the Delrin hardware ...

Best wishes


Warmsteel 27th Jun 2016 8:52 am

Re: HP8640B questions
Greetings Everyone

Thank to all for your help in this matter, It turns out that the information Nymrod121 (Guy) provided is just what is needed and perhaps this information will be helpful to others in the future.

I used 3/8 lengths and although these are a little long for the 4-40 it still does the job.

As for Radio Wranglers comments that the grub screws could be Bristol spline grub screws, I am unable to be sure since the heads are to badly damaged and I had to use 2 different sized torx drivers on each of the 6-32 grub screws, but thank you for pointing that out.
Also David, thank you for the added info of when it was built and where, it does help in my search for a manual, which I am still unsuccessful in locating. It seems that my serial number as fallen in to a large hole when it comes to the manual but I may be lucky yet?

So now all that is left to do is reattach the printed disk and make sure that the disc matches the attenuation!

Once again Thank you to all


Nymrod121 27th Jun 2016 10:48 am

Re: HP8640B questions
Nice one, John :thumbsup:

I can sympathise regarding your comments on serial numbers. My first 8640B (512MHz) is fitted with the wafer switch version of the deviation range vs. frequency tracking assembly (A9) and I've had to cross-reference several different versions of service manual circuit diagrams.

Much burning of midnight oil !:)



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