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Vintage HP 3721A Correlator : Repair, Restoration and Enhancement

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dazz1:

--- Quote from: factory on September 26, 2024, 07:22:32 pm ---
--- Quote from: Kean on September 26, 2024, 12:20:50 pm ---A nice design, but those wiring harnesses certainly would have been quite labour intensive.

I hope you are able to get it functioning, but I have no idea what it is used for.

--- End quote ---

If your interested there is a good write up about the HP 3720A, 3721A and 3722A in the HPSQF books, have a look at volume 2 pages 246 to 259 for more. One of the 3721A Correlators is now a museum piece, along with an early 3722A Noise Generator, these were donated to the National Museum of Scotland.

A link for the complete set HP SQF history books can be found on the hpmemoryproject website, the pdf printer editions were the only download links that worked for me.

https://www.hpmemoryproject.org/timeline/hugh_walker/hugh_walker_memoir.htm

--- End quote ---
Wow those history books are amazing.  They contain technical info as well as the usual historical stuff.  When combined with the referenced HP Journals, this is a great repository of information.
After reading the stuff about the HP 3720A Spectrum Display, it looks like that could be replicated entirely within a PC connected to the digital interface on the HP 3721A. 


--- Quote from: factory on September 25, 2024, 03:00:35 pm ---
I think you might need a newer edition of the manual, the prefix is 1850U, this is the design revision for week 50 of 1978 (this indicates it was made at or after that date), possibly amongst the last batch(es) made at the HP South Queensferry plant in Scotland.

They only tended to update the manuals after several major design changes, there would be a set change sheets to cover the prefixes not in the manual.
If the manual & change sheets are original to your 3721A, then the change sheets should cover the prefix of 1850U and there should be something covering the two options installed.

--- End quote ---

There are separate change sheets in the manual.  I have not studied them in detail.


--- Quote from: factory on September 25, 2024, 03:00:35 pm ---As there were only 610 made, the last serial number should be around 700, the first production models would serial numbers starting at approx 100, the HP SQF history book explains why, they didn't want the customer to think they had an early unit with possible problems. The pre-production batch (aka prototypes) would be a small batch (6-10 units) with random serials numbers between 1 to 99, I have seen them with no serial number too.

David

--- End quote ---
So if I am interpreting my S/N correctly, my HP3721A was number 685 out of 700.  So production unit number 585 of 610 (excluding prototypes?). 
Maybe they ran out of knobs because I now realise that the HP knobs on mine are too small compared to the text printed on the panels. 

Now that I know more about these things, I am more incentivized to get my HP 3721A working. 

dazz1:
I have done as many functional tests as I can.  To go any further, I need a HP 3722 Pseudo Random Noise Generator. 

I did think that a standard DSO could be converted wot a HP 3721A Correlator, but after studying more references, that would not be a good solution because the DSO would need to be gated, triggered and clocked with the pseudo random number generator.  This is required to allow the noise source to be replayed again and again without variation.  The statistics of true noise would affect correlation results. 

So the combination of a HP 3722 Pseudo Random Noise Generator and the HP 3721 Correlator would do a better job than kludging together a DSO and noise generator.

The photo shows the rear panel of the HP 3721 and the connectors that connect directly to the HP 3722. 

So I need an  HP 3722 Pseudo Random Noise Generator to complete functional checks, to calibrate the HP 3721 and to make it really useful.

dazz1:
Hi
I need a HP 3722 Pseudo Random Noise Generator to be used with my HP 3721 Correlator.  They were designed to be used together.

If you know where I might buy a HP 3722 at a reasonable price in good condition, please let me know.

dazz1:
During the Auto-correlation functional test, I noticed the single data point (1 of 100) rising.    This was a good confidence booster that HP 3721 is working end to end.  It doesn't prove that the Correlator is working fully, but it is encouraging.

dazz1:
Hi
I have part numbers for the broken knobs, but they don't appear to be valid.
I am wondering if the part numbers were for the big chunky knobs usually seen on photos of HP 3721A.
I have attached annotated photos of what I am looking for.

If you know of where I might source these, please let me know.

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