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Vintage HP 3721A Correlator : Repair, Restoration and Enhancement

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factory:

--- Quote from: dazz1 on October 07, 2024, 03:54:54 am ---Just looking a bit closer at the HP 372OA Spectrum Display.  It seems the HP 3720A qualifies as being very rare.  I could not find a modern photograph on Google.  There is no realistic chance of finding one to pair with the HP 3721A Correlator.

--- End quote ---

Here is a picture of one, it was archived by me from ePay.de, seller had both the 3720A & 3721A.



And a another picture to show the typical condition of used TE in the UK, yes it was a 3722A.  :-DD



David

dazz1:

--- Quote from: factory on October 07, 2024, 07:52:27 pm ---

Here is a picture of one, it was archived by me from ePay.de, seller had both the 3720A & 3721A.
...

And a another picture to show the typical condition of used TE in the UK, yes it was a 3722A.  :-DD
...
David

--- End quote ---

Wow,  so there is at least one surviving HP 3720A in the world, but that is paired with a HP 3721A and no sane person would separate them. 
I have played around with your photo to produce what you see attached.   

I would imagine that any orphan HP 3720A would probably look like the naturist HP 3722A in your photos, or worse. 

The HP3720A would have been the "portable" alternative to the fridge sized HP computers of the time.  Still my best chance of implementing a HP 3720A is to emulate one in software.  I have an HP laptop so making an interface to the HP 3721A would be a historically consistent thing to do.

abeyer:

--- Quote from: dazz1 on October 06, 2024, 10:46:01 am ---The cat has quickly adopted the top of the HP 3721A as the warmest place to be.

--- End quote ---

I feel like I must object to your choices in heat dissipation approach. Excessive floof makes for poor thermal interface material and contributes to overall high system Rth.

dazz1:
Just reading up on the HP 3721A Correlator and came across some details of the glass memory.

They managed to store 102 x 24bit words in a piece of glass using ultra-sonics.  The documentation says: "Information in the main store recirculates at a bit rate of approximately 18MHz, one complete [read/write] cycle taking about 135.6us.  Operation of the glass delay line is similar to shouting down a long tunnel, catching the sound at the other end, and feeding it back to the beginning."    Perhaps the first "large" capacity "high" speed dynamic RAM.

Also the dates on the ICs indicate the HP 3721A was built after March 1979.   The HP 3721A was sent to Australia Nov 1982, where the clock frequency was found to be off.  So I am now wondering if the HP 3721A was sent to Australia under a warranty repair/calibration.  If so, shipping cost (=$$$) and the risk of damage, would have been HPs responsibility.   

Just some things you never knew you wanted to know.

dazz1:
The fan hack on the Correlator looked like a cancerous tumor on the side.  Something had to be done.
The aim was to make a fan mod that looked like it could have been factory fitted.   I was originally planning on machining a flange mount from a chunk of plastic to use period methods.  That would have taken too much time/effort.  So I did a 3D print, sanded to give a slightly old looking finish.

I replaced the 240VAC tumor fan with a 12V 80x15 fan.  This required designing a mounting flange to support the fan.  The 12V supply would be used to drive a tape punch option if it was fitted.  So the 12V supply has spare capacity and is not sensitive to noise.

The new fan spins a lot faster than the old 240VAC version and appears to push more air. It is a bit noisy but no worse than the average PC.

I rate this mod as a success.

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