Hi
In 1985 I was at University and learned a method for tuning and characterising control loops using noise and statistical analysis. I didn't know that a specific item of test equipment was available to do that method, and more. A correlator is really good and dragging known signals from below the noise floor, and doing statistical analysis in the time and frequency domains.
Roll forward almost 40 years and some how I saw the HP 3721A Correlator on an auction site. The seller was located only 6km from my home with a low reserve price. At first I didn't realise what it did until I Google showed me a little information about it, including the operator manual. Then I realised what it could do. The manuals are dated May 1971, and for its time, it is a very capable and quite amazing item of test equipment.
I was the only bidder.
In 1976 the HP catalogue had the price of the HP 3721A listed at $USD10,125.
To get the most out of the HP 3721A also required the HP 3722A Pseudo-random noise generator priced at $USD3,600 and,
the HP 3720A Spectrum Analyser Display priced at $USD7,265.
In 1971 import duty was about 130% plus the excessive cost of shipping and fees. This trio of matching test equipment was priced at about the same as a nice house. Although a government department made the purchase, it seems it could only afford the HP 3721A. This was an expensive item of test equipment. I suspect it has only survived for so many years because it cost too much to throw away.
It powers up and the CRT is crisp. The only concerning thing is that the service manual has the parts section book marked. The manuals are dated 1971 but other documents show the HP 3721A was shipped to Australia to be calibrated in 1983. That would have been very expensive.
The fan mod is ugly but well executed.
This instrument is capable of a lot of things and learning to get the most from it will be a steep learning curve.
The HP 3721A was sold as faulty. I need to run through the 22 page performance check to see if it does have a fault.
The performance check and the calibration procedure specify the HP 3722A Noise Generator is required. I don't foresee any danger of obtaining an HP 3722A under any reasonable circumstances, so I will need to make an equivalent with modern parts.
There aren't many of those 3721A Correlators about, only 610 were made, between 1968 & 1979.
There are even less 3720A Spectrum Displays about, only 220 were made, this one was introduced four years later than the 3721A and discontinued at the same time.
But you might find the 3722A Noise Generator slightly easier to find, this one was the first of the trio, introduced in 1967 and discontinued in 1980, 1590 were made.
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.
The parts you reference are for an older version, a part number starting with five digits such as "00180" indicates the equipment that part was first used on, in this case "00180" would be the 180 series oscilloscope and the knobs are quite different to those on your later correlator.
Attached are some pictures of an older version (saved from ePay), if your 3721A had those "180" series knobs, they wouldn't be broken, the later winged knobs are some of the most common to find snapped off.
P.S. The delay offset switch might be an option (one of the two on the back?), I've seen at least two of them without this control. Edit: Three if you count the preserved 3721A, which is a museum piece now, along with a very early 3722A, both are in the National Museum of Scotland.
David
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