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Vintage HP 3721A Correlator : Repair, Restoration and Enhancement

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dazz1:

--- Quote from: tautech on October 05, 2024, 07:40:25 pm ---Eyes are immediately drawn to 3M9 R29.....  :horse:

--- End quote ---

OK so I replaced R29, and that fixed the problem with intensity control interacting with Y deflection.  Yahoo!  Thanks Tautech  :-+

Also the gain was restored to 100%, for about 30 minutes, then it wasn't.  R29 fixed one fault, but not the other.  So I have now confirmed I have multiple faults.

So what I am now seeing looks like unstable drive from one of the CRT  Y deflection plates.    I already know the deflection amps are working flawlessly, so I am going with the hypothesis that there is a bad connection to one Y deflection plate between the amp and the CRT.  That is a piece of wire.  A bad connection would be  consistent with both the instability and the stepped Y gain.

tautech:

--- Quote from: dazz1 on October 06, 2024, 05:16:37 am ---
--- Quote from: tautech on October 05, 2024, 07:40:25 pm ---Eyes are immediately drawn to 3M9 R29.....  :horse:

--- End quote ---

OK so I replaced R29, and that fixed the problem with intensity control interacting with Y deflection.  Yahoo!  Thanks Tautech  :-+

Also the gain was restored to 100%, for about 30 minutes, then it wasn't.  R29 fixed one fault, but not the other.  So I have now confirmed I have multiple faults.

So what I am now seeing looks like unstable drive from one of the CRT  Y deflection plates.    I already know the deflection amps are working flawlessly, so I am going with the hypothesis that there is a bad connection to one Y deflection plate between the amp and the CRT.  That is a piece of wire.  A bad connection would be  consistent with both the instability and the stepped Y gain.

--- End quote ---
:clap:
Okay so something still maybe heat related, time to bring out the Jewelers loupe and hunt for dry joints.

Edit to add.
Any socketed transistors ? Give them a reseat.
Clean PCB edge connectors gently with an eraser.

dazz1:
Hi
It's official.  The HP 3721A Correlator is correlating again.

I opened up the back of the CRT where all the connections are made.  The plan was to directly probe the pins of the CRT to see what signal the CRT was getting. 
I found that some sort of silicone compound had been applied to the low voltage (Vpk ~ 160V), but not on the HV side.  I suspected that the compound has degraded with time and is leaking electrons.
All the solder joints are good.
I cleaned out the silicone and directly probed the CRT Y deflection connection pins.  Interestingly the differential amp output is not symmetrical, but the displayed image on the CRT is exactly right.

So there was a problem with the wires and connectors between the Y deflection amp and the pins on the CRT.  I don't know why they siliconed the LV connections but I see no reason to replace it.   It could be that I dislodged some oxide that was acting as a diode, but whatever the reason, the fault is gone.

So I had two faults.  One correctly diagnosed by Tautech, leaving one for me to find and fix.

So if an oscilloscope looks at signals in the time domain, and a spectrum analyser looks at signals in the frequency domain, the Correlator looks at signals in the statistical domain.   It can do more that I know what to do with. I am going to have to spend some time learning its capabilities and limitations.

I do know it really needs to be partnered with a HP 3722A Pseudo Random Noise Generator to get to the really useful status.

The photos show:
The Y deflection signals applied to the CRT.
A test pattern showing 100% gain and a sharp image.  Each dot is 1 of 100 discrete data points.
The rear of the CRT showing the silicone applied to the LV connections.  The left side is up to 3kV.

dazz1:
Just some screen shots of the analysis of signals from my audio signal generator.

These images are unrelated to frequency or spectrum.
The display that looks like an FM spectrum is a pure sine wave.
The display that looks like exponential decay is the analysis of a swept sine wave.
The last three are cumulative density functions for:
sine
triangle
square waves.

The cat has quickly adopted the top of the HP 3721A as the warmest place to be.

dazz1:
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.

Looking at the available documentation, the 3720A is basically a data word driven spectrum analyser implementing the FFT.
There are a a few more features added, but it starts with two  100 word data records read from the HP 3721A Correlator digital port.  The HP 3721A uses trickery with delays to effectively add together multiple 100 point data records and widen the data window before applying FFT.  The HP 3721A data port is a 50 pin connector with a primitive control protocol.  Each word is 12bit and words are transferred at a rate of 7.4kHz.  That sort of processing would be easily achieved with a modern low-spec computer. 

There is no analog input.  The HP 3720A is completely useless without a HP 3721A to pair with.  It is highly unlikely that any isolated HP 3720A Spectrum Display has survived.

So while it might be impossible to find any remaining HP 3720 instruments, emulating one would be technically undemanding.  Helpfully the HP 3721A user manual includes source code, in assembler, for communicating with and controlling through the port.   The incentive for emulating the HP 3720A would be ending up with a spectrum analyser with a frequency range starting at 0.005Hz.  I have no idea what I would use that for, but it would be good for bragging rights. 

I already have too many projects to consider making a start on a HP 3720A emulator, but I do enjoy connecting modern tech to really old tech.

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