Author Topic: Vintage HP 3721A Correlator : Repair, Restoration and Enhancement  (Read 5437 times)

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Offline dazz1Topic starter

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Hi
I have just purchased a vintage HP 3721A Correlator.  An item of test equipment far more interesting and capable than its name suggests.
This one is in good condition except for some broken knobs and a developing fault, likely on the power supply for the CRT beam controls.
It is original except for an ugly cooling fan mod.

This topic is a continuation of this one.  https://www.eevblog.com/forum/testgear/test-equipment-anonymous-(tea)-group-therapy-thread/136750/
« Last Edit: September 26, 2024, 02:01:29 am by dazz1 »
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Offline KE5FX

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Re: Vintage HP 3721A Correlator : Repair, Restoration and Enhancement
« Reply #1 on: September 26, 2024, 01:56:26 am »
Fascinating piece of hardware.  I had to look that one up, it was a bit before my time.  Are you going to pair it with a 3720A and 3722A?
 
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Offline dazz1Topic starter

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Re: Vintage HP 3721A Correlator : Repair, Restoration and Enhancement
« Reply #2 on: September 26, 2024, 02:01:03 am »
Fascinating piece of hardware.  I had to look that one up, it was a bit before my time.  Are you going to pair it with a 3720A and 3722A?

Sadly there is practically zero chance of me finding or owning a HP 3720A or HP 3722A.
Dazz

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Re: Vintage HP 3721A Correlator : Repair, Restoration and Enhancement
« Reply #3 on: September 26, 2024, 07:14:06 am »
The asset sticker seems like a warning...  >:D
 
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Re: Vintage HP 3721A Correlator : Repair, Restoration and Enhancement
« Reply #4 on: September 26, 2024, 11:33:12 am »
Hi
This thing was made to be maintained.   The shelf opens out for access both sides and inside.  The connections are made by plugs so the shelf can be removed in a few minutes.  Very nice.  You could call it high quality or over-engineering.  Still nice.

The glass device is 1970's bit data storage.  I am not sure how many bits it can store.

I am about 1/3rd of the way through the functional tests.  All the front panel controls and indicators work. 
The CRT trace is almost perfectly steady in vertical. 
The clock output on the rear panel is not working as it should do.  This may be the first fault.  That is why I opened up the unit.

No sign of any rework inside. 
The power supply transistors heat up the rear panel and may be why the fan was installed.

I have fitted a couple of knobs as temporary until I get proper knobs.
« Last Edit: September 26, 2024, 11:54:45 am by dazz1 »
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Re: Vintage HP 3721A Correlator : Repair, Restoration and Enhancement
« Reply #5 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.
 

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Re: Vintage HP 3721A Correlator : Repair, Restoration and Enhancement
« Reply #6 on: September 26, 2024, 07:22:32 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.

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


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.

...
David

Hi
I know I am a geek when the date of a manual is an interesting topic  :-+

The serial number of the HP 3712A is: 1850U00685
The manual says it is for "Serials prefixed: U985 and 1051U"
"For instruments with Serial Prefixes lower than U985 refer to Section IX, Manual Changes"
"For instruments with Serial Prefixes higher than 1051U refer to the separate Manual Changes Sheet"

The manual includes a separate change sheet.

The manuals that came with the unit are genuine originals so in the absence of other information, I have to assume they are for my unit.  The manual was printed May 1971, so not sure yet how that compares to the date of the unit.

The auction listing included a photo of a dated PCB but this could be a replacement part.    It has a solder mask and no gold plating.

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.

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
 
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Offline KE5FX

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Re: Vintage HP 3721A Correlator : Repair, Restoration and Enhancement
« Reply #7 on: September 26, 2024, 08:15:46 pm »
Those books are amazing!  I couldn't document my own life at that level of detail, much less my workplace(s).
 

Offline dazz1Topic starter

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Re: Vintage HP 3721A Correlator : Repair, Restoration and Enhancement
« Reply #8 on: September 27, 2024, 07:33:20 am »
I hope you are able to get it functioning, but I have no idea what it is used for.

A correlator is a multipurpose instrument that looks for matches (correlation) between two input signals, which may include the same input signal offset by time. The correlator will dig out signals from under the noise floor.  OK so at this point I am confident what I have said is meaningless, so let me show you an example.

A common requirement for power supply design (or any closed loop control system) is to measure the frequency response and display this as a Bode plot.
The ideal Bode plot measures amplitude (y-axis) vs frequency (x-axis) from 0Hz to ∞Hz.  The Bode plot works in the frequency domain.

The Bode plot is the result of applying a unity function as and input, (often a constant amplitude input signal, amplitude = 1) to a control system and measuring the output signal.  When you apply a unity function as an input to a control loop, you get the frequency response of that control loop, the Bode plot. One of the ways of achieving that is to use a AWG to sweep from a very low frequency to a very high frequency and view the output (Bode plot) on a standard DSO.    That method is described in this link https://siglentna.com/application-note/power-supply-loop-response-bodeii/ using a Siglent AWG and DSO.  It works but is sensitive to noise and can't be used on a live system where there are significant control loop demand signals ie. real life.  The amplitude of the signal could be increased above the noise, but often the large signal response of a closed loop control system is different to the small signal response.  Things like slew rate limiting, amplitude limits etc.

Another input signal with a value of "1" across the entire frequency spectrum is the impulse function.  This is a single pulse with infinite amplitude and a pulse width approaching zero.  An impulse is a unity function.  When graphed in the time domain (like you would see on an oscilloscope), the area (energy) under the pulse = 1.     An ideal impulse is made up of frequencies from 0Hz to ∞Hz.  One of the ways to approximate a impulse is to apply an electrostatic discharge as the input signal.  Usually not a practical solution. 

The beautiful thing about the impulse function is that when you transform an impulse from the time domain (oscilloscope) to the frequency domain ( spectrum analyser), the signal has infinite bandwidth and near zero amplitude. The total area (energy) under the signal = "1".   It looks like white noise and it is practical to produce such a signal.   Note that viewing an ideal impulse would require a spectrum analyser with no noise floor.  In practice a true impulse will be buried in noise.

So now we know that applying a white noise input signal to a control loop will provide the Bode plot as an output.  That would be useful if the output signal was visible above the noise floor of the instrument.  The amplitude of the white noise could be increased but that would produce similar results to the Siglent method with the same issues and limitations. 

OK so what if the white noise input signal was sequentially applied, say 100 times.  If you summed the 100 control loop output signals, it would be like multiplying the input signal 100x.  In addition, if you sum true noise present in the control loop, the unwanted noise will be attenuated 100x.  That would give a signal to noise ratio gain of 100x100=10,000 (40dB) to lift the wanted output signal (Bode plot) out from beneath the noise floor. What is needed is an instrument that can read two input signals many times and sum the similarity (correlate) to attenuate the system noise and amplify the wanted white noise signal.   That is what the HP 3721A does.

It would be really convenient if the wanted white noise signal was replayed but true white noise is always random and is never repeated.  Fortunately pseudo white noise can be replayed and repeated any number of times.  The pseudo noise generator would need to be interfaced so the HP 3721A knows when the replay is started, and clocked to keep in sync.  That noise generator is the HP3722A pseudo noise generator.

If the noise generating bit sequence is very long, then the generated noise will include very low frequency components.  Much too low for any typical spectrum analyser to view.  HP created the HP 3720A with a bandwidth of 0.005 Hz to 250 kHz using the internal HP 3721A clock to sync.

So in this example, a control loop can be stimulated with a very low amplitude signal to produce a small-signal Bode plot that is resistant to true system noise when the HP 3721A and HP 3722A are combined.  The output can be viewed on the HP 3720A spectrum display.  The 3 instruments were made for each other and when combined, they can do some seemingly magic things.  They are especially good at separating known signals and lifting them from under the noise floor.


So ~40 years ago I learned the Siglent method, using noise and not an AWG,  for defining the frequency response of a control system.  I didn't know there was an instrument called a correlator.  I have never seen a working HP 3721A and until a couple of weeks ago, I didn't know this instrument existed, so I am looking forward to seeing what it can do in practice.  In the absence of a HP 3722A noise generator, I am already planning to build an equivalent pseudo noise source.   As a side note, adding basic correlation functionality to a Siglent DSO is probably just a factory software modification away.  No need for glass bit memory.

 When I saw this HP 3721A listed, I was just curious.  It was HP, big, heavy, the seller was close and it was cheap.  It wasn't until I started doing some research I figured out what it was capable of, especially when connected to the noise generator, spectrum display, and an early HP computer.  If you want to measure the frequency response of a suspension bridge or, retrieve sonar/radar return signals from far below the noise floor, these instruments will do the job.  This is an instrument I never new I needed until learned about it.  Unfortunately I don't think I will ever have the opportunity to obtain the 3 instruments and an original HP computer in one place to see them working together.

This is an example of one application where a correlator, and no other instrument, would excel in providing the desired output.  There is a whole lot I don't yet know about applications for this correlator.




« Last Edit: September 27, 2024, 09:46:28 am by dazz1 »
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Re: Vintage HP 3721A Correlator : Repair, Restoration and Enhancement
« Reply #9 on: September 27, 2024, 08:26:52 am »
Hi
While I was out walking the dog, another application came to mind.

The seller of the HP 3721A is into audio and purchased the correlator to match speakers so the outputs were in phase, and the correlator could be used for that.

A more interesting application would be mapping the frequency response of concert halls.  My understanding is this is normally done with the hall empty.  Starter guns or electric sparks generate sound impulses and these are used as the stimulating signal to record the Bode plot.   The testing requires minimum noise to get good results.   The problem is that the frequency response of an empty hall is different when people fill the seats.  The seats in the Sydney Opera House are designed to have the same frequency response as a person when the seats are empty.

The ideal solution would be to measure the frequency response of a hall when the seats are full of noisy people.  This would be the perfect application for the HP 3721 partnered with the HP 3722 Pseudo Random Noise Generator.  Low level noise would be played in the hall while it was full of people.  The people probably wouldn't even notice.  The correlator would drag the wanted signal out of the noise to produce the Bode plot.  I am not sure how reverberation would affect the results, and I am not going to try to figure that out.  A correlator would appear to be a good candidate for mapping a full hall.

« Last Edit: September 27, 2024, 09:44:37 am by dazz1 »
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Re: Vintage HP 3721A Correlator : Repair, Restoration and Enhancement
« Reply #10 on: September 27, 2024, 09:32:40 am »
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.

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
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. 


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.

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

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
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. 
Dazz

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Offline dazz1Topic starter

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Re: Vintage HP 3721A Correlator : Repair, Restoration and Enhancement
« Reply #11 on: September 29, 2024, 07:38:27 am »
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.
Dazz

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Re: Vintage HP 3721A Correlator : Need a HP 3722 Pseudo Random Noise Generator
« Reply #12 on: September 29, 2024, 07:49:16 am »
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.

Dazz

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Offline dazz1Topic starter

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Re: Vintage HP 3721A Correlator : Data point auto-correlation
« Reply #13 on: September 29, 2024, 08:16:19 am »
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.
Dazz

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Re: Vintage HP 3721A Correlator : Replacement Knobs ?
« Reply #14 on: September 29, 2024, 09:07:56 am »
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.
Dazz

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Re: Vintage HP 3721A Correlator : Replacement Knobs ?
« Reply #15 on: September 29, 2024, 04:12:55 pm »
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.

As I said in the TEA thread, the manual quotes part numbers for the knobs fitted to the earlier ones, they are valid for the oldest ones, remember this was designed in the late 60s, the fragile beige & dark grey knobs didn't exist then. The black knobs with aluminium trim discs were chosen, these were commonly found on the 180 series & a few other products introduced in the 1960s, then disappeared for most products (except the 180 scopes), to be updated to with those later beige/grey knobs.
If your change sheets cover the your instruments prefix, then you should have an updated parts list with these, probably won't help much, as used part dealers won't know the part numbers.

Of the half dozen 3721 I've seen on ePay over the years, only one was an early one with the black knobs, the rest were later ones with the newer beige/grey ones, also control legends were engraved on the first and printed on the newer ones.

It may take a while to find a 3722 at a fair price & in good condition, I hate to tell you, but they are known about by the modular synth crowd. You might want a H01 option, this is the one with the set of delay function thumbwheels in you picture, the internal picture is the standard version, H01 has an extra board.

David
« Last Edit: September 29, 2024, 04:14:29 pm by factory »
 

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Re: Vintage HP 3721A Correlator : Replacement Knobs ?
« Reply #16 on: September 30, 2024, 07:56:33 am »

As I said in the TEA thread, the manual quotes part numbers for the knobs fitted to the earlier ones, they are valid for the oldest ones, remember this was designed in the late 60s, the fragile beige & dark grey knobs didn't exist then. The black knobs with aluminium trim discs were chosen, these were commonly found on the 180 series & a few other products introduced in the 1960s, then disappeared for most products (except the 180 scopes), to be updated to with those later beige/grey knobs.
If your change sheets cover the your instruments prefix, then you should have an updated parts list with these, probably won't help much, as used part dealers won't know the part numbers.

Yes, I looked at the change pages and none of the numbers returned knobs on Google.  I did the annotated images to show what I am looking for rather than quote dead end part numbers.

It may take a while to find a 3722 at a fair price & in good condition, I hate to tell you, but they are known about by the modular synth crowd.

I am patient.  I will wait until I find what I am looking for. 

Google suggests other noise generators are being recommended ahead of the 3722. Maybe because of lack of supply.  I found one 6 year old listing here https://reverb.com/item/13307500-hewlett-packard-hp-3722a-noise-generator-oscillator-1965-grey for $USD35 or so.  I'd be happy to pay that for one in good condition.  There is an awful 3722 on e-bay at present.  No one is buying.

You might want a H01 option, this is the one with the set of delay function thumbwheels in you picture, the internal picture is the standard version, H01 has an extra board.
What does the delay function do?  Is it worth waiting for?
Dazz

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Re: Vintage HP 3721A Correlator : Replacement Knobs ?
« Reply #17 on: September 30, 2024, 08:26:33 pm »
You might want a H01 option, this is the one with the set of delay function thumbwheels in your picture, the internal picture is the standard version, H01 has an extra board.
What does the delay function do?  Is it worth waiting for?

I can't really answer this, I don't know how much use the additional "delayed binary output" would be, or if it's relevant for the options installed in your 3721. Information isn't really out there on the web, except for this application note, which only shows it used with a demonstration unit; https://www.hpmemoryproject.org/an/pdf/an_98-2.pdf

David
 

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Re: Vintage HP 3721A Correlator : Replacement Knobs ?
« Reply #18 on: September 30, 2024, 11:59:47 pm »
What does the delay function do?  Is it worth waiting for?

I can't really answer this, I don't know how much use the additional "delayed binary output" would be, or if it's relevant for the options installed in your 3721. Information isn't really out there on the web, except for this application note, which only shows it used with a demonstration unit; https://www.hpmemoryproject.org/an/pdf/an_98-2.pdf

David

It looks like the selectable delay would be used where there is a fixed delay in the signal path to the DUT as shown in Figure 2.  Adding delay would enable synchronization of the OUTPUT signal and the delayed prbs.  With modern circuits, it would be relatively easy to add an external delay that was clocked by the process control.    That would make the HP 01 option desirable, not essential.
Dazz

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Re: Vintage HP 3721A Correlator : Repair, Restoration and Enhancement
« Reply #19 on: October 02, 2024, 02:14:30 am »
Hi
Just did a bit of playing around with the HP 3721A connected to my very basic Elektor audio signal generator, that includes a frequency sweep (FM) output.
These were not calibration runs.  These were confidence checks of functionality.
Pleased to say that I saw what I expected.  The attached are summated probability distributions for sine, triangle and square waves.  Note that changing the signal frequency made no difference to the displayed output. 

Dazz

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Re: Vintage HP 3721A Correlator : Repair, Restoration and Enhancement
« Reply #20 on: October 02, 2024, 02:21:35 am »
Attached is the cross correlation of a swept (FM) frequency verses the triangle wave and sine wave output.

Note I didn't record any settings.  I was basically just trying different things.

I think I might have a fault on the HV for the CRT.  When I increase brightness, the Y deflection decreases.  I suspect a weak negative voltage somewhere.
Dazz

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Re: Vintage HP 3721A Correlator : Repair, Restoration and Enhancement
« Reply #21 on: October 05, 2024, 04:33:41 am »
Hi

The correlator has a fault revealed by the CRT intensity (brightness) adjustment.  I can' figure out what the cause is so putting this out to ask for help.
The symptoms are that that as CRT intensity is adjusted from zero, the trace moves vertically (Y) from the bottom of the screen.  At about 1/2 intensity the trace settles at the crt mid point, where it is supposed to be.
As the intensity is further increased, the trace stays in the middle, where it is supposed to be.  The fault is not linear across the range of intensity adjustment.

In addition, the crt trace can also be randomly unstable. 

Also the vertical gain varies in discrete steps.


When I vary the Intensity setting, the Y trace position does not change instantaneously with intensity adjustment.  There is a long RC delay.  The intensity setting does adjust the intensity. 

The X of the trace is unaffected and behaves normally regardless of the intensity adjustment.  Only the Y is affected.

When I connect up the X,Y,Z Correlator outputs to an oscilloscope, they are normal regardless of intensity.  The output of the Y deflection amplifier is rock solid on both legs of the output amplifiers.  This eliminates the X,Y deflection amplifiers as  the source of the fault.  I have my scope connected to the diff outputs of the vert deflection amp.  It is clean and working perfectly.

The only thing separating the Y deflection amp outputs and the crt is wire.

I don't have a HV probe to look directly at the CRT 3kV HV.
The heater circuit is floating at 3kV above ground, so not touching that with a meter.

Referring to the attached drawing:
The HT oscillator is the lower left part.
The CRT HT feedback for voltage regulation is the lower part.
Upper part is the Intensity and Z modulation. Both controls are working correctly.

R26 provides feedback to Q11, a constant current source.  Then there is a chain of transistors that end at Q8, which modifies the switching point of the HT oscillator Q6 & Q7.
All the test points are normal and are not affected by adjustments to the intensity.

I have just turned on the HP3721A and the crt trace was unstable in Y.  X is always OK.  I will make additional posts with photos.

I suspect leakage or breakdown of an HV part.  Unless I have missed something.
Dazz

Over Engineering: Why make something simple when you can make it really complicated AND get it to work?
 

Offline dazz1Topic starter

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Re: Vintage HP 3721A Correlator : Repair, Restoration and Enhancement
« Reply #22 on: October 05, 2024, 04:36:03 am »
The two attached images show the normal display and the unstable Y trace.
This Y displacement is the full height of the screen.  This is full scale, so I will call it a gain of 1.
Dazz

Over Engineering: Why make something simple when you can make it really complicated AND get it to work?
 

Offline dazz1Topic starter

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Re: Vintage HP 3721A Correlator : Repair, Restoration and Enhancement
« Reply #23 on: October 05, 2024, 04:40:12 am »
The attached photo shows a normal display (no noise or instability).
This shows the gain is about 40% of full scale.  It should be a gain of 100%.
Dazz

Over Engineering: Why make something simple when you can make it really complicated AND get it to work?
 

Online tautech

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Re: Vintage HP 3721A Correlator : Repair, Restoration and Enhancement
« Reply #24 on: October 05, 2024, 04:44:13 am »
Old drifted carbon comp high value resistors.  :horse:
Might as well replace the 4700pF caps too....
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