Vintage HP 3721A Correlator : Repair, Restoration and Enhancement  - Page 1
Forum section: electronics / testgear

author: dazz1
author_rank: Frequent Contributor
timestamp: September 26, 2024, 01:40:41 am
content: HiI 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/

author: KE5FX
author_rank: Super Contributor
timestamp: September 26, 2024, 01:56:26 am
content: Fascinating piece of hardware. I had tolook that one up, it was a bit before my time. Are you going to pair it with a 3720A and 3722A?

author: dazz1
author_rank: Frequent Contributor
timestamp: September 26, 2024, 02:01:03 am
content: Quote from: KE5FX on September 26, 2024, 01:56:26 amSadly there is practically zero chance of me finding or owning a HP 3720A or HP 3722A.

author: Kean
author_rank: Unknown
timestamp: September 26, 2024, 07:14:06 am
content: The asset sticker seems like a warning...

author: dazz1
author_rank: Frequent Contributor
timestamp: September 26, 2024, 11:33:12 am
content: HiThis 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.

author: Kean
author_rank: Unknown
timestamp: September 26, 2024, 12:20:50 pm
content: 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.

author: factory
author_rank: Super Contributor
timestamp: September 26, 2024, 07:22:32 pm
content: Quote from: Kean on September 26, 2024, 12:20:50 pmIf 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.htmQuote from: dazz1 on September 25, 2024, 11:20:03 pmThey 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

author: KE5FX
author_rank: Super Contributor
timestamp: September 26, 2024, 08:15:46 pm
content: Those books are amazing! I couldn't document my own life at that level of detail, much less my workplace(s).

author: dazz1
author_rank: Frequent Contributor
timestamp: September 27, 2024, 07:33:20 am
content: Quote from: Kean on September 26, 2024, 12:20:50 pmA 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 linkhttps://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.

author: dazz1
author_rank: Frequent Contributor
timestamp: September 27, 2024, 08:26:52 am
content: HiWhile 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.

author: dazz1
author_rank: Frequent Contributor
timestamp: September 27, 2024, 09:32:40 am
content: Quote from: factory on September 26, 2024, 07:22:32 pmWow 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 pmThere are separate change sheets in the manual. I have not studied them in detail.Quote from: factory on September 25, 2024, 03:00:35 pmSo 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.

author: dazz1
author_rank: Frequent Contributor
timestamp: September 29, 2024, 07:38:27 am
content: 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.

author: dazz1
author_rank: Frequent Contributor
timestamp: September 29, 2024, 07:49:16 am
content: HiI 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.

author: dazz1
author_rank: Frequent Contributor
timestamp: September 29, 2024, 08:16:19 am
content: 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.

author: dazz1
author_rank: Frequent Contributor
timestamp: September 29, 2024, 09:07:56 am
content: HiI 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.

author: factory
author_rank: Super Contributor
timestamp: September 29, 2024, 04:12:55 pm
content: Quote from: dazz1 on September 29, 2024, 09:07:56 amAs 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

author: dazz1
author_rank: Frequent Contributor
timestamp: September 30, 2024, 07:56:33 am
content: Quote from: factory on September 29, 2024, 04:12:55 pmYes, 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.Quote from: factory on September 29, 2024, 04:12:55 pmI 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 herehttps://reverb.com/item/13307500-hewlett-packard-hp-3722a-noise-generator-oscillator-1965-greyfor $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.Quote from: factory on September 29, 2024, 04:12:55 pmWhat does the delay function do? Is it worth waiting for?

author: factory
author_rank: Super Contributor
timestamp: September 30, 2024, 08:26:33 pm
content: Quote from: dazz1 on September 30, 2024, 07:56:33 amI 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.pdfDavid

author: dazz1
author_rank: Frequent Contributor
timestamp: September 30, 2024, 11:59:47 pm
content: Quote from: factory on September 30, 2024, 08:26:33 pmIt 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.

author: dazz1
author_rank: Frequent Contributor
timestamp: October 02, 2024, 02:14:30 am
content: HiJust 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.

author: dazz1
author_rank: Frequent Contributor
timestamp: October 02, 2024, 02:21:35 am
content: 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.

author: dazz1
author_rank: Frequent Contributor
timestamp: October 05, 2024, 04:33:41 am
content: HiThe 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.

author: dazz1
author_rank: Frequent Contributor
timestamp: October 05, 2024, 04:36:03 am
content: 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.

author: dazz1
author_rank: Frequent Contributor
timestamp: October 05, 2024, 04:40:12 am
content: 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%.

author: tautech
author_rank: Super Contributor
timestamp: October 05, 2024, 04:44:13 am
content: Old drifted carbon comp high value resistors.Might as well replace the 4700pF caps too....

author: dazz1
author_rank: Frequent Contributor
timestamp: October 05, 2024, 06:49:07 am
content: Quote from: tautech on October 05, 2024, 04:44:13 amThe resistors are all 1% metal film. Even the logic pull up resistors are 1%. The only engineering in this instrument is over engineering. It is almost like the Scottish design team were trying to show the US HP office how good the Scottish office were, rather than building a commercially viable test instrument.The big HV resistors look ceramic types.Today I washed the HV board with IPA to clean away any contaminants that might lead to arcing. That made no difference.I am leaning towards the fault being the wires/connections between the crt deflection plates and the Y deflection amplifier. Bad connections would be consistent with the step changes in Y gain I am seeing. Maybe a floating deflection electrode in the CRT would explain the interaction with intensity. Right now it is the only thing that fits the facts.I ran the HP 3721A for about 6 hours today to attempt flush out the fault. The 3721A gets HOT HOT HOT. I measured 51 deg C on the metal near the power transistors. Even with the ugly fan, the rear half of the 3721 was uncomfortably warm. I am going to fit TO-3 heat sinks to try and dump the heat into the air and not into the chassis. I am also going to replace the super ugly external fan with an internal 80 x 15 12V fan and make a nice bezel with a period correct grill to cover the hole through the side of the enclosure.I tried to video the HP 3721A fault seen on the crt, but I got the exposure completely wrong.

author: tautech
author_rank: Super Contributor
timestamp: October 05, 2024, 06:59:04 am
content: Quote from: dazz1 on October 05, 2024, 06:49:07 amFTFYStill, have you measured them ?IME any high value resistors near the grids for focus/intensity biasing are always the 1st suspects when CRTs misbehave.

author: dazz1
author_rank: Frequent Contributor
timestamp: October 05, 2024, 07:36:49 am
content: Quote from: tautech on October 05, 2024, 06:59:04 amOK I will take a closer look. I have been able to avoid faults on CRTs until now so still on the bottom left edge of the learning curve.

author: factory
author_rank: Super Contributor
timestamp: October 05, 2024, 09:32:09 am
content: Quote from: tautech on October 05, 2024, 06:59:04 amKnowing the age of this (late 1970s), there will be very few carbon comp resistors, by then they had swapped out these parts for film types, except for the few needed in special cases, such as high frequency areas.The precision film resistors do go bad, this is a problem sometimes found with the 140/1 & 180 series scopes, they tend to go high in value in PSU regulator circuits and other areas, sometimes this is obvious due to discolouration, but not always. The focus/intensity pots can go noisy, or open circuit too. Sometimes the small plastic transistors can fail in strange intermittent ways too, had this with the CRT HT regulation circuitry in the last 180A I worked on.Can you please add a picture of the board in question, as it's hidden by the cover in the previous pictures.David

author: dazz1
author_rank: Frequent Contributor
timestamp: October 05, 2024, 07:31:29 pm
content: HiHere are photos of the HV section for the HP 3721A as requested.

author: dazz1
author_rank: Frequent Contributor
timestamp: October 05, 2024, 07:33:50 pm
content: HiHV circuit board drawing and parts locater attached.There may be multiple faults.The gain steps about 25%, 40%, 100% which suggests two parts that affect the Y of the trace failing.The inconsistent instability of the vertical trace indicates a part partially failing.I have not seen an unstable trace at 100% gain.The symptoms only affect the vertical Y of the trace. The X of the trace is steady. The X is unaffected by Y gain steps or instability.I have done in-circuit testing of all the resistors. None of the measured values were far enough out of tolerance to justify lifting the part to test out of circuit.

author: tautech
author_rank: Super Contributor
timestamp: October 05, 2024, 07:40:25 pm
content: Eyes are immediately drawn to 3M9 R29.....

author: dazz1
author_rank: Frequent Contributor
timestamp: October 05, 2024, 07:50:59 pm
content: Here is the circuit diagram for the HV section.

author: dazz1
author_rank: Frequent Contributor
timestamp: October 05, 2024, 08:01:24 pm
content: Quote from: tautech on October 05, 2024, 07:40:25 pmOK R29 measures 4.3M out of circuit (pot disconnected).The FOCUS pot measures 5.3M.R30 measures 29M out of circuit.I can't probe R29 with power on. The volts are too high with R29 sitting at -3kV. R29 should only have about 126V across it, so well within limits.I will just change it out.

author: dazz1
author_rank: Frequent Contributor
timestamp: October 06, 2024, 02:03:46 am
content: R29 is rated to 0.5W and drawing 0.37W. Should be OK on paper, but my experience is that resistors should be operated well under their power rating.I am going to replace R29 with a 1W rated resistor.

author: dazz1
author_rank: Frequent Contributor
timestamp: October 06, 2024, 05:16:37 am
content: Quote from: tautech on October 05, 2024, 07:40:25 pmOK so I replaced R29, and that fixed the problem with intensity control interacting with Y deflection. Yahoo! Thanks TautechAlso 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.

author: tautech
author_rank: Super Contributor
timestamp: October 06, 2024, 05:54:30 am
content: Quote from: dazz1 on October 06, 2024, 05:16:37 amOkay 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.

author: dazz1
author_rank: Frequent Contributor
timestamp: October 06, 2024, 10:01:40 am
content: HiIt'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.

author: dazz1
author_rank: Frequent Contributor
timestamp: October 06, 2024, 10:46:01 am
content: 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:sinetrianglesquare waves.The cat has quickly adopted the top of the HP 3721A as the warmest place to be.

author: dazz1
author_rank: Frequent Contributor
timestamp: October 07, 2024, 03:54:54 am
content: 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.

author: factory
author_rank: Super Contributor
timestamp: October 07, 2024, 07:52:27 pm
content: Quote from: dazz1 on October 07, 2024, 03:54:54 amHere 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.David

author: dazz1
author_rank: Frequent Contributor
timestamp: October 07, 2024, 10:22:29 pm
content: Quote from: factory on October 07, 2024, 07:52:27 pmWow, 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.

author: abeyer
author_rank: Frequent Contributor
timestamp: October 07, 2024, 11:32:07 pm
content: Quote from: dazz1 on October 06, 2024, 10:46:01 amI 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.

author: dazz1
author_rank: Frequent Contributor
timestamp: October 08, 2024, 09:29:26 pm
content: 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.

author: dazz1
author_rank: Frequent Contributor
timestamp: October 19, 2024, 12:31:39 am
content: 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.

author: dazz1
author_rank: Frequent Contributor
timestamp: October 19, 2024, 12:53:12 am
content: Just a couple of photos showing the process of doing the fan mod.I used a wood router to carve away some of the aluminum chassis to allow the fan to fit behind the side panel. This was the only way I could make the fan fit behind the panel.I made a green fitting template to size and locate the holes in the panel.I have added a photo of the modified fan mod. I flame polished the 3D print and then sanded it smooth. I then applied some silicone spray. This hid most of the things that identify this as a 3D print. The result is a fan shroud that almost looks like a factory one.

author: Kean
author_rank: Unknown
timestamp: October 19, 2024, 11:04:16 am
content: Very nice work. That 3D print is very effective at hiding multiple sins.

author: dazz1
author_rank: Frequent Contributor
timestamp: October 23, 2024, 10:05:18 am
content: I ran the HP 3721A for a couple of hours today with my new fan installed.It ran a lot cooler so obviously a lot more air is being pushed through. It does make a bit of noise, so I might experiment with adding a in-line resistor to reduce the speed a bit.I am still planning to add TO-3 heat sinks as soon as I get a long enough list of parts to order without paying shipping costs.Unfortunately I suspect the HP 3721A has developed some new faults. This instrument has been sitting in storage for 8 years, and prior to that, probably something like 20 to 30 years sitting on a shelf. Latent defects are likely to be hiding in there. If I didn't have the manuals, I wouldn't even consider a repair effort.

author: dazz1
author_rank: Frequent Contributor
timestamp: November 13, 2024, 12:17:34 am
content: HiThis is a call for help. My HP 3721A has a fault that I can't fix.Symptoms are:CRT vertical deflection shows exponential response to step input signals.The displayed amplitude is about 2/3rds of what it should be.In addition, the displayed signal is sometimes jumpy.Things I have checked:Horizontal deflection is unaffected.The LV vertical deflection signals at the pins of the CRT are OK.The FOCUS voltage applied to the CRT tube is steady and OK.The LV power supply voltages applied to the PCB are OK.The INTENSITY adjustment is working OK.The Z-MOD input (connected to the same summing point as the INTENSITY) is OK.The ASTIG voltage is steady and OK.The feedback signals TP1, TP2 and TP3 are all OK with no sign of any errant signals.I suspected the connector on the rear of the CRT had corrosion. I removed the end cap and found traces of corrosion that I cleaned off.I re-melted all of the HV pcb solder joints to get rid of any bad joints. HP soldering is very good.I cannot measure the 3kV HV supply. None of my instrumentation is rated to that voltage.I can connect the LV Z-mod and x,y deflection signals to my scope, and they are displayed correctly. These are the LV signals that drive the CRT. These signals are not on the HV pcb.So it appears I have a component that fails on HV but the only part of the circuit I can't probe has the most likely candidates.All the components I can probe appear to be working.If it is a problem on the 3kV HV, then I am not seeing any error signal in the feedback.There are two separate issues being displayed.1. The exponential decaying response where there should be a straight line.2. The unsteady signal displayed on the CRT, consistent with something arcing over, or failing.Any suggestions would be much appreciated.

author: dazz1
author_rank: Frequent Contributor
timestamp: November 14, 2024, 09:19:06 am
content: HiI think I have two options.1. Replace the HV parts until it works,2. Buy a HV probe or,3. DIY a HV probe.Buying replacement parts in the hope that I end up replacing a faulty part is probably the cheapest path to a repair. I hate repair-by-replacement on principle.HV probes are an expensive item I would seldom use. The shipping cost only adds to the expense.The DIY option has some appeal. I already have the materials required to make the plastic items. I would only need the electronic passive parts, caps and resistors.

author: factory
author_rank: Super Contributor
timestamp: November 14, 2024, 08:05:23 pm
content: If the problem is vertical deflection I would be looking at the vertical circuitry. HV fault would affect the horizontal deflection, trace usually gets wider/longer if HV drops.As for measuring the HV, are there no AVO lamp foots over there? some of the model 8 have a 3kV range, probably enough for this as the HV isn't that high compared with higher frequency scopes. Only problem in finding good one, took me a few attempts, they go for about £10 here.David

author: dazz1
author_rank: Frequent Contributor
timestamp: November 14, 2024, 09:18:18 pm
content: HiYes, Avo's are available here but they are now rare enough to sell for prices 10x to 20x your £10.My highest rated meter is a Fluke 77 @ 1kV so well short of being useful. Even if it was rated higher, the 10Mohm meter input impedance would load down the 3kV supply.The HP 3721A has a 3kV supply with a tolerance of +/- 2v. That tolerance will provide accurate measurements off the CRT image, but it actually makes more sense to read the data from the pre-GPIB data bus that uses a 50pin Centronic connector. Rationally I should ignore the CRT fault and spend my time and money emulating a HP 3720A Spectrum Display.I think I have been dealing with at least 3 separate faults. There was the carbon compound resistor identified by TauTech. Cleaning the corrosion of the CRT pins seems to have fixed the Y deflection modulation by the brightness control, and apparent breakdown of a mystery HV component. None of the low voltage things I can measure correlate with what I see on the display.

author: dazz1
author_rank: Frequent Contributor
timestamp: November 15, 2024, 12:26:06 am
content: HiSo while a rational person might just leave the CRT fault as is, I never claimed to be rational.I took a harder look at the circuit diagram to figure out where a fault could exist that did not generate an error signal. I narrowed it down to R17, a 1Mohm LV resistor. When measured unpowered, this resistor had and does check out OK.So I changed that resistor out and changed the fault.The time constant significantly reduced and the Y-deflection went full scale. The unstable trace is now stable. In addition, adjusting the intensity no longer has a significant effect on the Y-deflection. Almost BINGO.The photos show the before and after changing R17.Based on these observations, I now believe that R17 was failing to a higher resistance state that created a significant time constant re-charging C7 and maintaining the grid voltage.I now suspect that C7 has or is failing to an open circuit. Without the full capacitance of C7, it takes time for the current to flow through R17 to build up the voltage on the control grid of the CRT.My next problem is obtaining a new HV cap. I just can't wander down to the local corner shop and pick one off the shelf. I have to wait until I have a big enough parts order to take advantage of the free shipping. Typically I order parts about twice a year. I end up with multiple projects on hold until I get parts to progress them. So expect radio silence on this topic for quite a while.

author: tautech
author_rank: Super Contributor
timestamp: November 15, 2024, 02:54:01 am
content: Quote from: dazz1 on November 15, 2024, 12:26:06 am2m search:https://www.aliexpress.com/item/1005006067184429.htm

author: dazz1
author_rank: Frequent Contributor
timestamp: November 15, 2024, 04:38:42 am
content: Quote from: tautech on November 15, 2024, 02:54:01 amI buy a lot of stuff of Aliexpress but I hadn't considered buying HV caps. I will look into that.

author: dazz1
author_rank: Frequent Contributor
timestamp: November 16, 2024, 04:41:07 am
content: Quote from: dazz1 on November 15, 2024, 04:38:42 amI have looked into the Aliexpress option for HV caps and decided not to go that path.I can buy 5x HV caps from Aliexpress for the price of 1x HV cap from element14.I only need 1x so buying 5x caps is not an advantage.I would have greater confidence in the quality of parts from element14, so the plan is to buy from element14.That won't stop me from buying a lot more stuff from Aliexpress.

author: tautech
author_rank: Super Contributor
timestamp: November 16, 2024, 05:07:04 am
content: Quote from: dazz1 on November 16, 2024, 04:41:07 amYet there are 5x 4700pF caps in that HV circuit......Back when I repaired CRO's I sourced all the HV caps required from Ali....YMMV

author: factory
author_rank: Super Contributor
timestamp: November 16, 2024, 12:07:20 pm
content: Trouble with parts from aliexpress is you never know if they will meet the spec (if there is a spec at all, can't view ali for some reason), they may send good parts one day, crap the next time.If you haven't got a way of measuring 3kV, then I'm guessing you probably don't have a way of testing HV disc capacitors either. One thing to note with ceramic disc caps, is the capacitance changing with voltage applied, the capacitance drops with increasing voltage.David

author: dazz1
author_rank: Frequent Contributor
timestamp: November 17, 2024, 09:54:36 am
content: HiIf there was a lot of $ at stake here, I would definitely give Aliexpress a go. The key to successful buying off Aliexpress is to buy the most expensive version, which is invariably heaps cheaper than a locally available version. With Aliexpress, more money = more quality. In this case, there is no cost saving compared to element14 because either way, I still have to pay out about the same amount of $.There are multiple HV caps of the same value/voltage on my HP 3721A, but only one has failed. The failed one has led an arduous life because it has pulse current due to the 100 blips on each x deflection. The other HV caps basically sit at DC.Another issue is the physical size. The existing caps have a pin spacing of about 15mm, but replacement options have spacing down to 5mm. I don't think 3kV across a 5mm gap is a recipe for long term reliability because of the risk of dirt leading to leakage, or even arc-over.With no cash incentive to buy Aliexpress, the assumed quality and known physical dimensions of the element14 range win the day.

author: dazz1
author_rank: Frequent Contributor
timestamp: December 06, 2024, 09:14:51 am
content: HiI figured it would be too difficult and too expensive to replace the missing and broken original late model HP knobs. I would have to buy from multiple suppliers and the total shipping costs would eye-watering. Also genuine HP knobs only seem to be over-priced. The broken and missing knobs were really annoying me. After a lot of research I found some close equivalents to the old model HP knobs. These work better than the late model knobs because the front panel art work was made for them. The late model knobs are a little too small.So the new knobs were not cheap and cost me twice as much as I paid for the HP 3721A but I think they are worth it.I had to modify knobs to fit the double decker control switch "Delay Offset" but the finished job almost looks factory. I am going to paint the pointer line and the brushed aluminium bits blue to match the panel.I have attached a photo of an original HP with black knobs and my copy of the HP black knobs.

author: ch_scr
author_rank: Frequent Contributor
timestamp: December 06, 2024, 10:16:25 am
content: Great job, looks factory

author: dazz1
author_rank: Frequent Contributor
timestamp: December 07, 2024, 07:40:44 pm
content: HiNow working on improving the thermal management. Cooling might have been sufficient in the chill of Queensferry in the north of Scotland, but not so good in a temperate climate. It is summer here.I have fitted heat sinks to the TO-3 power transistors on the rear. The aim being to dump heat outside the enclosure before it gets into the chassis. The heat sinks will fit under the factory cover.I have also fitted a thermostatic controller to the fan so it only runs as fast as necessary.I have also replaced the HV cap but now have an issue with power supply fuses blowing. I can't yet test the fix. Before you ask, the heat sinks are electrically isolated as they should be.The problem is on the 5V supply which draws a LOT of power on this thing. One of the stud diodes on the 5V rectifier has previously been replaced so I am not the first to have a problem.There is a before and after fitting the heat sinks.Also a photo showing the fan controller fitted to a 3D printed bracket, wiring not yet fully loomed-in.

author: dazz1
author_rank: Frequent Contributor
timestamp: December 11, 2024, 07:08:24 am
content: HiI went through and did a lot of static checks on the power supplies to make sure there were no obvious broken parts.The power supplies have a cascading protection scheme starting with the 50V supply. The +24VDC supply requires a 50V input to the reference. If the 50V supply fails and goes to zero, the 24VDC reference is zero and the 24VDC supply goes to 0VDC.The +12VDC supply reference also depends on the 50VDC supply. The +5VDC supply reference is driven by the +12VDC supply. Other supplies also have dependencies.As a result of all this interconnection, if one fuse blows, multiple power supplies drop out. It looked really bad, until I dragged out the schematics to see how everything works. Once I had completed static checks of the key components in the power supplies, I refitted/replaced the fuses as required, and flicked the power switch. It all burst back into life again as if it was just another day.The +5VDC power supply has some interesting features, if you are a power supply geek. It runs 2x 2N3055 power transistors in parallel because one was obviously not enough. That is a lot of power. The transistor that supplies the base currents is itself a power transistor. Being all linear power supplies, there is a lot of waste heat.The newly fitted heat sinks are definitely doing their job. The heat sinks combined with the fan controller are working well together. I can't quantify how much heat is being dumped by the heat sinks, but it is significant. I have tuned the fan controller to start accelerating from a low base speed at 25 deg C, and go to full speed at 50 deg C.With the power supplies now fully operational, I checked the repair to the CRT power supply (new HV cap). The good news is that the symptoms consistent with a failed HV capacitor have gone. The bad news is that the power supply output is unstable, consistent with arc-over somewhere. That brings the tally to 4 defective/replaced components and at least one more to discover. I now have a used HV 1000:1 probe so I can now poke around to see new things I could not see before.Three of the rotary switches were loosely mounted, so I tightened those up.To the HP employee that left the message "FIXIT" 55 years ago please note, where ever you are, I am trying.So the next step is to figure out where the remaining HV fault(s) is. So far no magic smoke has been observed during test flights.

author: dazz1
author_rank: Frequent Contributor
timestamp: December 13, 2024, 05:09:39 am
content: HiPhoto below IMG_3185 shows the crt display before the HV cap and resistor were replaced. The cap value had degraded. The "dot" are near vertical as the HV supply dropped with the increased current required to draw a dot.Photo IMG_3316 is the after the fix. The dots are closer to being dots. Vertical deflection is still about half what it should be so still a fault there to fix. There is no droop in the line of dots. The line is straight.

author: dazz1
author_rank: Frequent Contributor
timestamp: April 05, 2025, 07:35:53 am
content: HiI have been looking for a HP 3722A Pseudo Random Noise Generator to partner with my HP 3721A Correlator. They were designed to be used together.I have found a HP 3722A in very good condition at a tiny auction house in Houston USA. It is exactly what I am looking for. I would sell my neighbours kids for lab experiments to own it. All I need to do is bid and win, and there would be a good chance I would be the only bidder.Then my problems would only start. The auction staff won't pack it for me. Even if they did, I would have to pay for it to be shipped to my freight forwarder in Portland OR (not too many $$).The absolute show stopper is the cost of getting it from the USA to New Zealand. The cost is eye watering. What is worse is shipping cost is dead money. Paying shipping cost does not increase the value of the kit I want to buy. So as much as I would like to, I won't be bidding for the HP 3722A.So for anyone else interested in an HP3722A, here it is.https://directbids.com/medical-equipment-auctions/hewlett-packard-hp-3722a-noise-generator-ye4698szI have been looking at the option of using a modern dsp to emulate an HP 3722A. Specifically I have been looking at the new Microchip DSPic devices. I think I could use the exact same hardware to emulate the HP 3720A Spectrum Analyser.

author: Kean
author_rank: Unknown
timestamp: April 05, 2025, 09:28:31 am
content: Quote from: dazz1 on April 05, 2025, 07:35:53 amYeah, 5-10 years ago I might have considered getting something like this shipped from USA to AU, and have indeed done it a number of times. Today the typical quoted costs are crazy.Quote from: dazz1 on April 05, 2025, 07:35:53 amI'd be very interested to see that. I have used DSPIC parts, but only as a drop in alternative to some PIC24 parts during the global part shortages. Never played with the DSP features, nor do I have much inclination to learn how.

author: dazz1
author_rank: Frequent Contributor
timestamp: April 05, 2025, 09:56:00 am
content: Quote from: Kean on April 05, 2025, 09:28:31 amI have only been looking at options. One of the issues is that dsp integrated ADCs usually max out at 44kHz. The HP 3722A went up to 250kHz. So I would need an external 8 or 12bit ADC.Most of the work will be in the controlling/interfacing software, not the dsp part. The same is true for both the noise generator and the spectrum analyser. It looks like practically standard code blocks can be used for noise and spectrum analysis. I would consider adding a general purpose MCU (eg esp32) for control and interfacing.The cool thing about the spectrum analyser would be its speed. The HP 3720A spanned 0.005Hz to 250kHz to match my HP 3721A. That would make it suitable for things like buildings and bridges.Microchip DSPic parts are very cheap, but the PicKit 5 programmer is about $200 for me.Other manufacturers make faster "better" devices, but they all seem to be BGA. I don't need that many IO pins and I don't want to try BGA soldering.

author: dazz1
author_rank: Frequent Contributor
timestamp: April 05, 2025, 08:07:07 pm
content: Quote from: Kean on April 05, 2025, 09:28:31 amI have looked and a one-way economy flight from Auckland to Houston is about the same as shipping a HP 3722A from Houston to Auckland. Ouch.What makes is worse is that I would have to pay 15% tax on the shipping cost as well as the HP 3722A when it arrived in Auckland.Years ago I brought back a complete set of tyres, brake discs and pads as check-in hand luggage. Took a lot of planning but saved me a lot of $$$

author: dazz1
author_rank: Frequent Contributor
timestamp: July 24, 2025, 11:12:28 am
content: Hi[EDITED to add more info 25 June 2025)I have circled back to the HP 3721A CRT HV supply/driver fault.I figured that one or more HV caps on the 3kV and 3.050kV supplies was breaking down, but I wasn't sure because my 5kV scope probe drags down the HV and my 20kV 10Gohm input probe us DC only.I brought a few HV 4.7nF 6kV caps from Aliexpress and fitted those a few days ago. Didn't make much difference. There was only one HV cap left, 1.5nF but I didn't buy one of those.I strung together 3x 4.7nF caps to get a measured 1.53nF capacitor and tried that in place of the factory fitted cap. A temporary kludge for testing.That fixed the break down fault. I am left with a low frequency decay type curve that looks like a cap is not doing it's job.So it looks like the fault is a combination of HV cap breakdown and low voltage cap failure (high ESR, less than spec capacitance etc)The orange highlighting in the attached circ diagram shows the offending parts.The 1500pF HV cap was breaking down but the low voltage smoothing caps were hiding this. At least one of the LV caps is not capacitating properly.One point to note about the 6kV blue Aliexpress caps is how small they are compared to the element14 6kV orange cap. I suspect the Aliexpress caps have a lot less headroom to reach failure, but 10x Aliexpress caps are less $$ than on element14 cap.The plan is to replace all of the low voltage caps. Only about 7x so not too hard.I have already checked the LV caps in circuit, but if I take them out to test them, I might as well just replace them.I will fit a new 1.5nF HV cap.

author: Stray Electron
author_rank: Super Contributor
timestamp: July 24, 2025, 01:00:33 pm
content: Quote from: dazz1 on September 26, 2024, 02:01:03 amAre you user? Have you looked at (the US) Ebay? There is a front panel for a HP 3722A on there right now (#293082478664) and two manuals for a 3722. I've been able to find just about anything that I wanted sooner or later on Ebay. Just set up a search and save it and have them E-mail you when something shows up that matches the search.

author: dazz1
author_rank: Frequent Contributor
timestamp: July 24, 2025, 09:46:42 pm
content: HiI have a search setup on ebay but the reality it is not economic to buy. There is a tired looking HP 3722A listed on ebay at present. The asking price is stupid. The shipping cost to my freight forwarder in Portland is more than the asking price. The cost of shipping from Portland to New Zealand is about $USD700 or so. Then I have to pay 15% tax on all of that. Even if the asking price was $1, it would not be worth buying/importing a HP 3722A from the USA.I am looking at emulating a HP 3722A with a DSPIC. I can get ready made dev boards from Aliexpress and I would just need to make up an interface board, to interface to the HP 3721A.The same setup would allow me to emulate the matching HP 3720A spectrum analyser. There were only 610 HP 3721A manufactured. I'd be surprised if more than 60 HP 3720A were built.

author: somejoe
author_rank: Contributor
timestamp: July 29, 2025, 03:48:28 am
content: Quote from: dazz1 on July 24, 2025, 09:46:42 pmdazz1,Instead of the DSPIC I think it would be faster to implement the 3722A logic on an FPGA or CPLD to mimic the logic of the original unit. This would be fairly straightforward as all the logic block diagrams and schematics are included in the manual. The HP-3721A can be duplicated the same way, adding the necessary logic to drive an oscilloscope instead of the built-in CRT.While I ponder the project and my time availability (I'm a working hardware/software engineer) - is there a way to get the rest of the 3721A manual (hopefully a complete copy)?Cheers,

author: dazz1
author_rank: Frequent Contributor
timestamp: July 29, 2025, 04:24:32 am
content: Quote from: somejoe on July 29, 2025, 03:48:28 amHiThanks for the advice. I don't have prior experience with DSPIC, FPGA or CPLD. I have looked at the time and effort to climb the learning curves. The DSPIC appears to be the shortest path to a solution because I have previously done some development for microcontrollers, including assembler. So while the block diagrams are a good guide, the learning curve is a significant hurdle.Scanning the manuals for the HP3721A would take a significant effort. There are three volumes. Only 610x HP3721A were manufactured and few remain. I would need a really good reason to justify the effort to scan the manuals.I have compared the digital interfaces to the HP 3720A, HP 3721A and HP 3722A. The 3720 and 3721 play nicely together. The interface is just a straight cable between 50 pin Centronics connectors with 12.5V logic levels. Easy stuff. The interface to the HP 3722A is not compatible and would require something like an HP 2100 computer to translate. Not a big issue, just needs some design work and translation software.I am at least a year, probably 2 years, away from making any significant start on a HP 3722A and HP 3720A emulators. I suspect the biggest effort would be writing the user interface, probably as a web page server to an attached PC. I think the same hardware could emulate the 3720, 3721 and 3722. I also think these instruments still have valid use-cases.

author: somejoe
author_rank: Contributor
timestamp: July 29, 2025, 01:34:44 pm
content: dazz,I did not realize there were two separate threads on this topic, so I'll keep it brief.QuoteI did not know it was a three volume set. Can you post the titles and HP part numbers in case I come across them? Also,as far as scanning goes, no need to do it yourself. Most large copy 'n' print stores have scanning capability as well - albeit for a fee of course.QuoteThe rear 36-pin connector on the 3722A is designed to mirror the front panel controls. Can you post more details how the 3721A links to the 3722A? Is it merely taking the output from the 3722A or does it control the operation of the unit? Since I do not have the 3721A manual, a description and pin-out of the interfaces would be helpful.QuoteAlthough the correlation and spectrum display functions can be done on a single PC, I agree that these instruments still have valid use cases. I'm myself interested in a functional 3721 design (minus the CRT), but would need specs and an operations manual before I can begin working on it.Cheers,

author: factory
author_rank: Super Contributor
timestamp: July 29, 2025, 07:31:51 pm
content: Quote from: dazz1 on July 24, 2025, 09:46:42 pmSales of the 3720A between 1972 & 1979 were 220 units, 3721A between 1968 & 1979 were 610 units and 3722A between 1967 & 1980 were 1590 units, no need to guess, the info was in the HPSQF history book Vol 2.Your were very picky with the condition of the 3722A recently sold, it didn't look that bad, covers just needed panel beating, or if that bothers you replacing with some from a dead 141T.You of course have competition from the analog test gear music crowd for the 3722, as well as other collectors, I doubt many people would want the others, most HP test gear of this age isn't selling well, even more so for the larger items.That shipping quote of $700 seems excessive, I had an oversize 35.25 lb parcel sent from the US to the UK for only $170 via shipito (using DHL express), distance from LA to New Zealand is just over 21% further than LA to the UK according to a web search.And why are you concerned about increasing the value of kit you buy, is this a hobby or a business?David

author: dazz1
author_rank: Frequent Contributor
timestamp: July 29, 2025, 09:30:55 pm
content: Quote from: factory on July 29, 2025, 07:31:51 pmHiThe cost of shipping and tax is dead money. I get better value for money spent if I buy expensive (high quality) items. I did find a not e-bay HP 3722 in good condition at a good price but they would only allow pick-up.I hate e-bay and I only use it as the option of last resort. Thankfully e-bay completely failed to get a foothold in New Zealand.The cost of shipping is not by distance, it is by capacity. There is a lot of capacity by air and sea over the Atlantic. In the Pacific not so much. Shipping stuff across the USA border into the South Pacific is expensive.The HP 3721A I have is one of a pair imported into New Zealand by the Govt. One went to an agricultural research lab, and the other went to a University. I don't know if the second one still exists.I am never going to make any money out of any of this. The replacement knobs cost twice as much as I paid for the HP 3721A. I just think it is a really interesting piece of kit but it really needs the HP 3720A and HP 3722A to achieve its full potential. I studied the application of noise and correlation to control systems when I was at University, but I didn't know HP made test equipment to do it.