Author Topic: Vintage HP 3721A Correlator : Repair, Restoration and Enhancement  (Read 5433 times)

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Offline dazz1Topic starter

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Re: Vintage HP 3721A Correlator : Fault finding & Running Hot
« Reply #25 on: October 05, 2024, 06:49:07 am »
Old drifted carbon comp high value resistors.  :horse:
Might as well replace the 4700pF caps too....

The 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.
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Re: Vintage HP 3721A Correlator : Fault finding & Running Hot
« Reply #26 on: October 05, 2024, 06:59:04 am »
Old drifted carbon comp high value resistors.  :horse:
Might as well replace the 4700pF caps too....

The resistors are all banded 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.
FTFY

Still, have you measured them ?
IME any high value resistors near the grids for focus/intensity biasing are always the 1st suspects when CRTs misbehave.
« Last Edit: October 05, 2024, 07:56:55 am by tautech »
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Offline dazz1Topic starter

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Re: Vintage HP 3721A Correlator : Fault finding & Running Hot
« Reply #27 on: October 05, 2024, 07:36:49 am »

The 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.
Still, have you measured them ?
IME any high value resistors near the grids for focus/intensity biasing are always the 1st suspects when CRTs misbehave.

OK 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.
Dazz

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Offline factory

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Re: Vintage HP 3721A Correlator : Fault finding & Running Hot
« Reply #28 on: October 05, 2024, 09:32:09 am »
Old drifted carbon comp high value resistors.  :horse:
Might as well replace the 4700pF caps too....

The resistors are all banded 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.
FTFY

Still, have you measured them ?
IME any high value resistors near the grids for focus/intensity biasing are always the 1st suspects when CRTs misbehave.

Knowing 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
« Last Edit: October 05, 2024, 09:33:55 am by factory »
 

Offline dazz1Topic starter

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Re: Vintage HP 3721A Correlator : Repair, Restoration and Enhancement
« Reply #29 on: October 05, 2024, 07:31:29 pm »
Hi
Here are photos of the HV section for the HP 3721A as requested.
Dazz

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Re: Vintage HP 3721A Correlator : Repair, Restoration and Enhancement
« Reply #30 on: October 05, 2024, 07:33:50 pm »
Hi
HV 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.


« Last Edit: October 05, 2024, 07:49:39 pm by dazz1 »
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Re: Vintage HP 3721A Correlator : Repair, Restoration and Enhancement
« Reply #31 on: October 05, 2024, 07:40:25 pm »
Eyes are immediately drawn to 3M9 R29.....  :horse:
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Offline dazz1Topic starter

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Re: Vintage HP 3721A Correlator : Repair, Restoration and Enhancement
« Reply #32 on: October 05, 2024, 07:50:59 pm »
Here is the circuit diagram for the HV section.
Dazz

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Offline dazz1Topic starter

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Re: Vintage HP 3721A Correlator : Repair, Restoration and Enhancement
« Reply #33 on: October 05, 2024, 08:01:24 pm »
Eyes are immediately drawn to 3M9 R29.....  :horse:

OK 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. 

Dazz

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Re: Vintage HP 3721A Correlator : Repair, Restoration and Enhancement
« Reply #34 on: October 06, 2024, 02:03:46 am »
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.
Dazz

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Offline dazz1Topic starter

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Re: Vintage HP 3721A Correlator : Repair, Restoration and Enhancement
« Reply #35 on: October 06, 2024, 05:16:37 am »
Eyes are immediately drawn to 3M9 R29.....  :horse:

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

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

So what I am now seeing looks like unstable drive from one of the CRT  Y deflection plates.    I already know the deflection amps are working flawlessly, so I am going with the hypothesis that there is a bad connection to one Y deflection plate between the amp and the CRT.  That is a piece of wire.  A bad connection would be  consistent with both the instability and the stepped Y gain.
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Re: Vintage HP 3721A Correlator : Repair, Restoration and Enhancement
« Reply #36 on: October 06, 2024, 05:54:30 am »
Eyes are immediately drawn to 3M9 R29.....  :horse:

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

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

So what I am now seeing looks like unstable drive from one of the CRT  Y deflection plates.    I already know the deflection amps are working flawlessly, so I am going with the hypothesis that there is a bad connection to one Y deflection plate between the amp and the CRT.  That is a piece of wire.  A bad connection would be  consistent with both the instability and the stepped Y gain.
:clap:
Okay so something still maybe heat related, time to bring out the Jewelers loupe and hunt for dry joints.

Edit to add.
Any socketed transistors ? Give them a reseat.
Clean PCB edge connectors gently with an eraser.
« Last Edit: October 06, 2024, 06:11:25 am by tautech »
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Re: Vintage HP 3721A Correlator : Repair, Restoration and Enhancement
« Reply #37 on: October 06, 2024, 10:01:40 am »
Hi
It's official.  The HP 3721A Correlator is correlating again.

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

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

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

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

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

The photos show:
The Y deflection signals applied to the CRT.
A test pattern showing 100% gain and a sharp image.  Each dot is 1 of 100 discrete data points.
The rear of the CRT showing the silicone applied to the LV connections.  The left side is up to 3kV.
« Last Edit: October 06, 2024, 10:05:01 am by dazz1 »
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Re: Vintage HP 3721A Correlator : Repair, Restoration and Enhancement
« Reply #38 on: October 06, 2024, 10:46:01 am »
Just some screen shots of the analysis of signals from my audio signal generator.

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

The cat has quickly adopted the top of the HP 3721A as the warmest place to be.
« Last Edit: October 07, 2024, 01:49:58 am by dazz1 »
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Re: Vintage HP 3721A Correlator : HP 3720A Spectrum Display Emulator?
« Reply #39 on: October 07, 2024, 03:54:54 am »
Just looking a bit closer at the HP 372OA Spectrum Display.  It seems the HP 3720A qualifies as being very rare.  I could not find a modern photograph on Google.  There is no realistic chance of finding one to pair with the HP 3721A Correlator.

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.
« Last Edit: October 07, 2024, 07:49:46 pm by dazz1 »
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Re: Vintage HP 3721A Correlator : HP 3720A Spectrum Display Emulator?
« Reply #40 on: October 07, 2024, 07:52:27 pm »
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.

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



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



David
« Last Edit: October 07, 2024, 07:54:35 pm by factory »
 

Offline dazz1Topic starter

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Re: Vintage HP 3721A Correlator : HP 3720A Spectrum Display Emulator?
« Reply #41 on: October 07, 2024, 10:22:29 pm »


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

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

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

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

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

Dazz

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Re: Vintage HP 3721A Correlator : Repair, Restoration and Enhancement
« Reply #42 on: October 07, 2024, 11:32:07 pm »
The cat has quickly adopted the top of the HP 3721A as the warmest place to be.

I feel like I must object to your choices in heat dissipation approach. Excessive floof makes for poor thermal interface material and contributes to overall high system Rth.
 
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Offline dazz1Topic starter

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Re: Vintage HP 3721A Correlator : Glass Dynamic RAM
« Reply #43 on: October 08, 2024, 09:29:26 pm »
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.
« Last Edit: October 08, 2024, 09:56:58 pm by dazz1 »
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Re: Vintage HP 3721A Correlator : Repair, Restoration and Enhancement
« Reply #44 on: October 19, 2024, 12:31:39 am »
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.

Dazz

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Offline dazz1Topic starter

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Re: Vintage HP 3721A Correlator : Repair, Restoration and Enhancement
« Reply #45 on: October 19, 2024, 12:53:12 am »
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. 
« Last Edit: October 23, 2024, 02:13:19 am by dazz1 »
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Re: Vintage HP 3721A Correlator : Repair, Restoration and Enhancement
« Reply #46 on: October 19, 2024, 11:04:16 am »
Very nice work.  That 3D print is very effective at hiding multiple sins.
 
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Re: Vintage HP 3721A Correlator : Repair, Restoration and Enhancement
« Reply #47 on: October 23, 2024, 10:05:18 am »
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.
Dazz

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Re: Vintage HP 3721A Correlator : CRT HV PSU Fault
« Reply #48 on: November 13, 2024, 12:17:34 am »
Hi
This 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.
« Last Edit: November 13, 2024, 12:28:24 am by dazz1 »
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Re: Vintage HP 3721A Correlator : Repair, Restoration and Enhancement
« Reply #49 on: November 14, 2024, 09:19:06 am »
Hi
I 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.

Dazz

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