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The following items have been added to the Files area of the [email protected] group.

By: Philip Freidin <philip@...>

Description:
Service notes for E3610
Re: HP8757A Display Problem (Scalar Network Analyzer)

 

Hmmmm?? messages passing in the night (or day)
It looks like you were typing message 151895 while I was typing 151896.
Hopefully my info will be helpful.
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Philip
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Re: HP8757A Display Problem (Scalar Network Analyzer)

 

I think the 8757A uses an HP 1345A display module, which is used in multiple
different instruments. You can isolate the display subsystem by unplugging the
26 signal flat flex cable, at either end. This will cause the 1345A to go into test
pattern mode, that should look like this:
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The service manual for the 1345A is available online from the usual sources.
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I recently repaired one of these displays that apparently had a functional Y axis, but
the X axis was dead, thus just a vertical line with varying intensity. As part of my debug
process, I took scope measurements of all the nodes in the Y axis starting at the
pre-amp chip at U28 and continuing to the final transistors that drive the deflection
plates. All the scope images, test pattern, and faulty display can be accesses at:
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Many of these signals have a significant DC offset (10V to 100V) with an AC
component (the actual display data) of 20mV to 40mV P2P. My scope traces
either show that in two separate pictures, or on the same picture with two
channels, one showing average DC level, and the other trace shows the AC
coupled version of the signal with 20 mV/Div .
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From your description, it sounds like your issues are earlier in the signal path,
but this test pattern is an easy starting point to localize the fault to be either in
the display section, or in the 8757A supplying display commands over the
flat flex cable.
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None of the analog signals representing frequency or amplitude are directly
connected to the display. They are processed in the rest of the analyzer
and a list of display commands (a little like HPGL) are sent over the flat flex.
Depending on the options for the 1345A display, it ether has a display memory
that contains the display command list, and repeatably plays it over and over,
or it doesn't have this memory, and it is the responsibility of the host instrument
(8757A) to continuously send the data to refresh the screen.
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With display memory, the host instrument only has to send the display commands
when the display has to change. This is best for generally static displays. Without
the memory, the host instrument processor has to continuously send display
commands, so a heavier load, but more appropriate for a continuously changing
display.
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Philip
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Re: HP8757A Display Problem (Scalar Network Analyzer)

 

This afternoon I read more of the manual. I then checked the "X-Y-Z Analog" board that drives the video - gently "banging" (tapping with a plastic rod!) the rear corner of that board suddenly but intermittently changed the vertical position of the display. I dismounted this board and I saw that someone in the past had re-soldered 4 of the preset adjustable pots - that's an indication that they also maybe had this problem in the past, so I checked around them and properly cleaned those joints and re-soldered them together with the pins for the wites that connect to the backside of the board.
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This solved the problem of the display being off-centre - it now appears in the correct place (also with the test pattern selected), although the problem of the sequencing/cycling of the 4 different 'video' channels being far too slow is still there.
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I checked the CPU board and the battery - the battery is still good, and I can store the setup and recall it perfectly okay.
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So I think the problem that remains is the speed of the 'frame sequencing' if that's the correct terminology, and maybe that is on the CPU board since that sends the outputs to the display board. I have no further ideas.
Re: HP 478A Thermistor Sensor. Input return loss and matching

 

When I fitted a ferrite bead to my early 478A I found that the VSWR curve became smoother but it degraded from about 1.068:1 at 50 MHz to about 1.075:1 with the ferrite added. This is unavoidable as the resonance was slightly improving the VSWR at 50 MHz.
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A VSWR of 1.075:1 is still OK for measuring the output of a typical 1mW reference at 50 MHz but I had a go at improving this by fitting an external matching network at 50 MHz. Obviously, the matching network should provide minimal insertion loss.
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The complex input impedance of the early sensor at 50 MHz is about 1000pF in series with 50 ohms. So the simplest matching network would be a series inductor of about 11nH.
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See the plot below that shows the input VSWR of the early 478A sensor (complete with an added ferrite) in the unmatched state
against what happens when matched with an external series 11nH inductor with an unloaded Q of about 55 at 50 MHz.
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You can see that the response is about the same if I set port 2 of the simulator to have the same port impedance as my early 478A sensor with the ferrite added. Without the matching network, the insertion loss is about 0.005dB.
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The resistive loss of the matching inductor means that matching it makes no improvement in the insertion loss but there should be an improvement in mismatch uncertainty.
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By changing port 1 to 52R (VSWR of 1.04:1) and adding a long lossless 50R transmission line I can graphically proved an indication of the mismatch uncertainty caused by a 1mW source that might have a VSWR of 1.04:1. See the second plot.
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You can now see there is a tiny advantage in having the matching network as there is much less uncertainty ripple (at 50 MHz) in the red trace compared to the green trace. However, in reality, I think a typical 1mW source from an HP power meter will have a fairly accurate 50R resistive part of its source impedance. So I'm not convinced that there will be much advantage in having this matching network inline. This is because of the way the 1mW 50 MHz reference circuit is designed in a typical HP power meter. It's likely to look like (maybe just over?) 50R resistive in series with a tiny reactance.
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However, I have gone ahead and built a matching network and I can describe and show it it a later post. I hope this stuff is interesting and not too nerdy :)
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Regards
Jeremy
Re: HP 478A Thermistor Sensor. Input return loss and matching

 

On Sat, May 17, 2025 at 03:14 PM, Ed Marciniak wrote:
It would be reasonable to try 61 mix NiZn ferrite with a 300C curie temperature, and mu of 125
Thanks. I tried lots of ferrite types on a VNA and quite a few were OK to use but I chose one that gave at least 70R ESR across 50 MHz to several GHz that was fairly stable on a VNA when heated. However, I would much rather find out what HP used and fit this instead. Maybe someone here may know what ferrite was used. I think the ESR at 50 MHz has to be at least 50 ohms as this is 'big' compared to the 2.7R of R1.
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Regards
Jeremy
Re: HP8757A Display Problem (Scalar Network Analyzer)

 

It sounds like it could be in the analog display area, the video and sweep drive to the CRT. Check the horizontal and vertical drive to the CRT during the analog portion of the sweep. With no signal input the vertical should be a negative voltage (check the manual for values), the horizontal voltage should be equal positive and negative values (check the manual). Yours should not be horizontally balanced, if it is offset. Trace that signal back through drive circuitry until it changes to a balanced signal.

Don Bitters
Re: HP 478A Thermistor Sensor. Input return loss and matching

 

Here's a comparison showing the VSWR of an early sensor (VNA measurement of my HP 478A) against a model based on the schematic of the later sensor. You can see that the VSWR at 50 MHz should improve to about 1.05:1 with the later sensor type and there is no resonant blip at 53 MHz.
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I do have the later type of sensor here but it is still factory sealed (see the image below to see it has never been used) and it seems a shame to disturb it just to measure the VSWR. You can see in the image that the red internal washer for the N connector is supplied external to the sensor. I'm not sure why this is. I doubt it has fallen out on its own but it is lying flat just below the black outer of the N connector. I think this sensor is actually over 30 years old but never been unpacked and used.?
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I also have a brand new unused (also made over 30 years ago) 432A meter with sealed cable and paperwork from HP and I keep this carefully stored for the day my other 432A or 478A finally fail BER.
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You can also see that the efficiency chart on the side of the sensor shows the efficiency of the later sensor type is very close to 100% at 0.01GHz and the efficiency drops to about 98% by 2 GHz.
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My early sensor also has a minor VSWR wiggle and (in)efficiency bump at about 850 MHz but I've not investigated this as it is almost certainly caused by something inside the metal body containing the two thermistors and I don't want to disturb this.
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Regards
Jeremy
Re: HP 478A Thermistor Sensor. Input return loss and matching

 

¿ªÔÆÌåÓý

It would be reasonable to try 61 mix NiZn ferrite with a 300C curie temperature, and mu of 125.

From: [email protected] <[email protected]> on behalf of jmr via groups.io <jmrhzu@...>
Sent: Saturday, May 17, 2025 8:34:17 AM
To: [email protected] <[email protected]>
Subject: Re: [HP-Agilent-Keysight-equipment] HP 478A Thermistor Sensor. Input return loss and matching
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The reason the efficiency also suffers with a blip at about 40-60 MHz is that some of the incident RF from a 1mW reference source will end up in the 2.7R damper resistor via the resonant path to R1. This means the DC substitution (used by the HP 432A meter) can't correctly indicate the incident power in this frequency range unless the efficiency change is known across this tight frequency range. It's the equivalent of the sensor losing efficiency and in my case this was a loss of over 1% efficiency at the peak of the bump. This was always really annoying for me because it meant I had to have an efficiency chart showing how the efficiency sharply changed by over 1% across 40-60 MHz and this required lots of cal points.
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Adding the ferrite achieves two things. It removes the bump in the input VSWR and it also flattens the efficiency. At first, it might appear that adding the ferrite would suck even more energy from the thermistors as a ferrite bead looks resistive up at VHF.? This would make the efficiency even worse. However, as long as the ferrite introduces 'lots' of series resistance (ESR), there will be a big mismatch loss at 50 MHz so only a tiny amount of incident RF will get burned up in the high resistance of the ferrite. The efficiency won't be perfectly flat but there should be a big improvement in efficiency across 40-60 MHz and I was able to achieve this.
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Obviously, the choice of ferrite is fairly critical here as it has to provide a high ESR at 50 MHz and it also has to be fairly immune to temperature changes. This means that the initial permeability needs to be reasonably stable wrt temperature changes across about 10-50degC.
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The latest manual for the 478A sensor lists this ferrite as E1 but it doesn't give a part number for it. It would be really nice to know what the official ferrite material is.
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Regards
Jeremy
Re: HP 8566B repair

 

It almost seems like the display is in a test mode. This first pic shows the 0-10V ramp of the first band sweep, the second pic shows the ramp voltage for each of the separate bands B, C, D, E, but not the accumulated sweep (it should be 0-10V also, with steps like the 2nd pic). You definitely have a problem in the 85662A A3 video section. Double check the SMB cable routing in this area. One cable routing is for ¡°Test¡° mode. Also check for pinched cables. There is a ¡°Test¡± push button switch on one of the brds. in this section, check to see if it is functioning. Another good idea would be to pull the A3 cover and pull out the A3 brds one by one and inspect the brds. for issues, and edge connectors for clean, shiny connectors. If any are white filmed, clean them with IPA and a soft cloth. When required I have also used an India Rubber eraser to clean them. If you find more than a few filmed contacts, clean all the A3 brds.

Don Bitters
Re: HP 478A Thermistor Sensor. Input return loss and matching

 

The reason the efficiency also suffers with a blip at about 40-60 MHz is that some of the incident RF from a 1mW reference source will end up in the 2.7R damper resistor via the resonant path to R1. This means the DC substitution (used by the HP 432A meter) can't correctly indicate the incident power in this frequency range unless the efficiency change is known across this tight frequency range. It's the equivalent of the sensor losing efficiency and in my case this was a loss of over 1% efficiency at the peak of the bump. This was always really annoying for me because it meant I had to have an efficiency chart showing how the efficiency sharply changed by over 1% across 40-60 MHz and this required lots of cal points.
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Adding the ferrite achieves two things. It removes the bump in the input VSWR and it also flattens the efficiency. At first, it might appear that adding the ferrite would suck even more energy from the thermistors as a ferrite bead looks resistive up at VHF.? This would make the efficiency even worse. However, as long as the ferrite introduces 'lots' of series resistance (ESR), there will be a big mismatch loss at 50 MHz so only a tiny amount of incident RF will get burned up in the high resistance of the ferrite. The efficiency won't be perfectly flat but there should be a big improvement in efficiency across 40-60 MHz and I was able to achieve this.
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Obviously, the choice of ferrite is fairly critical here as it has to provide a high ESR at 50 MHz and it also has to be fairly immune to temperature changes. This means that the initial permeability needs to be reasonably stable wrt temperature changes across about 10-50degC.
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The latest manual for the 478A sensor lists this ferrite as E1 but it doesn't give a part number for it. It would be really nice to know what the official ferrite material is.
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--
Regards
Jeremy
Re: HP 478A Thermistor Sensor. Input return loss and matching

 

Here's the schematic of the early 478A sensor. You can see there is a 460pF capacitor C3? in series with a 2.7R damping resistor R1.
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I've added some extra detail to show that there is a short red wire that connects between C1 (inside the metal sensor block) and C3 on the PCB on the back of the block. This wire is probably an inch or so long.
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Sadly, this causes a resonance near 50 MHz and this is the cause of the blip in the VSWR. I believe that R1 is there to damp out this resonance. To demonstrate this, I've put together a very crude model of the sensor in Genesys and modelled the red wire as a 22nH inductor. I modelled with and without the 2.7R damping resistor R1 and the results are shown below. I also included the real VNA measurement of a real (early) 478A sensor.
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You can see that the VSWR has a huge resonant wiggle at about 53 MHz when R1 is removed (see the green trace). So I think R1 was added to try and damp out this resonance.
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Later sensor variants appear to deal with this issue by fitting a ferrite bead over the red wire and the damper network is no longer needed. I've tried upgrading my sensor with a ferrite bead and can confirm that it is well worth doing as it completely damps out the resonance, leaving a nice smooth VSWR curve.
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I hope the simulation plot is readable. I've enlarged the fonts to try and improve things.
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Regards
Jeremy
HP 478A Thermistor Sensor. Input return loss and matching

 

In the last few weeks, I've had my old HP 432A power meter on the bench along with its 478A thermistor sensor. I normally only use it about once a year to check the 1mW (50 MHz) reference on another power meter.
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However, I've spent a bit more time looking at the 478A sensor and wondered if it was worth sharing some of the knowledge gained and also some of the options for reducing the measurement uncertainty compared to using a standard sensor at 50 MHz.
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I've looked at the input match, the efficiency and also at how the 478A has had some changes internally over the years. One obvious option to improve the input match at 50 MHz is to buy a special version with option H75 or option H76. These are probably quite rare and expensive. Another option is to improve the input match of a standard sensor by adding a low loss (external) matching section inline with the standard sensor. I have done this and have achieved good results. I'll show the design process and test results in this thread.
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The standard sensor has a fairly grim spec for input VSWR at 50 MHz of about 1.3:1. However, it shouldn't be this poor at 50 MHz if you look at what is inside the sensor and model the VSWR. In theory, an older version of the 478A (like mine) should manage a VSWR of about 1.07:1 at 50 MHz. The later versions might be closer to 1.05:1 at 50 MHz. So in this sense, there probably isn't much point trying to improve the match if the aim is to measure a 1mW reference from a typical HP power meter. This is because the source VSWR of the 1mW reference is likely to be much lower than its spec of about 1.06:1. So the mismatch uncertainty should be negligible. Only a calibration house should really be concerned here, but I thought I would have a go at improving my sensor anyway.
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The following posts show the thought process and the modelling and the test results.
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I've attached a VSWR plot of the input VSWR of an early HP 478A sensor. This was taken with a lab VNA set to a really slow sweep time of about 20 seconds, although slowing the sweep time from less than 1 second to 20 seconds didn't seem to make much difference.
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You can see there is a kink in the VSWR at 50 MHz and this has always bugged me because it causes a bump in the efficiency across about 40-60 MHz. I've lived with this 'bump' issue ever since I first got this sensor back in the 1990s. But now I've managed to get rid of it and I'll show what causes it and how to upgrade an older sensor like mine to get rid of it.
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Regards
Jeremy
Recap 6205B

 

I am seeking replacements or substitutes for these caps:
C10?? HP p/n 0180-1852 x2
C14?? HP p/n 0180-1888 x2
C20?? HP p/n 0180-1851 x2
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I have tried a variety of cobbled together solutions and they have not worked.? I do not want to scrap this device but I am out of ideas.
Thanks
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Joe White
KW4YW
Re: 1 mw - 0dbm lab calibration

 

¿ªÔÆÌåÓý

Hi Jeremy,

Nice thread on old stuf I hardly ever use. But now I got encouraged to do some tests as well.

You certainly discovered that Appendix II of the O/S Manual of the HP432A gives instructions to install long cable options. So HP is very much aware of the total resistance values. Mine has Option 009 (10 feet cable) but I choose to disregard for the moment, I have the standard cable. The calibration procedure 2 is quite convenient making use of my HP34401 DVM. You need a long stabilization time.? I was wondering: how about thermocouple effects of the different connections in the chain from the thermistors to the internal bridges.

For fun I measured the return loss of one of my HP8478B (1833A13571) sensors. Quite good actually and certainly within spec. See below.

Thanks for the nice work and observations. Best 73 de Harke


On 8-5-2025 22:14, jmr via groups.io wrote:

Hi Paul my name is Jeremy.
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From a metrology point of view, one key limitation of the Wheatstone bridge system used in the 432A is that the thermistor is at the end of a long and flexible sensor cable. This adds resistance to the bridge as shown in blue in the diagram below.? This introduces a tiny error in the power calculation unless the resistance is known. On my 432A the internal mount resistance is virtually spot on 200R within just a few milliohms. However, when I include the resistance of the sensor cable this increases to about 200.2R. This is a 0.1% error which is not that relevant to most users of the 432A. However, when doing DC substitution this resistance needs to be known and entered into the equation for the power calculation. Otherwise, this contributes about a 0.1% error to the power calculation. You can see in my youtube video I entered 200.2 ohms rather than 200 ohms. I think this assumes the mount resistance is an accurate 200R. I check both the mount and the mount plus cable resistance.
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The Tegam meter automatically measures the mount resistance with a 4 wire system and so I think it claims an advantage here. Unless the operator of the 432A measures this resistance with a very accurate ohmmeter then this error can't be managed properly when carrying out a DC substitution measurement. I use a Keithley 2015 in 4 wire mode to measure this resistance using a special set of 4 wire test leads. I don't think there is a sense return wire in the 432A sensor lead and presumably the sensor wire will be made of wire that is designed to be flexible and reliable. So this probably means the wire has a bit more resistance than you might otherwise expect. I think the ground return wire/shield back up the cable will have very low resistance and so this doesn't really contribute much in the way of an error. I don't really see this need to measure the cable resistance as a significant problem. It never seems to change year on year and the error it introduces if ignored is tiny at about 0.1% in my case.
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I think things might be slightly different for the sensor cable used in the N432A and the Tegam meter. This cable may well have an additional sense return wire in it that connects right at the thermistor. So this may alter how this potential error is managed. But I'm really just guessing. It does make sense to have this extra sense wire with the Tegam system.
Regards
Jeremy
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HP8478B_RL.gif

how to convert HP83711B to 83712B!

 

Hi folks,
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I have converted my HP83711B signal generator into an HP83712B and have added options.
See here how I did it:
Maybe useful to someone or just fun to read.
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Tom
HP8757A Display Problem (Scalar Network Analyzer)

 

Hi all,
I have a HP8757A scalar network analyzer with a display problem which has appeared after some 6 months of no use. It was previously working just fine, with a HP8350B sweep generator and the 10MHz - 20GHz plug-in.
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What happens is that on switch-on, it goes thru the self-test and then reverts to a display which seems to contain some correct information such as frequency span, key button names (at the right), and a 'noisy' scan waveform (as expected) - BUT the display refresh is very slow and is well off-centre - it is clearly cycling through the 4 'display channels sources' but it is nearly unreadable (certainly unusable).
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I have another 8757A which I use with another sweeper which works well, and by using this as a 'test reference' I can determine which buttons to the right of the screen to use to perform some tests. In this way, I have called up the test pattern display (SYSTEM -> MORE -> SERVICE -> DISPLAY -> TEST PATTERN) and this generates the wanted test pattern BUT this is also well off-centre and refreshed slowly.
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I have checked the error-code LEDs (removed the top cover of course) and they are all off (zero) after switch-on or after "PRESET" which indicates no error found, and also checked the +15V, -15V and +5V on the small power board with the fuses. The +105V rail there is also correct.
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This problem is independent of whether the 8350B sweeper is on or off, and if it is on the PRESET button initializes both the 8757 and the 8350 as it should. I have compared the problem unit with the working unit and the basic functionality would appear to be present. I am wondering if the battery on the CPU board (backing up some constants?) could be a reason, but I doubt it - I can replace this tomorrow if I think it might be.
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I know that the refresh frequency of the display will be lower with the more channels that are being scanned (A, B, C, R) but this does not appear to change the problem on the unit that I am dealing with.
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For what it's worth, I have a comprehensive workshop and lab at home and I am used to working with both microprocessor designs and (more so) with microwave / ATV designs. I have read the service manual and operating manual but with so many pages and different references I am not sure where I should be looking.
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Would anyone here have a good idea of where the problem could be?
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My sincere thanks in advance.
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Re: HP DC Power Supply modification

 

Hi Nick,
That is a computer power supply not really related to this test equipment group. Have a websearch adding "lithium charger" they are popular with people charging big lithiums and some of their forums may have info. There are a number of possibilites including a simple adjustment, resistor change, software controlled adjustment or putting two or three silcon diodes in series with the sense connection.
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Robert. G8RPI
Re: Looking for service manual/schematic for early E3611A DC Power Supply

 

¿ªÔÆÌåÓý

I have started a new folder and uploaded all I have???

New folder?? E 3610,1,2 A

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Paul

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From: [email protected] [mailto:[email protected]] On Behalf Of Philip Freidin via groups.io
Sent: 16 May 2025 17:18
To: [email protected]
Subject: Re: [HP-Agilent-Keysight-equipment] Looking for service manual/schematic for early E3611A DC Power Supply

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You didn't specify which version of the schematic you had.

I have 5959-5304? Edition 6 , October 2007.? Is this earlier than what you have?

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The document does not have a detailed component list, and many transistors,

diodes, and op-amps on the schematic have no part numbers. The schematic

does not include the meter section.

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Cheers,
Philip

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Re: Looking for service manual/schematic for early E3611A DC Power Supply

 

This seems to have the correct schematic and parts list...
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HP E3610A, 11A, 12A Operating _ Service.pdf
HP E3610A, 11A, 12A Operating _ Service.pdf

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