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- HP8924-Family-Spectrum-Analyzers
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looks like time to re-cap my 8935
My 8935 suddenly won't turn on. I get nothing, no fans,,,like the power cable is bad (isn't) so assuming I need to replace some caps. Looking at the -90015 manual and see its the A3 assembly, but there are no pictures and I'm curious which boards I need to fiddle with. Looks like a "power supply" and a "regulator" board. Is the work on one of these boards or both. Just curious what I'm getting into. I have two other 8935's but this my favorite.?
GeorgeC W2DB |
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Re: 8920A PS recapping
normally I now just do the output caps as well as the one small cap in the high voltage area ( 47uf 63V IIRC )? Opinions vary of course but when you have done as many as I have you start adjusting to maximize efficiency .
The high voltage cap and the DC input caps are usually fine ...never seen one of those leak.? The small board? is the DC mode switcher and hardly ever powered in most units. I always proactively re-cap the 892X supplies and usually it;s the +5 volt out that start leaking but the other output caps have leaked as well. The spacer plate for the ANT and DUPLEX ports are only on the early version ...later versions have a revised front panel casting. The push rods have become very fragile with age and I am extremely cautious and gentile removing that part .? The RF I/O assembly has to be pulled and I use a medium screw driver to separate the rod from the switch. I have had issues with the AC on/off switch and often will pull the switch and clean as well as swap it with the AC/DC switch .? Same switch and rarely gets used . I did post the schematics which were contributed on the old yahoo page The supplies were OEM'd but specific to the 8920/21? |
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Re: 8920A PS recapping
Gidday All, Bill VK6II I just did My 8921A PSU and read the manual to see how to extract it. Slide off the cover to reveal the works inside, undo the nuts on the BNC connectors to the Spec An section (Front Panel). Don't lose the spacer . Carefully remove the SMA connectors to the Spec-An. Undo the Torx screws on the right side and on top of the spec -An . Lift it a bit and gently pry the DB9 connector underneath, inside left, and lift it out of the crate. Pull the On/Off knob off the push rod from the front panel. Access the rod from the bottom of the crate and pry the rod from the push switch on the PSU. Extract the rod by moving it back a bit to clear the frame and lift it out up & forwards where the Spec-An was. Undo the screws that hold the PSU and it will come out thru the bottom of the crate, with a pause half way out to pull off the multi pin output connector. The PSU main board will need to be removed from its chassis. ?The power transistors attached to the side are held by spring clips. Be careful not to break them. Replace the (heatsink compound) Unplug the small regulator board that is plugged into the base board. Remove the back panel of the PSU, it unplugs from the base board. The base / mother board, of the PSU is held by nylon snap pegs. A pain to get them to all release. It's just a struggle with care. I suggest replacing all electro paps, as the repair process is quite a task, and it is probably not made to do too often. There is a poly cap that is next to a heat sink (sort of hidden), in the middle rear back part , it usually explodes and is easily seen. Hell of a thing to get to. Replace all similar type capacitors while you are into it. When finished and your are satisfied that all your work is "top notch", reassemble the PSU and it can be run up on the bench as a stand alone unit. Measure all the outputs, taking care not to short adjacent pins. NB. Be aware that some screws in the instrument are longer than others. Make notes as you dismantle the instrument and mark them to go back where they came from. You can blow something up with a long wrong screw :( All the best "Family", Regards (Doing this on OZ is very expensive, US$10 for a part +US$56 shipping is a killer. We get very innovative) On 30/10/2022 7:28 pm, george edmonds via groups.io wrote:
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Re: 8920A PS recapping
Hi again Mike Look at the groups file area, most of what you want is there including the 8920A CLIP. The assembly level service manual may not be, but was available on the Keysight web site, if not I have it. 73 George G6HIG Dover UK On Sunday, 30 October 2022, 05:41:02 GMT, Mike / K5JR <k5jr.lists@...> wrote: So, my trusty 8920A decided to groan and stink now when it¡¯s turned on versus booting into normal operating mode. Leads me to believe it¡¯s cap replacement time and/or a few other parts. So, extracting the power supply module appears to be hindered due to the On/Off switch extension ¡°pushrod¡± going to the front panel. What¡¯s the magic trick to disengage it and get it out of the way? I didn¡¯t fiddle with it, lest I break it, thinking the collective on here has been through this several (many?) times. Or box it up and ship it to Rick? I¡¯m perfectly comfortable with installing new caps if I can get the darn module out to work on it. tnx Mike / K5JR Alpharetta GA |
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Re: 8920A PS recapping
Hi Mike Yes do not try to remove the power supply without first removing the push rod, if you do you will certainly damage or destroy the push-rod, the instructions are in the service manual which is online. It is about four years ago that I last did one so my memory of how to do so is fading, but it essentially involves removing some components from the front of the instrument. There is a standard problem with these supplies in that some resistors go high, this prevents the power supply turning on. I do have a partial schematic for the supply. 73 George G6HIG Dover UK On Sunday, 30 October 2022, 05:41:02 GMT, Mike / K5JR <k5jr.lists@...> wrote: So, my trusty 8920A decided to groan and stink now when it¡¯s turned on versus booting into normal operating mode. Leads me to believe it¡¯s cap replacement time and/or a few other parts. So, extracting the power supply module appears to be hindered due to the On/Off switch extension ¡°pushrod¡± going to the front panel. What¡¯s the magic trick to disengage it and get it out of the way? I didn¡¯t fiddle with it, lest I break it, thinking the collective on here has been through this several (many?) times. Or box it up and ship it to Rick? I¡¯m perfectly comfortable with installing new caps if I can get the darn module out to work on it. tnx Mike / K5JR Alpharetta GA |
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Re: 8920A PS recapping
Rick does a great job and picks up all the widely known issues. I shipped mine to him from the UK and back.
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His Retirement may not be so long away, don¡¯t wait too long¡. V 73, Colin G4CWH On 30 Oct 2022, at 05:41, Mike / K5JR via groups.io <k5jr.lists@...> wrote: |
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8920A PS recapping
So, my trusty 8920A decided to groan and stink now when it¡¯s turned on versus booting into normal operating mode.
Leads me to believe it¡¯s cap replacement time and/or a few other parts. So, extracting the power supply module appears to be hindered due to the On/Off switch extension ¡°pushrod¡± going to the front panel. What¡¯s the magic trick to disengage it and get it out of the way? I didn¡¯t fiddle with it, lest I break it, thinking the collective on here has been through this several (many?) times. Or box it up and ship it to Rick? I¡¯m perfectly comfortable with installing new caps if I can get the darn module out to work on it. tnx Mike / K5JR Alpharetta GA |
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Re: HP8920A Per-Cal Proceedure
Thanks to all who replied to my question. I had success setting the DAC's and running Per-Cal on my 8920A. It went from 2445 HZ off at 950MHZ to 6 HZ off. Neat capability!
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Re: Program to retrieve HP8920 calibration data...
My interest in all this was to be able to back up saved screen setups to my computer HD, but it seems I can only write them one at a time to a card,
or to format a card (LIF?) with all the saved screens at once.? Also to be able to load other tests, but was only able to load the ASCII text files.? t would be great to know how or what comms package to use to move the binary files back and forth.? Supposedly, if the card is formatted DOS the machine will know to convert all files writing to that card to a DOS compatible file. I left my cal dump in the HP8920 tools folder (with s/n goofed).? I can't figure out how to restore any of the data without having an alignment card to properly load them.? Interesting how fast the 8920A Epson card question ran to the B PCMCIA card.? I never did figure out how to dump binary files in LIF (or DOS) into any meaningful file dump from a Epson card to back up to a dos file and back to the instrument to another Epson card via some ASCII serial comms package.? A text file is no problem but binary files get corrupted trying upload unprintable strings back to a card. ? In addition to that, there is a notation on the board schematic that infers that some memory areas load differently??? So I would guess that letting the machine do the writing to card via rs232 works.? At one time I was able to reverse engineer ONE of my 32k Epson cards and jumper it to enable unused memory to 128k, but others I looked at were just blobs on the board - no joy.? After some waiting and watching though I was able to find some cheap 1M Epson cards and snapped them up. |
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Re: HP8920A Per-Cal Proceedure
Per_Cal means periodic calibration and Ref osc is one that should be checked every 6 mo. and maybe every couple of days at first to make
sure everything is OK.? Yes your GPSDO is fine so long as the signal level and quality meets the expectation of the Per_cal routine, but you should read up the whole instrument calibration procedure and realize if you don't have accurate enough equipment for one of the steps, you might wind up screwing up a relatively good calibration.? Make sure you verify you have a DVM that is real accurate.? The 8920 may be more accurate than the rest of your equipment.? An adjustment there could throw off other adjustments.? A good idea to check things a variety of ways to get an idea what you can do with what you have. Focus on calibrations that you can make with the equipment you have and understand the relationship between each adjustment and interactions with other calibrations. |
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Re: HP8920A Per-Cal Proceedure
Jim, you GPSDO is fine for this. I turn mine on for a day or 2 before I intend to use it. Most come from CDMA cell sites so have to lock pretty quick for power supply cuts and such.
I think PER_CAL also needed an accurate DC 4 !/2 digit DVM to set +/- 5V? on the measurement unit. Might be another procedure but if your setting up frequency you should also set the measurement unit 5V rails. The PER_CAL procedure itself is pretty easy step by step - just follow on screen prompts. Do a dry run if you like and don't change anything. If you get to a point you confused or not sure, just power off, wait a minute then power on. I've done it heaps of time on a 8924, 8935 & E8285. 73 de Chris ZL1CVD |
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Re: HP8920A Per-Cal Proceedure
I would suggest you first find a stable signal source that will allow you to map the warmup characteristics of your instrument over time.? It is easier to get an idea from a signal at 500Mhz as opposed to a signal at 10Mhz.? You can try with internal vs your external ref. It will be different depending on if you have the standard TCXO or the optional OCXO.? This will also give you an idea how long you should wait for a warmup and what you might expect as a "before" calibration.? There is always some drift and there is a difference between a TCXO at lab temp and the OCXO that must be heated up to temp.? You could beat your head against a wall over a few Hertz if you don't understand it will be only that accurate if you always exactly duplicate the warmup time and ambient.? I have used the ATSC pilot signal from off-air TV stations in saved screens for evaluating the warmup of the instrument.? Although they generally don't have enough signal to meet the standard of PER_CAL function, they are a quick check.? My 8920a has an optional OCXO so I have calibrated it after 2 hours warmup -+1 hz and I consider it ready after 30min for accurate -+10hz frequency readings.? I have another IFR with a battery and a TCXO that I consider "ready enough" for -+20hz at 10 minutes. There is also a manual adjustment that can be made that will be lost at reboot that can be used.? They input directly to the memory positions for the coarse and fine adjustments of the ref-osc.? There is a sheet that tells of that adjustment on Three documents of interest: 8920A-B-and-8921A-Test-Set-Assembly-Level-Repair-08920-90168.pdf???? File name may be abbreviated to the HP/Agilant # 08920-90168 The procedures, either manual or semi-automated from an external reference are on page 121.? It is very simple to do.? All you do is feed a strong stable signal into the front input (rather than the REF input in the rear) that meets the minimum and follow the instructions on the screen.? In the TESTS menu find Per_CAL.? It is I BASIC program that is included in the instrument.? You essentially run the program and follow the directions.? If you screw up you can always do it again. Also look in the FILES/HP-8920A and HP8920B related/ folder in this group and get: 08920-90126.pdf?? ??? Applications manual and- 8920A-RF-Communications-Test-Set-Users-Guide-(Apr00)-08920-90219.pdf ? the User manual. There are several other documents there that might be of interest such as the "Quick User Guide".? It was included in the lid of unit and any of the copies I have seen are kind of crude and low res but handy printed and put in a 3 ring binder. |
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HP8920A Per-Cal Proceedure
I have an HP 8920A that I would like to run the Per-Cal proceedure on but I don't have a good description about how to do it. The manual says very little about what all the steps are. I have another calibrated HP8920A and a 10 MHZ GPSDO available. I understand the basic idea of what is happening but am unclear about how to do it. I would pay for a ZOOM session to get some instruction. Thanks!
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Re: Program to retrieve HP8920 calibration data...
Hi Ade,
I have downloaded your Data.txt, and wondered how you did it ? I am also working my way through a the HP8921A. I also found C7 in the power supply blown. I bit the bullet and replaced all the electro capacitors and the poly caps. The C7 capacitor is a 0.047 uF, which I replaced with the same value but a bit higher voltage 630V. I ran the PSU outside the instrument, i.e., bare on the bench and measured the outputs no load. The HP displayed "Unable to read the Memory Rom, which I now know is that the memory battery is flat. It is located on the Memory board, which is the far right hand board . (instrument facing you). My HP has a PCB mounted LIPO Battery. I found a "VARTA" 3V non-rechargeable PCB mount in OZ from Queensland, which is coming now in the Post. As a temporary measure, I bought an ordinary LIPO from my local hobby electronics store, which got the screen up and revealed more errors. The 4 LEDs on the edge of the ?CPU board, gave me a clue. read the Assembly level manual and it will give a list of the errors in hexadecimal. My HP indicated the receiver board was faulty and that it had a bus fault. Fortunately, I have another HP8920A, and I marked the modules before I started swapping, then swapped the receiver module. All is now ? working better, but still an error 0200 is there, so the bus is not happy. Like yourself, I do not have any PMCIA cards. I am thinking af how to dump the ROM data into a PC and archive it for future use. I am also thinking about how to adapt a normal PMCIA to USB card to permit communication I/O . My HP has the RS232 port on the back, which I may be able to communicate via too. I have an old Win XP Toshiba laptop with a DB25 Connector, perhaps that can become a terminal using some "Basic" language . I see that HP use IBASIC language. So I think I have found a download of the program, which I will play with in coming weeks. So we should dialogue some more on this quest Ade. Regards Bill M.? (VK6II) |
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Re: HP 8935 memory backup battery replacement
Photo of actual board with backup battery soldered in. Looks to be the original battery.
Thanks Zl1cvd for your photos showing a battery holder. I was wondering if a holder would raise the overall height of the battery such as it would interfere with the adjacent board, but your photo seem to indicate that sufficient spacing exists for? a holder. Now to order the correct holder. |
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Re: HP 8935 memory backup battery replacement
Hank,
Thank you for the information. This should point me in the right direction. I'll post a photo of the memory board when I get it out.? Seems like the CR2477 would be in a socket already, but of course I haven't seen it yet. And yes, a socket is the only way to go. Shane Ke6jz |
Chris, ZL1CVD
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