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8920A PS recapping
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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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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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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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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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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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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? |