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Thanks James and John,
This is why I asked,? I don't know where I got the idea that the B model supply was more like a computer supply.? I didn't know if any or all of the 8920 modules are interchangeable with the 8924 but I will have to revisit a certain website with pictures that show the numbers of what HE had.? This was part of my original question.? I don't have a schematic on anything about the 8924, so I didn't know what was powered to keep it warm, and I still don't.? The OPT Ref module in the CLIP shows it likely "could" be powered to warm it if the line to the oven were isolated from the rest of the 8920.

My next step would be to get into my OPT REF module and see what model OCXO is really there and look for data and do tests on that unit to see if this is feasible.? I can always revert to what I am doing now.?? Testing has shown that I can be within 4 hz after 30 min warmup time, and best after 1 hr for setting high stability ref osc for multi site repeaters on 800.? It would be better to have GPSDSO units at each site, but I have a feeling someone would borrow one generating a service call.? In any case we already have the Hi Stab options.? I do have a portable OCXO that stays powered all the time for a third opinion.? This is mostly for verification as they don't wander too much.? A TCXO I find useful for 2m 440 but less accurate than I need for 800 multi-cast.

Hi JB
As per the posts before, the power supply between the 8920A / B /D and 8921A are the same. There was small changes to the power supply during the life of the 8920 series test set (as with all of the boards in these test sets) . They didn’t have a standby voltage line for the oven and the design doesn’t allow for it. I think you are getting confused with the 8924 series which has a standby crystal oven. I believe the REF board and 8924 is made different to accommodate this.
The 8924 series and 8920 series power supplies are not interchangeable.

James G

I did re-post the 8920/21 power supply schematics? in the files section ( forget who initially posted this to the old forum ) ...thought the 12V AUX was for the fan and CRT? ...it may have been used to run the heater in the OCXO but never looked .? Assumed it was for all the noisy loads to isolate from the analog supplies.? Never really paid much attention to the internal power distribution though

The 8920/21 power supply are completely killed by the on/off switch including DC mode.

The 8920/21 will run of off 12VDC so you could just power the whole unit up and turn it on ...note you have to switch the power supply to DC mode from the rear switch.? No modification needed.

This would help up reduce the warm up time on-site

Often sites will have a GPS locked master? reference that may be a better alternative? ...just bring a long BNC jumper and plug into the distribution panel ...usually used on the 700/800 trunking systems

Of course the other though is to just install one for your equipment ....most synthesized equipment could leverage that and one less thing to align.?

On Tue, Jan 31, 2023 at 03:02 PM, jyelmgren wrote:

See?08920-90168 page 510? ? ...there is no standby voltage in the 8920/21 for the oven ....the 8924 does have a standby 15v which does provide? for keeping the oven on.

I have the 1996 version of that (Assy Level Repair) document and there is nothing on that page regarding the discussion, but there is a block of the PS on 513.? There is a +12v_aux from which the +12v_oven is derived, but there may be other circuits that are fed by that voltage that I need to verify, so it looks like I will be isolating any external voltage from the +12v_aux line for safety sake.? The Assy Level documents have no actual schematic, only partials for the purpose of their discussion.? All I have is what I found in the FILES section.

The core change with the 8920B was the processor board along with the memory board and using PCMCIA instead of EPSON

Not sure where the standby voltage ever came up in the context of the 8920 /21

The standby voltage came up in the context of my original post with the questions.

Because I don't have a schematic for "whatever" model actually has the standby 15v (thanks) or any other than the 8920A CLIP 08920-90107.? That document does not have PS schematics or any other than the EPSON memory board.? I Have never been inside a B or any other model.? I had thought the 8920B had the standby voltage.? I guess I was wrong about that.? Again, I don't know if any of it applies to the oven in my specific 08920-61035 OPT REFERENCE module, because I don't know if it is interchangeable with any other models that are standby powered.? I will likely have to open it up and see which OXCO module is there and look it up.? I have no time right now, but I do open the unit for dusting and inspection at least yearly so I'm planning ahead at this point.

Back to context of the original post questions, I was hoping to be able to modify my 8920A to be able to provide 12v directly to the oven (perhaps diode isolated) so it would be 30 minutes warm by the time I get to the site.? There is a separate oven voltage on the oven.? From experiments with other OXCO modules, the actual voltage doesn't always matter as long as it is above the drop-out voltage of the regulator and thermostatic circuits in the oven.? I imagine (but don't know for sure) that the oscillator circuit might not have to be powered to keep the oven warm, but that depends on how the thermostatic circuits are powered.? It looks like I could simply modify the Opt Ref module with diode isolation of the 12v_oven line and a jack in the back for a separate wall wort or 12v source for temporary use.? The only other thing would be to do a timed check to plot the warm oven stabilization after the main power is activated and the unit boots.

See?08920-90168 page 510? ? ...there is no standby voltage in the 8920/21 for the oven ....the 8924 does have a standby 15v which does provide? for keeping the oven on.

The core change with the 8920B was the processor board along with the memory board and using PCMCIA instead of EPSON

Not sure where the standby voltage ever came up in the context of the 8920 /21

On Sun, Jan 22, 2023 at 04:15 PM, jyelmgren wrote:

"The power supplies in the 8920/21 A/B are the essentially the same."

There must be a difference or the 8920A would have standby oven voltage like the "B".? Just saying.
Focus.

The power supplies in the 8920/21 A/B are the essentially the same.? You can inter change the power supplies between those models.? There are no differences at the power supply level? in those models.

I assumed you were talking about bench use....OCXO based units for the field are usually a bad fit due to exactly what you are talking about ..the long warm up time.

Usually m experience is that it takes at least 5 minutes and closer to 15 minutes for service monitors using that type of reference to become usable.

Normally I would rather have a TCXO based unit which is more effective in the field and on battery operation and use and external reference on the bench.

On Sat, Jan 21, 2023 at 01:27 PM, jyelmgren wrote:

There are no differences in the 8920/21/A/B power supplies at a high level.??

I don't know what that means.? I don't think they are directly replaceable, or are they?? I thought the whole "on-off" and standby voltages were different.? Murphy would say "go ahead and plug them in and see what happens".? I have a 08920-61035 OPT REFERENCE module.? There are several different model # I have seen in pictures.? It would be nice to know if they can be swapped around without mods or smoke.

If you really want to avoid the OCXO warm issue I would just use an external reference ..lots of GPS locked ones out there that are cheap and effective.?

I have an external OCXO that is always powered by a wide range of either battery or external power.? It is something else to carry around though.?

The down side to powering the OCXO all the time is that the? standby supplies will eventually fail ( see the 8924 for it's major issue ) or in some cases the OCXO finally burns up from age and use ( IFR COM_120 was an example of that )

It's a shame IFR cooked their own goose.? I used a FM/AM 1000s, then the 1200s for many years in the field.? Then they got so big and awkward, you had to make your own wheelchair access, tie downs and boom trucks for them.? How did they transport the NAZI super tanks that were too big for the roads?? Maybe air dropped from a cargo plane?

get a set of replacement EPROM's and re-program.

The exact type and version depends on what memory card you? have.

What version shows on the screen? when you start up ?

It could also be the memory card or the controller that are failing as well ....easiest is to just do the firmware chips

The images used to be in the files area

There are no differences in the 8920/21/A/B power supplies at a high level.? ?There may be some version /vendor differences since it was OEM'd but they are roughly all made to the same spec AFIK

If you really want to avoid the OCXO warm issue I would just use an external reference ..lots of GPS locked ones out there that are cheap and effective.

The down side to powering the OCXO all the time is that the? standby supplies will eventually fail ( see the 8924 for it's major issue ) or in some cases the OCXO finally burns up from age and use ( IFR COM_120 was an example of that )

OK so I will formally ask the questions.? The 8920A has no power on all the time to keep the OCXO warm and it takes at least 15 min to to become stable enough for 2m or 440 and it reaches maximum stability after about an hour.? I hear the 8920B has that voltage on all the time it is plugged in.?

Questions:?
-? What is the voltage?? I find that it is called out as +12v_oven and becomes +12v_aux on the motherboard, so is this the +12v or the +14.0v output from a 8920B supply?? I'm not sure if this is used in the 8920A model at all as I don't have the 8920A power supply schematic, so maybe +12v_aux gets supplied from the +12v(F) somewhere??

- Has anyone out there tried to remotely power the +12v_oven to keep the 8920A OCXO warm and was it useful?

- For those who own the 8920B and models that have standby voltage to the OCXO, are they ready to go with max stability at bootup or is there still a warmup curve?

Typically this is all a moot point, 'cause if I fire up the 8920A to check something, by the time 15min has elapsed, I am off to another thing and never get back to it until several hours later anyway.

--

I believe the 8920/21 supplies are roughly the same.? ?there may be some variations but HP never really made those and pretty sure the specs never? changed.

The monitors are the same story ...except the monitor also was used in the 8924 as well.

Those were OEM'd as well from various vendors like SOny and Panasonic IIRC

There were lots of version changes along the way in the 892X family ....major delta there was on the CPU board and the memory /PCMCIA/ Epson board.

Thought some of that was in the available CLIP's

Not sure I have had any problems repairing that family of products with the info available ...glad we have as much as we do

Of course it is frustrating that we have schematics for the 8920a monitor, and 2 of the supplies for 8920b but no
schematic for the supply for the 8920a and no CLIP for the 8920b.? Hard to tell what is different, but we know there
must be something...?? I don't know they even ever existed.? Someone will have to be the idiot that reverse engineers
in order to build these documents.

Try updating the firmware.
--

Best Regards

?
Box

Maybe we should put the schematics in our files???
Are they the correct ones???

Other information;
If I cycle the power just after the message , the CRT shows a message of error 223 as you can see on the picture.
The only way to work normally its wait 10 or 15 minutes before power on.

Hi friends
My HP8921A show the message of firmware erro after few minutes powered on.
Then , if I cycle the power and the instrument doesnt back to work imediatelly . The only way is to wait for 10 minutes before turn on again .
Looks something are getting hot and need some time .
Any advice?
Thanks in advance?