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I have seen this failure a few times where it was not the RX module as well.

It's been awhile but IIRC it actually may have been an issue on the generate output level and may have been a blown port on the RF I/O module.

The bad port may have been the DUPLEX or the ANTENNA,? You could run the MISC DIAGS as a start.

Not sure that was it ...like said its been awhile but the RX Module was not the problem in? a few of those failures.

I do recall swapping a number of other assemblies like the measurement assembly and the audio processing section.?

Hi Rick

Thanks for pointing me into that direction. Will check Q512 on the RX modules 08920-61102 that I have.

I am a little bit confused though since the failing RX modules in the "non-working unit" works fine in my "main" unit.
If Q512 actually is bad, shouldn't I get?"Squelch Fixed High Level and Squelch Open High Level" fails on the RF_Diags on my main unit as well?

Regarding HP 8920A problems where the diagnostics report "Squelch Fixed High Level and Squelch Open High Level" fail, this was because of a failure of Q512 in the receiver module. This failure mainly occurred with earlier receiver module numbers 08920-61028, not the newer receiver - 08920-61102. Q512 is a BFT-93 leadless transistor. With this failure, you usually see a 10 - 15 dB drop in receiver sensitivity. Good luck!? rick

?

Alright, I have now received 2 other receiver modules (08920-61102) and when I use them in my first and working unit they both pass all tests including the dreaded receiver and spectrum analyzer tests.
On of the RX modules actually IMPROVES the signal levels into specs (+/- 2.5 dBm) when monitoring the dBm level of signal in SA mode.

As soon as I put any of those two RX modules into my "new" 2nd unit that had 5-6 errors in the RF_Diag-tests for its original SA and RX modules then all RF_Diag-tests are passed except on the
"Squelch Fixed, High Level" that should show around 0.9 VDC (same as "Squelch Open, High Level".

So, now I am wondering what can cause this error since the RX modules works fine in my "main" unit.
I have cleaned the squelch pot and it works as it should, i.e. opens the RX when I turn it down and closes RX when I turn it up.

When I read the RF_Diag test description on page 650 it says that the internal RF generator is set to -80 dBm with FM modulation at 1kHz with 10kHz peak deviation and "Squelch Fixed".
I am not sure what "Squelch Fixed" actually means and how that is set to "Fixed"?
Measurement point is "DVM:PRE_NOTCH_RMS" and I should be able to see that value in the latches menu but is there a test point where I can measure this myself?

Should I use an external RF generator and feed in that signal into P1-3 on the RX module and measure the voltage level (in case the internal RF generator has some kind of problems)? But where is the measurement point for this?

TN209 which is pin 6 of U514B and/or TN162 on pin 5 of U514??

(What does TNxxx mean that can be seen on the schematics?)

See attached picture one with the full description of the squelchtest sequence.

Picture 2 shows the voltage levels for the Squelch-test on the failing unit.
Picture 3 shows the voltage levels for the Squelch-test on my working unit using the SAME RX module.

Hi Rick,

Note that it is the MAR-7SM that is the correct replacement, not the MAR2SM.

https://www.minicircuits.com/pdfs/MAR-7SM+.pdf

Thank you!

Hi Alessio,

? Thank YOU for the information on the MAR-2SM+ being a replacement for the MSA0786 used in the 8920 / 8924 input module counter circuit. We have a stock of the "0786" but should be using the replacement. Good luck to you ! Rick

Sorry for the multiple edits.
Updated my post with attachments instead of inline pictures.

(I had to delete my previous post in order to be able to remove the inline pictures and instead add them as attachments.)


While waiting for components (MSA0786, MAS0686 and others), connectors (DB9 and 2x20 pin connectors), ribbon cables and the two "new" receiver modules to arrive I did some digging in my storage where I have some electronic equipment and holy below! I found two receiver modules and two spectrum analyzer modules where one of the SA modules have been opened up and I remember it had some issues and I got a replacement from the eBay seller from whom I bought a 8920A few years ago.

Anyway, when I tested the "failing" unit with this receiver module and the SA module that was initially in this 8920A it actually passes the SA module test in the RF_Diag tests!
Only fail I get now is still in the receiver module but this time it is NOT the downconverter tests but?

Squelch Fixed, High Level that shows actual value of 0.001 VDC when the range allowed is 0.754 - 1.131 VDC.
See attached picture number one.

Note that both the Squelch Fixed, High Level and Low Level shows the same value. Could this squelch problem be the Squelch section or completely somewhere else:
Looking at the block schematic and actual schematics I am wondering where to start measuring (or should I look completely elsewhere).
See attached picture number 2.


Maybe I should start measuring around U514A which is the Fixed Comparator for the "Squelch Fixed Level" high and low?
See attached picture number 3.

?

I have also done a "flatness" test of the SA module.
I connected the Duplex Out to the Ant In and ran a trace from approx 15Mhz up to 995Mhz with -20 dBm output and reference level at -10 dBm and it looks pretty flat to me:
-19.69 dBm at 14.5MHz
-22.04 dBm at 500 MHz
-21.85 dBm at 995 MHz

It looks pretty flat with a variation from 14.5MHz to 995MHz of approx 2.16 dBm:
See attached picture number 4.

I expect to receive my two "new" receiver modules within a week or two and hopefully at least one of them will work good enough so that the RF_Diags tests complete without errors and then I spend time on fault searching the non-working modules.

Following...

Ohm out Pot-to-Board
Pot Cleaner and 'Wiper' 1-3min. to clean em off.
Re-Run Diag.

'You know my methods, Watson'

73 DE MATT

I shoulda say MAR 7SM+ instead of MAR 2SM+

When its disconnected from the unit, the green light is on (also on when connected to unit) and i have 15v
on the white wire, I wish i knew howto turn the PS on when out of the unit before i removed a bunch of caps,
but i wasnt sure how the switch worked with the PS to turn it on.? With the PSU out of the unit, i used my
DMM to check for shorts on the connections on the unit where the PSU connects and wasnt able to find any
shorts on them. Perhaps im doing this wrong?? Hopefully not as id hate to think the psu failed and took out
the unit too.

I think im chasing a red herring, Sadly my knowledge of SMPS's isnt great so im not
sure where else to go from here. I have pulled and checked all the electrolytics that were showing shorts
while in circuit and they all test good, i pulled?a few of the 1uf ceramics and they check good as well.? ?The
short i thought i had on the output on J11 was?actually a 15ohm reading, i was tired and forgot to switch
to resistance mode, after seeing a 15 ohm resistor inline with it, i felt really dumb and went to bed lol. I DO
have a short on both C59&63, both are ceramics and the short is still on them after removing them. Im
wondering if maybe the transformer didnt short? Though that seems like itd be pretty rare to have happen.
And if it was, im sure id be dead in the water, i highly doubt id be able to find a replacement.?

Any ideas or where to possibly go from here would be wonderful if anyone is able educate me some etc
and id be most grateful! :)

**update** So i got home and decided to check again the power rail on the unit where j11 connects, and
i show about 77-80ohms give or take on the 5.2v connections.? Would this be normal or close enough to consider
it a short?? Ill be home for the next few weeks as im now awaiting neck surgery the 1st of april so i?
should have more time to test etc from here forward to april.?

Thanks Robert!
David

Hi Rick,
Thank you so much again, I have check the mmic one of them U5 have some strange voltage, like 6 volts on output and 2.5 volt on the imput, MSA0786 as you know is obsolete, in a big italian supplier specilized in rf components i found a minicircuit MAR-7SM+ as exact replacement for the msa (only a noise figure is way better), if this component is good for this application, i think to change all of this amps, this chip cost only 2,30€.

Thanks Robert,

Yes, you are correct, i got a few minutes to myself earlier today and removed it from the housing and did some probing.? I definitely have a short on the 3.3v rail on J11. All the schottkeys tested good so im under the assumption its either an electrolytic that shorted or with my luck, a ceramic.? Thinking if im going to be pulling caps to test them, i may as well replace them at the same time due to age, though i was considering possibly using a thermal camera and seeing if injecting 1-2v into that rail may show up or not, though im not sure if thatd be a safe method to use in this instance.?

I probed the connections on the unit too but didnt see anything that showed up as a short so i think i may have dodged a bullet there! lol

I'll try to narrow it down tomorrow, sure would be nice if it was the first cap i check LOL when does that ever happen though??


David

Dear Alessio,

?? That looks like you have a bad mmic amp on the control board inside your input module. These amps supply RF to the counter. Check U4, 5 and 6. These are MSA0786 amps. Likely one is bad. Measure the voltage on them. You should see 4VDC on the output and 1.6VDC on the input.? An alternative is to just replace all 3 amps.? rick

A ticking switch mode power supply almost certainly has a short circuit on the output. This could be on the power supply itself or elsewhere in the instrument.

What does it do if you unplug the output? If the PSU works when disconnected check for shorts on the supply rails in the instrument. Un-plugging modules can help isolate the fault.

Robert

Recap all.

8924C PSU

Took the PS out of the chassis and it looks like the 47uf cap was already replaced in my PSU unit, the green light is on, but when the power button is pressed, the supply makes a clicking noise and the unit doesnt power up.? I dont have much switching PSU supply experience though im thinking a mosfet or something along those lines?

Any help troubleshooting this would be much appreciated!

David

Gents,

A quick update.
I am in the process of building a DB9 male to female ribbon cable and trying to find my riser cards that I bought a few years ago in order to be able to do the measurements properly.

I am also waiting for 2 receiver modules to arrive (hopefully next week) so that I can do some testing with swapping modules.

One thing I have noticed is that when I first turn on my 2nd (and faulty) 8920A BUT with my RX and SA module from my "working" 8920A it passes all RF Diag tests for the first 15-20 minutes the "failing" HP 8920A actually passes all RF Diag tests but then after approx 15 minutes it fails on the Down Converter-test for 500 MHz, 114.3 MHz IF with value -3.275 dBm when the lower range is -3.000 dBm.

I'll report back as soon as I have either been able to measure as suggested earlier in this thread or when I have received the 2 "new" RX modules.

Hi guys !!
Suddenly I have this problem on the test set, what could it be?
I'm not going to spend a lot of money on it, can it be used with sufficient accuracy, or is it very seriously ill?
Greetings
Alessio IZ5RGO