Good morning from Pinellas County, Fl. Just was wondering where would be the best place to sell my HP8924C (working condition). Power supply caps are good to go and the unit is fully functioning. Thank you in advance for any suggestions.

-73? KQ4KIO Tally Tech and Troubleshooting

[Moderator note:? Rick's email was sent around noontime (US EDT) Wed.; actual posting is therefore two days later and out of order]

I would first check the rf level coming out of the input module Disconnect the smc connector at either the input or where it connects to the downconverter. If I remember , you should see about 10 dB less than what you are generating into the antenna port. If that's good, and If you have an 8920A, first try swapping the receiver module. If your spectrum shows the same, it's not the receiver, if it shows fairly close, your problem is in the receiver. A common failure in the receiver that causes the spectrum levels to be low is U4 or the supply voltage to U4. On U4, you should see about 4V DC on the output and 1.6VDC on the input. Even if these DC voltages are good, the amp could still be bad. U4 is an MSA0786. Rick

Hi! I am new to this site.

Joined because my 8935 just wouldn't power up one day. I did not have it plugged in or turned on for several months, and when I went to use it, it was dead. Would not turn on at all.? I eventually found this site and read about the 47uf 50 volt cap on a little stand up board on the main power supply board. I found two 47uf 50v caps on different little standup boards and replaced both of them because I was in there and because I did not know which one was the culprit. I had very little faith that this would actually fix my 8935, but it was the only thing to try because a long search for schematics turned up empty and I was flying blind. I was able to replace both caps without removing the stand up boards. It was a bit tedious but went well. One of the caps I mounted on the back side of the board because it was easier to do. Anyway, bottom line it it worked! I couldn't believe it! I am so happy! if I had not found this group I would not have known what to do.

Thank you! Thank you!

Chris Bolkan

The receiver for the earlier 8920A units is a bit different but will still work in the 8920B. It may give errors because you can't transfer the cal data too. What you need to do is just swap the receiver module and see if your 8920B spectrum analyzer level is in the "ballpark" of where it should be. If so, You have a bad signal path through the receiver module.

Your next step is to check U4 (MSA0786) amp. Check the dc voltages (4V and 1.6 -input) on the U4. This is generally the problem with low levels to the spectrum. I have never seen a bad downconverter. Rick

If you look at the RF I/O you should see an MMIC or two used for the higher ranges.

Those may be weak in both units.

You could look at the RF I/O output? using single step on the RX diag section and a external SA.? ?The mixer assemblies are pretty reliable ...have maybe replaced one or two over the years.? Not much to break.

I usually loop the Antenna to Duplex in the SA screen and use the tracking gen across the full 1Ghz to check the response ...should be pretty flat across the range? ...like may +- 4 db ripple max IIRC

Chances are without the SA both units pass all the RF diags because the tests for the absolute levels are skipped ...three tests.? Usually the low frequency passes.

?

?I read page 645 in the Assembly Level documentation which describes the Receiver Assembly test:




I should be able to measure the 114.3 and 614.3 MHz-signal levels on various places on these modules to verify very the signal level is lost?

Ok so before I swapped the three units RX, RX Synt and SA from (what I thought was) my working unit I run the complete RF_Diags and I was surprised that I now got ONE failure on the test and it was the 900 MHz, 114.3 MHz IF test that measures -2.306 dBm while the lower limit is -2 dBm.... Dough.... this unit used to work well without errors on the RF_Diag test.

Anyway, I swapped the three units RX, RX Synt and SA into the non-working unit and now I get only ONE error on the failing unit instead of 6 failures and now the only fail is in the RX assembly test and it is on the Down Converter test but now for the 500M MHz, 114.3 MHz if showing -3.275 dBm compared to the fail on the working unit where it fails on the 900 MHz frequency.

Am i correct in my conclusion now that it is a problem somewhere in the Down Converter path since I ONLY get one error in RF_Diag running the full RF_Diag tests and no errors in the SA tests? (The SA module in the failing unit is still having problems since if I put back that SA module I get more errors.)







I just realised that I forgot the results of running the RF_Diags WITHOUT the SA module installed. Will do that tonight and report back.

So instead of looking for only a SA module to test with (before attempting a repair of the SA module) I need to find a working RX module to test with.?

I am a little bit confused of the part number for the Receiver Module...
I see?08920-61828 in the Assembly Level Repair document but I have also seen?
08920-61028
08920-61102

Will all those part numbers work or are those two other ones different in some way?

The SA can be easily extend using a 9 pin D extender and I also use the SA terminator ( used when the option is NOT installed ) with the resistors removed to extend out the two RF connections.

Aaaah! I did not realize that there is a DB-9 connector on the bottom of the SA module between the P1 and P3 connectors! I see that now when I look at the schematics and the picture showing the physical board layout.
Thanks for the tip to use that in combination with termination module! ?

(I do think that I have a proper extender card lying somewhere in my boxes though.)

Do I understand it correctly that instead of using the termination module with resistors removed and feed the RF signals from there into the SA module I can just feed in the 20MHz reference-signal and the 114.30MHz IF-signal using external RF generators and only connect the SA module to the motherboard using the DB9-extender??

I can read in the schematics that the 20MHz reference signal should be +3dBm but I am not sure from the schematic if the 114.30MHz IF-signal should be -1dBm or something else?

?

During the weekend I will test with swapping the SA module and RX module from my working unit into the failing unit and run the RF_Diags again and see if errors disappear. I will also run the MISC_Diags to verify the three external RF ports.

U15 in the SA LO was what I was referring to.? This is the LO that is actually swept for the SA function.

The SA can be easily extend using a 9 pin D extender and I also use the SA terminator ( used when the option is NOT installed ) with the resistors removed to extend out the two RF connections.

Only the reference is really needed for troubleshooting ...the IF input can come directly from an external sig gen at 114.3 Mhz.

The reference could be external as well using ( per the above schematic ) a 20Mhz +3dbm signal

the 9 pin D connector extender can be easily built using? readily available connectors or cables ...wouldn't go too long though ...just enough to lay the card flat on an inuslator.

Thanks for valuable comments John ?

I will try to verify the Power Supply levels as well as inspect the caps on it first to make sure the caps in the PSU are not bad/getting bad.

The power meter is on the RF I/O? input only.? ? You can get an input level on the ANTENNA but that's usually with the SA .....at least that's how I normally do it.

Thast is how I also do it when using ANT IN port.
However, when I read page 65 in the "Assembly Level Repair" document at

it suggests a TX power reading available when using the ANT IN port when diagnosing faults in the RF Analyzer section:

To isolate the RF Analyzer:

? On the Test Set:

? Press PRESET.

? Press TX.

? Set Tune Mode to Manual. .

? Set Tune Freq to 100 MHz.

? Set Input Port to Ant.

? On the external RF signal generator:

? Set the frequency to 100 MHz CW.

? Set the amplitude to 0 dBm.

? Connect the output to the Test Set’s ANT IN connector.

? Set the RF signal generator frequency to 100, 500, and 900 MHz. For each frequency,

the TX TEST display on the Test Set should read as follows:

? TX POWER should read approximately 0.001 ? for each frequency.

? TX FREQUENCY should read 100, 500, and 900 MHz respectively.

? If the Test Set has a Spectrum Analyzer, temporarily select SPEC ANL and note the

level and frequency of the signal’s fundamental component. (Press TX to return to

the TX TEST screen.)

However, as the last paragraph says, TX level has to be chosen to see TX power if the unit has the SA module installed?

I usually? run the MISC DIAGS? as well to verify all three RF ports and often pull the RF I/O module and open to inspect for any over power if I have the unit open.

Ok will test that as well tonight. Will inspect the RF I/O module during the weekend.

The diagnostic mostly uses the calibration signal internal to the card to test.? May have a bad LO output MMIC on the card.? Sometimes the RF chokes open up as well.

Ok, do you mean a bad LO outut MMIC on the SA module itself?
Ah! Reading page 643 in the Assembly Level document indicates that this calibration signal is on the SA card.

I am looking in the schematic for the SA module from the CLIP-files for the 8920A and I see the section of the schematic for the calibration signal but not sure which of the IC:s there would be LO output MMIC?
Would it be U3A, U3B and/or the U10A?

you may have multiple issues.

The SA card looks very suspect since it fails the diagnostic section for the SA ...looks like it has a major failure in the signal path. internally ...probably repairable.
The diagnostic mostly uses the calibration signal internal to the card to test.? May have a bad LO output MMIC on the card.? Sometimes the RF chokes open up as well.

That failure is causing the IF failure in the RX section test.

The other failures shouldn't be there for an SA issue.

You can pull the SA and run the diagnostics ....normally you would have a termination card installed instead but doubt that will affect much ...just has the proper termination for the IF input and reference to the SA ...no big deal.

It's almost like your SA and RX modules have issues ...may want to verify your power supply ...look at the caps to see if they are starting to go ...common issue.

You can swap those two cards and they should pass the diagnostics ...calibration data will not usually change the diagnostic pass fail.

Understand the SA taps off the signal at the IF frequency so even though it's reading low the RX in this case is getting the full signal to properly? detect the frequency and modulation..

The power meter is on the RF I/O? input only.? ? You can get an input level on the ANTENNA but that's usually with the SA .....at least that's how I normally do it.

Normally I would never input +10 dbm into an Antenna port ...try to limit that to -20dbm.? The 8920 should take it but try to error on the side of caution.

I usually? run the MISC DIAGS? as well to verify all three RF ports and often pull the RF I/O module and open to inspect for any over power if I have the unit open.

Hi Rick

Yes I have run the full RF_diags suite and it reports in total 6 failures, 5 of them in the receiver assembly test and one in the spectrum analyzer test.

SA module has one fail (I have option 002) on the vertical sensivity test:


Receiver assembly has the following five fails. 3 of them on the IF down converters:


and 2 of them on the high squelch levels:



As I mentioned in my first post I do have a working unit and I am a little bit surprised that when I measure TX power on the RF IN/OUT on both units I get proper readings on both the working unit and the "failing" unit. However, BOTH units shows dashes ("-----") when feeding a 0 dBm signal into the ANT IN port. Looking at the block diagram it looks like TX power measurement is not possible on the ANT IN port?

Anyway, using the "failing" unit and feeding a 0 dBm, 100MHz 30% 1.25 kHz AM modulated signal into the ANT IN I do get proper reading of frequency, modulation level and modulation frequency readings (I have 10MHz GPS frequency standard reference feeding to all my instruments):


Feeding a +10dBm, 100MHz unmodulated signal into the ANT IN port on the failing unit displays as a -74dBm signal on the SA:


Feeding the same signal into the RF IN/OUT port shows similar low level of -74 dBm:


Same signal looks correct on my working HP 8920A feeding into either the RF IN or ANT IN ports.
It shows -2.43 dBm which is within specs (I think it is +/- 3dBm):


So the question is where is the signal for the SA measurements "lost" and is the SA and/or input module(s) bad?
Looking at the test set overall block diagram, shouldn't I get low/bad readings for the TX frequency as well as modulation readings if the ant in is broken?
The block diagram indicates also that TX power readings are not possible via the ANT IN port.



And looking at the block diagram for Input Module A23 it is even more clear that TX power is not possible to measure on ANT IN port. However, if ANT IN was overpowered I should still get proper SA level readings using the RF IN/OUT port?


If the ANT IN port was bad due to overpower I believe I should not get proper readings for modulation level, TX frequency and modulation frequency?

Question now is if the next natural step would be to temporarily swap the SA module from my working unit into the failing unit?

Did you run all the RF_diags? Is that the only failure? If the input module was overpowered at the antenna port, you would get that failure.

开云体育

Is there any way to link the posted files to the discussion in which they were associated?

Thanks,

Mike

Hank and all,

The video I mentioned is the same as: (Hp8920a Rf Communication Set-1.wmv).

Mike

That file was mine, created with help of another some time ago. The original format is VHS. I still have the tape. It is a bit noisy or was during the transcription on that particular player..

Mike,

I believe that is the same HP training video that is presented John Higham. It is available on YouTube as a four part series titled "HP 8920A 8920B RF Communications Test Set - 1 of 4"