HP 8920A repair

[papabear69]

Hi everybody,
I have an HP 8920A which worked just fine until some time ago. It starts to present noise on the demodulation of the signals. This noise makes the measurement of the deviation and AF signal impossible. As well, the scope is showing me noise instead of clear CTCSS or audio signals like 1000 HZ. Please give me some ideas how to solve this issue. Where to look? Which module should I verify?

[Tony_G]

I don't have one of these but the assembly level repair manual from Keysight should get you to the offending assembly:

https://www.keysight.com/upload/cmc_upload/All/EPSG072204.pdf

From there you'll need to find the appropriate assembly CLIP to get schematics.

TonyG

[TheSteve]

Run through the self tests several times, see if anything fails.

[david_ww_jd]

Hi!
I have problems with the power supply from my HP8920A, some components are burnt and can't read the values. Where can I find component level manual for Power Supply? I've downloaded the component level manual from https://www.keysight.com/en/pd-1000002342%3Aepsg%3Apro-pn-8920A/rf-communications-test-set?pm=PL&nid=-536900808.536880153&cc=US&lc=eng, but it's missing the A9 Power Supply chapter.
Thanks.

[George Edmonds]

Hi all

There are NO schematics for the power supply in the CLIP as it was a proprietary bought in item by HP.

The damaged power supply section is the feedback/driver  circuit to the main switching device which has very probably gone short circuit.  It is an absolute PITA tp work on, I know as I have repaired two of these supplies in the past, admittedly not with the amount of damage shown.

Even with a schematic they would be very difficult to repair unless you have a lot of experience with SMPSU's and can easily be lethal as there is some 350VDC at high current used in them.

George G6HIG 

[George Edmonds]

HI again all

I would not be surprised if the OP fault was also a SMPSU fault.  They are basically a 120VAC design, I know some will argue this, and don't like operating on 230VAC.

George G6HIG

[Wallace Gasiewicz]

Perhaps someone on this group can help:
https://groups.io/g/HP8924-Family-Spectrum-Analyzers/topics

In response to papabear;
If the spectrum analyzer works fine I would look at the audio/scope portion of the unit. Possibly you could replace one of the boards and be fine.

Wally KC9INK

[George Edmonds]

Hi Wally

Unfortunately in Europe we have different constraints on us, on your side of the pond you have Amtronix Instruments of Lakewood NY who provide support for HP test sets such as the 8920A, there is no such an operation in Europe.

Spares in Europe are a major problem, for example Amtronix list a PSU for a 8924 (looks like there none available for a 8920A) at $350 exchange, the critical word being exchange, given the ludicrous shipping fees across the pond plus customs and other charges both ways you could easily treble the $350.  At this point the  cost of a single item is almost 100% of the current value of the instrument.

Spares for HP test sets are sometimes available from Israel, but again prices are high as are shipping charges.

Incidentally the posting that you are replying to was made in 2018. hope the OP has some resolution by now

George G6HIG


 

[Wallace Gasiewicz]

Thanks George, guess I should look at the dates more often

I really dislike working on SMPS, any little thing that is wrong leads to disaster.
If the main switching device is out, that could be replaced, but figuring out the control feedback is far more difficult.
Would it be possible to use another PS??  Or several ? (one for each voltage)
I do not know how many different volts are supplied.

Wally

[George Edmonds]

Hi Wally

It was a bit naughty for the second OP not to have opened a new thread.

In my opinion it would be virtually impossible to replace the 8920A power supply internally with anything else, it is a custom unit made to fit a very tight space.  To remove it without damaging the plastic mains switch operating plastic rod means quite a lot of work.

It is a particularly nasty design by today's standards.  The problems are two fold, it uses a transformer to provide isolation in the feedback path, this means that a blow up around the switching stage can be transmitted back into the low voltage stage regulators, current designs mainly use opto isolators  to prevent this.  Secondly it is a flyback design which if it losses the feedback loop it will almost certainly totally self destruct, which appears to be the case with this one.

For myself I would repair it, but I will not do so for anyone else as almost invariably they fail again in short order due to hidden damage.  There is also the chance that the insulation within the main switching transformer has been compromised.

In Europe there is some 370VDC, mains derived at low impedance around the switching stage, this can readily cause molten copper to be produced,  also unless working in a safe environment using an isolation transformer there a severe Health and Safety issues.

There are safe ways of working on these supplies, if you have been trained to do so and are experienced. 

My best regards to you.

George   

[g7kbr67]

Hi all

I have been given a HP8920A and its works ok, but the other night it went bang, make a nasty smell.  After taking out the PSU I can see a very stressed cap.  Wonder if anyone can confirm its working voltage?  See attached photos

Regards

[tunk]

Most likely a Rifa, e.g.:
https://www.eevblog.com/forum/chat/old-rifa-capacitors-and-a-disaster-story/

[George Edmonds]

Hi

These capacitors are X and Y rated and MUST be replaced with the correct ones as there are safety issues involved

George

[g7kbr67]

Hi George

OK on the class type, I have ordered this from  rs https://uk.rs-online.com/web/p/paper-capacitors/1734955/

It's a X1

Regards

Paul

[py5lf]

This capacitor is a 47nF , 630V . I had the same problem and replace it with 2 100nF 400V in serial connection. 73

[Wallace Gasiewicz]

I hate to say this but, here is a you tube video that has the schematics for the 8920;



And here is the Groups.io link for the power supply schematics

https://groups.io/g/HP-Agilent-Keysight-equipment/message/99647

Guess the schematics were always in the neighbor group from 9-30-19