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Tektronix 492BP Spectrum Analyser Repair: Only fundamental 0-1.8GHz band works

adil malik
 

Hi Everyone,

I have got my hands on a 21 GHz Tek 492BP and was hoping it would be my first SA for university RF work :( . The instrument seemed to be in good nick, with original cal stickers from 2005 still sealed. I tested the instrument and apart from a dim CRT it seemed operational.

However, I noticed that the instrument only displays signals in the fundamental band of 0-1.8GHz. When I switch to other bands, the noise floor jumps up but I cannot see any input signals displayed, it is as if the input is disconnected. The good sign is that the fundamental band 0-1.8G looks fine with decent amplitude accuracy indicating the front end mixer is operational.

To start debugging, I inspected the 2nd LO used for the higher bands, and it correctly turns on in the appropriate bands despite not responding to any input signals. This tells me the 2nd LO circuit is operational.

Any ideas about how to proceed debugging?

Thanks

John Miles
 

To start debugging, I inspected the 2nd LO used for the higher bands, and
it
correctly turns on in the appropriate bands despite not responding to any
input
signals. This tells me the 2nd LO circuit is operational.

Any ideas about how to proceed debugging?
I'd start by checking the preselector. Hit the blue-shift PEAK MENU button
and put it in KNOB mode, apply a strong signal at +10 dBm or so, and see if
you can find it by adjusting the MANUAL PEAK control back and forth slowly.


If you can peak it manually, you can either go through the tracking
calibration steps in the manual, or just use it in manual-peaking mode.
Worst-case, you can bypass the preselector and use the analyzer that way,
but the preselector itself seldom fails.

-- john, KE5FX

adil malik
 

Hi John,

Thanks for your help.

Is there a section in the manual that explains what the peak/average knob does? Im not quite sure what I am testing with what you suggested above.

Thanks

Chuck Harris
 

The PEAK John was talking about is a knob that allows you to
tune the center frequency of the YIG Tuned Filter to peak at
the frequency of the YIG Tuned Oscillator. In other words, make
the two YIG's track each other more perfectly.

It is simply adjusted for maximum signal amplitude in the swept frequency
range of interest.

The PEAK/AVERAGE knob, if I recall correctly, divides the display
into an upper region, and a lower region. The signals displayed
above the dividing line are using a peak detector, and the signals
displayed below the dividing line are using an averaging detector.

It has nothing to do with what John was suggesting.

-Chuck Harris

adil malik wrote:

Hi John,

Thanks for your help.

Is there a section in the manual that explains what the peak/average knob does? Im not quite sure what I am testing with what you suggested above.

Thanks



Thomas S. Knutsen
 

What options does the analyzer have? there is a sticker on the back
outlining that.

BR. Thomas.

tor. 1. aug. 2019 kl. 18:48 skrev adil malik <adilmalik_n@...>:


Hi Everyone,

I have got my hands on a 21 GHz Tek 492BP and was hoping it would be my first SA for university RF work :( . The instrument seemed to be in good nick, with original cal stickers from 2005 still sealed. I tested the instrument and apart from a dim CRT it seemed operational.

However, I noticed that the instrument only displays signals in the fundamental band of 0-1.8GHz. When I switch to other bands, the noise floor jumps up but I cannot see any input signals displayed, it is as if the input is disconnected. The good sign is that the fundamental band 0-1.8G looks fine with decent amplitude accuracy indicating the front end mixer is operational.

To start debugging, I inspected the 2nd LO used for the higher bands, and it correctly turns on in the appropriate bands despite not responding to any input signals. This tells me the 2nd LO circuit is operational.

Any ideas about how to proceed debugging?

Thanks


--
With Best regards, Thomas S. Knutsen.

Please avoid sending me Word or PowerPoint attachments.

adil malik
 

Ok thanks for that. I will try what John suggests but the preselector or its relays feel like the fault to me.

adil malik
 

The sticker is blank unfortunately. But I have opened it up to have a look and the YIG preselector is present and seems to be hooked correctly.

Chuck Harris
 

But that is the preselector that you are tuning.

-Chuck Harris

adil malik wrote:

Ok thanks for that. I will try what John suggests but the preselector or its relays feel like the fault to me.



adil malik
 

Yes, what I meant was I think the path is dead completely.

adil malik
 

Thanks for your help.

I tried what you suggested, and it works! So does that mean the preselector path is OK and the fault is with the YIG preselector sweep/sync circuitry?

John Miles
 

There may not be anything really wrong with it, depending on whether the stored peaking constants are valid. Do a search in the manual for the term 'REPEAK', and it should give you an idea what to try.

If your internal lithium or silver-oxide battery is dead, the instrument may not be capable of retaining the peaking constants. So you'd need to either replace the battery or leave it in manual-peaking mode in that case. (No internal calibration data is saved in the battery-backed RAM, but the front-panel calibration and peaking information is.)

And yes, you can conclude that the preselector and its associated RF plumbing itself are most likely OK, if the knob works. It's possible that the knob is active by default at power-up time -- not 100% sure about that -- and if so, your stored constants may be OK. Someone may have just left the knob at an inappropriate setting for the frequency you were looking at.

-- john, KE5FX

Thanks for your help.

I tried what you suggested, and it works! So does that mean the preselector
path is OK and the fault is with the YIG preselector sweep/sync circuitry?