EEVblog Electronics Community Forum
Electronics => Repair => Topic started by: Bjoern on July 10, 2018, 06:16:05 am
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After I replaced two capacitors on the analog board I got the unit up and running. However the calibration routine seems to have issues with finding or using the calibration signal.
I made a YouTube video showing the issue and would very much appreciate your input:
https://www.youtube.com/watch?v=Wj0fqtFWRZo (https://www.youtube.com/watch?v=Wj0fqtFWRZo)
Many Thanks !!
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Can you specify which capacitors have you changed?
Maybe some loose connector? The calibrator output is -30dBm.
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Hi and many thanks for the prompt reply !
I do not have a schematic of the instrument therefore its a bit hard to say which of the caps I changed without opening it up again.
When I bought it the +15v were down and I found that on the analog board two of the tantalum caps were shorted. I assume they are filtering the main 15v supply of the board. When I changed them to electrolytic the SA powered up.
Do you know on which board the calibration signal is generated ?
Would I have to find the calibration signal that is accessible from the back or the one from the front panel ?
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Luckily I found the schematics within a large manual.
I remember the Caps to be C40 and C50 (see schematics) which seems to be on part of the board that might cause the error.
Maybe the OP-Amps did suffer ?
What do you think ?
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I think your general idea where the problem is (Yig Coil-Drivers) is correct. An unmodulated signal shouldn't look like it does on your screen. Which means either the coil-driver itself, or the control circuits for the driver are bad / horribly out of calibration. Have you tried adjusting any potentiometers or measured them? What do the control-voltages for the YIG look like?
Btw. How are the Supply-Voltages doing? Voltage ok? Excessive Ripple? If one Tantalum has gone bad, there's a chance others are causing problems as well.
Also with the Cal-Signal supposed to be -30dBm, it's quite a bit lower on your screen. With strong harmonics (don't know if they are supposed to be there). I'd suggest you build a simple diode-detector (if you don't have any equipment to measure the cal-signal) and measure the output-power. Doesn't have to be precise, just a rough estimate to see if the signal is in the right area.
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I am just a few days away from finishing my Signal Tracer project.
Its based on Carlson's Super Probe (Youtube) and I hope that it will help me find some faulty cap's.
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The level of the calibration signal is -20dBm; my apologies.
Your analyzer settings do not allow you to view the 300MHz calibration signal; the start frequency is 400MHz; there is also a spurious signal at about 410 MHz.
(http://[url=https://islandlabs.eu/8590.jpg]https://islandlabs.eu/8590.jpg[/url])
Can you post a photo at full span?
This is my analyzer's screenshoot
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Hi attached is my screenshot, does it give any clues of were to search ?
I hope that my BNC cable connection on the back panel (RF in/out) is acceptable ?
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Hi
Have you a signal generator for testing the frequecy response? Also the local oscillator feedtrough is very low, check the amplitude and stability of LO. First check all power suppy.
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I checked all the voltages which are OK.
I checked the cal output on the backside which gives a nice 10Mhz signal and is OK as well.
Attached are more screenshots of the calibration process, I hope that it gives further clues what might be at fault.
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I am thinking, that maybe not the calibration signal has an issue but the input of the signal analyzer.
I connected my Signal Generator and fed a simple 10 Mhz none modulated signal into the SA input.
Attached are the screenshots which show that the unit definitely has issues.
Probably these issues do not allow a proper calibration, or do you think that the misreadings are due to a failed calibration ?
Where could I start searching, are there modules I could skip (inside) in order to sort of trace my 10 Mhz signal ?
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Can you post a screenshot with start -5MHz e stop 45MHz and center 10MHz, span 100khz and RB 3khz? Also you have a lock off message
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I tried to match your request as good as possible but some settings change others.
I start by setting the center f to 10Mhz when I set the start -5 and end 45 then the center has changed.
The attached is a close match.
What is the RB 100khz setting I cant find it (what does RB stay for ?).
I noticed the Lock Message but I am not sure why it is there or how to get rid of it.
Hope the photo helps identify the error.
Highly appreciate your support !!!
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Sorry I noticed that the reference BNC cable on the back was disconnected.
I did the same test again with the signal connected at the back.
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The signal dont look bad.
Start -5 stop 45 is the same as center 20 span 50. RB is resolution bandwith under the button SWP/BW.
New setting:
Centre 20 span 50. AT 50 dB; take a shot.
Centre 10M, span 100khz; screenshot. Wait 60 minutes or more for a warmup
You can say tanks with the button in the right upper window
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OK I will do the other measurements after warmup,
can I ask why you think the signal doesnt look bad, I connected a 10 Mhz signal gen (sine wave, no modulation) to the input,
I would expect low (no) noise floor and a sharp peak at 10 Mhz instead I see all this mess ??!!
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The image of post #12 look good.
Try with a signal generator a 10MHz O dBm. SA setting AT 20 .
Seem as you have some problem with lock as there is a lot of modulation
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I took your recommended settings.
For the 10Mhz center where we do not see the noise floor the image is not stationary, the spikes are moving from right to left.
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Hi, I am not sure I can follow.
QUOTE:
Try with a signal generator a 10MHz O dBm. SA setting AT 20 .
Seem as you have some problem with lock as there is a lot of modulation
My Signal Generator doesn't allow amplitude settings in dBm (or do I set this at the SA?).
Which setting do you want me to set to 20 ? The Amplitude ?
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This is the screenshot
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I found this regarding the LOCK OFF msg:
φ
LOCK OFF appears briefly during
the CAL FREQ routine, during instrument preset, or when the
frequency value is changed; this is normal and does not indicate a
problem.
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But it seems to stay and to me is an indication some PLL has a problem. You should look in the schematic where they have a PLL.
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In the post #12 the level of the 10 MHz signal seems OK; maybe you also have problems with the attenuator or bandwidth filters and PLL of course
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I did a CONF TEST and the following attached errors were displayed,
would that point me to the right board to look for hardware errors ?
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When was the last time you saw the analyzer work or is it a recent purchase?
Have you checked that all the voltage are between the limits?
It seems to me that you are not very familiar with the analyser
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You are absolutely right, I never had a working SA. Not familiar with the operations but I got some good news today that a friend of mine got access to a 500Mhz scope and hopefully we will be able to trace the signal.
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A friend of mine lent me his 500Mhz scope so I could do some more testing today.
I measured the T- signal that goes into the YIG oscillator and I get a good 122khz signal with a span adjustment above 10Mhz.
Below or equal 10Mhz I get a jittery signal which looks faulty.
I disconnected the YIG from on the RF side to measure any output but there is nothing.
Now I am not sure, if the YIG is damaged, and in addition I have a problem below 10Mhz Span or if the analog board doesnt provide the correct signals to the YIG.
The voltages on the YIG are OK.
The T+ signal shows nothing really.
The heater works.
Does anybody understand what signals I should be able to measure in order for this module to work properly ??
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I have also a problem with my YIG. I measure the following on the connector:
FM- = -3.8mV
FM+ = -1.2mV the resistance between FM- and FM+ is 0.48 Ohm !
When I disconnect the connector FM+ measures +7.5 V (with an 139khz signal on top of the DC), this is with a span of >10Mhz. If the span is <=10Mhz the signal jumps hence the DC is unstable (coming from the analogue board)
Main Coil has 13 Ohm resistance and 13V as well as 14.95 V respectively
I cant get a signal on FM- with or without connection.
Do you see any indication that the YIG is definitely damaged (too low resistance on FM?)?
It seems I might have two issues, one on the YIG with FM and another with the <>10Mhz span fed from the analogue board ?
Would be great to get your insides. BTW my HP is the 8590L, I attach the relevant schematics.
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I did some more work on the Spectrum Analyzer and noticed that above a SPAN of 100khz I seem to have a good signal versus a SPAN of <100khz where it becomes distorted.
Could you point me to the circuit that could be potentially responsible for this error ?
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Hello Bjoern,
I am facing a similar issue with my SA and I kindly ask if you is you could share the whole schematics file or the shortcut for this instrument.
Despite my model is different , I think it will be a start reference....
Thank you very much!
Marcelo
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Hi I tried to send to you but each PDF is around 15Mbyte large and I cant attached it here.
Any suggestions ?
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Upload to Dropbox or Google Drive and send the Links by PM or post here would be a good idea, I think.
Were you able to fix it in the end btw. ?