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Problem with 70900A Spectrum Analyzer LO module.

Question asked by Adrian on Jul 2, 2007
Latest reply on Jul 23, 2007 by Adrian

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Hello,

I have a lovely 71100 spectrum analyzer that has little mileage on it. Unfortunately, the 70900A LO plugin has now died. Seems the YTO (5086-7747) is dead as there is no output signal. I would be very happy to get the required information to fix it. Could you please let me know how I can test it separately on the bench? I'd just need a connection diagram w/ voltages and the oscillator transistor model.

Thanks,
Adrian

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jborges3 Jul 3, 2007 4:12 PM
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Hello Adrian,

Welcome to the forum!

I was able to locate an old 70900A Service Guide which I have scanned and posted to the web for you. It will show up on the Agilent.com 70900A product page in the next 24 hours. Refer to the troubleshooting sections in chapter 5.

I have also posted the manual at the following temporary link for your convenance. http://sa.tm.agilent.com/mms/70900A-Service-Guide.pdf

Unfortunately, 70900A is out of support so Agilent can not supply replacement parts.

Best Regards,

Joseph Borges
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Adrian Jul 3, 2007 4:50 PM
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Hello Joseph,

many thanks for the service manual.
Maybe you could also locate the service manual vol. 2 and 3 that contain chapter 9 (component level)?

Meanwhile I've extended the YTO ribbon cable, so I can put the oscillator on the bench. As far as I've found out, the oscillator is running at 5 volts, and there is a ramping voltage on the tuning coil but no output. So, the problem seems to be inside the YTO and I'm guessing it might be the oscillator transistor. I would appreciate any info about the oscillator circuit, particularly the transistor model number.

Best regards,
Adrian
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jborges3 Jul 3, 2007 5:58 PM
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Hello Adrian,

The Component Level volumes do not contain the information you have requested. (and they are mostly full of large fold out pages that I wouldn't be able to scan)

Even back in the days when HP did a lot of component level support, micro-circuits always had an assembly level replacment strategy. Inside the YTO can is an open dye micro bondwire circuit full of custom HP/Agilent parts that are no longer available. Most YIG cans were sealed so they could never be opened, they are not servicable.

I took a look at Agilent's online part store and found that there are some of these parts in stock, you can see them by searching for 5086-7747 at www.parts.agilent.com

Best Regards,
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Adrian Jul 4, 2007 3:28 AM
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Hello Joseph,

that was the missing link.

Btw. I'm just wondering if there is any forum on the web that deals with troubleshooting of older instruments?

Thank you very much again.

Best Regards,
Adrian
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JohnMiles Jul 9, 2007 5:28 PM
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Hi, Adrian --

Try the hp_agilent_equipment group at http://groups.yahoo.com . This is a fairly-active group, and it can be downright indispensible for owners of used/surplus gear that is no longer directly supported.

Re: the 70900A YTO, none of the YIG oscillators that I have seen are specified to run on 5 volts. Are you sure it's not supposed to operate from 15 VDC?

There is at least one fellow (in the EU) on the Yahoo group who has his own wire-bonding machine at home. He has fixed various "unrepairable" mixers and YIG-tuned devices for other people. Apparently open bond wires are the #1 thing that goes wrong with YIGs.

Failing that, many YIG oscillators are actually fairly interchangeable. If you can get any output at all from yours, you may be able to determine its main-coil and FM-coil tuning sensitivities, and substitute a $50 oscillator from eBay that will be "close enough for government work." I have a feeling you can buy another 70900A plugin for what Agilent will ask for a replacement YTO.
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Adrian Jul 10, 2007 11:30 AM
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Hi John,

many thanks for your advice. I'll check over there at the yahoo group.
Frankly, I'd be happy to get it fixed w/o buying a new YTO. Just thought it might sound inappropriate to insist on more details.
I haven't yet fully figured out the oscillator circuit, so I might be wrong about the voltage. There are 3 external pins connected to the circuit though...

Thanks,
Adrian
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Adrian Jul 23, 2007 12:55 PM
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Hello,

in the meantime I bought another 70900A that however showed excessive noise sidebands, so I decided to swap the YTO into my other LO unit. I adjusted the YTO frequency endpoints by centering each pot within the YTO locking range, since I don't have the module verification program.

I couldn't find any informations about the required HPIB commands in the manuals, that would be very helpful to do the alignments properly. I think I'd rather pick the commands from a printout and enter them by VEE than trying to find an old series 200.

Is there any chance to get an ASCII copy of the 70900A (etc.) module verification program, or a scanned printout?

Regards,
Adrian
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Adrian Jul 23, 2007 12:56 PM
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Hello,

in the meantime I bought another 70900A that however showed excessive noise sidebands, so I decided to swap the YTO into my other LO unit. I adjusted the YTO frequency endpoints by centering each pot within the YTO locking range, since I don't have the module verification program.

I couldn't find any informations about the required HPIB commands in the manuals, that would be very helpful to do the alignments properly. I think I'd rather pick the commands from a printout and enter them by VEE than trying to find an old series 200.

Is there any chance to get an ASCII copy of the 70900A (etc.) module verification program, or a scanned printout?

Regards,
Adrian
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Problem with 70900A Spectrum Analyzer LO module.

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