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 Post subject: Tektronix 496 half deaf
PostPosted: Dec Fri 08, 2017 1:22 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 396
Location: Green Valley, AZ, USA
Just got one at an area hamfest, was told it worked but didn't have any power or time to check. Was told it displayed 2 meter handhelds with a paper clip for an antenna. Very nice physical shape not beat up, good cry display.
Got it home and Cal out -20dBm 100mHz shows about 2.5 grid squares while on my signal generator set for 0.0 dBm it shows about 3.2 grid squares across its 1.8 GHz spectrum on 10dB/div and doesn't increase at 0.0dBm or +10dBm. Response flattens out 3.2 grid square. Wild guess is that the mixers might be OK since it reads the same up tp 1.8 GHz.and the fault is in the front end either attenuator or whatever it uses for an amp. If this is the case are parts or good replacement PCB assemblies available? The free PDFs are not easy to read, so I'll see if I can find an original paper manual.
I haven't opened it up yet, still trying to make sure I'm using it correctly
Any tips or suggestions appreciated.

Hank WD5JFR


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 Post subject: Re: Tektronix 496 half deaf
PostPosted: Dec Fri 08, 2017 7:40 am 
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Joined: Nov Mon 02, 2009 7:01 am
Posts: 2316
Location: Lincoln City, OR
Greetings to Hank and the Forum:

The problem you describe could be almost anywhere including the vertical amplifier. Tek gear is very finicky as to correct power supply voltages. This is the place to start. I agree that a paper manual, if you can find one, is the way to go. Once you have suitable information, start by measuring all the supply voltages. Also check for ripple.

After that, you will need a good scope to see if the vertical amplifier circuits are working, and probably another spectrum analyzer to see if the mixer and IF circuits are correct. The manual will give you an idea of the test equipment you will need. Hopefully, the problem will be in the vertical amplifier circuitry, which will be easier to troubleshoot and doesn't contain any exotic 1.8 GHz capable parts.

Regards,

_________________
Jim T.
KB6GM


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 Post subject: Re: Tektronix 496 half deaf
PostPosted: Dec Fri 08, 2017 8:18 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 396
Location: Green Valley, AZ, USA
Hey Jim
Thanks...good points. I've got a fairly good Anritsu MS 2670A in storage. As I recall it has a Warning 0VDC. I'll have to get it and make a DC block before I use it. I guess a 100 mmfd in series in an old scope probe should be OK. The 496 has no warning for DC on its input???
Hank


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 Post subject: Re: Tektronix 496 half deaf
PostPosted: Dec Sun 10, 2017 2:04 am 
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Joined: Nov Mon 02, 2009 7:01 am
Posts: 2316
Location: Lincoln City, OR
Greetings to Hank and the Forum:

You wrote:
Quote:
The 496 has no warning for DC on its input???


I'm sure it has a maximum value for DC; it apparently is not placcarded under the input connector. As a general rule, spectrum analyzers should NEVER have DC applied to their input terminals; a good flat response DC block is a very important part of your test lead collection, or should be. The scope probe trick works fairly well; bear in mind that it is not very flat in frequency response.... especially at VHF and above.

Regards,

_________________
Jim T.
KB6GM


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 Post subject: Re: Tektronix 496 half deaf
PostPosted: Dec Sun 10, 2017 6:08 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 396
Location: Green Valley, AZ, USA
Reading the 496 PDF manual OVDC input is indicated, interesting that that doesn't show as a warning at the input as it does on many other SAs.. I'm curious if an the DC voltage from digital Fluke VOM 289 would have any effect, I measured 1.8VDC and it sinks 1 milliamp into 0-5maA panel meter..

Hank wd5jfr


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