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Tek 492 - Possible YIG failure?
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I am trying to fault find a 492 which will not display the calibrator signal @ 100MHz.
After checking further it seems that the 1st local oscillator output @ 2172 MHz (I set the SA to 100MHz and zero span for the test, which should give the 2171MHz LO) is not there when I test the output on the front panel. I have checked the signals to the YIG and all seem to be good. It seems also that the main coil current changes accordingly when the frequency is changed. Does it sound like the YIG is faulty? Is this a common problem people have found with the 492? I will try to measure the YIG output directly soon to eliminate the possibility of issues with the power divider that supplies the 1st mixer and the front panel connector. Also out of interest the serial number is 101080. This machine is very old and seems has many modifications that were done to the circuits after it was built (almost too many to be a production model!). I am wondering if it is one of the 1st 492 ever produced? Thanks, Chris |
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What are you using to test the 1st LO? The SA can't see it's own 1st LO, so you need to measure it with another instrument. Another SA or counter should do. Also, check for the 2nd LO frequencies. As I recall, the 2182 MHz is always present, while the 719 MHz is only added in certain bands. The dual frequencies may confuse a counter, so an SA is best for checking these.
Of course, you should first check the calibrator signal with a scope or counter to make sure it's actually there, if that's the only test signal being used. It also helps to know whether this unit was working and just crapped out, or you just got it, and don't know its condition. If so, maybe a previous repair attempt left something missing or wrong. Ed |
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Hi Ed,
Thanks for your comments. I tested the 492 with another spectrum analyser and yes the calibrator signal was present. Further testing directly at the YIG yielded no signal output that I could measure. I then removed the YIG for inspection and noted there was one blown resistor (this seemed to be part of a filter for the 15V1 supply to the YIG) on the YIG interface board, I replaced the resistor and found that this now overloaded the power supply. I then disconnected the connector to the YIG supplying the 15V1 and found that the power supply was back working again. So now I am fairly sure that the YIG is no good. The machine was supposedly working but who knows how it was tested. I don't think it has been messed about with much. There was a ding it the cabinet which meant it was almost impossible to take the cover off. I thought that the YIG would normally be connected to an external circuit to produce the oscillator and buffer amplifier but it seems that in this case the YIG contains the oscillator and the buffer amplifier. Is this correct? Any comments about the serial number 101080. I am more and more convinced that this unit is NOT a production unit. My feeling is that this machine may have been a "engineering prototype". Thanks, Chris |
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Hi Chris,
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Can you post a pic or two of the yig oscillator? Is there a PC board with power filtering mounted on the YIG body? If son I would check for a shorted tantalum capacitor just sfter the smoked resistor. Regards ----- Original Message -----
From: cmbath@... [TekScopes] To: TekScopes@... Sent: Monday, July 27, 2015 7:21 PM Subject: [TekScopes] Re: Tek 492 - Possible YIG failure? Hi Ed, Thanks for your comments. I tested the 492 with another spectrum analyser and yes the calibrator signal was present. Further testing directly at the YIG yielded no signal output that I could measure. I then removed the YIG for inspection and noted there was one blown resistor (this seemed to be part of a filter for the 15V1 supply to the YIG) on the YIG interface board, I replaced the resistor and found that this now overloaded the power supply. I then disconnected the connector to the YIG supplying the 15V1 and found that the power supply was back working again. So now I am fairly sure that the YIG is no good. The machine was supposedly working but who knows how it was tested. I don't think it has been messed about with much. There was a ding it the cabinet which meant it was almost impossible to take the cover off. I thought that the YIG would normally be connected to an external circuit to produce the oscillator and buffer amplifier but it seems that in this case the YIG contains the oscillator and the buffer amplifier. Is this correct? Any comments about the serial number 101080. I am more and more convinced that this unit is NOT a production unit. My feeling is that this machine may have been a "engineering prototype". Thanks, Chris |
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YTOs often have clamp Zener diodes mounted right at the power pins for OV protection. It's possible the Zener is shorted rather than the YTO - that's a much better outcome. It may not be shown in the schematic since the YTO is treated as a module. If the Zener is bad, replace it with a 1W unit rated just above the normal supply voltage. Also, as someone mentioned, if there are any Ta bypass caps on the supplies, they are suspect too.
Remember that the YTO must not only have proper supplies, but the main coil current has to be in the normal range too, in order to get any output. As I recall, there's also a main coil filter cap (510 uF or so, wet Ta type cap) that's switched in by a relay when in CW or narrow band sweeps. A failure in the coil driver circuit or this filter could cause main coil current to be wrong. If you confirm that the supplies are right, and you're actually getting the right coil current, then the YTO is likely bad if there's no output. Ed |
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On Mon, Jul 27, 2015 at 5:21 PM, <cmbath@...> wrote:
Any comments about the serial number 101080. I am more and more convinced Serial prefix 1 indicates a Guernsey assembled product. They didn't build prototypes there. |
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Hey ..... YIGs almost never die !!!
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Herbert Am 29.07.2015 00:02, schrieb cmbath@... [TekScopes]:
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