7L12 spur at 210MHZ - Request for test results or suggestions?


Cliff Carrie
 

For 7L12ers: Can you run a quick test, please?

I have been restoring a 7L12 for a while now. First was the Weinschel input attenuator, toasted by excessive input. A long rebuild using parts cannibalized from a similar but physically incompatible Weinschel unit. Next was a loss of 20dB through the 30KHz bandpass filter which responded to vibration around the prefilters (possibly cracked solderwork - gone for now),

The unit now passes all performance tests except one, the test for spurs (Item 7, Page 3-4 of the manual). There is only one spur above the -100dB noise floor, -80 to -85dB at 210MHz (2nd harmonic of the105MHz IF). It does not respond to changes in the input attenuator setting but does respond accurately to changes in the Gain Selector or Variable Gain controls. Switching in the 300Hz video filter drops the noise floor to about -106db and suppresses the spur to about 2db above this (-104db). The other video filters and the video processor have no effect. It's not an image (tunes in the correct direction), and it seems not to be a harmonic (tunes at the normal rate). Note that the dB figures refer to the 1st mixer input and are 30db lower than the displayed Reference Level (per the Tek manual).

Could anyone with a 7L12 and ideally in a 7854 run this test from Page 3-4 and look for a spur at 210MHz (or at other frequencies as well) and post the results? My 7L12 is stock, SN B192xxx, with the LED Reference Level readout and LO output jacks.

The setup is: No input cables, RF attenuator at 30db, gain selector fully CCW, and variable gain in CAL detent. The Reference Level should show 0dbm on screen and in the LEDs. Set the Span/Div to 1MHz, the Resolution Bandwidth to 30KHz, and the Time/Div to Spectrum and Cal. In 2db/Div, set the baseline to the bottom graticule line. Switch to 10db/Div and tune to around 210MHz, looking for a spur. Also, do a manual tuning scan from 0 to 1800MHz looking for other spurs.

Results and comments gratefully received.

Cliff


Cliff Carrie
 

Has anyone seen the 7L12 spur at 210MHz as described above? I'm about to launch into a full recal of my unit to see if I can eliminate it. Before doing that, I would love to know if this is a common occurrence. If it is, I may just live with it rather than go through a long recal with the risk of no improvement. Tek says no spurs should be seen above -100db at any frequency setting and this spur reaches -80 on my unit. It's the only spur it has.

Regards, Cliff Carrie
________________________________
From: TekScopes@groups.io <TekScopes@groups.io> on behalf of Cliff Carrie <cliffcarrie@hotmail.com>
Sent: Sunday, April 4, 2021 10:58 PM
To: TekScopes@groups.io <TekScopes@groups.io>
Subject: [TekScopes] 7L12 spur at 210MHZ - Request for test results or suggestions?

For 7L12ers: Can you run a quick test, please?

I have been restoring a 7L12 for a while now. First was the Weinschel input attenuator, toasted by excessive input. A long rebuild using parts cannibalized from a similar but physically incompatible Weinschel unit. Next was a loss of 20dB through the 30KHz bandpass filter which responded to vibration around the prefilters (possibly cracked solderwork - gone for now),

The unit now passes all performance tests except one, the test for spurs (Item 7, Page 3-4 of the manual). There is only one spur above the -100dB noise floor, -80 to -85dB at 210MHz (2nd harmonic of the105MHz IF). It does not respond to changes in the input attenuator setting but does respond accurately to changes in the Gain Selector or Variable Gain controls. Switching in the 300Hz video filter drops the noise floor to about -106db and suppresses the spur to about 2db above this (-104db). The other video filters and the video processor have no effect. It's not an image (tunes in the correct direction), and it seems not to be a harmonic (tunes at the normal rate). Note that the dB figures refer to the 1st mixer input and are 30db lower than the displayed Reference Level (per the Tek manual).

Could anyone with a 7L12 and ideally in a 7854 run this test from Page 3-4 and look for a spur at 210MHz (or at other frequencies as well) and post the results? My 7L12 is stock, SN B192xxx, with the LED Reference Level readout and LO output jacks.

The setup is: No input cables, RF attenuator at 30db, gain selector fully CCW, and variable gain in CAL detent. The Reference Level should show 0dbm on screen and in the LEDs. Set the Span/Div to 1MHz, the Resolution Bandwidth to 30KHz, and the Time/Div to Spectrum and Cal. In 2db/Div, set the baseline to the bottom graticule line. Switch to 10db/Div and tune to around 210MHz, looking for a spur. Also, do a manual tuning scan from 0 to 1800MHz looking for other spurs.

Results and comments gratefully received.

Cliff


Ed Breya
 

Don't know if this helps, but 210 MHz is twice 105 MHz, which is as I recall one of the LOs or IFs in the frequency plan. If it's one of the LOs, there could be leakage somewhere getting into the front end. It may be as simple as a loose connector or ground or shielding - or not.

Ed


Jean-Paul
 

Cher Cliff I have a fine 7L12, used in 7904 or 7603 mainframe and never noticed any spurs at all.

I do not have a 7584 mainframe and do not seen how the mainframe can affect my spur.

Please uploading a screen capture of the spur, diagram of setup, and photos of the front panel set-up, so easier to check this on my unit.

Bon journée, Best Regards


Jon


Jean-Paul
 

Cliff I just setup the 7L12 and used the HP 8640B RF gen setup at 201 MHz and levels from -90 dBm to -30 dBm. Past -100 dBm the noise floor was obscuring.

The signal was clearly visible and no spurs were seen at 210 MHz or elsewhere.

The SN of my 7L12 is B2139xx and it has the LED ref lev readout.

Hope this is useful.....

Jon

PS: I can only marvel at the electronics, packaging and mechanical design of the classic SA plugins.


Jean-Paul
 

correction at 210 MHz I can see -100 dBm, above noise, no spurs at all anywhere 0-1500 MHz

Jon