COM-120A/B ...Bug or broken ?

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jry
Posts: 481
Joined: Fri Feb 06, 2004 3:14 pm

COM-120A/B ...Bug or broken ?

Post by jry »

Was working on a COM-120A that has had the controller/processor upgraded to the B version with main firmware at 4.12 IIRC

When doing the DVM calibration the screen and procedure in the manual shows looking for 1 ,10 and 100VAC along with 2VDC.

During the cal it will fail everything but the 100VAC using these levels.
The DVM works fine but the calibration routine appears to be setting the 10X and 100X attenuators at the wrong points in the calibration process.

So for the 1 VAC calibration you actually need to input 10VAC . For the 2VDC you need 20VDC input and the 10VAC needs a 100VAC calibrated level.

If you go directly into the baseband card the 1VAC /2VDC level will work.

it's not the analyzer or front panel since both were swapped and as I mentioned the DVM itself works fine and the DVM attenuator switches at the correct ranges.
In theory the controller /uP and baseband modules appear to be functioning correctly with the only issue being during the calibration which is firmware.

The controller was upgraded to the B vintage and looks like it got a later digital module ( still has the 2nd var tone gen upgrade connector ).

Everything else in the unit appears to work after a full re-cap ( baseband, front panel and digital trays )

Ended up down this path as a result of a typical intermittent ( only when cold ) distortion meter selftest failure that persisted even after the re-cap. That actually ended up being a receiver issue. Looks like the selfttest loops the audio gen through the generator to the receiver and then to the distortion /sinad meter. It appears to measure triangle wave vs. sine wave distortion . The interesting thing here was that the distortion meter would always work fine even when cold directly from the audio gen.

So is this a known bug ( last COM120B/C firmware was 4.19 that I know of ) in the 4.12 code or something broken on the controller ?

Thanks
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