The RF signal goes through the 3 converters and then goes to the A11 filter module, then to A12 Amplitude control, A13 filter.

At this point we have the AUX IF signal available at the back of the instrument at 21.4MHz.

From this point, the signal can take two different paths: the one from A14 LOG-LIN amplifier/detector for the standard resBW from 5MHz to 1kHz, and finally the second path, option 130 which contains narrowband filters (300Hz,100Hz,30Hz...).

Option 130 has its own LOG-LIN amplifier/detector.

The output of A14 and option 130, is the AUX VIDEO signal available at the back of the instrument.

Depending on the selection of the resBW, a multiplexer will choose one or the other.

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So, the question arises, since the signal for option 130 follows the same path to the input of A14 (AUX, IF) and option 130 does not have a level problem, then logically the problem would be with A14 LOG amplifier/detector.

This would be true, if no other conditions change from the previous circuits when switching between option 130 and normal mode with A14.

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I checked all the data we took, and the data at TP5 gave us some similarities and some not.

But our probes did not have the same impact on TP5 (hot spot). I would say that TP5 may not be the best point of comparison.

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The probe effect:

Your probe

mkr level with probe on tp5 mkr level no probe

Log -14.2 -4.3

Lin -21.6 -20.2

You get 9.9 dB with or without probe for LOG mode, and 1.4 for LIN mode.

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In my case

mkr level with probe on tp5 mkr level no probe

Log -25.9 -20.0

Lin -23.1 -21.2

I get 5.9dB with or without probe for LOG mode, and 1.9dB for LIN mode.

The probe used TEK P6138 10MOhms 10pF.

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I studied your comments about the two SELFCAL videos.

On my Spectrum, the video display limiter was enabled, that's why the limiting level is different. But in the following image, I have disabled it.

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I take the example here of the 1MHz resBW, but the remarks apply to all filters between 5MHz and 1kHz. During the amplitude calibration, the filters are aligned to have the correct filter width.

We see that the signal is limited at the top of the screen, but this is not the real limit, by putting a marker we can find the limiting point. With this configuration for the 1MHz filter, in my case it is -6.57dBm. I used an external signal to find this limitation. When the 1MHz filter is aligned, the measured level is -11.63 dBm, so the filter is aligned 5dB before the compression point. The firmware searches for the peak, then the two points at -3dB. The filter is readjusted in width to have a value within the specifications, then switches to linear mode for the level adjusted to -20dBm.

With your 8591E, the level reaches -4.74dBm, it's high, but it seems that the filter is reaching an alignment.

Starting check 1.06MHz

tests#1 560kHz

tests#1 780kHz

tests#1 880kHz

tests#1 960kHz

tests#1 1.01MHz

goes to LIN mode for the level at -20dBm

So, the level at 21.4MHz would be stronger on your Spectrum, as well as the limiting point since you have -4.74dBm, without knowing where the real limiting level is.

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So, as you pointed out, is this an imbalance between LIN and LOG MODE. You start from -4.74dBm to -20dBm, or 15.3dB. But on my Spectrum, I go from -11.63 to -20dBm or 8.4dB.

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But there is no error message, and you are left with an error of 15dB after the SELFCAL, while in my case everything is correct with a level of -20dBm as expected.

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