Re: 7A26 attenuator compensation problem

Fabio Trevisan
 

On Mon, Jan 28, 2019 at 06:16 PM, Nenad Filipovic wrote:

Thank you all for your hints, but still no go.
Yes, 10mV and 20mV are pass-through...
Hello Nenad,
Arrived a little bit later to this, but I see there might be a confusion or misunderstanding that can ruin all the process down the road.
I do own a 7A26 myself and 10 and 20mV ARE NOT the ranges where all the high impedance attenuators are BYPASSED.
They're 5mV and 10mV / div, being that 10mV/div is the attenuation that shares the same preamplifier GAIN with all the other (higher) attenuation factors, while the 5mV/div setting stands alone on its own, as it employs no input attenuation (just as the 10mV does) and doubles the gain of the pre-amplifier.
If you're performing the preamplifier's LF and HF response adjustments at 10mV/div you're fine but, if you're performing them using the 20mV range, believing this range is a BYPASS range, then you will be compensating on the preamplifier's AC response adjustments, whatever deviation your X2 attenuator block might have, and this is not right.

So - risking to digress from Tektronix a little bit - basically what you would want is:
1. Go through the checks of the DC attenuation factors, to rule-out that there might be some bad contacts on the attenuator blocks and/or the contact leaves.
2. Using a low impedance source (e.g. going through a 50 Ohm past-through terminator), and using the 10mV range, perform all the preamp AC adjustments (LF and HF step response).
3. Changing over from the 50Ohm terminator to the Input normalizer, adjust the pre-amlipfier's input capacitance (C130 alone or C130 and C134, depending on the serial number of your 7A26).
4. Then perform each attenuator block's Frequency compensations and Input capacitances in the following order:

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