On 31/10/18 21:48, tekscopegroup@... wrote:
On Tue, Oct 30, 2018 at 01:11 PM, Chuck Harris wrote:Yes. IIRC all the indicators are driven by a shift register; one changes and all stages of the shift register have to be loaded serially.For what it's worth, the power cycle counter, and on time counterHi Chuck, this certainly makes a lot of sense. Thanks for clarifying it, never heard this simple and logical explanation from anyone else, other that people being very happy when they see low numbers on those counters on a newly acquired scope. This one does not have any past cal stickers at all, so wonder if it ever was calibrated after being purchased.
Also, under certain conditions, specially when using the x10 horizontal magnifier and on the lower vertical v/div ranges (10mV and bellow), the traces seem somewhat blurry. But without changing the v/div range they clear up nicely and look very sharp when I engage the 20MHz BW limit, tough. Would this possibly mean HF power supply noise or power line ripple creeping into the circuits? (aging filter caps that need to be replaced?). I have not yet gotten to checking all the power rails at J119, will do so when the Mouser parts order arrives and the cover needs to come off again.Measure the ripple and replace the capacitors where beneficial. Replace them one at a time; in some manuals two caps are swapped compared to the actual boards. That is well documented elsewhere.
Don't forget that there is a lot more thermal noise in a 350MHz bandwidth than a 20MHz bandwidth - you may simply be seeing that thermal noise.
The other issue I've seen is that the automatic frequency measurement (press the measure button and then select 1 for frequency) is off usually by at least 10% or so, sometimes even more. Accuracy of the internal 10MHz reference oscillator not really being that accurate? If so wish there was a way to retrofit an external 10MHz ref input, which I understand is one of the standard options. I read somewhere that it is fairly easy to do it, but found no detailed information. As a comparison, my 2247A will always read the frequency very accurately, and its off by just 71Hz tested against a freshly calibrated rubidium oscillator.Highly unlikely the 10MHz is incorrect - but you can always measure it on your other scope :)
What's more likely is that the A5 voltages and/or calibration values are invalid. Before tweaking anything, make sure the reference and DAC output voltages are as specified in the service manual.