Re: Tek 2467B Erratic Intensity controls


Chuck Harris <cfharris@...>
 

The processor has no knowledge of how fast the DAC is working.
It simply sends the DAC a value, and assumes it does its job.

I can think of a scenario that could be in play. All of the
controls are digitized, processed, made analog, and have their
analog value stored in a sample-and-hold gate.

Because the sample-and-hold gate is a capacitor that is holding
a charge, the CPU has to refresh that charge at regular intervals,
or it will leak down.

Suppose the MUX that charges the sample-and-hold capacitor is
broken so that it is high resistance, instead of its usual very
low resistance.

The CPU will set the DAC to the appropriate voltage, and the
broken MUX will charge it a little, then at each refresh interval,
the CPU will set the DAC to the same value, and the broken MUX
will charge it a little more... each refresh putting a little
more charge into the cap until it eventually reaches full charge.

You will see this as the control moving very slowly.

-Chuck Harris

peter bunge wrote:

Yes they are 'sticky' especially if I move ch2 then ch1. Also the digital
display of the DC trigger point is sluggish and often does not change with
the +/- level knob. It acts like a delay to the ADC which is not a
component but a complex group.
It could be a sluggish DAC U2101 sending to the sequencer via U2530 and U
2630 but I can't understand how (Analog Control tab 2). I am about to
'scope them. It is interesting that there is signs of work done around the
DAC, perhaps a broken trace but it is difficult to tell hence 'scoping to
look for a suspicious signal to follow. It could be an address line
floating but I would expect more problems. They look clean. pins 19 & 18
look messy but they are current O/P so look at U2630 pins 3 and 12. The
voltage here does not follow the intensity pots but is delayed. It jumps
suddenly but is clean. Moving on......
Peter.

On Sun, Dec 1, 2019 at 3:35 PM Bob Koller via Groups.Io <testtech=
yahoo.com@groups.io> wrote:

Join TekScopes@groups.io to automatically receive all group messages.