Re: 7834 readout problems


Paul
 

Hi Albert - thanks for joining in the fun! I did check those voltages, and they're both hovering at 15.04v.

The voltage at the base of Q3264 should be 3.1 volts.
got 3.12 - close enough?

The CR321x and R32xx parts are the switching network which disables various
readout elements based on which plug-ins are used and which plug-ins are
selected via the mode switches. The force readout signal from the plug-ins is
used to prevent the mode switches from disabling a specific readout so plug-ins
like counters and multimeters do not need to be selected to use the readout.

On other 7000 series oscilloscopes, you can disconnect the connector P2118 going
between the readout board and the mode switches and force readout signals to
force all of the readouts to be active.
Oh, OK, I see what you mean about the force readout - I found P43 on the main interface board which when disconnected does exactly that.

I considered suggesting this early on as it is likely to improve the situation
or at least reveal something but did not because I think it would fix the wrong
problem.
Yeah, that's what I suspected, as specially given that I'm seeing the correct character in one of the characters (the '<' for uncal). I'm going to try it anyway, though that may be Tomorrow Paul's problem.

I have never before studied the detailed differences between the earlier readout
boards and the later ones like in the 7834. This game is fun.
yeah man, thanks for playing! There was an even later version on the 7834 readout that I think that drew the characters raster style, like the character generator on the DF01. What an ingeniously odd circuit this is…

(*) This points to either Q3264 or Q3406 being the problem because Q3419 can
only increase the current.
Ahh, interesting. Q3264 is listed as 'Selected' so I was also suspicious that this might be the culprit. Unfortunately, I swapped Q3234 & Q3264 and no change. That leaves Q3406. Are they in the same package for thermal matching? I swapped it with the one from my 7603. no change.
Q3416 maybe?
Ooof - back at it tomorrow. Thanks everyone for following me down this rabbit hole.

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