On 17 Sep 2014 20:40:49 -0700, you wrote:
Hi Albert - thanks for joining in the fun! I did check those voltages, and they're both hovering at 15.04v.If it was very far off, then Q3264 could be the cause but a value of 3.12 isThe voltage at the base of Q3264 should be 3.1 volts.got 3.12 - close enough?
ambiguous. 0.1 milliamps if Q3264 was very bad would only cause a shift of 70
millivolts so I was hoping for something more.
The force readout signals allow the plug-ins to override the mode switches asThe CR321x and R32xx parts are the switching network which disables various
far as the readout is concerned.
If it makes for a quick fix though, then maybe it is the way to go. It is notI considered suggesting this early on as it is likely to improve the situationYeah, 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.
uncommon to trade off gain and zero errors in analog to digital converters to
minimize peak or RMS error which *is* what is needed here.
I know the late 7903, 7844, 7104, and 7904A used a microcontroller driven vectorI have never before studied the detailed differences between the earlier readoutyeah 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…
readout as shown in a change notice in the scan of the 7904A service manual that
I have. The late 7633 or 7632B or both used either the same board or
something else again. I do not have a late service manual for either of those.
The late 7934 had something that looks like a hybrid of the original readout
design and the late 7904A readout design. It does not have a microcontroller
but cleverly swapped an EPROM for the analog character ROMs.
My guess is that the readout board evolved to use up old readout parts that
Tektronix still had on hand after they stopped producing them.
The 7854 of course generates its own readout as part of a full raster display.
All of my mainframes have the original analog ROM readout design in one form or
another so I have not studied the later ones.
It might have been selected for high gain.(*) This points to either Q3264 or Q3406 being the problem because Q3419 canAhh, 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.Dual transistors like Q3406 are usually employed where Vbe matching is important
but that does not make all that much sense to me here since the voltage steps at
the base of Q3406A are about 500 millivolts. I assume they needed the extra
Q3416 maybe?It is worth checking since we are out of good options but I doubt it. I think
the signal into pin 9 is digital. If the voltage level is far enough off which
I believe you measured to be the case, then it might cause U3418 to behave
I do not think there is anything special about Q3416 so your favorite general
purpose PNP should work in its place. The line which drives pin 4 goes to some
other places through CR3416 which should be checked.
I would try adding a pullup resistor to pin 9 to raise the voltage a little and
see if that changes anything.
While difficult, what I would do is carefully measure the readout voltage levels
at pin 10 compared to the +15 volt reference. TP3445 off of the timeslot
counter can be used to trigger an oscilloscope and produce a synchronized
display. Careful measurement at that point should reveal if there is an offset.