Re: Tek 7623A - Readout omits 2nd zeroes while cold

Fabio Trevisan
 

Hi Roger,
My answers next to your posts / questions...

On Thu, Apr 26, 2018 at 04:10 am, Roger Evans wrote:
I have looked at the pins you mention on U2232, particularly at pin 12, C1
which is involved in adding zeros. The behaviour of all the pins is
consistent with a logic low of around 0.5V and a high level around 3V. The
high level has a lot of switching noise looking like analogue ramps with an
amplitude of close to 1V and the low level is clean but of very short duration
so that you may not see it depending on your sweep speed and trigger settings.
The narrow spikes carry the readout information and move and disappear when
you change eg from 500mV to 1V per division, the 'noise' on the high logic
level is largely unchanged.
Thank you for taking the time to look at it! What you describe is exactly what I see on mine.
By the way, the column in question here is actually C2 (pin 13 of U2232). C1 is involved only for the single zero and C2 for the double zero.
Now that you confirmed that yours look the same as mine I take it as a confirmation that's nothing wrong in this department (i.e. Column decoded signals are OK).
It's weird though, because the Row decoded signals, are beautiful signals with lows almost at 0V and highs of flat, noiseless 3V.
On the Column decoded lines however, it's not easy to distinguish which line as active at any time slot, because of the distracting garbage on the higher part of the waveform, and the low spikes don't quite fit the bill of discrete logic levels.
Even more difficult to understand is the narrow spikes, the only portion of the Column decoded signal that comes closer to 0V, are not actually generated by the column decoder!
They are artifacts coming from the load of those lines (all 5 character generators, the spikes reflect the current being drawn by the character generators while "plotting" )
So, if we disregard those spikes, the "low" actually being generated by the decoder IC are the two plateaus immediately before and after the spikes... and they're barely distinguishable (in level) from the surrounding "high" garble...
It's a miracle that U2232 can pick any "logic level" from those column decoded lines lines!
Anyway, it is what it is.

I just posted the photos from mine: https://groups.io/g/TekScopes/album?id=46025

Having had two more evenings to sleep on the problem, I think I can conclude the issue is really within U2232, and not that it's just more sensitive - while cold - to some marginal signal at its inputs...
The evidence to that is the First Zero that indeed shows up, irrespective of temperature.
The whole thing is confusing and we tend to think that the first zero is generated by an active level detected at C1, and the 2nd zero generated by an active level detected at C2 (during Time Slot 1).
But there is only one column active at TS1... either it's C1, for which only one zero is generated (later, at TS5), or it's C2, for which the double zero is generated, respectively at TS5 and TS6.
So, for the sake of assessing if the issue would be caused by C2 being at a marginal logic level which U2232 **could** be misinterpreting it as inactive (while cold)...
If that would be the case, the TWO zeroes would disappear, not just the second one...
It's the same C2 that's being sampled at TS1 and being memorized at that moment who is going to trigger the first **and** the second zeros to appear, each one at the correct time slot...

Have you tried removing U2232 (and maybe U2244 and U2185 the row and column
decoders) and cleaning pins? You may have the notorious TC IC sockets which
are very prone to corrosion problems.
Yep, not only removed them and scraped the pins to eliminate any oxidation (from all the 3 ICs), but I also swapped U2244 with U2185 to rule out the Column decoder could be generating poor signals.
Indeed, the dreaded Texas sockets are all around but, at least this time, they don't seem to be the cause, as I confirmed continuity from each pin.

So now, I`m left with 3 options:
1. I live with the problem because it really takes little time to get on its feet (maybe 5 minutes). The only down side is if the IC degrades further in the future and maybe I won't find a replacement then.
2. I put up some external circuitry together (maybe glue logic or uC), to emulate the broken part of the IC, or maybe the whole IC. It would be fun but I have other priorities.
3. I find myself another U2232 and grab it. If I would be in the US, I would have done it already... but being in Brazil, with the added complication of customs sticking their nose into every tiny package. Only thinking of it makes me sigh already.
One time my daughter bought a $8.50 wig (cos-play stuff) from China... It got stuck in customs... they charged me twice what the wig costed and I lost almost an afternoon to attend to the place and take it out...
Damn bureaucrats!

Many thanks again and Rgrds,

Fabio

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