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7854 - No RO in SCOPE + WF modulates RO in STORED

Ram
 

Hi folks,
I was working on a 7T11 --> 7T11A mod for my recently acquired 7854. Got some intelligible traces @ 100kHz (albeit dotty) on the 7854 in SCOPE mode. However after trying these and a 7A24/7b92 combo I seem to have a problem with the horizontal switching circuits as apparent from this:
(a) There is no readout in SCOPE mode at all though waveforms show up fairly clean
(b) If I press the STORE button soon after startup (startup tests pass) I see nice readouts though the expected horizontal default trace appears after some silence and some glitches
(c) If in STORE mode I choose a vert channel which has a plug-in, the vertical waveform seems to "modulate" the readout and garbles it up - the modulation is indeed proportional to the signal level.
(d) If in STORE mode I choose a horiz channel which has a plugin a similar thing happens - the readout gets "modulated" horizontally and a series of dancing horizontal lines appears where the readout was

I seem to think this is a problem with whatever circuitry does the horizontal channel selection (like the U800 in the 24xx). Looking through the 7854 schematics I am wondering if this means there is a problem with the signal RO-WFMX at J12, diagram 10, which appears from U1720 (LF13333). I see older threads here which refer this chip going bad.

Any other candidates to suspect based on the observations above? Further, how is it that the default stored readout appears clean? Does it not pass through LF13333?
Any inputs or hypotheses gratefully accepted.
Thanks! - Ram
David C. Partridge
 

This has been done before (by me) <http://perdrix.co.uk/7T11Conversion/index.htm>

David

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Ram
Sent: 30 November 2020 08:37
To: TekScopes@groups.io
Subject: [TekScopes] 7854 - No RO in SCOPE + WF modulates RO in STORED

Hi folks,
I was working on a 7T11 --> 7T11A mod for my recently acquired 7854. Got some intelligible traces @ 100kHz (albeit dotty) on the 7854 in SCOPE mode. However after trying these and a 7A24/7b92 combo I seem to have a problem with the horizontal switching circuits as apparent from this:
(a) There is no readout in SCOPE mode at all though waveforms show up fairly clean
(b) If I press the STORE button soon after startup (startup tests pass) I see nice readouts though the expected horizontal default trace appears after some silence and some glitches
(c) If in STORE mode I choose a vert channel which has a plug-in, the vertical waveform seems to "modulate" the readout and garbles it up - the modulation is indeed proportional to the signal level.
(d) If in STORE mode I choose a horiz channel which has a plugin a similar thing happens - the readout gets "modulated" horizontally and a series of dancing horizontal lines appears where the readout was

I seem to think this is a problem with whatever circuitry does the horizontal channel selection (like the U800 in the 24xx). Looking through the 7854 schematics I am wondering if this means there is a problem with the signal RO-WFMX at J12, diagram 10, which appears from U1720 (LF13333). I see older threads here which refer this chip going bad.

Any other candidates to suspect based on the observations above? Further, how is it that the default stored readout appears clean? Does it not pass through LF13333?
Any inputs or hypotheses gratefully accepted.
Thanks! - Ram
Ram
 

Yes, thank you David. I did use your excellent instructions to perform the board-level surgery though for the blanking board itself I used a general purpose PCB. However my post is not about the 7T11 conversion, it's an unrelated problem.
Albert Otten
 

Hi Ram,

Just to be sure that it's not related to the 7B92/7854 combination I tried this. Is fine. (There is another issue mentioned in the operators guide.)
At 100 kHz I also get several more or less random dots at the curve in stead of a solid curve. The curve gets filled up in the stored waveform. IIRC it's because of the sampling method used at the lower 7T11 ranges.
You probably found already that you can only stop AQR using the STOP key and then clear the error by pressing another valid key.

Albert
Ram
 

Let me clarify a couple of points that were probably vague in the original post. The observed problem with the readouts appeared with the 7b92/7a24 plugins and with the 7s11/7t11mod used *separately* (i.e., only 2 plugins at a time). I did this just to be sure the readout issue is not because of the plugin. I also tried one plugin at a time to see what happens to the readout and I can see, e.g., that with the 7b92a for example, when I merely select the slot containing the plugin, the readout gets "modulated" by the plugin waveform. The same thing happens no matter which of the 4 plugins I use. So it's not to do with the plugin. The fact is that the waveform is finding its way into the readout signal which seems to indicate that the horizontal channel switching is where the issue resides. Does the problem and my reasoning sound clearer now?
Best - Ram
Ram
 

...and sorry for another post but I forgot to mention that the AQR function does not work at this point - it always results in an error. I indeed wanted to use AQR to get nice results fro my modded 7T11 :-(. I believe this is a consequence of the readout switching being faulty and there being no readout in SCOPE mode.
Best - Ram
Albert Otten
 

Hi Ram,

In general the readout position influenced by the signal and sweep in 7000 series scopes is well known and due to bad low frequency response (in turn perhaps thermal effects) of the vertical and/or horizontal amplifier chain. Search "wobbly readout" in old messages. I would first exclude that part of the present problem is related to this.
You might try (with only ordinary plugins) a sharp-edged square wave with very low repetition rate. Observe how fast the trace stabilizes at the new vertical level after a jump. The effect (also in readout) will increase with amplitude. Horizontal sweep position effect on Readout can best be seen at very slow sweep speed; you don't need a vertical signal for that.

Albert

On Mon, Nov 30, 2020 at 04:26 PM, Ram wrote:


Let me clarify a couple of points that were probably vague in the original
post. The observed problem with the readouts appeared with the 7b92/7a24
plugins and with the 7s11/7t11mod used *separately* (i.e., only 2 plugins at a
time). I did this just to be sure the readout issue is not because of the
plugin. I also tried one plugin at a time to see what happens to the readout
and I can see, e.g., that with the 7b92a for example, when I merely select the
slot containing the plugin, the readout gets "modulated" by the plugin
waveform. The same thing happens no matter which of the 4 plugins I use. So
it's not to do with the plugin. The fact is that the waveform is finding its
way into the readout signal which seems to indicate that the horizontal
channel switching is where the issue resides. Does the problem and my
reasoning sound clearer now?
Best - Ram
Ram
 

Thanks Albert. I'm wondering if the fact that there is absolutely no readout in SCOPE mode (rather than a wobbly readout) would direct me to a different starting point.
Best - Ram
Albert Otten
 

On Tue, Dec 1, 2020 at 01:54 AM, Ram wrote:

Thanks Albert. I'm wondering if the fact that there is absolutely no readout
in SCOPE mode (rather than a wobbly readout) would direct me to a different
starting point.
Best - Ram
No, in hindsight (after reading your messages once more) I see that your 'scope very likely has another problem.
After some searching in the manual I found U1720 at diagram 25, board A29. It would have been faster to search for U1720 in the old messages. There I saw it's 9 years ago that I looked at that schematic! At this moment I'd rather not repeat that because it's distracting me too much from other Tektronix "jobs" I'm doing.
Success,
Albert
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