Persuading a 7S12 to play nice with a 7934.


Richard Steedman
 

I've encountered a rather annoying issue when trying to use my 7S12 in tandem with other vertical and horizonal plugins in a 7934 mainframe. The dot blanking signal which drives the mainframe Z axis is leaking out of the 7S12 during those times when it shouldn't be driving the display.

I've posted some pictures here to illustrate the problem. The 7S12 is in the middle two slots of the mainframe (obviously) and the other two slots are occupied by a 7A26 and a 7B92A. When the ALT horizontal mode is selected, everything is fine but when CHOP or LEFT horizontal mode is selected, the 7A26's trace intensity is modulated by the dot-blanking signal of the 7S12. The 7S12's Q694 is not shutting off the dot-blanking signal completely.

(In case you're wondering, the top trace is looking at the trigger out of a S-54 pulse generator in the 7S12. The trace is too dim and the pulse too narrow to be seen. The fact that the 7A26/7B92A are triggering off this pulse (which is generated by the 7S12 to align with its sweep) is the reason why the Z-axis modulation of the top trace is stationary.)

I've also posted some shots of the Z-axis signal in the ALT and LEFT horizontal mode. I've traced three signals: The top trace is the Sweep Out provided on the front panel of the 7S12. The middle trace is the Z-axis voltage. The bottom trace is the 7934's "Display Right" signal which drives pin B7 (MF chan switch) on the edge connectors of the vertical plug-ins. Photo Z-blanking_Alt.jpg shows the dot blanking being shut off nicely when it is the left slot's turn to drive the display (just before the 7S12's sweep flies back). Photo Z-blanking_Alt_Mag.jpg shows the same thing with the Z-axis signal magified to 20mV/div. The Z-axis is at 10mV when the left vertical plug-in is active. Photo Z-blanking_Left.jpg shows what happens when LEFT vertical mode is selected on the mainframe. The 7S12's dot blanking output leak through Q694, swinging from 10mV to 30mV.

I've taken a look at the voltage at the base of Q694 and it swings nicely between 0 and 5V in ALT mode but oscillates between 4V and 5V in LEFT mode. Why a steady low voltage (-0.4V) coming in on the B7 input to the 7S12 (together with 0V on A16) should be causing the 4V/5V at the base of Q694 is a mystery. I can make the issue go away by strengthening the pull-up resistor R693 (reducing it from 15kΩ to 6kΩ). See photo 7934_Horiz_Chop_fixed.jpg. I'm wondering, however, whether there is an issue with the circuit driving the B7 control (i.e. Q4382 and Q4382 on the mainframe's logic board)? I've yet to try probing around there.


Albert Otten
 

Hi Richard,
W.r.t. to your final question: Which manual are you using? In my paper manual (rev. March 1984) the labels Q690 and Q694 at <7> are wrong (interchanged). The internet pdf (Rev. March 1985) is correct and also shows P600 for interdot enable/disable. Albert


Albert Otten
 

... or it was wrong in the Rev. 1985 manual? There the schematic says Q694 is NPN while the parts list says PNP. It's long ago that I ran into this. Albert


Richard Steedman
 

The manual I am working off is this one which lives in the 'smb' folder on Kurt's w140.com website. You are right - Q690 and Q694 are reversed on its diagram 7 and CR692 is in the wrong place. So all references to Q694 in my inital post should read Q690. Sorry.

Agreed, the 1985 manual on the BAMA website looks correct but I don't appear to have a P600 on my unit (SN in the B14nnnn range).


Albert Otten
 

To make it more confusing at my A1 board (version -06) jumper P600 is connected differently.
Anyway I thought to remember the same problem. I tried a 7704A with 7S12 and 7A26/7B80. With P600 in "as is" position (must be enabled) the interdot blanking pulses are always effective. The other position works fine. The same happens wit a 7854. So the problem is not restricted to the 7934.
You probably don't observe the blanking pulses when you only select Left and B. But that's because the sweep rate and dot rate rates are unrelated. Now trigger the 7Bxx externally from the 7S12 vertical output or sweep output. At 20 us/div you will recognize the between samples periods and the blanked parts therein.

Albert


Richard Steedman
 

Yes, the intensity modulation of the 7A26 trace by the dot-blanking leakage is only noticeable when looking at a signal which is related in frequency to the 7S12 sweep. In my example, I'm looking at the trigger output of a S-54 pulse generator in the 7S12 and the trigger is generated by the 7S12. There is still an issue, however, when looking at signals asynchronous to the 7S12 in that the dot-blanking leakage dims the 7A26 trace significantly, especially noticeable when using delayed triggering.


Albert Otten
 

The solution with reduced value of R693 seems to have disadvantages also, otherwise Tek would have done so I think.
That might be the reason that P600 has been introduced to enable/disable interdot blanking. In the open position the PNP 2N2907A is shut off completely. In the closed position the situation is as before. (I avoid Q690/Q694 numbering.)
The next version of P600 is wired differently. When closed it it ties the base of the 2N2907A to GND. The connection of the base to R593 is deleted. In the closed position interdot blanking works, and works always, i.e. independent of what happens at A16 and B7. When P600 is open the situation is the same as with the previous P600 version. This version explains why I see the dot blanking in other traces so clearly. About 50% blank of between samples period is completely blanked.

Albert


Albert Otten
 

The connection of the base to R593 is deleted.
It's strange. With a DMM I could not find a connection of 2N2907A-base to any other component in the neighborhood or a A/B contact. But then the pair of NPN transistors with their bases to the B7 and A16 inputs would be useless. Do I overlook something?
Albert


On Thu, Mar 25, 2021 at 09:00 PM, Albert Otten wrote:


The solution with reduced value of R693 seems to have disadvantages also,
otherwise Tek would have done so I think.
That might be the reason that P600 has been introduced to enable/disable
interdot blanking. In the open position the PNP 2N2907A is shut off
completely. In the closed position the situation is as before. (I avoid
Q690/Q694 numbering.)
The next version of P600 is wired differently. When closed it it ties the
base of the 2N2907A to GND. The connection of the base to R593 is deleted. In
the closed position interdot blanking works, and works always, i.e.
independent of what happens at A16 and B7. When P600 is open the situation is
the same as with the previous P600 version. This version explains why I see
the dot blanking in other traces so clearly. About 50% blank of between
samples period is completely blanked.

Albert


Richard Steedman
 

Thanks for your insight on this Albert.

By coincidence, a paper copy of the 7S12 manual which I purchased on eBay a while back arrived today, dated June 1986. So now I have yet another reference to consult. It shows the same arrangement as the BAMA manual - P600 connects the base of the 2N2907A either to ground or to the collectors of the transistors which perform the (B7 XOR A16) function.

It's strange. With a DMM I could not find a connection of 2N2907A-base to any other component in the neighborhood or a A/B contact. But then the pair of NPN transistors with their bases to the B7 and A16 inputs would be useless. Do I overlook something?

You didn't accidentally have your P600 set such that the link between the NPNs and the 2N2907A-base was open?

As I said, my board has no P600. The collectors of the XOR transistors and the base of the 2N2907A are directly connected by a trace on the non-component side of the board. Where is P600 located on your board? I don't see it depicted on the board illustrations of any of the manuals I have looked at.

This circuit seems to have undergone many revisions over the years - perhaps reflecting issues encountered with newer generation mainframes. I share your unease about reducing the pullup resistor to 6kΩ. My other thought was to put the B7 input through an opto-isolator to get a proper 0V/5V swing and see if that cleans things up. This is easily done by lifting one end of R691 (the 430Ω resistor) off the board.

Richard


Albert Otten
 

P600 connects the base of the 2N2907A either to ground or to the collectors of
the transistors which perform the (B7 XOR A16) function.
Thanks a lot Richard! It really looked as if the third pcb pin was isolated from the rest and was only there to keep the jumper save in non-gnd position. So I simply left the jumper out when trying things! This also means that interdot blanking enabled/disabled is not the correct description for the P600 positions. It's always enabled (except when left out) and enables/disables B7/A16 function you mentioned.
Now I put P600 in the non-gnd position and tried things again. And yes, now I get interdot blanking leakage in the 7A26 trace. I added a photo to your album.
As I said, my board has no P600. The collectors of the XOR transistors and the base of the 2N2907A are directly connected by a trace on the non-component side of the board.
Where is P600 located on your board? I don't see it depicted on the board illustrations of any of the manuals I have looked at.
For this I also added a photo.

Albert


Richard Steedman
 

I've added a photo of the left interface board of my 7S12 so you can compare.


Albert Otten
 

On Thu, Mar 25, 2021 at 05:56 PM, Richard Steedman wrote:


Yes, the intensity modulation of the 7A26 trace by the dot-blanking leakage is
only noticeable when looking at a signal which is related in frequency to the
7S12 sweep. In my example, I'm looking at the trigger output of a S-54 pulse
generator in the 7S12 and the trigger is generated by the 7S12. There is
still an issue, however, when looking at signals asynchronous to the 7S12 in
that the dot-blanking leakage dims the 7A26 trace significantly, especially
noticeable when using delayed triggering.
Hi Richard,

Let me guess. You looked at CH1 of the 7A26. What if you use CH2?
Of course this suggestion looks weird. But after many waveform measurements I run into something I didn't know of before. The Q690/692/694 circuit Z-axis modulation causes interdot blanking. Even when only Left and B are selected. BUT then this modulation is only present each second sweep of the 7A26. Moreover, when the 7A26 is set to ALT it's always the CH1 sweep which is affected. I don't know yet what the logical background might be.
I measured about 0.5 mV more positive level of A17 during the active parts of interdot blanking. That doesn't look impressive, but the effect of 0.5 mV change in Z-axis is very well visible.
The ripple indeed vanishes when R693 is reduced, as you noted previously.

Not having 2 flexible extenders for the 7S12, I placed the 7A26 on a flexible extender. There is enough space above and below the flat cable to guide wires to measurements points in the 7S12. I could grab A8 (+5V) and A17 (Aux Z-axis) at the 7704A interface board.

Albert


Albert Otten
 

On Wed, Mar 31, 2021 at 04:07 PM, Albert Otten wrote:


I measured about 0.5 mV more positive level of A17 during the active parts of
interdot blanking. That doesn't look impressive, but the effect of 0.5 mV
change in Z-axis is very well visible.
Sorry, it was 5 mV, not 0.5 mV. Albert.


Richard Steedman
 

I see intensity modulation on both CH1 and CH2 of my 7A26 in its ALT and CHOP modes but in ALT mode, the CH2 has a brighter intensity offset. See this photo. This is consistent with the Z-axis signal in my photo Z-blanking_Left.jpg where, as you say, there are different amounts of leakage on alternate sweeps. I guess in your case, the leakage on one of the sweeps is small enough not to be noticeable. I'm pretty sure the reason for the different leakage levels on alternate sweeps is (in my case) noise from the mainframe on the B7 input. In my Z-blanking_Left.jpg picture, I traced that signal (bottom trace of the three) but at a scale of 1V/div and the noise isn't visible. Here's a magnified trace and you can see a shift of about 40mV on alternate sweeps.


Albert Otten
 

Tomorrow I will try to better understand your new photos. Today I spent a lot of time with 7A26/7S12/7B80 in a 7904A. The 7904A might come closer to your 7934 than the 7704A I used so far.
With the 7904A the behavior is quite different (more pronounced). In all 7A26 modes the trace or traces are completely blanked during the interdot blanking periods. No surprise: the B7 line goes more negative than in the 7704A. Ignoring noise I measured about -0.75 V for CH1 and --0.65 V for CH2. (BTW I don't trust your -0.4 V). That noise actually appears to be more or less square wave with frequency about 1 MHz, so likely coming from a CHOP signal.
Albert


Richard Steedman
 

I re-checked the voltage level of the B7 input and you are correct - It's not -0.4V at all, rather it's shifting between -0.66V and -0.7V. I must have mis-read the display when I first looked at it. I'm pretty sure it's shifting levels on alternate sweeps rather than from chop signal noise. The shift occurs every 250µs and the timebase in the 7934 is set to 20µs/div. The chop frequencies of a 7934 are roughly 1MHz for vertical and 200kHz for horizontal. I really appreciate the time you're spending to verify my observations. It's reassuring to find that this behavior is not unique to my mainframe.


Albert Otten
 

Both effects are present in my opinion, shift between alternate sweeps and chop noise. I added a picture "A17 noise" that might convince you. The noise frequency corresponds tp the chop rate (judged by eye, line A5 signal). With the 7704A there was a third "noise" source", the interdot blanking pulses. I don't understand why interdot blanking periods in my 7S12 last so long, some 10 us. The manual says 3 us but that might have changed with the new Q690 etc. circuit.
The alternate and chop choice might be due to bad noise immunity in the main frame "Display Right" circuits.
I'd like to postpone further comments (on your previous findings) and investigations until I can place the 7S12 on extenders. I still have 2 kits somewhere.

Albert

On Sat, Apr 3, 2021 at 02:50 PM, Richard Steedman wrote:


I re-checked the voltage level of the B7 input and you are correct - It's not
-0.4V at all, rather it's shifting between -0.66V and -0.7V. I must have
mis-read the display when I first looked at it. I'm pretty sure it's shifting
levels on alternate sweeps rather than from chop signal noise. The shift
occurs every 250µs and the timebase in the 7934 is set to 20µs/div. The
chop frequencies of a 7934 are roughly 1MHz for vertical and 200kHz for
horizontal.
I really appreciate the time you're spending to verify my observations. It's
reassuring to find that this behavior is not unique to my mainframe.


Richard Steedman
 

While you hunt for your extenders, I've been taking a bunch of observations and have written up my findings in a PDF posted here. I have just one of John Griessen's flexible extenders and put my vertical amp on that which let me probe the left interface board of the 7S12 in situ. I've also gone back through the various manuals and written up the documentation discrepancies as I see them. One thing that was confusing me previously is that the page on Karl's wiki describing the 7000 series plug-in interface had incorrect voltage levels for pin B7. The wiki stated that B7 and A16 were both 0V/+5V but that's not the case. Checking the actual Tek documentation revealed B7 to be spec'ed as -0.6V/+1.1V with a +/-0.2V tolerance. So the signal from my 7934 is well with spec - the 7S12 just can't deal with it. I've corrected the wiki. If you read my document you'll see that another issue with the NPN transistor is its slow switch-off time, due to lack of a Baker-clamp in the circuit. I might try substituting it with a 2N2369 which has gold doping and faster switch-off time and see if that improves things.


Richard Steedman
 

I tried replacing the 2N3904 with a 2N2369A and the result was only a marginal improvement. I have updated my notes here. I think I shall just go with reducing the pullup resistor to 6kΩ.


Jim Ford
 

Hi, Richard.

If you want a working 7S12, I have one for sale (it does have a crack in the Time/Distance crank knob though), along with a 7S11, two S-4 sampling heads, S-51, S-52, two S-53s, and a 012-0124-00 sampling head extender. I'm asking $200 for the whole kit, all working properly, but definitely negotiable.

I'm in Laguna Hills, California, roughly halfway between Los Angeles and San Diego.

Make me an offer!

Jim Ford

------ Original Message ------
From: "Richard Steedman" <richard.steedman@gmail.com>
To: TekScopes@groups.io
Sent: 5/25/2021 9:25:44 AM
Subject: Re: [TekScopes] Persuading a 7S12 to play nice with a 7934.

I tried replacing the 2N3904 with a 2N2369A and the result was only a marginal improvement. I have updated my notes [here](https://groups.io/g/TekScopes/files/7S12%20in%207934/7S12%20in%207934%20v2.pdf). I think I shall just go with reducing the pullup resistor to 6kΩ.