Topics

7B92A time-base plug-in odd behaviour

Colin Herbert
 

I have a 7B92A which is puzzling me. When turned on, it will only show the "ALT" time-base, which isn't altered by the time-base Time/Div knob, but after warming-up for probably only a minute or so, everything seems to work correctly. I have a feeling that this is associated with one or other of the switches on the "Pull for Alt / Push for Dly'd" switch. I am a bit puzzled about these two switches and the Service Manual isn't enlightening me too well. There is a micro-switch close to the front-panel, which I _think_ is S490 (the mechanical parts list refers to it as "SWITCH, SENS:(SEE S490 REPL)). This only operates when the "Time/Div or Dly Time" knob is pulled out with the two time-bases locked. It then stays in that position until the delaying and delayed time-bases are at the same Time/Div again and the knob pushed in. It seems to be connecting something to chassis ground when the knob is pushed in, but I can't locate it anywhere in the manual. The other switch that is operated by pulling/pushing the time-base knob looks to be a slider-switch on the Readout board, but it is on the inner face of this board and difficult to see without removing the board. I can't find out what it does, as I can't locate it in the manual.
Can anyone enlighten me as to the identity of these two switches and their function? Also, some idea as to what might be causing the odd behaviour before the time-base plug-in warms up?
TIA, Colin.

Albert Otten
 

Hi Colin,

S490 is shown at diagram <4> top-right (in my manual Rev. D, April 1976). Switching contact goes to GND, one choice is open, the other(LOCKED) connects it to P911 at/to the A6 Interface Board. From there also to R334 at <3>. Somewhat vague, as if R334 is in the wire , then to P936 and P204. I didn't look at the function (yet).

Albert

On Wed, May 22, 2019 at 02:10 PM, Colin Herbert wrote:


I have a 7B92A which is puzzling me. When turned on, it will only show the
"ALT" time-base, which isn't altered by the time-base Time/Div knob, but after
warming-up for probably only a minute or so, everything seems to work
correctly. I have a feeling that this is associated with one or other of the
switches on the "Pull for Alt / Push for Dly'd" switch. I am a bit puzzled
about these two switches and the Service Manual isn't enlightening me too
well. There is a micro-switch close to the front-panel, which I _think_ is
S490 (the mechanical parts list refers to it as "SWITCH, SENS:(SEE S490
REPL)). This only operates when the "Time/Div or Dly Time" knob is pulled out
with the two time-bases locked. It then stays in that position until the
delaying and delayed time-bases are at the same Time/Div again and the knob
pushed in. It seems to be connecting something to chassis ground when the knob
is pushed in, but I can't locate it anywhere in the manual. The other switch
that is operated by pulling/pushing the time-base knob looks to be a
slider-switch on the Readout board, but it is on the inner face of this board
and difficult to see without removing the board. I can't find out what it
does, as I can't locate it in the manual.
Can anyone enlighten me as to the identity of these two switches and their
function? Also, some idea as to what might be causing the odd behaviour before
the time-base plug-in warms up?
TIA, Colin.

Roger Evans
 

On my 7B92A, the microswitch near the front panel is connected by a single wire to P911 on the interface board. P911 is to be found on schematic <4>, 'Delaying Sweep', near the top right corner. It is marked as going to R334 on schematic <3> and the switch is marked as S490. S800 on 'Horizontal Logic', schematic <7> is marked with 'pull for alt, push for delay' and is surely the second switch on that shaft, it connects through to the Readout board.

I noticed while investigating my 7B92A that the whte plastic 'cotton reel' that activates the microswitch didn't seem to be an entirely tight fit on the shaft but settled down happily after a few iterations. I hadn't noticed any odd behaviour when it is operating in the mainframe.

Regards,

Roger

Colin Herbert
 

Thanks for the info, guys. I have now located where these switches go, but I am unclear as to what their functions are. Perhaps I don't really need to know, after all.

I am still perplexed by the odd behaviour of the plug-in in terms of the behaviour before warm-up. Once the scope has warmed-up, the time-base is still showing some instability which can be induced/stopped by wiggling the time-base "Time/Div" knob. I am now thinking that it might be caused by poor contact on one or more of the cam-switches. This looks like it will involve taking the cam-switch assembly out and doing the "paper-soaked-in-IPA" procedure. As there are about 50 of these, has anyone any idea which might be the dodgy one(s)?

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Roger Evans via Groups.Io
Sent: 22 May 2019 16:44
To: TekScopes@groups.io
Subject: Re: [TekScopes] 7B92A time-base plug-in odd behaviour
On my 7B92A, the microswitch near the front panel is connected by a single wire to P911 on the interface board. P911 is to be found on schematic <4>, 'Delaying Sweep', near the top right >corner. It is marked as going to R334 on schematic <3> and the switch is marked as S490. S800 on 'Horizontal Logic', schematic <7> is marked with 'pull for alt, push for delay' and is surely the >second switch on that shaft, it connects through to the Readout board.
I noticed while investigating my 7B92A that the whte plastic 'cotton reel' that activates the microswitch didn't seem to be an entirely tight fit on the shaft but settled down happily after a few >iterations. I hadn't noticed any odd behaviour when it is operating in the mainframe.
Regards,
Roger

Albert Otten
 

Hi Colin,

Since most of the problem disappears after warming up I guess it's more like a semiconductor issue.
Do you have the model with "Alt Off" at the ccw position of the TRACE SEP knob, S/N B06-up? There are several ICs involved in processing the Alt Off position, like U635<5> and U856<7>.

Albert

On Fri, May 24, 2019 at 03:24 PM, Colin Herbert wrote:


Thanks for the info, guys. I have now located where these switches go, but I
am unclear as to what their functions are. Perhaps I don't really need to
know, after all.

I am still perplexed by the odd behaviour of the plug-in in terms of the
behaviour before warm-up. Once the scope has warmed-up, the time-base is still
showing some instability which can be induced/stopped by wiggling the
time-base "Time/Div" knob. I am now thinking that it might be caused by poor
contact on one or more of the cam-switches. This looks like it will involve
taking the cam-switch assembly out and doing the "paper-soaked-in-IPA"
procedure. As there are about 50 of these, has anyone any idea which might be
the dodgy one(s)?

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Roger
Evans via Groups.Io
Sent: 22 May 2019 16:44
To: TekScopes@groups.io
Subject: Re: [TekScopes] 7B92A time-base plug-in odd behaviour
On my 7B92A, the microswitch near the front panel is connected by a single
wire to P911 on the interface board. P911 is to be found on schematic <4>,
'Delaying Sweep', near the top right >corner. It is marked as going to R334 on
schematic <3> and the switch is marked as S490. S800 on 'Horizontal Logic',
schematic <7> is marked with 'pull for alt, push for delay' and is surely the
second switch on that shaft, it connects through to the Readout board.
I noticed while investigating my 7B92A that the whte plastic 'cotton reel'
that activates the microswitch didn't seem to be an entirely tight fit on the
shaft but settled down happily after a few >iterations. I hadn't noticed any
odd behaviour when it is operating in the mainframe.

Regards,
Roger



Colin Herbert
 

Not totally the case, Albert. As I said, the instability can be present after warm-up and can be stopped or induced by "wiggling" the Time/Div knob. Specifically, if the delaying or delayed time-base is showing (i.e. the Time/Div knob is "IN") and the Time/Div knob is pushed in a little more firmly, the time-base stops and a vertical line or dot (depending on the Y input) at the left-hand side is shown.

Yes, It is the model with the "Alt Off" position on the Trace Sep knob; the S/N is B096566.

Colin.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Albert Otten
Sent: 24 May 2019 15:31
To: TekScopes@groups.io
Subject: Re: [TekScopes] 7B92A time-base plug-in odd behaviour

Hi Colin,

Since most of the problem disappears after warming up I guess it's more like a semiconductor issue.
Do you have the model with "Alt Off" at the ccw position of the TRACE SEP knob, S/N B06-up? There are several ICs involved in processing the Alt Off position, like U635<5> and U856<7>.

Albert

On Fri, May 24, 2019 at 03:24 PM, Colin Herbert wrote:


Thanks for the info, guys. I have now located where these switches go, but I
am unclear as to what their functions are. Perhaps I don't really need to
know, after all.

I am still perplexed by the odd behaviour of the plug-in in terms of the
behaviour before warm-up. Once the scope has warmed-up, the time-base is still
showing some instability which can be induced/stopped by wiggling the
time-base "Time/Div" knob. I am now thinking that it might be caused by poor
contact on one or more of the cam-switches. This looks like it will involve
taking the cam-switch assembly out and doing the "paper-soaked-in-IPA"
procedure. As there are about 50 of these, has anyone any idea which might be
the dodgy one(s)?

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Roger
Evans via Groups.Io
Sent: 22 May 2019 16:44
To: TekScopes@groups.io
Subject: Re: [TekScopes] 7B92A time-base plug-in odd behaviour
On my 7B92A, the microswitch near the front panel is connected by a single
wire to P911 on the interface board. P911 is to be found on schematic <4>,
'Delaying Sweep', near the top right >corner. It is marked as going to R334 on
schematic <3> and the switch is marked as S490. S800 on 'Horizontal Logic',
schematic <7> is marked with 'pull for alt, push for delay' and is surely the
second switch on that shaft, it connects through to the Readout board.
I noticed while investigating my 7B92A that the whte plastic 'cotton reel'
that activates the microswitch didn't seem to be an entirely tight fit on the
shaft but settled down happily after a few >iterations. I hadn't noticed any
odd behaviour when it is operating in the mainframe.

Regards,
Roger



Colin Herbert
 

Has no-one any ideas about this? Surely it can't be that unusual a problem?
Colin.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Colin Herbert via Groups.Io
Sent: 24 May 2019 16:49
To: TekScopes@groups.io
Subject: Re: [TekScopes] 7B92A time-base plug-in odd behaviour

Not totally the case, Albert. As I said, the instability can be present after warm-up and can be stopped or induced by "wiggling" the Time/Div knob. Specifically, if the delaying or delayed time-base is showing (i.e. the Time/Div knob is "IN") and the Time/Div knob is pushed in a little more firmly, the time-base stops and a vertical line or dot (depending on the Y input) at the left-hand side is shown.

Yes, It is the model with the "Alt Off" position on the Trace Sep knob; the S/N is B096566.

Colin.

Albert Otten
 

Hi Colin,

When the sweep stops I would expect a blanked screen. You still see the signal at the start position of the sweep. This could perhaps mean that the sweep is running but that the sweep output to the horizontal amplifier is interrupted.
Is the screen blanked when you don't trigger the sweep(s)?
Perhaps you should choose the lowest possible sweep rate. If you then wiggle the time/knob and the sweep reappears, does it start then at the left side or does it start "random" at some horizontal position, as if the sweep was running normally but with sweep output interrupted? Maybe you can also recognize a still normally running sweep from the trigger light blinking.
(Just an idea. I have not encountered this problem.)

Albert

On Thu, May 30, 2019 at 07:01 PM, Colin Herbert wrote:


Has no-one any ideas about this? Surely it can't be that unusual a problem?
Colin.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Colin
Herbert via Groups.Io
Sent: 24 May 2019 16:49
To: TekScopes@groups.io
Subject: Re: [TekScopes] 7B92A time-base plug-in odd behaviour

Not totally the case, Albert. As I said, the instability can be present after
warm-up and can be stopped or induced by "wiggling" the Time/Div knob.
Specifically, if the delaying or delayed time-base is showing (i.e. the
Time/Div knob is "IN") and the Time/Div knob is pushed in a little more
firmly, the time-base stops and a vertical line or dot (depending on the Y
input) at the left-hand side is shown.

Yes, It is the model with the "Alt Off" position on the Trace Sep knob; the
S/N is B096566.

Colin.


Roger Evans
 

Colin,
Can you not eliminate the mechanically operated switches eg S490 by either removing the single wire from its mating pin or solidly jumpering it to ground so that it is definitely open or definitely closed and should not change on warm up?

It looks like S800 should only affect the readout but you can check with the 'theory of operation' in the manual.

Roger

Colin Herbert
 

Hi,
In continuation of this, when the scope is turned on and the CRT has warmed up, all I see is a vertical line (or a spot) on the left of the screen, corresponding to the Y input. The "TRIG'D" light is illuminated, but there is no horizontal sweep. If I pull out the "TIME/DIV" knob to show the "ALT" time-base display, I get a sweep but this is not altered by the "TIME/DIV" knob, but _is_ altered by the transparent skirt. Again, the "TRIG'D" light is illuminated. After some warm-up period, a horizontal sweep with the "TIME/DIV" knobs locked can be seen, but it is unstable and doesn't always travel the entire width of the display. What _does_ sometimes have an effect on the horizontal sweep is the "VARIABLE" knob, but this also alters the width of the sweep; is this supposed to happen? If I use "NORM" triggering mode with no Y input, the sweep is blanked as you would expect.
I realise that there are jumpers on the boards to alter the "TIME/DIV" selector from delaying sweep to delayed sweep and for compatibility with different mainframes. As far as I can tell, these are set for variable delaying sweep and for 7400, 7600 and 7700 mainframes (mine is a 7623A). I am wondering if S800 has a problem (it is difficult to access without removing a few things, or if the switch associated with the "VARIABLE" knob is faulty. I have ascertained that switch S490 is _not_ causing any of these problems (by wiggling it when the time-base is connected using a rigid extender - the time-base doesn't seem to work with either of my flexible extenders and I'm sure that they're good) .
I am also thinking that the time-base cam switches might benefit from cleaning, but as I said before, there are around 50 of them and that sounds like a big job, unless someone can tell me where I might try first.
I have cleaned the PCB edge-contacts which connect to the mainframe with no improvement (although some blue stuff came off some of the contacts) and I have tried another time-base plug-in (my 7B53A) which works fine.
Head-scratching time....

TIA, Colin.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Albert Otten
Sent: 30 May 2019 19:34
To: TekScopes@groups.io
Subject: Re: [TekScopes] 7B92A time-base plug-in odd behaviour

Hi Colin,

When the sweep stops I would expect a blanked screen. You still see the signal at the start position of the sweep. This could perhaps mean that the sweep is running but that the sweep output to the horizontal amplifier is interrupted.
Is the screen blanked when you don't trigger the sweep(s)?
Perhaps you should choose the lowest possible sweep rate. If you then wiggle the time/knob and the sweep reappears, does it start then at the left side or does it start "random" at some horizontal position, as if the sweep was running normally but with sweep output interrupted? Maybe you can also recognize a still normally running sweep from the trigger light blinking.
(Just an idea. I have not encountered this problem.)

Albert



On Thu, May 30, 2019 at 07:01 PM, Colin Herbert wrote:


Has no-one any ideas about this? Surely it can't be that unusual a problem?
Colin.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Colin
Herbert via Groups.Io
Sent: 24 May 2019 16:49
To: TekScopes@groups.io
Subject: Re: [TekScopes] 7B92A time-base plug-in odd behaviour

Not totally the case, Albert. As I said, the instability can be present after
warm-up and can be stopped or induced by "wiggling" the Time/Div knob.
Specifically, if the delaying or delayed time-base is showing (i.e. the
Time/Div knob is "IN") and the Time/Div knob is pushed in a little more
firmly, the time-base stops and a vertical line or dot (depending on the Y
input) at the left-hand side is shown.

Yes, It is the model with the "Alt Off" position on the Trace Sep knob; the
S/N is B096566.

Colin.


Albert Otten
 

Hi Colin,

I hardly use my 7B92A and I had to play with it some time to re-familiarize myself with how it operates. That must be difficult when the 7B92A is faulty and new to you. Here are some "features" that might or might not help you.
- In Normal sweep mode (locked knobs) the displayed sweep actually is generated as delayed sweep, not as delaying sweep.
- The upper readout is for the delaying sweep, the lower readout is for the delayed sweep. Note how the readout jumps from lower to upper when from Normal sweep you pull the Time/Div knob.
- The delaying sweep goes up to 10 ns/div even when the knob is at a faster position. (Just try the previous remark at 1 ns/div).
- I have VAR set for Delayed sweep. So VAR also affets the sweep rate in Normal sweep mode. VAR can reduce sweep speed and at the same time reduces sweep length (like you seemed to notice).
- ALT mode is set by pulling the Time/Div knob *and* setting Trace Sep not fully ccw. Is that wheat you meant by ALT? Then 2 traces are displayed. The upper (delaying, with intensified zone) trace intensity is controlled by the 7B92A Intensity(!) and probably invisible when you set that Intensity fully ccw previously. The lower (delayed) trace listens to the mainframe Intensity.
- If you turn the transparent skirt in ALT mode then you change the delaying time/div. This will shift the delayed t race (unless Delay Time Multiplier is fully ccw). Is that what you observed?
- The trigger light is on during the delayed sweep (in Normal sweep mode) or during the delaying sweep mode (in all not-Normal modes). This can easily be recognized at a very slow time/div and even slower trigger signal.

Altogether I have the impression the your delaying sweep is working and displayed in non-Normal modes but that the delayed sweep is interrupted somewhere. I guess that the delayed sweep generator itself is working correctly since otherwise triggering would be blocked I think. It's difficult to follow in the schematics when triggering is blocked and given free again. The same holds for (un)blanking signals.

Albert

On Mon, Jun 3, 2019 at 06:42 PM, Colin Herbert wrote:


Hi,
In continuation of this, when the scope is turned on and the CRT has warmed
up, all I see is a vertical line (or a spot) on the left of the screen,
corresponding to the Y input. The "TRIG'D" light is illuminated, but there is
no horizontal sweep. If I pull out the "TIME/DIV" knob to show the "ALT"
time-base display, I get a sweep but this is not altered by the "TIME/DIV"
knob, but _is_ altered by the transparent skirt. Again, the "TRIG'D" light is
illuminated. After some warm-up period, a horizontal sweep with the "TIME/DIV"
knobs locked can be seen, but it is unstable and doesn't always travel the
entire width of the display. What _does_ sometimes have an effect on the
horizontal sweep is the "VARIABLE" knob, but this also alters the width of the
sweep; is this supposed to happen? If I use "NORM" triggering mode with no Y
input, the sweep is blanked as you would expect.
I realise that there are jumpers on the boards to alter the "TIME/DIV"
selector from delaying sweep to delayed sweep and for compatibility with
different mainframes. As far as I can tell, these are set for variable
delaying sweep and for 7400, 7600 and 7700 mainframes (mine is a 7623A). I am
wondering if S800 has a problem (it is difficult to access without removing a
few things, or if the switch associated with the "VARIABLE" knob is faulty. I
have ascertained that switch S490 is _not_ causing any of these problems (by
wiggling it when the time-base is connected using a rigid extender - the
time-base doesn't seem to work with either of my flexible extenders and I'm
sure that they're good) .
I am also thinking that the time-base cam switches might benefit from
cleaning, but as I said before, there are around 50 of them and that sounds
like a big job, unless someone can tell me where I might try first.
I have cleaned the PCB edge-contacts which connect to the mainframe with no
improvement (although some blue stuff came off some of the contacts) and I
have tried another time-base plug-in (my 7B53A) which works fine.
Head-scratching time....

TIA, Colin.