Timebase Question : Auto vs Norm Mode

#188722

Hi Ian,

I think this is the page you're looking for:

Thanks,
Barry - N4BUQ

Show quoted text

----- Original Message -----
From: "tinkera123" <iacbell@...>
To: "tekscopes" <[email protected]>
Sent: Monday, December 13, 2021 11:12:30 PM
Subject: Re: [TekScopes] Timebase Question : Auto vs Norm ModeHi all.
I am trying to follow along this discussion, but not sure if I have the correct
Block diagram and Schematic for U520.
Is it this one on Tek Wiki ....  155-0109-01 ...
    ???
If not, web address would be appreciated.

--
Cheers,
Ian,
Melbourne, Australia

#188726

Expanding on this a bit, when my 7B53N is in AUTO, when a sweep ends, the Sweep Stop Comparator causes the Main Sweep Holdoff to send a pulse to the Main Sweep Trigger (Q454 and Q466).  When this occurs, I see the waveform at TP475 that's shown in the 7B52 manual for that same TP.

If I switch to NORM, the triggering waveform is present at the emitter of Q434 but that stops at the other side of both C451 and C461 and I'm wondering if the TDs are holding that side of both diodes low.  Note that HOLDOFF is also held LOW at this point (presumably due to no sweep), which could contribute to there being no triggering waveform at TP475.

Since the HOLDOFF waveform is present at both Q454 and Q466 when in AUTO, then I presume that's responsible for producing the waveform I'm seeing at TP475 but I'm not understanding that if the time-base is in NORM, why the waveform at the emitter of Q434 doesn't generate that waveform at TP475 but, again, I suspect the TDs.

Using my Fluke 27 DMM in Diode Check mode, CR470 and CR475 appear to check shorted but I don't know if that's expected with a DMM check.  BTW, I checked the Fluke 27's current demand in Diode Check mode against my Fluke 25 and it shows only about 350uA which I presume should not harm the TD when forward biased; however, would that be enough to harm it in reverse bias?  I ask because the Delayed Trigger Generator has the same two TDs and, before I check those with the DMM (just to see if they exhibit the same readings), I would like to make sure I'm not going to harm them in the process.

Thanks,
Barry - N4BUQ

Show quoted text

----- Original Message -----
From: "Barry" <n4buq@...>
To: "tekscopes" <[email protected]>
Sent: Saturday, December 11, 2021 8:00:05 PM
Subject: Timebase Question : Auto vs Norm ModeI'm working on an 7B53N but I think this question applies to many/most scopes
and/or time-bases.

In AUTO mode, the time-base is in a "free-run" mode which generates a ramp at
the selected sweep rate and, at the end of the ramp, generates another ramp.
This continues whether a trigger is present or not.

In NORM, the time-base waits for a triggering signal before executing a sweep
and, at the end of the sweep, waits for another trigger signal before
generating another ramp.

My question is with respect to triggering in AUTO.  Since the ramp will be
generated regardless of a trigger, how does the system perform triggering when
in AUTO mode?

I know this is a very basic question, but I don't seem to be finding a good
explanation in the manual so I thought I'd ask here.

Thanks,
Barry - N4BUQ

#189109

Just to follow up on this thread, With Ozan's help, CR430 was bad.  The originals are 1S2200 (10mA, 8pF max) and I replaced it with a 1S2199 (10mA, 2pF) and the triggering is working now.

After fixing the rear drum mechanism, for the most part, the plugin is working as it should.

Particular thanks again to Ozan for all his help but, also, to everyone else that responded with tips.

Barry - N4BUQ

Show quoted text

----- Original Message -----
From: "Barry" <n4buq@...>
To: "tekscopes" <[email protected]>
Sent: Tuesday, December 14, 2021 10:48:58 AM
Subject: Re: Timebase Question : Auto vs Norm ModeExpanding on this a bit, when my 7B53N is in AUTO, when a sweep ends, the Sweep
Stop Comparator causes the Main Sweep Holdoff to send a pulse to the Main Sweep
Trigger (Q454 and Q466).  When this occurs, I see the waveform at TP475 that's
shown in the 7B52 manual for that same TP.

If I switch to NORM, the triggering waveform is present at the emitter of Q434
but that stops at the other side of both C451 and C461 and I'm wondering if the
TDs are holding that side of both diodes low.  Note that HOLDOFF is also held
LOW at this point (presumably due to no sweep), which could contribute to there
being no triggering waveform at TP475.

Since the HOLDOFF waveform is present at both Q454 and Q466 when in AUTO, then I
presume that's responsible for producing the waveform I'm seeing at TP475 but
I'm not understanding that if the time-base is in NORM, why the waveform at the
emitter of Q434 doesn't generate that waveform at TP475 but, again, I suspect
the TDs.

Using my Fluke 27 DMM in Diode Check mode, CR470 and CR475 appear to check
shorted but I don't know if that's expected with a DMM check.  BTW, I checked
the Fluke 27's current demand in Diode Check mode against my Fluke 25 and it
shows only about 350uA which I presume should not harm the TD when forward
biased; however, would that be enough to harm it in reverse bias?  I ask
because the Delayed Trigger Generator has the same two TDs and, before I check
those with the DMM (just to see if they exhibit the same readings), I would
like to make sure I'm not going to harm them in the process.

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Barry" <n4buq@...>
To: "tekscopes" <[email protected]>
Sent: Saturday, December 11, 2021 8:00:05 PM
Subject: Timebase Question : Auto vs Norm ModeI'm working on an 7B53N but I think this question applies to many/most scopes
and/or time-bases.

In AUTO mode, the time-base is in a "free-run" mode which generates a ramp at
the selected sweep rate and, at the end of the ramp, generates another ramp.
This continues whether a trigger is present or not.

In NORM, the time-base waits for a triggering signal before executing a sweep
and, at the end of the sweep, waits for another trigger signal before
generating another ramp.

My question is with respect to triggering in AUTO.  Since the ramp will be
generated regardless of a trigger, how does the system perform triggering when
in AUTO mode?

I know this is a very basic question, but I don't seem to be finding a good
explanation in the manual so I thought I'd ask here.

Thanks,
Barry - N4BUQ

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Timebase Question : Auto vs Norm Mode