7704A - A Trace Oddity


n4buq
 

I'm noticing something about the trace of my 7704A that I've not seen before in my other scopes (mostly Teks). The trace appears to be "snake-like" in that, for most settings, it appears as a dashed line where the dashes move back in forth along the trace in random motions. If I had to guess, I would suspect the Z-Axis signal is being pulsed - perhaps as a result of unwanted oscillations - but that's only a guess.

At first, I suspected it might be the system's attempt to draw the readout data but, in my scope, the readout does not work (yet another issued to track down) but even if I disconnect the readout lines from the vertical and horizontal boards, the snaky traces persist.

I notice that this odd display seems to be exacerbated by how much the graticule intensity is turned up. It doesn't go away if it is all the way off, but it does seem to get worse the brighter I make the graticule lamps.

Any ideas where to look for the cause(s) of this?

Thanks,
Barry - N4BUQ


Albert Otten
 

Hi Barry,
Could it be related to the about 20 kHz frequency of the HV primary supply?
You can try to trigger on this f.i. with a probe tip in the neighbourhood of HV circuits and see if that stops the pattern from running.
Albert


n4buq
 

Hi Albert,

It appears that was it. Probing near the HV line, I could trigger on that and, although that trace is a bit fuzzy (likely due to the poor health of plugins I have), I couldn't see the broken, moving traces there.

Since I've been seeing that with all the top covers removed, I replaced the cover over the HV section and that seems to have taken care of it. The sine wave from my signal generator is relatively clean now.

Thanks so much for that! I should've known that operating it that way can/will have weird consequences and just didn't think about that for this problem.

Barry - N4BUQ

----- Original Message -----
From: "Albert Otten" <aodiversen@concepts.nl>
To: "tekscopes" <TekScopes@groups.io>
Sent: Sunday, September 26, 2021 1:37:25 PM
Subject: Re: [TekScopes] 7704A - A Trace Oddity
Hi Barry,
Could it be related to the about 20 kHz frequency of the HV primary supply?
You can try to trigger on this f.i. with a probe tip in the neighbourhood of HV
circuits and see if that stops the pattern from running.
Albert



n4buq
 

Readout is working now. Someone had unplugged P03 from the readout board. That jack/plug is a bit hidden underneath the other cables for that board and I hadn't noticed it before now.

It's looking more and more like a real 7704 now!

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Barry" <n4buq@knology.net>
To: "tekscopes" <tekscopes@groups.io>
Sent: Sunday, September 26, 2021 12:51:22 PM
Subject: 7704A - A Trace Oddity
I'm noticing something about the trace of my 7704A that I've not seen before in
my other scopes (mostly Teks). The trace appears to be "snake-like" in that,
for most settings, it appears as a dashed line where the dashes move back in
forth along the trace in random motions. If I had to guess, I would suspect
the Z-Axis signal is being pulsed - perhaps as a result of unwanted
oscillations - but that's only a guess.

At first, I suspected it might be the system's attempt to draw the readout data
but, in my scope, the readout does not work (yet another issued to track down)
but even if I disconnect the readout lines from the vertical and horizontal
boards, the snaky traces persist.

I notice that this odd display seems to be exacerbated by how much the graticule
intensity is turned up. It doesn't go away if it is all the way off, but it
does seem to get worse the brighter I make the graticule lamps.

Any ideas where to look for the cause(s) of this?

Thanks,
Barry - N4BUQ


n4buq
 

I thought this issue was resolved but I'm still seeing remnants of it. If I use my function generator to sweep past 20kHz to the vertical amp, I can see the interaction of that frequency with the HV's 20kHz frequency. It's particularly noticeable around 40kHz and I think I saw artifacts of it at 80kHz as well.

The anode cable runs from the inverter to the front of the scope, then on the other side, it loops back at least half-way and then forward again to the anode connection. Where it loops back is right next to the horizontal board which, I presume, could cause some of this. Is it possible that cable is routed incorrectly, shielding that might be missing, or is it an indication of something wrong in the HV supply (or something else)?

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Barry" <n4buq@knology.net>
To: "tekscopes" <TekScopes@groups.io>
Sent: Sunday, September 26, 2021 2:21:23 PM
Subject: Re: [TekScopes] 7704A - A Trace Oddity
Hi Albert,

It appears that was it. Probing near the HV line, I could trigger on that and,
although that trace is a bit fuzzy (likely due to the poor health of plugins I
have), I couldn't see the broken, moving traces there.

Since I've been seeing that with all the top covers removed, I replaced the
cover over the HV section and that seems to have taken care of it. The sine
wave from my signal generator is relatively clean now.

Thanks so much for that! I should've known that operating it that way can/will
have weird consequences and just didn't think about that for this problem.

Barry - N4BUQ

----- Original Message -----
From: "Albert Otten" <aodiversen@concepts.nl>
To: "tekscopes" <TekScopes@groups.io>
Sent: Sunday, September 26, 2021 1:37:25 PM
Subject: Re: [TekScopes] 7704A - A Trace Oddity
Hi Barry,
Could it be related to the about 20 kHz frequency of the HV primary supply?
You can try to trigger on this f.i. with a probe tip in the neighbourhood of HV
circuits and see if that stops the pattern from running.
Albert



Mark Vincent
 

Barry,

Move the anode wire away from the horizontal board to see if that makes any difference. You may need new electrolytics in the high voltage section. If the 8,2mfd is going bad, that will allow ripple and be seen on the screen. The harmonics you are seeing make me suspect of too little filtering. Mouser currently has 10, 12 and 18mfd 250V ULD in stock to replace the 8,2mfd. If you decide to use the 18mfd, the additional capacitance will not hurt. Replacing the two 100mfd for the graticule lights can be done also. The 100mfd 16V of the same series are in stock. Your decoupling on the vertical and horizontal boards may also be going bad. Those can be raised to 47mfd 25V in the ULD series for the 5V and 15V supplies. At the age of the originals and being general purpose 85C types that would also be seen in consumer grade products, replacement would be a good idea. The life of the originals would be 2000-3000 hours.

Mark


n4buq
 

Mark,

I moved the anode lead a lot further away from the horizontal board and that didn't change anything so I presume this is a case of more tired electrolytics. I have several plugins and will be working my way through them so those will be addressed. I'll also look at the caps in the HV inverter too.

Thanks again,
Barry - N4BUQ

----- Original Message -----
From: "Mark Vincent" <orangeglowaudio@gmail.com>
To: "tekscopes" <TekScopes@groups.io>
Sent: Thursday, September 30, 2021 10:12:59 AM
Subject: Re: [TekScopes] 7704A - A Trace Oddity
Barry,

Move the anode wire away from the horizontal board to see if that makes any
difference. You may need new electrolytics in the high voltage section. If the
8,2mfd is going bad, that will allow ripple and be seen on the screen. The
harmonics you are seeing make me suspect of too little filtering. Mouser
currently has 10, 12 and 18mfd 250V ULD in stock to replace the 8,2mfd. If you
decide to use the 18mfd, the additional capacitance will not hurt. Replacing
the two 100mfd for the graticule lights can be done also. The 100mfd 16V of the
same series are in stock. Your decoupling on the vertical and horizontal boards
may also be going bad. Those can be raised to 47mfd 25V in the ULD series for
the 5V and 15V supplies. At the age of the originals and being general purpose
85C types that would also be seen in consumer grade products, replacement would
be a good idea. The life of the originals would be 2000-3000 hours.

Mark



Albert Otten
 

Barry,

You might inspect the voltage at Z-azis TP41119 for presence of 20 kHz ripple.
Best done in XY-mode since then there is no (un)blanking pattern in that voltage.
If no ripple present then at least you can again exclude a possible source.

Albert


n4buq
 

Albert,

I can take a look at that. I'm not sure, though, how to use XY with this scope. Does that require a timebase that provides that mode?

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Albert Otten" <aodiversen@concepts.nl>
To: "tekscopes" <TekScopes@groups.io>
Sent: Thursday, September 30, 2021 2:54:35 PM
Subject: Re: [TekScopes] 7704A - A Trace Oddity
Barry,

You might inspect the voltage at Z-azis TP41119 for presence of 20 kHz ripple.
Best done in XY-mode since then there is no (un)blanking pattern in that
voltage.
If no ripple present then at least you can again exclude a possible source.

Albert



Tom Lee
 

One way to think about it is that the left two bays control up and down, the right two bays control left and right. So, to get xy mode, put an amplifier plug-in in both.

--Cheers
Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 9/30/2021 13:11, n4buq wrote:
Albert,

I can take a look at that. I'm not sure, though, how to use XY with this scope. Does that require a timebase that provides that mode?

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Albert Otten" <aodiversen@concepts.nl>
To: "tekscopes" <TekScopes@groups.io>
Sent: Thursday, September 30, 2021 2:54:35 PM
Subject: Re: [TekScopes] 7704A - A Trace Oddity
Barry,

You might inspect the voltage at Z-azis TP41119 for presence of 20 kHz ripple.
Best done in XY-mode since then there is no (un)blanking pattern in that
voltage.
If no ripple present then at least you can again exclude a possible source.

Albert




n4buq
 

That certainly makes sense! I wasn't sure if time-bases and amplifiers could occupy any slot but apparently so.

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Tom Lee" <tomlee@ee.stanford.edu>
To: "tekscopes" <TekScopes@groups.io>
Sent: Thursday, September 30, 2021 3:18:47 PM
Subject: Re: [TekScopes] 7704A - A Trace Oddity
One way to think about it is that the left two bays control up and down,
the right two bays control left and right. So, to get xy mode, put an
amplifier plug-in in both.

--Cheers
Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 9/30/2021 13:11, n4buq wrote:
Albert,

I can take a look at that. I'm not sure, though, how to use XY with this scope.
Does that require a timebase that provides that mode?

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Albert Otten" <aodiversen@concepts.nl>
To: "tekscopes" <TekScopes@groups.io>
Sent: Thursday, September 30, 2021 2:54:35 PM
Subject: Re: [TekScopes] 7704A - A Trace Oddity
Barry,

You might inspect the voltage at Z-azis TP41119 for presence of 20 kHz ripple.
Best done in XY-mode since then there is no (un)blanking pattern in that
voltage.
If no ripple present then at least you can again exclude a possible source.

Albert







Ozan
 

In addition your 7B53A has horizontal input mode when time/div is set to AMPL and trigger source set to Ext (for 100mV/div) or Ext/10 (for 1V/div).
Ozan

On Thu, Sep 30, 2021 at 01:44 PM, n4buq wrote:


That certainly makes sense! I wasn't sure if time-bases and amplifiers could
occupy any slot but apparently so.

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Tom Lee" <tomlee@ee.stanford.edu>
To: "tekscopes" <TekScopes@groups.io>
Sent: Thursday, September 30, 2021 3:18:47 PM
Subject: Re: [TekScopes] 7704A - A Trace Oddity
One way to think about it is that the left two bays control up and down,
the right two bays control left and right. So, to get xy mode, put an
amplifier plug-in in both.

--Cheers
Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 9/30/2021 13:11, n4buq wrote:
Albert,

I can take a look at that. I'm not sure, though, how to use XY with this
scope.
Does that require a timebase that provides that mode?

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Albert Otten" <aodiversen@concepts.nl>
To: "tekscopes" <TekScopes@groups.io>
Sent: Thursday, September 30, 2021 2:54:35 PM
Subject: Re: [TekScopes] 7704A - A Trace Oddity
Barry,

You might inspect the voltage at Z-azis TP41119 for presence of 20 kHz
ripple.
Best done in XY-mode since then there is no (un)blanking pattern in that
voltage.
If no ripple present then at least you can again exclude a possible
source.

Albert







n4buq
 

Hi Ozan,

I noticed that after I'd asked about it. :)

Thanks!
Barry - N4BUQ

----- Original Message -----
From: "Ozan" <ozan_g@erdogan.us>
To: "tekscopes" <TekScopes@groups.io>
Sent: Thursday, September 30, 2021 4:20:44 PM
Subject: Re: [TekScopes] 7704A - A Trace Oddity
In addition your 7B53A has horizontal input mode when time/div is set to AMPL
and trigger source set to Ext (for 100mV/div) or Ext/10 (for 1V/div).
Ozan



On Thu, Sep 30, 2021 at 01:44 PM, n4buq wrote:


That certainly makes sense! I wasn't sure if time-bases and amplifiers could
occupy any slot but apparently so.

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Tom Lee" <tomlee@ee.stanford.edu>
To: "tekscopes" <TekScopes@groups.io>
Sent: Thursday, September 30, 2021 3:18:47 PM
Subject: Re: [TekScopes] 7704A - A Trace Oddity
One way to think about it is that the left two bays control up and down,
the right two bays control left and right. So, to get xy mode, put an
amplifier plug-in in both.

--Cheers
Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 9/30/2021 13:11, n4buq wrote:
Albert,

I can take a look at that. I'm not sure, though, how to use XY with this
scope.
Does that require a timebase that provides that mode?

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Albert Otten" <aodiversen@concepts.nl>
To: "tekscopes" <TekScopes@groups.io>
Sent: Thursday, September 30, 2021 2:54:35 PM
Subject: Re: [TekScopes] 7704A - A Trace Oddity
Barry,

You might inspect the voltage at Z-azis TP41119 for presence of 20 kHz
ripple.
Best done in XY-mode since then there is no (un)blanking pattern in that
voltage.
If no ripple present then at least you can again exclude a possible
source.

Albert








Jean-Paul
 

Hello again

If the scope has a crt digital readout the caractère generation requires a time div slot, and produces a chopper or flickering effet at some timebase and plugin settings.

Jon


n4buq
 

I did get the READOUT function working after I observed the flicker appeared to be virtually gone. I'll have to check it with that turned off.

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Jean-Paul" <jonpaul@ix.netcom.com>
To: "tekscopes" <TekScopes@groups.io>
Sent: Friday, October 1, 2021 2:19:31 PM
Subject: Re: [TekScopes] 7704A - A Trace Oddity
Hello again

If the scope has a crt digital readout the caractère generation requires a time
div slot, and produces a chopper or flickering effet at some timebase and
plugin settings.

Jon



Harvey White
 

There's a setting, on the readout board, I think, that controls when the beam is switched over to the readout.  it's either time multiplexed at any time or synchronized with the sweep (during retrace time).

If the scope is used mostly for slow sweeps, time multiplexing is likely best.  If fast repetition sweeps, then the synchronized sweep may give the best results.

Harvey

On 10/1/2021 3:35 PM, n4buq wrote:
I did get the READOUT function working after I observed the flicker appeared to be virtually gone. I'll have to check it with that turned off.

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Jean-Paul" <jonpaul@ix.netcom.com>
To: "tekscopes" <TekScopes@groups.io>
Sent: Friday, October 1, 2021 2:19:31 PM
Subject: Re: [TekScopes] 7704A - A Trace Oddity
Hello again

If the scope has a crt digital readout the caractère generation requires a time
div slot, and produces a chopper or flickering effet at some timebase and
plugin settings.

Jon





Mark Vincent
 

Harvey,

You are thinking about the chop frequency switch on the logic board of 200kc or 2mc. The higher frequency models have the feature to change the readout timing that is switched off the readout board. The readout is normally on retrace.

Mark


n4buq
 

Hi Albert,

Are you referring to TP41120? I don't see a TP41119; however, I put my 465B on TP41120 and there is a 200mV p-p, 20kHz signal at that point. The 20kHz waveform is a very "narrow" spike; however, inside that 50us span, there are other "noise" waveforms at about 100mV p-p occurring at about triple the 20kHz frequency and those signals are not the fast spiked waveforms like the 20kHz.

Is the magnitude of that 20kHz waveform excessive for that point in the circuit?

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Albert Otten" <aodiversen@concepts.nl>
To: "tekscopes" <TekScopes@groups.io>
Sent: Thursday, September 30, 2021 2:54:35 PM
Subject: Re: [TekScopes] 7704A - A Trace Oddity
Barry,

You might inspect the voltage at Z-azis TP41119 for presence of 20 kHz ripple.
Best done in XY-mode since then there is no (un)blanking pattern in that
voltage.
If no ripple present then at least you can again exclude a possible source.

Albert



Albert Otten
 

Hi Barry,

TP41120 indeed, I read the number from R4119. I think 200 mV pp is visible but at this moment my bench is too full to test this. Probably not when the disturbances are spikes. You could apply a signal to the the external Z-axis INPUT, and set the generator such that this signal also produces 200 mV pp at the testpoint and so on.

Albert

On Sat, Oct 2, 2021 at 05:38 PM, n4buq wrote:


Hi Albert,

Are you referring to TP41120? I don't see a TP41119; however, I put my 465B
on TP41120 and there is a 200mV p-p, 20kHz signal at that point. The 20kHz
waveform is a very "narrow" spike; however, inside that 50us span, there are
other "noise" waveforms at about 100mV p-p occurring at about triple the 20kHz
frequency and those signals are not the fast spiked waveforms like the 20kHz.

Is the magnitude of that 20kHz waveform excessive for that point in the
circuit?

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Albert Otten" <aodiversen@concepts.nl>
To: "tekscopes" <TekScopes@groups.io>
Sent: Thursday, September 30, 2021 2:54:35 PM
Subject: Re: [TekScopes] 7704A - A Trace Oddity
Barry,

You might inspect the voltage at Z-azis TP41119 for presence of 20 kHz
ripple.
Best done in XY-mode since then there is no (un)blanking pattern in that
voltage.
If no ripple present then at least you can again exclude a possible source.

Albert