Date   
Re: Just got a Tektronix 561A, HV woes...

 

Brenda, and Roy (and others):

I am late to the party insofar as responding to this thread- so please
bear with me. Brenda, am I correct in understanding that you have
measured the CRT's internal filament/heater resistance (between pins 1
and 14 of the CRT, with the 561A's CRT connector unplugged from the CRT,
and with the 561A unplugged from AC power), and measured about 5
megohms? Can you double check this? It seems really strange- an open
CRT filament should result in a measurement of (I think) a lot more than
5 megohms; likely beyond the range of any commonly available meter. And
a still-intact filament should measure something (I think) in the 20 to
500 ohm range.

If the CRT filament is toast, then I suspect that the CRT itself is
toast. However- a discontinuity in the internal CRT's filament circuit
might be right at pin 1 or 14. You might want to examine pins 1 and 14
of the CRT to determine if maybe the internal CRT wires have somehow
separated (bad solder joint?) from the pins- and if this has occurred
inside the pin(s), a touchup with a soldering iron might restore
filament continuity. Maybe a (very light) tap on the pins might restore
some degree of continuity, which you could observe on an analog ohmmeter
("twitching" of the needle). Someone else on this listserver- please
correct me if this is incorrect.

I have a functioning 561A, and I previously had a non-functioning 504
scope. I repaired the 504; it had a short-to-ground in the 60 hz power
transformer winding that feeds the CRT filament, and I repaired it by
adding a supplemental 6.3VAC power transformer and transferring the CRT
filament winding wiring to the new 6.3 VAC transformer that I added.
The 504 and 503 scopes are prone to failure of this nature, and I
suspect that the 561A might have similar issues, as the "architecture"
(powering the CRT filament from a 6.3VAC winding on the 60hz power
transformer) is similar. There are certainly many emails dealing with
the 503 and 504 problems.

But before we go there... Brenda, could you double-check your resistance
measurement for the CRT filament/heater?

Thanks!

Mike Dinolfo N4MWP

On 11/10/18 5:08 PM, Roy Morgan wrote:
Brenda,

My apologies for not reading all the replies you got so far, but:

The CRT HV transformers in those scopes can give trouble.
One hopeful cure is to heat the transformer with a light bulb for a day or so BEFORE trying to run the scope.

Good luck.. very nice scopes in my opinion. I’m not sure I have enough of one and plug-ins to get one running anymore.

Roy

On Nov 8, 2018, at 9:10 PM, brendda75 via Groups.Io <brendda75=yahoo.com@groups.io> wrote:

Hello everyone,

I just got my hands on a nice looking Tektronix 561A, and has some problems. I did see some searches on the HV issue with the power transformer for the CRT heater.
Roy Morgan
K1LKY since 1958
k1lky68@...






Re: Just got a Tektronix 561A, HV woes...

Harvey White
 

On Sat, 10 Nov 2018 09:54:54 -0800, you wrote:

Thanks so much Harvey for your input!
Quite welcome.

I have a few more details which may help others help me with this issue. According to the schematic that I have downloaded from tekwiki site, the 5642 rectifier that is arcing when the socket is on the CRT is V822, which connects to the CRT cathode. The 2 neon lamps that light up like crazy is connected between the CRT cathode (pin 2) and one of the grids (pin 3). Pin 3 connects to the intensity control potentiometer.
Those neons are not supposed to light, they're supposed to light to
protect the CRT and other circuits.

I have also checked for shorts with a meter in the CRT itself. But the CRT heater is open, measured about 5.22M between pins 1 and 14, so that there, the CRT is bad.
Hmmm, did you check between the intensity control to ground?

But the funny thing is that when I had the 561A running trying to do some troubleshooting and testing voltage points, the area of the CRT shield where the heater would be did get warm.
Without the CRT plugged in?

You could put in a small light bulb in series with the heater, it
ought to light, say same thing as the CRT, 6.3 volt max as a guess.

But I just did another test (Not a true test by any means whatsoever) I left the CRT unplugged and powered the scope back up. Both 5642 rectifiers had a glow, and since I did not have a HV meter, I used a neon lamp and brought it close to the connections for the CRT heater from the power transformer and got a glow. No arcing anywhere.
Then that indicates that the transformer is oscillating and producing
a magnetic field around it. Good test for older flybacks and HV
transformers.



This is sounding to be more like a fault with the CRT.
Doesn't it, though. You may have a good spares unit for your existing
651A. I have two 7904s, and about 3 units that had been somewhat
cannibalized (by others, I must add....) but are available for spare
parts. Already got a readout generator board from one.

Spares can be good things. You may have (and I don't know) a real
need for an HV transformer in the future.

Harvey


Brenda


Re: ( the price of) printed original manuals

Kevin Oconnor
 

USPS shipping.... U picked an open wound with me. US small mail customers are getting screwed by international trade agreements. Cheapest <13oz 1st class parcel for me is ~$5-7 to lower 48.
I get a epacket from China for <$2. Amazon delivers, via USPS, a $5 item in an 6x6x10 in box for free. And the USPS is still going broke.

Sent from kjo iPhone

Re: 468 still trouble shooting Horizontal jitter

tom jobe <tomjobe@...>
 

How about breaking the circuit(s) in various places?
I'd guess that the 468 has some components in sockets you could remove for testing.
There has to be some way to solve this problem.
tom jobe...

On 11/10/2018 3:58 PM, lop pol via Groups.Io wrote:
On Fri, Nov 9, 2018 at 06:51 AM, tom jobe wrote:

Hi Lop Pol,
You don't seem to be getting anywhere with the technique(s) you are
using, which makes me wonder if you could simply follow the signal going
backwards from the CRT connections to see where the horizontal jitter
first gets into the signal?
If that seems wrong... how about following the signal from its origin
and see where the jitter first gets into the signal?
This kind of simple approach has worked well for me a few times, as it
pulls you through the circuit on the schematic increasing your
understanding as you go.
tom jobe...




On 11/8/2018 6:53 PM, lop pol via Groups.Io wrote:
On Sat, Nov 3, 2018 at 12:25 PM, lop pol wrote:

On Sun, Oct 28, 2018 at 05:25 PM, lop pol wrote:

I have cleaned up the power supplies with new caps all around. Still have
a
horizontal jitter. Its almost unnoticeable during A sweep. Its most
noticeable
in delayed B starts after delay and inten A, it almost stops when I
switch
to
Norm.
happens in X-Y mode when feeding it an out of phase sine wave most
noticeable
in 10X mag the lissajous pattern will jerk horizontally. Occasionally
when
turning the delay time position pot in inten A the highlighted section
will
move further than what it should and come back on its own to the correct
Still troubleshooting this issue. This is what I have found so far. If you
are
familiar with the 468 please let me know where i may have made some
mistakes.
Pin 6 of U445 and pin 6 of U240 and the output of U250 that feeds pin 6 of
U445 and U250 (U250 being a -5V voltage regulator) are not stable when
viewing from another scope. Also My schematic is not very clear but pin 6
appears to be VBE of these microchips. When DC coupled at .5 volts per
division i see several 100mV jumps. These are schematic 6 on the A12 A&B
trigger generator board. I have ordered a -5V voltage regulator from mouser
but could this be the cause for my trigger and horizontal jitter? Thanks
guys.
I replaced U250 still no good. Checking the- 8V line on the A12 trigger
board it get a funny waveform.
https://groups.io/g/TekScopes/photo/76822/1?p=Name,,,20,1,0,0 the amplitude
increases when increasing the intensity and changes when changing the time/div
also the amplitude increases when using A inten or B delay. Weird stuff.

Thats part of the problem every point I check has jitter and it leads me in a total circle back to where I started. There is jitter when there is no trigger and no signal, the trace jitters horizontally and just about every test point wave form shows the same jitter, and you are right I'm getting nowhere.

Re: Beam modulation on 2465B

Siggi
 

On Fri, 9 Nov 2018 at 11:22 Roger Evans via Groups.Io <very_fuzzy_logic=
yahoo.com@groups.io> wrote:

Actually I can (sort of) replicate Rolf's observation but it is very, very
difficult to home in on it. I tried my own suggestion looking at the A
gate out signal from the 2465B and found the signal frequency where the
delay between sweeps would jitter between N and (N+1) signal periods. Then
by making tiny changes to the frequency I could sometimes get the flicker
to appear but never with the severity that Rolf sees. Maybe someone with a
really stable signal generator can try this. Since Rolf turned off his
readout display I don't know exactly what parameters to try and replicate.
Ah - I have a synthesized RF generator I can try this with. Looks like at
3MHz I have 10mHz resolution, whodathunkit. My 2467's MCP CRT may gloss
over the modulation though.

Re: Beam modulation on 2465B

Siggi
 

On Fri, 9 Nov 2018 at 11:02 Roger Evans via Groups.Io <very_fuzzy_logic=
yahoo.com@groups.io> wrote:

I believe the cpu is constantly refreshing all the parameters that the DAC
controls including the trigger level and holdoff, so maybe if the trigger
level is being refreshed just at the time when it should trigger then the
sweep will be delayed by one period of the the input signal and the trace
will appear fainter because of the longer wait time. You might be able to
see some jitter on the duration of the 'A gate low'.
I plumbed the A gate of my 2467 into my TDS784D to test this theory.
Doesn't pan out, not in NORM nor AUTO trigger modes, there's less than
0.01us jitter on the A gate low - basically I either trigger on all of them
or none of them with a time-based trigger. The MPU is supposed to check up
on things every 3.3ms according to the service manual, but it doesn't look
like the checking is in any way shape or form intrusive to job #1, which is
displaying a signal :).
I didn't think to try AUTO LVL, though.

Re: 468 still trouble shooting Horizontal jitter

Brendan
 

On Fri, Nov 9, 2018 at 06:51 AM, tom jobe wrote:


Hi Lop Pol,
You don't seem to be getting anywhere with the technique(s) you are
using, which makes me wonder if you could simply follow the signal going
backwards from the CRT connections to see where the horizontal jitter
first gets into the signal?
If that seems wrong... how about following the signal from its origin
and see where the jitter first gets into the signal?
This kind of simple approach has worked well for me a few times, as it
pulls you through the circuit on the schematic increasing your
understanding as you go.
tom jobe...




On 11/8/2018 6:53 PM, lop pol via Groups.Io wrote:
On Sat, Nov 3, 2018 at 12:25 PM, lop pol wrote:

On Sun, Oct 28, 2018 at 05:25 PM, lop pol wrote:

I have cleaned up the power supplies with new caps all around. Still have
a
horizontal jitter. Its almost unnoticeable during A sweep. Its most
noticeable
in delayed B starts after delay and inten A, it almost stops when I
switch
to
Norm.
happens in X-Y mode when feeding it an out of phase sine wave most
noticeable
in 10X mag the lissajous pattern will jerk horizontally. Occasionally
when
turning the delay time position pot in inten A the highlighted section
will
move further than what it should and come back on its own to the correct
Still troubleshooting this issue. This is what I have found so far. If you
are
familiar with the 468 please let me know where i may have made some
mistakes.
Pin 6 of U445 and pin 6 of U240 and the output of U250 that feeds pin 6 of
U445 and U250 (U250 being a -5V voltage regulator) are not stable when
viewing from another scope. Also My schematic is not very clear but pin 6
appears to be VBE of these microchips. When DC coupled at .5 volts per
division i see several 100mV jumps. These are schematic 6 on the A12 A&B
trigger generator board. I have ordered a -5V voltage regulator from mouser
but could this be the cause for my trigger and horizontal jitter? Thanks
guys.
I replaced U250 still no good. Checking the- 8V line on the A12 trigger
board it get a funny waveform.
https://groups.io/g/TekScopes/photo/76822/1?p=Name,,,20,1,0,0 the amplitude
increases when increasing the intensity and changes when changing the time/div
also the amplitude increases when using A inten or B delay. Weird stuff.


Thats part of the problem every point I check has jitter and it leads me in a total circle back to where I started. There is jitter when there is no trigger and no signal, the trace jitters horizontally and just about every test point wave form shows the same jitter, and you are right I'm getting nowhere.

Re: Just got a Tektronix 561A, HV woes...

Roy Morgan
 

Brenda,

My apologies for not reading all the replies you got so far, but:

The CRT HV transformers in those scopes can give trouble.
One hopeful cure is to heat the transformer with a light bulb for a day or so BEFORE trying to run the scope.

Good luck.. very nice scopes in my opinion. I’m not sure I have enough of one and plug-ins to get one running anymore.

Roy

On Nov 8, 2018, at 9:10 PM, brendda75 via Groups.Io <brendda75=yahoo.com@groups.io> wrote:

Hello everyone,

I just got my hands on a nice looking Tektronix 561A, and has some problems. I did see some searches on the HV issue with the power transformer for the CRT heater.
Roy Morgan
K1LKY since 1958
k1lky68@...

Re: Scanning software

Brad Thompson
 

On 11/10/2018 2:01 AM, Brian Symons wrote:
Hi Brad,
What VueScan settings do you use?
I did get VueScan some years ago & paid for the lifetime updates.
I was impressed that it is reasonably priced.
Hello, Brian--

I pretty much leave the settings at "auto" or "default" until
I need something special done (e.g., grain reduction or color balance
adjustment).

I've been running VueScan for over four years, and I too signed up for
lifetime upgrades.

73--

Brad AA1IP

475 No Sweep: DC Voltages and Waveforms around U600

agwnorden@...
 

I have a 475 which has performed faultlessly for about 10 years and has now lost its Sweep. It will not A Trigger on any input and AUTO trigger is dead. The A Arm TD is permanently armed in Single Sweep and will not Fire.

The DC and Waveforms around U600 look wrong, Waveforms at 0.1mS/div (DC measured in Single Sweep) show:on Pin 3 (+Auto Gate) approx 5 MHz `pulses` of 300 mV (0.167V DC), Pin 8 is 3.65 DC, Pin 17 is 1.81DC, At Pin 6,AUTO Timing terminal no significant waveform and 5.43V DC; R609 and C609 measured good (ESR not checked on C609). Pin 16 (holdoff start) is 2.43 V DC and Pin 18 is 0.01V DC. (The 475 was my main `scope so cannot definitively describe pulses on Pin 3 dues to poor rise time).

I am unsure whether U600 is in permanent Holdoff/is oscillating (Pin 3 excepted to show very much slower square wave) or both?

Is U600 dead?

Thank you for advice?

Re: Just got a Tektronix 561A, HV woes...

Brenda
 

Thanks so much Harvey for your input!

I have a few more details which may help others help me with this issue. According to the schematic that I have downloaded from tekwiki site, the 5642 rectifier that is arcing when the socket is on the CRT is V822, which connects to the CRT cathode. The 2 neon lamps that light up like crazy is connected between the CRT cathode (pin 2) and one of the grids (pin 3). Pin 3 connects to the intensity control potentiometer.

I have also checked for shorts with a meter in the CRT itself. But the CRT heater is open, measured about 5.22M between pins 1 and 14, so that there, the CRT is bad. But the funny thing is that when I had the 561A running trying to do some troubleshooting and testing voltage points, the area of the CRT shield where the heater would be did get warm. But I just did another test (Not a true test by any means whatsoever) I left the CRT unplugged and powered the scope back up. Both 5642 rectifiers had a glow, and since I did not have a HV meter, I used a neon lamp and brought it close to the connections for the CRT heater from the power transformer and got a glow. No arcing anywhere.

This is sounding to be more like a fault with the CRT.

Brenda

Re: More 2712 Spectrum Analyzer fun. Backup battery replacement?

Chuck Harris
 

I had a major catastrophe with Lithium Iodide cells in
a tektronix 1240 logic analyzer. When the stainless
steel enclosed hermetically sealed pacemaker cells
got sufficiently discharged, they blew a hole through
the side of the stainless steel, and spewed a hot
liquid in the case. The liquid etched a hole through
the aluminum chassis (14ga).

I worry about letting Lithium Iodide cells getting too
far discharged.

-Chuck Harris

Harvey White wrote:

On Sat, 10 Nov 2018 10:15:27 -0500, you wrote:

Randy

Haven't totally thought this suggestion through ( i.e. the proof is left
to the student as we used to say in college), but you might consider
adding another diode in series with 3.6 volt battery bringing it down to
3V which is closer to the original 2.8. I am guessing this would also
lower the current drain/extend the life of the 3.6 volt battery ???
Only if the current drain of the device is proportional to voltage.
May not be that significant.

However, you may want to check the device it's powering. In the
DM5010, the RAM that has the calibration constants is powered by 3.6
volts (3 nicads that self destruct), and could be replaced by a
lithium battery with a diode in series. The RAM is good down to 2.5
volts. The reduced voltage will work.

May work this way, or you may not need the diode unless the original
battery is supposed to be charged. In the DM5010, the diode is
absolutely needed.



I am guessing the use of the high grade battery was nod in those days
towards preventing leakage.
A thought that Tektronix would have been wise to consider.

Harvey

Re: More 2712 Spectrum Analyzer fun. Backup battery replacement?

Harvey White
 

On Sat, 10 Nov 2018 10:15:27 -0500, you wrote:

Randy

Haven't totally thought this suggestion through ( i.e. the proof is left
to the student as we used to say in college), but you might consider
adding another diode in series with 3.6 volt battery bringing it down to
3V which is closer to the original 2.8. I am guessing this would also
lower the current drain/extend the life of the 3.6 volt battery ???
Only if the current drain of the device is proportional to voltage.
May not be that significant.

However, you may want to check the device it's powering. In the
DM5010, the RAM that has the calibration constants is powered by 3.6
volts (3 nicads that self destruct), and could be replaced by a
lithium battery with a diode in series. The RAM is good down to 2.5
volts. The reduced voltage will work.

May work this way, or you may not need the diode unless the original
battery is supposed to be charged. In the DM5010, the diode is
absolutely needed.



I am guessing the use of the high grade battery was nod in those days
towards preventing leakage.
A thought that Tektronix would have been wise to consider.

Harvey


My $.05 worth
Dave
NR1DX


On 11/10/2018 9:26 AM, Randy.AB9GO wrote:
Good morning and thanks for the reply. I downloaded the service manual all
600-plus pages of it and looked at the schematic. They're using a 43256
sram integrated circuit for the memory on the storage board. It already has
a diode in series with the battery dropping .7 volts forward. The
interesting thing I did note is that there was a modification done to the
board to add a DS1210 battery backup chip to the digital storage board.
It's the same chip that they're using on the rs-232 board for the rest of
the 43256 sram. It must have been an addition too late production units
as there is no mention of this on the schematic. I still think I'll be okay
with the 3.6 volt battery replacing the 2.8 lithium iodide original used. I
also purchased a couple of the 2032 battery holders in case I run into a
spot where the 1/2 aa just won't fit. I'll keep you updated on how it turns
out. from what I've seen the battery that they used before was an
excellent long-life battery unfortunately it doesn't seem to be available
anymore. Welded stainless steel Construction and glass feed-through
insulators for the negative contact. there's no doubt this thing had at
least a 10 to 20 year lifespan potential. It's the same chemistry used in
pacemaker batteries in the 70s and 80s and from the same manufacturer.

Randy.ab9go



Re: More 2712 Spectrum Analyzer fun. Backup battery replacement?

Chuck Harris
 

Lithium iodide makes a 3.6V cell. When it gets to 2.8V
it is done.

The cell you are describing is still available, but is
now from Eagle-Pitcher, and is called a "Keeper" and
is part number LTC-7, and can be had from OSI Batteries
<https://www.osibatteries.com/> for about $15.

-Chuck Harris



Randy.AB9GO wrote:

Good morning and thanks for the reply. I downloaded the service manual all
600-plus pages of it and looked at the schematic. They're using a 43256
sram integrated circuit for the memory on the storage board. It already has
a diode in series with the battery dropping .7 volts forward. The
interesting thing I did note is that there was a modification done to the
board to add a DS1210 battery backup chip to the digital storage board.
It's the same chip that they're using on the rs-232 board for the rest of
the 43256 sram. It must have been an addition too late production units
as there is no mention of this on the schematic. I still think I'll be okay
with the 3.6 volt battery replacing the 2.8 lithium iodide original used. I
also purchased a couple of the 2032 battery holders in case I run into a
spot where the 1/2 aa just won't fit. I'll keep you updated on how it turns
out. from what I've seen the battery that they used before was an
excellent long-life battery unfortunately it doesn't seem to be available
anymore. Welded stainless steel Construction and glass feed-through
insulators for the negative contact. there's no doubt this thing had at
least a 10 to 20 year lifespan potential. It's the same chemistry used in
pacemaker batteries in the 70s and 80s and from the same manufacturer.

Randy.ab9go

Re: More 2712 Spectrum Analyzer fun. Backup battery replacement?

ArtekManuals
 

Randy

Haven't totally thought this suggestion through ( i.e. the proof is left to the student as we used to say in college), but you might consider adding another diode in series with 3.6 volt battery bringing it down to 3V which is closer to the original 2.8. I am guessing this would also lower the current drain/extend the life of the 3.6 volt battery ???

I am guessing the use of the high grade battery was nod in those days towards preventing leakage.

My $.05 worth
Dave
NR1DX

On 11/10/2018 9:26 AM, Randy.AB9GO wrote:
Good morning and thanks for the reply. I downloaded the service manual all
600-plus pages of it and looked at the schematic. They're using a 43256
sram integrated circuit for the memory on the storage board. It already has
a diode in series with the battery dropping .7 volts forward. The
interesting thing I did note is that there was a modification done to the
board to add a DS1210 battery backup chip to the digital storage board.
It's the same chip that they're using on the rs-232 board for the rest of
the 43256 sram. It must have been an addition too late production units
as there is no mention of this on the schematic. I still think I'll be okay
with the 3.6 volt battery replacing the 2.8 lithium iodide original used. I
also purchased a couple of the 2032 battery holders in case I run into a
spot where the 1/2 aa just won't fit. I'll keep you updated on how it turns
out. from what I've seen the battery that they used before was an
excellent long-life battery unfortunately it doesn't seem to be available
anymore. Welded stainless steel Construction and glass feed-through
insulators for the negative contact. there's no doubt this thing had at
least a 10 to 20 year lifespan potential. It's the same chemistry used in
pacemaker batteries in the 70s and 80s and from the same manufacturer.

Randy.ab9go


--
Dave
Manuals@...
www.ArtekManuals.com

Re: More 2712 Spectrum Analyzer fun. Backup battery replacement?

Randy.AB9GO
 

Good morning and thanks for the reply. I downloaded the service manual all
600-plus pages of it and looked at the schematic. They're using a 43256
sram integrated circuit for the memory on the storage board. It already has
a diode in series with the battery dropping .7 volts forward. The
interesting thing I did note is that there was a modification done to the
board to add a DS1210 battery backup chip to the digital storage board.
It's the same chip that they're using on the rs-232 board for the rest of
the 43256 sram. It must have been an addition too late production units
as there is no mention of this on the schematic. I still think I'll be okay
with the 3.6 volt battery replacing the 2.8 lithium iodide original used. I
also purchased a couple of the 2032 battery holders in case I run into a
spot where the 1/2 aa just won't fit. I'll keep you updated on how it turns
out. from what I've seen the battery that they used before was an
excellent long-life battery unfortunately it doesn't seem to be available
anymore. Welded stainless steel Construction and glass feed-through
insulators for the negative contact. there's no doubt this thing had at
least a 10 to 20 year lifespan potential. It's the same chemistry used in
pacemaker batteries in the 70s and 80s and from the same manufacturer.

Randy.ab9go

Re: More 2712 Spectrum Analyzer fun. Backup battery replacement?

Ed
 

If the memory they are backing up is rated for 3.6V, go for it. Otherwise, I would put the diode in series, maybe with a 10nF cap to ground at the memory device if there is not one there already.

Re: Beam modulation on 2465B

Tom Gardner
 

I'm not sure which of my two messages you are replying to, since you didn't quote the context.

It appeared from the video that there is a intensity modulation, i.e. a z-axis modulation. Hence it would be useful to determine the source of that modulation.

If you can use a "spare" channel to probe around the scope, you may be able to find a signal voltage that is going up and down at the same frequency as the z-axis modulation. Any such effect would be easy to see if voltage and intensity are displayed on the same CRT, more difficult if they are on different CRTs/scopes.

The intensity is determined by a voltage that is derived from various sources, e.g. A timebase, B timebase, readout, blanking etc. I mentioned U950 simply because the various z-axis control waveforms pass through u950 on the way to the A9 HV board. For the details of the various input voltages to u950, see the manual.

If the intensity modulation is a beat-frequency effect, then the beat frequency seen on the screen is the difference between two other frequencies. As such is it very sensitive to the absolute values of each of the other frequencies. For example, if f1=3,000,099 and f2=3,000,100 then the resultant frequency is 1Hz. But if you change f1 by 1ppm=3Hz, then the beat frequency would be 4Hz. The difference between a 1Hz and 4Hz intensity modulation should be easy to seem but possibly not easy to video.

The crystal oscillator on the control board won't be stable to 1ppm over the temperature range, since nothing in the scope requires that level of stability. Hence cooling the oscillator may simply have shifted f1 by a few ppm, and hence the beat frequency so much that intensity modulation is no longer visible.

On 10/11/18 10:45, BUR wrote:
Hello Tom
Yes, for instance I always checked an offset of 100Hz.
To test at which Shift the modulation stops a have gone from 1Hz up to 100Hz. 20Hz is not enough Hz by Hz up the modulation disappears from 35Hz upwards.
I didn’t check any voltage around U950. What do you mean "the waveforms around u950»?
I have checked also on CH3 and 4 same result.
50, 100 or 150Hz doesn’t make any difference.
I did one more test. Using the TEK 013-0156-00 “floating ground on BNC” just to see if we have any ground loop problems. But there is no difference with the floating BNC or direct coupling.
Rolf

Re: Beam modulation on 2465B

Roger Evans
 

I am afraid I am struggling to see the phenomenon that Rolf is concerned about. In the first video the only feature that to me seemed unexpected was the flickering, and that seemed to affect the whole trace at the same time rather than be a moving pattern. In the second video, with delayed sweep, there seemed to be a slight vertical jitter, again affecting the whole trace at the same time. At the end of the second video the readout display is pulsing slowly, I guess this is an artifact of the video frame rate.

Anyway there is a stimulus that makes a consistent change which is cooling the CPU. Aside from the general issue of intermittent dry joints there are two electrical effects. The CPU frequency will change, but it is a crystal oscillator so this may be small compared with the 500Hz needed to make the effect go away ( and there is a division of 8 so 500Hz at the CPU is 4kHz at the crystal). Also the CPU will consume less power since the switching speed becomes faster at low temperatures (I am fairly sure this happens for CMOS and less sure about NMOS). It might be worth looking at ripple and switching noise on +5VD in case this is causing marginal behaviour somewhere on the digital side.

Roger

Re: Beam modulation on 2465B

BUR
 

Hello Tom
Yes, for instance I always checked an offset of 100Hz.
To test at which Shift the modulation stops a have gone from 1Hz up to 100Hz. 20Hz is not enough Hz by Hz up the modulation disappears from 35Hz upwards.
I didn’t check any voltage around U950. What do you mean "the waveforms around u950»?
I have checked also on CH3 and 4 same result.
50, 100 or 150Hz doesn’t make any difference.
I did one more test. Using the TEK 013-0156-00 “floating ground on BNC” just to see if we have any ground loop problems. But there is no difference with the floating BNC or direct coupling.
Rolf