Date   
Re: Procedure to measure rf watts

Jim Potter
 

Randy,

The technique is to use calibrated attenuators to bring the power level down to 10 mW or so. 10 mW is 1V peak or 2V Pk-pk. Ideally you should calibrate the scope, but to do that you need a calibrated signal generator and a 50 Ohm load. If you frequency is well within the bandwidth of the scope you can calibrate by other means or accept the scope calibration as is.

If you have 10 W you need about 30 dB of attenuation. Its not critical as long as you get the signal into the amplitude range of the scope. It's best to be at the few volts pk-pk not down in the mV. You also need to use the scope at 50 Ohms input impedance. If it doesn't have a 50 Ohm setting then a Tee and a terminator will do for frequencies below 20 or 30 MHz. Above that you need an accurate 50 Ohm in-line terminator.

You then measure the pk-pk volts on the scope. The power into the scope is Ps =1/4 Vpp^2/R where R is 50 Ohms. The power is P = 1/2 V^2/R where V is the peak voltage and Vpp = 2*V is the pk-pk voltage.

Now you need to correct for the attenuator. The attenuation factor is 10^(A/10) where A is the attenuator attenuation. The power from your amplifier is then Pa = Ps * 10^(A/10).

One thing to be careful of is harmonics. If harmonics are an issue you need an appropriate low pass filter. It needs to be calibrated at your operating frequency and it's attenuation needs to be added to the attenuator attenuation.

If you are not used to working with rf this may be too brief an explanation.

I use a Tek 2467B to measure peak power of rf pulses at 425 MHz all the time. This is outside the scope bandwidth, so I calibrate the scope with a CW signal and an average power meter. The 2467B reads about 0.7 V for a 1 V signal at 425 MHz. This lets me get accurate peak power readings with out buying a peak power meter. Used Gigatronics peak power meters and heads run about $4k total used. The 2467B is under a $1000. (That's a guess. It's been a few years since I've purchased one.)

de K9GXC, Jim

At 07:25 PM 5/22/2019, you wrote:
On Wed, May 22, 2019 at 04:00 PM, Randy.AB9GO wrote:


Hello everyone,

I have a need to check the calibration on some low power watt
meters (10 watts
or less, 21 megahertz or less) and was wondering if anybody had a favorite
procedure using their scope to measure peak-to-peak rf voltage
without letting
out the magic smoke. My thought is to use a t connector, hook one
side to the
transmitter, one side to a dummy load and then the center
connector straight
to the 10X oscilloscope probe. Any other precautions I should take?

Thank you,
Randy.
I will say above anything else, 10W is way too much power to be putting into most test equipment, especially spectrum analyzers. You *will* burn up something in the frontend and then you'll be sad. You want something like an RF sampling tee that will couple a much smaller version of your TX signal into your instrument.

Sean

James M. Potter, PhD, President
JP Accelerator Works, Inc.
2245 47th Street
Los Alamos, NM 87544

TEL: 505-690-8701

Re: Procedure to measure rf watts

Jim Potter
 

If you use a directional coupler you need to be sure that it is calibrated properly. Unless you have a fully calibrated 50 Ohm system your measurement is nothing more than a WAG (Wild A$$ Guess). The higher the frequency the more complicated it is to get accurate measurements.


At 07:54 PM 5/22/2019, Jim Ford wrote:
Definitely use a directional coupler or an attenuator.I assume that the flow-through wattmeters like those made by Bird have couplers inside those plug-in modules.Jim Ford Sent from my Verizon, Samsung Galaxy smartphone --------
Original message --------From: sdturne@q.com Date: 5/22/19 6:25 PM (GMT-08:00) To: TekScopes@groups.io Subject: Re: [TekScopes] Procedure to measure rf watts On Wed, May 22, 2019 at 04:00 PM, Randy.AB9GO wrote:>> Hello everyone,> > I have a need to check the calibration on some low power watt meters (10 watts> or less, 21 megahertz or less) and was wondering if anybody had a favorite> procedure using their scope to measure peak-to-peak rf voltage without letting> out the magic smoke. My thought is to use a t connector, hook one side to the> transmitter, one side to a dummy load and then the center connector straight> to the 10X oscilloscope probe. Any other precautions I should take?> > Thank you,> Randy.>I will say above anything else, 10W is way too much power to be putting into most test equipment, especially spectrum analyzers. You *will* burn up something in the frontend and then you'll be sad. You want something like an RF sampling tee that will couple a much smaller version of your TX signal into your instrument.Sean
James M. Potter, PhD, President
JP Accelerator Works, Inc.
2245 47th Street
Los Alamos, NM 87544

TEL: 505-690-8701

Re: 7904 excessive shadow/flare

Chuck Harris
 

The 7904, because of its very high anode voltage, and its
mesh lens is one of the worst scopes in existence for reflections
and flare inside of the CRT. But I like it anyway.

-Chuck Harris

unclebanjoman wrote:

No, the artifact is visible with any type of signal.
It also appears using the signal standardizer. I used the latter today to perform some calibrations on my 7904 and I was very irritated by this halo / flare effect.
With sweep rates of 0.1-0.5 sec and no signal applied is clearly visible the usual "globus" leading/lagging the spot.
Already with moderate brightness the artifact is visible.

Max



Re: Procedure to measure rf watts

Bob Albert
 

Richard I pretty much agree with your comments.  But I'd like to add that, at HF, precision connectors and fancy adapters are overkill.  I have a T connector, UHF type, in the line from the rig.  I plug into that an adapter from UHF to BNC and another adapter from there to binding posts.  I connect the HP 410B to the binding posts.
Now of course I am aware that this is not good practice.  But I suggest that, for frequecies below 30 MHz, it's close enough.  There are no discontinuities more than an inch or so, much less than what rule of thumb (one tenth wavelength) says.  My readings are close to what I expect.  And if there is an error, well it can't be great and I don't think FCC will be breathing down my neck for running too much power.
Should I desire to work at 2 meters, then I might be more rigorous.  (I am reminded of an old exam question for ham license that suggested using an HF SWR meter at 2 meters if that's all you have, and it's close enough.)
Bob K6DDX

On Wednesday, May 22, 2019, 6:46:39 PM PDT, Richard Knoppow <dickburk@...> wrote:

    At communication frequencies I use a General Radio 1800A
mainly because I have the proper adapter for the probe. The 410B
is fine but its adapters for any of the more common coaxial
connectors are pretty scarce. Otherwise its fine. The 1800A takes
a  shell for a GR 874 connector which screws into the end of the
probe, the banana plug acting as the center conductor. I then use
an N type T with UHF adaptors on two ends and the probe in the
center. I have a couple of dummy loads which have been measured
on a GR RF bridge so I know their actual impedance at the
measurement frequency. In general a DC resistance measurement
comes close and they are pretty non-reactive. I can then
calculate the power with reasonable accuracy.
    Both the 1800A and  410B have single diode rectifiers for RF
and both have the usual characteristic of being square law at
small voltages transitioning to peak reading at high voltages.
The scales are calibrated in the RMS value of a sine wave and
both have different scales for low and high voltages as required
by the characteristic of the diode.  I have calibrated my MFJ
tuner/SWR meter/power meter using this arrangement. It seems to
be reasonably accurate. A scope could be used but as you say
requires some calculation because it reads peak-to-peak.
    Nothing is as simple as it seems or as you would prefer.
    OTOH, a scope can tell you an awful lot about what is going
on in the transmitter.

On 5/22/2019 5:39 PM, Bob Albert via Groups.Io wrote:
  I use my venerable HP410B.  The ac probe has wide bandwidth and the most sensitive range of 1 V should suffice for all but the weakest power from a transmitter.  Full scale corresponds to 20 mW.  Half scale, or 0.5 Volt, 5 mW.
Sure you can use an oscilloscope.  You will have some calculation to do.  Further, you would be able to ascertain whether you are working with something close to a sine wave.
Ideally, use both instruments, one for checking waveform and the other for a quantitative reading.  The load, of course, needs to be purely resistive.  If not, your 'power' reading will be wrong.
In any case, an oscilloscope isn't the best tool for precise measurements.  The trace width and screen nonlinearities need to be considered.  A spectrum analyzer might be a good idea, since it would show each frequency component and allow one to decide if any are large enough to influence the result.  Most spectrum analyzers have 50 or 75 Ohm input already in place; the pitfall there is to make sure you don't burn out the termination.  Ten Watts is too much, and most aren't safe above 1 Watt or less.  A dummy load and attenuator might be a good idea.
When I measure the power out of my ham transmitter I can get an accurate reading with the HP voltmeter.  I use either a dummy load or an antenna with close to 1:1 SWR.  Without the linear amplifier I get around 65-70 V reading (100 W or so) and with the amplifier about 220-250 Volts (over 1 kW), depending on tuning and load quality.  The HP pointer movement is so well damped that I can get peak readings when operating pulsed, such as a series of CW dots.
I also have an oscilloscope to see the wave; I couple it to the system with a loop pickup.  I connect a coaxial cable to the 'scope and short the other end around one of my voltmeter leads.  Thus, inductive pickup.
When I use audio modulation I can measure peak envelope power on the voltmeter.  The 'scope will enable me to adjust the modulation such that I don't get clipping.  I can also verify the performance of the speech processor.
Bob
      On Wednesday, May 22, 2019, 4:01:03 PM PDT, Randy.AB9GO <@AB9GO> wrote:
 
  Hello everyone,

I have a need to check the calibration on some low power watt meters (10 watts or less, 21 megahertz or less) and was wondering if anybody had a favorite procedure using their scope to measure peak-to-peak rf voltage without letting out the magic smoke.  My thought is to use a t connector, hook one side to the transmitter, one side to a dummy load and then the center connector straight to the 10X oscilloscope probe. Any other precautions I should take?

Thank you,
Randy.

--
Richard Knoppow
dickburk@...
WB6KBL

Re: Photo Storage Space Solution, was IMPORTANT: Photos are eating up our storage

 

Hi Harvey,
Not that I know of. You may be thinking of TekScopes2 and there was another one devoted to Tek Archives but the name escapes me. It may have been TekScopesArc or something like that.
Dennis Tillman W7PF

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Harvey White
Sent: Wednesday, May 22, 2019 4:24 PM
Subject: Re: [TekScopes] Photo Storage Space Solution, was IMPORTANT: Photos are eating up our storage

Didn't tekscopes have some other groups that were created solely for image storage back on Yahoo?
Harvey


On 5/22/2019 7:02 PM, nonIonizing EMF wrote:
I was wondering if moving the images and details related like including the link to the groups.io or other post to Tekwiki might be a solution. I'm not sure about the server limitations and expense there however.



--
Dennis Tillman W7PF
TekScopes Moderator

Re: 7904 excessive shadow/flare

Roger Evans
 

To my eye each of the shadow peaks and troughs of the sine wave lies on a straight line joining the real peak/trough to the CRT centre, but farther out. This should be very obvious at say 100msec/div. I would guess the probable cause is the CRT cathode voltage dropping out of regulation due to a HV capacitor failure. If you don't have a HV probe you should be able to see the ripple on the feedback to the regulator.

Regards,

Roger

Re: Procedure to measure rf watts

 

Hi Randy,
How about a Bird Wattmeter. They are the first thing a ham thinks of when they need to measure RF power.
Dennis Tillman W7PF

On Wednesday, May 22, 2019, 4:01:03 PM PDT, Randy.AB9GO <@AB9GO> wrote:

Hello everyone,

I have a need to check the calibration on some low power watt meters (10 watts or less, 21 megahertz or less) and was wondering if anybody had a favorite procedure using their scope to measure peak-to-peak rf voltage without letting out the magic smoke. My thought is to use a t connector, hook one side to the transmitter, one side to a dummy load and then the center connector straight to the 10X oscilloscope probe. Any other precautions I should take?

Thank you,
Randy.
--
Dennis Tillman W7PF
TekScopes Moderator

Re: Procedure to measure rf watts

Jim Potter
 

Bob,

Good advice. A lot of people don't think through what they are doing. There's some distance between theory and "what works". Sometimes at HF a light bulb tells you all you need to know.

I once rebuilt a BC-610, 250TH in the final. The load was five 100 W incandescent bulbs in parallel. You could guesstimate the power by the brightness of the bulbs. On 10m you could see a standing wave. The bulbs were not all the same brightness.

Jim

On May 22, 2019 10:48:09 PM MDT, "Bob Albert via Groups.Io" <bob91343=yahoo.com@groups.io> wrote:
Richard I pretty much agree with your comments.  But I'd like to add
that, at HF, precision connectors and fancy adapters are overkill.  I
have a T connector, UHF type, in the line from the rig.  I plug into
that an adapter from UHF to BNC and another adapter from there to
binding posts.  I connect the HP 410B to the binding posts.
Now of course I am aware that this is not good practice.  But I suggest
that, for frequecies below 30 MHz, it's close enough.  There are no
discontinuities more than an inch or so, much less than what rule of
thumb (one tenth wavelength) says.  My readings are close to what I
expect.  And if there is an error, well it can't be great and I don't
think FCC will be breathing down my neck for running too much power.
Should I desire to work at 2 meters, then I might be more rigorous.  (I
am reminded of an old exam question for ham license that suggested
using an HF SWR meter at 2 meters if that's all you have, and it's
close enough.)
Bob K6DDX
On Wednesday, May 22, 2019, 6:46:39 PM PDT, Richard Knoppow
<dickburk@...> wrote:

    At communication frequencies I use a General Radio 1800A
mainly because I have the proper adapter for the probe. The 410B
is fine but its adapters for any of the more common coaxial
connectors are pretty scarce. Otherwise its fine. The 1800A takes
a  shell for a GR 874 connector which screws into the end of the
probe, the banana plug acting as the center conductor. I then use
an N type T with UHF adaptors on two ends and the probe in the
center. I have a couple of dummy loads which have been measured
on a GR RF bridge so I know their actual impedance at the
measurement frequency. In general a DC resistance measurement
comes close and they are pretty non-reactive. I can then
calculate the power with reasonable accuracy.
    Both the 1800A and  410B have single diode rectifiers for RF
and both have the usual characteristic of being square law at
small voltages transitioning to peak reading at high voltages.
The scales are calibrated in the RMS value of a sine wave and
both have different scales for low and high voltages as required
by the characteristic of the diode.  I have calibrated my MFJ
tuner/SWR meter/power meter using this arrangement. It seems to
be reasonably accurate. A scope could be used but as you say
requires some calculation because it reads peak-to-peak.
    Nothing is as simple as it seems or as you would prefer.
    OTOH, a scope can tell you an awful lot about what is going
on in the transmitter.

On 5/22/2019 5:39 PM, Bob Albert via Groups.Io wrote:
  I use my venerable HP410B.  The ac probe has wide bandwidth and the
most sensitive range of 1 V should suffice for all but the weakest
power from a transmitter.  Full scale corresponds to 20 mW.  Half
scale, or 0.5 Volt, 5 mW.
Sure you can use an oscilloscope.  You will have some calculation to
do.  Further, you would be able to ascertain whether you are working
with something close to a sine wave.
Ideally, use both instruments, one for checking waveform and the
other for a quantitative reading.  The load, of course, needs to be
purely resistive.  If not, your 'power' reading will be wrong.
In any case, an oscilloscope isn't the best tool for precise
measurements.  The trace width and screen nonlinearities need to be
considered.  A spectrum analyzer might be a good idea, since it would
show each frequency component and allow one to decide if any are large
enough to influence the result.  Most spectrum analyzers have 50 or 75
Ohm input already in place; the pitfall there is to make sure you don't
burn out the termination.  Ten Watts is too much, and most aren't safe
above 1 Watt or less.  A dummy load and attenuator might be a good
idea.
When I measure the power out of my ham transmitter I can get an
accurate reading with the HP voltmeter.  I use either a dummy load or
an antenna with close to 1:1 SWR.  Without the linear amplifier I get
around 65-70 V reading (100 W or so) and with the amplifier about
220-250 Volts (over 1 kW), depending on tuning and load quality.  The
HP pointer movement is so well damped that I can get peak readings when
operating pulsed, such as a series of CW dots.
I also have an oscilloscope to see the wave; I couple it to the
system with a loop pickup.  I connect a coaxial cable to the 'scope and
short the other end around one of my voltmeter leads.  Thus, inductive
pickup.
When I use audio modulation I can measure peak envelope power on the
voltmeter.  The 'scope will enable me to adjust the modulation such
that I don't get clipping.  I can also verify the performance of the
speech processor.
Bob
      On Wednesday, May 22, 2019, 4:01:03 PM PDT, Randy.AB9GO
<@AB9GO> wrote:
 
  Hello everyone,

I have a need to check the calibration on some low power watt meters
(10 watts or less, 21 megahertz or less) and was wondering if anybody
had a favorite procedure using their scope to measure peak-to-peak rf
voltage without letting out the magic smoke.  My thought is to use a t
connector, hook one side to the transmitter, one side to a dummy load
and then the center connector straight to the 10X oscilloscope probe.
Any other precautions I should take?

Thank you,
Randy.

--
Richard Knoppow
dickburk@...
WB6KBL




--
Sent from my Android device with K-9 Mail. Please excuse my brevity.

Re: Photo Storage Space Solution, was IMPORTANT: Photos are eating up our storage

Dave Daniel
 

I believe it is TekScopesArc, still on Yahoo.

DaveD

Sent from a small flat thingy

On May 23, 2019, at 02:28, Dennis Tillman W7PF <@Dennis_Tillman_W7PF> wrote:

Hi Harvey,
Not that I know of. You may be thinking of TekScopes2 and there was another one devoted to Tek Archives but the name escapes me. It may have been TekScopesArc or something like that.
Dennis Tillman W7PF

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Harvey White
Sent: Wednesday, May 22, 2019 4:24 PM
Subject: Re: [TekScopes] Photo Storage Space Solution, was IMPORTANT: Photos are eating up our storage

Didn't tekscopes have some other groups that were created solely for image storage back on Yahoo?
Harvey


On 5/22/2019 7:02 PM, nonIonizing EMF wrote:
I was wondering if moving the images and details related like including the link to the groups.io or other post to Tekwiki might be a solution. I'm not sure about the server limitations and expense there however.



--
Dennis Tillman W7PF
TekScopes Moderator


Re: 7904 excessive shadow/flare

unclebanjoman
 

@Chuck:
I agree that. But 7854 and 7904 share the same CRT model (154-0644-05).
Why in my 7904 the exhibits more, more halo/flare effects than my 7854???
There's really a big difference between the two.
It's really the first time I've ever been disturbed by such an effect. In all my other oscilloscopes (465, 475, 7633, 7854) this effect is almost imperceptible.


@Roger:
As I stated previously, all voltages (high and low) are perfetly O.K. (I recapped ALL the SMPS capacitors with new Nichicon PWM grade ones).
Checking the -2960V catode supply (HV test point) with my Fluke H.V. probe I measured -2992V. It seems somewhat reasonable value to me, since Tek specs stated that this voltage should be -2960 V +/- 1%.
I'm unable to check H.V ripple because I don't know where to probe. The manual doesn't mention that check.

Max

Re: 497P frequency counter question? How to turn on?

radioconnection@...
 

Well, two odd things. The switches are set for a 497P. I didn't fully understand how the counter works. It apparently is enabled in this particular 497P, regardless of the fact that the switches are set for the correct model.

Re: 7904 excessive shadow/flare

unclebanjoman
 

Just to show what happens, I made this short video. Intensity controls are set to a very moderate brightness but the effects is noticeable:

https://www.dropbox.com/s/tfy91d8tq1b6ql9/7904_shadow_flare%20003.AVI?dl=0

Max

Re: Latest firmware for TDS210 and TDS2CM?

 

On Thu, May 23, 2019 at 03:58 AM, Jared Cabot wrote:

is there any info on that programming fixture?
The P/N is 067-0388-00 and it uses the same cover as the other extension modules. The serial port is still there but the other
connectors are not. In the hole for the GPIB connector is a IC socket for the master ROM and in the hole for the Centronics
connector is a small LED. The P/N for the master ROM v2.03 is 163-1368-02.
Although that I have one I can't recall ever using it and I haven't found any user info on it. Either it starts the flashing right
at power on or a start command has to be sent through RS232.

Are there any later versions than these?
I guess the only one that can tell what the last or latest versions are is Tek itself.

/Håkan

Re: 7904 excessive shadow/flare

Chuck Harris
 

I know that, but my 7904's beat my 7854 in the flare department
by easily double. They also have much brighter maximum intensity
than does the 7854.

I haven't explored the differences, but the 7904's all have a worse
time than *any* other scope I have ever seen.

As to what it looks like, if you go to a slow sweep, something on
the order of 20ms/div, and allow a single bright spot to draw a
center-line trace, you will see several other spots moving both
faster, more diffuse, and towards the outer reaches of the CRT screen.

It is clear that they all are "tethered" at the center of the
screen.

It is the inevitable cost of having such a high anode voltage with
a mesh lens. The mesh lens is an imperfect lens. It produces a
repeating series of ghosts images that are at intervals from the
desired image. The intervals are related to the beam size, and the
mesh's pitch. I think they are like the wavelets you see when a
collimated light source is aimed through a narrow slit... which is
exactly how a mesh (window screen) lens works.

-Chuck Harris

unclebanjoman wrote:

@Chuck:
I agree that. But 7854 and 7904 share the same CRT model (154-0644-05).
Why in my 7904 the exhibits more, more halo/flare effects than my 7854???
There's really a big difference between the two.
It's really the first time I've ever been disturbed by such an effect. In all my other oscilloscopes (465, 475, 7633, 7854) this effect is almost imperceptible.


@Roger:
As I stated previously, all voltages (high and low) are perfetly O.K. (I recapped ALL the SMPS capacitors with new Nichicon PWM grade ones).
Checking the -2960V catode supply (HV test point) with my Fluke H.V. probe I measured -2992V. It seems somewhat reasonable value to me, since Tek specs stated that this voltage should be -2960 V +/- 1%.
I'm unable to check H.V ripple because I don't know where to probe. The manual doesn't mention that check.

Max



Re: 7904 excessive shadow/flare

Chuck Harris
 

That is a 100% normal image for a 7904 scope.

-Chuck Harris

unclebanjoman wrote:

Just to show what happens, I made this short video. Intensity controls are set to a very moderate brightness but the effects is noticeable:

https://www.dropbox.com/s/tfy91d8tq1b6ql9/7904_shadow_flare%20003.AVI?dl=0

Max



Re: 7904 excessive shadow/flare

Chuck Harris
 

That's wonderful, but the flare exists when there is only the single
spot on the screen. Why in your theory, does the center spot not
change?

This is a completely normal effect caused by the mesh lens, and the
high energy electron beam.

The high energy beam allows this diffraction to be high enough energy
to light up the phosphor. The slow speed allows the phosphor even
more time to bank up its electrons and make light.

This is normal for the 7904 CRT. It happens on my 7904's, and my
7854 to exactly the same degree.

If you don't like it, turn the intensity down to a more reasonable
value.

-Chuck Harris

Roger Evans via Groups.Io wrote:

To my eye each of the shadow peaks and troughs of the sine wave lies on a straight line joining the real peak/trough to the CRT centre, but farther out. This should be very obvious at say 100msec/div. I would guess the probable cause is the CRT cathode voltage dropping out of regulation due to a HV capacitor failure. If you don't have a HV probe you should be able to see the ripple on the feedback to the regulator.

Regards,

Roger



Re: Procedure to measure rf watts

Chuck Harris
 

The venerable bird wattmeter is really quite awful in terms of
accuracy. At its rated best, it is +/- 5% of full scale.

So, if you are using a 10W slug, its accuracy band is +/- 0.5W.
Ok, if you are measuring 10 W, but it gives you a 50% uncertainty
if you are measuring 1W.... 20% if you are measuring 5W.

Your scope can do much better than that.

-Chuck Harris

Dennis Tillman W7PF wrote:

Hi Randy,
How about a Bird Wattmeter. They are the first thing a ham thinks of when they need to measure RF power.
Dennis Tillman W7PF

On Wednesday, May 22, 2019, 4:01:03 PM PDT, Randy.AB9GO <@AB9GO> wrote:

Hello everyone,

I have a need to check the calibration on some low power watt meters (10 watts or less, 21 megahertz or less) and was wondering if anybody had a favorite procedure using their scope to measure peak-to-peak rf voltage without letting out the magic smoke. My thought is to use a t connector, hook one side to the transmitter, one side to a dummy load and then the center connector straight to the 10X oscilloscope probe. Any other precautions I should take?

Thank you,
Randy.



Re: Procedure to measure rf watts

Mark Goldberg
 

I built one of these:

ae6pm.com/SCCARA-GRAM_Articles/A_40_dB_Power_Tap.pdf

Of course you have to calibrate it using a SA and TG or a VNA because the
attenuation is not exact, but I have had good results through HF feeding
the output to a SA or 50 ohm scope input.

Regards,

Mark
W7MLG

Re: 7904 excessive shadow/flare

Roger Evans
 

My comments about cathode voltage regulation were based on seeing a similar problem (and fixing it) in a much lower bandwidth scope without PDA.

I hardly ever use my 7904 at low sweep speeds, but lo and behold I get a similar displaced, diffuse arc (almost a complete circle) at 200msec/div. Poor regulation of the cathode voltage gives a diagonal line (or just a bright spot and a weaker diffuse spot) pointing to the CRT centre. When using the 7904 normally. I am used to a little 'flare' if the brightness is too high but it is never a problem and never a clearly defined second trace.

For what it is worth the HV regulator test point is TP1635, connected to pin 3 of the op-amp U1635 (schematc <11> CRT Circuit). It should be very close to 0V DC, it is difficult to say what ripple is excessive unless you can find a reference in the manual. The AC divider ratio at TP 1635 is very different to the DC divider ratio because of R1642/C1642. A DC measurement of the cathode voltage may not show a problem, especially if you have a digital meter, it mostly samples the correct, plateau, voltage and occasionally catches the dropoff before the capacitors charge again. An analogue meter would show the average voltage to be slightly low.

Roger

Re: 7904 excessive shadow/flare

unclebanjoman
 

On Thu, May 23, 2019 at 02:42 PM, Chuck Harris wrote:


I know that, but my 7904's beat my 7854 in the flare department
by easily double. They also have much brighter maximum intensity
than does the 7854.

I haven't explored the differences, but the 7904's all have a worse
time than *any* other scope I have ever seen.
O.K. indeed the artifact is two times more pronounced with my 7904 with respect to my 7854 (grossly speaking).
The question that arise is: why?
Both the two models share the same tube type.
The post acceleration voltage is the same (21 kV).
Catode voltage is the same.(I presume).

If this is due to the dome mesh, where is it located on the diagram?
is there a regulation that affects it?
On the 7904 scheme I also see an adjustment for the preset shield volt (I adjusted as per manual): what's the use?

Max