468 Digital Storage scope


Dr. Manfred K. Zeller
 

I need your help:
I am a newcomer-
all power rails are ok, but the tube heating has only 1.2 Volt?
I have no idea?
Regards and thank you,
Manfred


Mark Vincent
 

Dr. Zeller,

Welcome to the group.

Since the heater is at 1,2V, two things are likely without any further information, the resistor from the transformer to heater has gone high in resistance and/or bad solder joint(s) from the transformer to the socket. If the voltages on the secondaries of the high voltage transformer are right, the sentence before is the likely cause. If the resistor is bad, it can be replaced with a higher wattage if you want, e.g. 1/4W to 1/2W. If the output voltages are low, then check the oscillator stage including the regulation circuit.

Mark


 

Do you have a copy of the service manual? My first guess would be the HV multiplier. Can you check the voltage at TP 328, should be -2450 volts.

http://www.ko4bb.com/getsimple/index.php?id=download&file=Tektronix/Tektronix_-_468_Digital_Storage_Oscilloscope/Tektronix_468_Digital_Storage_Oscilloscope_Service_Manual_Volume_2.pdf

Regards,

Tom

On 4/18/2021 10:09 AM, Dr. Manfred K. Zeller via groups.io wrote:
I need your help:
I am a newcomer-
all power rails are ok, but the tube heating has only 1.2 Volt?
I have no idea?
Regards and thank you,
Manfred



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This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus


Dr. Manfred K. Zeller
 

Hi Mark,
thank you very much helping me.
A really wonderful group - thanks to all.
Regards
Manfred


Dr. Manfred K. Zeller
 

Hi Mark, could it also be, that is something wrong with the tube?
Regards and thank you,
Manfred


Dr. Manfred K. Zeller
 

The oscillator stage is ok and also the line between transformer and tube
socket. I checked the heater with external measurement and did something
wrong. The heater was gone. Installing a new tube - no change on the
oscillator circuit, no change between transformer and tube socket. Now,
with the new tube there are newly signs on the screen. Only for the xy mode
and pushing the beam finder button?
What does this mean?
Thanks
Manfred


Dr. Manfred K. Zeller
 

Thank you Tom - the 2450 are there.


 

On Tue, Apr 20, 2021 at 06:31 PM, Dr. Manfred K. Zeller wrote:


all power rails are ok, but the tube heating has only 1.2 Volt?
Forgive me for asking: Do you have at least basic knowledge about 'scopes and the high voltages that are present in them, often for a long time after switching them off? I'm asking because measuring the filament voltage isn't the first thing one normally does in these instruments, for several reasons.

Did you measure the CRT filament voltage by just touching the connections with your test wires, holding them in your hands and if so, did you realize that the filament is at -2450 V against ground on a functioning 'scope? If so, you may have been extremely lucky that the high voltage is not working... because that's what is most likely yyour situation.
Apart from a problem in the CRT causing this, which is unlikely, many other faults are far more likely, like a defective HV multiplier, as Tom M. suggests.
I am not aware of the existence of a resistor between the transformer and the heater filament, like Mark V. suggests. The transformer that Mark refers to most likely is T216 (dwg11), not the mains transformer.

You didn't tell us what symptoms you observe, like possibly a "fully functioning" instrument, with vertical attenuator, trigger and graticule lights working or not. If, as you say, all low voltages are OK, this could well be the case.

If at least some of the above is ok, the first thing I'd do is check F809, the inverter fuse in the HV area (after disconnecting the mains supply) and report back.

Raymond


 

On Tue, Apr 20, 2021 at 10:57 PM, Dr. Manfred K. Zeller wrote:


The oscillator stage is ok and also the line between transformer and tube
socket. I checked the heater with external measurement and did something
wrong. The heater was gone. Installing a new tube - no change on the
oscillator circuit, no change between transformer and tube socket. Now,
with the new tube there are newly signs on the screen. Only for the xy mode
and pushing the beam finder button?
What does this mean?
Aah, we got our messages crossed. Most of what I wrote is obsolete by now it seems.

Raymond


 

On Tue, Apr 20, 2021 at 10:57 PM, Dr. Manfred K. Zeller wrote:


no change between transformer and tube socket.
What do you mean by that?

Raymond


Dr. Manfred K. Zeller
 

What I could not understand:
2450 Volts are there and all all power rails correct.
1,2 Volts Heater Voltage with the former tube.
No signal on the screen.
oscillator stage ok and connection to the tube ok.
Then: changing the tube
Signal: xy mode and beam finder pressing
The signal only points on the right side of the screen?
With the former tube - no signal on the screen.

Service Manual is there.
Thanks a lot!
I am a fan of this wonderful piece of technique.
Great to see and a lot of respect for people who invented this.


 

On Tue, Apr 20, 2021 at 11:15 PM, Dr. Manfred K. Zeller wrote:


What I could not understand:
2450 Volts are there and all all power rails correct.
1,2 Volts Heater Voltage with the former tube.
No signal on the screen.
oscillator stage ok and connection to the tube ok.
Then: changing the tube
Signal: xy mode and beam finder pressing
The signal only points on the right side of the screen?
With the former tube - no signal on the screen.
Are you sure you made a correct measurement of the filament voltage? It's a > 20 kHz (I think) AC voltage, not DC or 50 Hz! Your meter may not be equipped to measure it correctly.
You said "the heater was gone", meaning open circuit?
Apart from whether your filament measurement was correct, without a functioning filament, there won't be light on the screen. With the now functioning CRT, it is able to show something.
Image only on left or right side of screen indicates loose connection to horizontal plate and/or broken horizontal output side. A short between one of the horizontal connections and ground (chassis) will likely damage the horizontal output.

Raymond


Dr. Manfred K. Zeller
 

It is now like it is / pressing the beam finder in the xy mode on the time
base I have now a point on the right side of the screen.
Otherwise the screen is empty.
What do you recommand doing the next step?
Thanks a lot!
Manfred


 

On Tue, Apr 20, 2021 at 11:34 PM, Dr. Manfred K. Zeller wrote:


It is now like it is / pressing the beam finder in the xy mode on the time
base I have now a point on the right side of the screen.
Otherwise the screen is empty.
From your earlier message I concluded that "the signal" extends across some part at the right side of the screen, not that it's just a point, as you now describe.

- Are you familiar with operating this instrument if it's healthy? I'm trying to exclude operating error here. I'm asking this because in XY-mode, without any signal applied, a point is all you should see... and you have to pay attention to the exact settings and where and how you connect your signals.

- Can you move the point on the screen by operating horizontal and vertical position controls?

- What do you see in non-XY mode, i.e. YT-mode?

- Without pressing beam finder (BF), the screen shows nothing? Did you try increasing the intensity setting?

- The position of the image (point in your case apparently) while pressing BF gives an indication of whether deflection is extreme (outside normal limits). BF tries to set image brightness to visible and decreases deflection to move image on screen so operating BF should give at least some indication of what's going on.

- Did you correctly reconnect the horizontal and vertical plate connections?

Raymond


 

On Tue, Apr 20, 2021 at 11:34 PM, Dr. Manfred K. Zeller wrote:


pressing the beam finder in the xy mode on the time
base
I don't understand what you mean because in xy mode there's no time base present.

Raymond


Dr. Manfred K. Zeller
 

Thank you Raymond for helping me - I did not know that the heater is
connected to 2450 Volt.
Thank you very much for your help!
Manfred


Tom Lee
 

Yes, electrostatically-deflected crts have to operate the plates near ground potential to prevent distortions due to other objects near the tube (such as your finger resting on the screen). That forces the cathode end to be kV below ground. And, since you can’t have too great a voltage difference between heater and cathode (to avoid breakdown), the heater has to be at about the same potential as the cathode.

TV crts are magnetically deflected, so their cathodes are near ground potential, while the anodes are very positive. Those with experience only with TV crt circuits often get, er, surprised by scope crt circuits. Glad you survived your encounter!

— Cheers,
Tom

Sent from my iThing, so please forgive typos and brevity.

On Apr 20, 2021, at 2:54 PM, Dr. Manfred K. Zeller via groups.io <drmanfredzeller=googlemail.com@groups.io> wrote:

Thank you Raymond for helping me - I did not know that the heater is
connected to 2450 Volt.
Thank you very much for your help!
Manfred





Mark Vincent
 

Raymond and Dr. Zeller,

The resistor I mentioned is commonly there in many models. I know not all have a heater resistor. I do not have the schematic in front of me. What I said was an assumption of a resistor being there. What has been suggested as a likely cause is the high voltage transformer being bad. What is said about measuring the heater voltage is correct. Use a good VOM or DMM that will respond to the frequency of the oscillator circuit and measure across the heater pins, 1 and 14 of the base. It should be 6,3VAC. Keep your fingers away from the tips when the scope is on. You can measure the resistance with the socket removed to confirm a complete circuit, not open. There will be a up to a couple of ohms of resistance due to the winding resistance. Read and do what the other posters have said. They will be spot on about your problem.

Mark


Mark Vincent
 

What I said about the transformer being bad was wrong. The multiplier is suspect. My mistake. The multipliers will go bad causing your problem. I apologize for any confusion.

Mark


Michael W. Lynch
 

On Tue, Apr 20, 2021 at 04:34 PM, Dr. Manfred K. Zeller wrote:


It is now like it is / pressing the beam finder in the xy mode on the time
base I have now a point on the right side of the screen.
Otherwise the screen is empty.
What do you recommand doing the next step?
Are you operating the scope in the Non-Store or Storage Mode? You should be working in NON-Store mode until you confirm that the analog portion is working correctly.

--
Michael Lynch
Dardanelle, AR