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Tektronix 475


Stephen
 

On Sun, Aug 9, 2020 at 05:58 AM, <tenareze32@...> wrote:


Here in SW France it cools a bit at night. We have AC in the house, but not in
the workshop. 37°C at 18:00 today.
Simon
39deg C in Paris. Cools down to 29 at night. AC died on us.


 

If I understand correctly, you have a brightness and a (time base) deflection problem. There may be two problems or they may be two results of one (power supply?) fault.
Did you try readjusting the CRT bias and check for any light? CRT bias isn't critical, as long as you keep brightness within limits. Just make sure that once everything is ok, you readjust CRT bias so that the screen can be set completely dark with intensity fully CCW at intermediary timebase speeds (or X_Y).

Raymond


 

On Sun, Aug 9, 2020 at 07:35 PM, Raymond Domp Frank wrote:


readjusting the CRT bias
Obviously, I meant Grid bias (3x).

Raymond


Simon
 

I was able to check the cathode voltage (-2450V) using a VTVM which goes up to 3000V. I have some chunky test probes, but it makes me sweat in this heat. I figured that the PDA voltage must be fairly high to light a neon bulb through a a 22 MΩ resistor. Now that I have the CRT out I might try the arc test, but I am not too keen. My assumption was that the high voltage multiplier module would fail terminally. I suppose it could limp along at a lower voltage, but that should have been enough to get a spot on the screen.
I cannot understand why the beam finder did not produce a spot with 50 V on both X plates and 10 V on both Y plates. Apart from reducing the X and Y gains it also shorts the Z blanking signal to -8 V and should bias the tube into the right ballpark.
I will have to check the X and Y amplifiers methodically.
Simon


Simon
 

Some progress, I tested the PDA voltage with an ignition lead and spark plug. It produced a good spark that would certainly start my lawnmower, and I put the CRT back. The time base is working and I have an extremely bright trace. If I connect the Y leads to the tube, I get a fuzzy vertical stripe which responds to the time base and X position and magnifier. The voltage on one Y lead responds to a test signal on both channels, while the other rises to +10V and stays there.
Incidentally I do not recommend piercing the PDA/HT lead with a hypodermic needle as it leaves a path for the electrons to escape. It is much safer to push an ignition lead into the socket and connect it to a spark plug.
I don’t know why the trace is so bright (even on minimum intensity, also responds to focus) and I don’t understand why attaching the Y leads give a fuzzy vertical stripe that goes from top to bottom with only a few volts on the Y plates.
Thanks to everyone for pointing me in the right direction. This pastime is rewarding as it forces one to revise one’s hypotheses in the light of evidence and other people’s input.
Simon


Roger Evans
 

Some possibilties for the very bright trace:

1. One pole of the beam finder (S450B on schematic <12>) is stuck

2. The grid bias R1375 might need adjustment, possibly someone tried to compensate for the previously weak display.

3. If the brightness control has no effect then maybe there is a fault in the Z axis amplifier.

Your measurements on the Y plates show there is a fault in the vertical amplifier, it may have gone into oscillation.

Roger


Simon
 

I have -20 V grid bias and the bias pot has no effect either on the grid voltage or brightness of the trace so I will check components around it.
Simon


Chuck Harris
 

Tektronix occasionally had some issues with welds inside of
the CRT's. The last time I saw a CRT that was very bright,
and didn't respond to intensity, grid bias, or any Z-axis
changes, it turned out to be a broken weld inside of the CRT
that opened the control grid, leaving about a 1/32" gap between
the wire that went to the tube base, and the strap that
connected to the control grid.

I have heard of at least one other instance (on this group)
where this exact failure happened on a 465/475 scope.

Similarly, a bad socket or a bad connection to the grid
would do the same thing.

Connect a floating DVM to the grid lead, and the cathode
lead of the CRT.... it must be insulated to withstand 2KV,
and don't touch it. Adjust the intensity, and look for
a corresponding change in the grid cathode voltage.

This is going directly to the horse's mouth.

-Chuck Harris

tenareze32@... wrote:

Some progress, I tested the PDA voltage with an ignition lead and spark plug. It produced a good spark that would certainly start my lawnmower, and I put the CRT back. The time base is working and I have an extremely bright trace. If I connect the Y leads to the tube, I get a fuzzy vertical stripe which responds to the time base and X position and magnifier. The voltage on one Y lead responds to a test signal on both channels, while the other rises to +10V and stays there.
Incidentally I do not recommend piercing the PDA/HT lead with a hypodermic needle as it leaves a path for the electrons to escape. It is much safer to push an ignition lead into the socket and connect it to a spark plug.
I don’t know why the trace is so bright (even on minimum intensity, also responds to focus) and I don’t understand why attaching the Y leads give a fuzzy vertical stripe that goes from top to bottom with only a few volts on the Y plates.
Thanks to everyone for pointing me in the right direction. This pastime is rewarding as it forces one to revise one’s hypotheses in the light of evidence and other people’s input.
Simon




Simon
 

I am beginning to suspect the CRT again as the instrument had obviously been dropped. I have not been able to increase the grid bias to more than -20 V. I will check the grid voltage again with my VTVM and see if I can change it.
I can set the voltage at TP 1364 to 15 V with the intensity control, and increase it to 26 V by pressing the beam finder as described in the manual The trace is very bright at all settings. but beam blanking is working.
I do not understand how I have gone from no trace to too much trace, the whole screen glows green even with the sweep off. All I did was take out and test three transistors on the horizontal board connecting the sweep section to the horizontal amplifier section. Maybe re-seating the transistors got the time base working normally.
Also connecting the Y leads to the tube produced a wide vertical stripe of much reduced intensity, so I think there is something odd going on inside the tube.
Simon


Tom Gardner
 

Have a look at the ground-referenced version of the blanking pulse. It should be less than 150V or so, so some care is needed.

The AC component is the important part, and the Vac reading shown on a multimeter will depend on the timebase, the holdoff, the highlighted B timebase portion (if any). Note how the AC value changes as you fiddle with the controls.

Now transpose the meter so that is measures the grid-cathode voltage, and the AC component there should be the same as the ground referenced blanking waveform. If not, then there is a fault in the resistor-capacitor-diode network that transposes the blanking waveform up to the cathode voltage.

Obviously extreme care is required since the meter and leads are 2-3kV away from ground, even though the differential voltage is only 150V or so.

On 11/08/20 15:06, tenareze32@... wrote:
I am beginning to suspect the CRT again as the instrument had obviously been dropped. I have not been able to increase the grid bias to more than -20 V. I will check the grid voltage again with my VTVM and see if I can change it.
I can set the voltage at TP 1364 to 15 V with the intensity control, and increase it to 26 V by pressing the beam finder as described in the manual The trace is very bright at all settings. but beam blanking is working.
I do not understand how I have gone from no trace to too much trace, the whole screen glows green even with the sweep off. All I did was take out and test three transistors on the horizontal board connecting the sweep section to the horizontal amplifier section. Maybe re-seating the transistors got the time base working normally.
Also connecting the Y leads to the tube produced a wide vertical stripe of much reduced intensity, so I think there is something odd going on inside the tube.
Simon


Tom Gardner
 

You too; I would have thought that was a rare failure.
https://entertaininghacks.files.wordpress.com/2016/03/crtgrid.jpg

In my case the CRT was a torch sufficient to illuminate a room well enough to move around, but not well enough to read.

On 11/08/20 14:13, Chuck Harris wrote:
Tektronix occasionally had some issues with welds inside of
the CRT's. The last time I saw a CRT that was very bright,
and didn't respond to intensity, grid bias, or any Z-axis
changes, it turned out to be a broken weld inside of the CRT
that opened the control grid, leaving about a 1/32" gap between
the wire that went to the tube base, and the strap that
connected to the control grid.

I have heard of at least one other instance (on this group)
where this exact failure happened on a 465/475 scope.


Simon
 

The maximum grid-cathode voltage I can attain is -18 V which I reckon should cut off the beam, but I don’t have the specification for this tube. The grid bias pot does reduce this voltage at certain settings of intensity as does the intensity control. These adjustments would increase the brightness, but it is already too bright. All the components in the DC restore circuitry test OK and the extra winding on the HT transformer is producing about 40 V p-p (though 47 pF and 390 kΩ), which I assume produces the maximum negative bias for the grid.
I’ll troubleshoot the Y amplifier to see whether it might be worth replacing the tube.
Simon


Chuck Harris
 

CRT's take rather a lot of negative grid voltage, WRT cathode,
to cut the beam off. Most take from -50 to -100V.

-18V would be full brilliance.

Looks like your CRT is off the hook!

Invest some time in the circuit descriptions, starting on p3-16.

It's time to fix your bad DC restorer circuit.

-Chuck Harris

tenareze32@... wrote:

The maximum grid-cathode voltage I can attain is -18 V which I reckon should cut off the beam, but I don’t have the specification for this tube. The grid bias pot does reduce this voltage at certain settings of intensity as does the intensity control. These adjustments would increase the brightness, but it is already too bright. All the components in the DC restore circuitry test OK and the extra winding on the HT transformer is producing about 40 V p-p (though 47 pF and 390 kΩ), which I assume produces the maximum negative bias for the grid.
I’ll troubleshoot the Y amplifier to see whether it might be worth replacing the tube.
Simon




Tom Gardner
 

On 12/08/20 15:05, Chuck Harris wrote:
CRT's take rather a lot of negative grid voltage, WRT cathode,
to cut the beam off. Most take from -50 to -100V.

-18V would be full brilliance.
That corresponds with my recollection (and the ~150V uses in the Z axis circuit), but it has been a few years since I looked.

I recall the peak-to-peak voltage being ~100V, but the AC RMS voltage measured with a meter varies widely depending on the timebase, trigger, and holdoff controls.

Looks like your CRT is off the hook!

Invest some time in the circuit descriptions, starting on p3-16.

It's time to fix your bad DC restorer circuit.
Only after the ground referenced Z axis waveform has been shown to be correct.

Then, if the AC RMS component  of the ground-referenced and HT waveforms differ, the  DC restorer is likely faulty.

With my CRT's broken grid they were the same, which encouraged me to extract the CRT and find the fault.


tenareze32@... wrote:
The maximum grid-cathode voltage I can attain is -18 V which I reckon should cut off the beam, but I don’t have the specification for this tube. The grid bias pot does reduce this voltage at certain settings of intensity as does the intensity control. These adjustments would increase the brightness, but it is already too bright. All the components in the DC restore circuitry test OK and the extra winding on the HT transformer is producing about 40 V p-p (though 47 pF and 390 kΩ), which I assume produces the maximum negative bias for the grid.
I’ll troubleshoot the Y amplifier to see whether it might be worth replacing the tube.
Simon


Simon
 

I came to the same conclusion after reading more about CRTs.-18 V is not enough to cut off the beam. Most specs I have seen require 40-50 V, so something is wrong with the negative bias circuitry, but all components in the DC restorer circuit seem OK. Back to basics I guess, something is amiss in the generation of the negative potential.
I can trace the Y signal up to U470 in the vertical output amplifier. I will attempt further testing as it is a custom part (155-0082-00), but I believe it can still be obtained. I also checked the Y deflection directly on the plates using a 9 V battery, beam goes up and down as expected.
Simon


Simon
 

I am not sure what you mean by the ground referenced Z axis waveform. Can I put a scope (DC coupled) on the Z axis amplifier output? Is this the waveform that produces a negative grid potential?
Many thanks
Simon


Tom Gardner
 

OK, I've just dug out the schematic.

The Z axis amplifer PSU is 0V to 110V at CR1360 on sheet 12. I.e. /not/ the 150V I remembered. Hence the max peak-peak voltage would be 110V, but in reality it would be noticeably less.

It looks like the output is available at TP1364, and maybe an AC coupled version at TP1366. Provided your probe and scope are rated accordingly, and neither the probe nor the shield slips, it should be safe to probe those TPs with a scope.

The Z axis amplifier is described on p3-19, and voltages on p5-26.

Pages are from the PDF it /looks like/ I got directly from Tektronix.

On 12/08/20 15:25, tenareze32@... wrote:
I am not sure what you mean by the ground referenced Z axis waveform. Can I put a scope (DC coupled) on the Z axis amplifier output? Is this the waveform that produces a negative grid potential?
Many thanks
Simon


Simon
 

I have read both those sections, but my understanding is lacking. I carried out the procedure described in 5-26 and it checked out OK, although the trace was very bright all the time.
I will check the Z-axis amplifier as it does not appear to be producing enough negative potential.
I was not keen to buy another tube for a scope I bought fairly cheaply for parts/repair. I first checked all the power supply voltages and they were within specification so I was dismayed not to get a spot or trace with the beam finder. I still don’t know why it appeared, but the X amplifier is flaky and I sometimes get a spot and sometimes a trace. I should probably re-seat some more transistors.
Simon


Tom Gardner
 

On 12/08/20 16:10, tenareze32@... wrote:
I have read both those sections, but my understanding is lacking. I carried out the procedure described in 5-26 and it checked out OK, although the trace was very bright all the time.
I will check the Z-axis amplifier as it does not appear to be producing enough negative potential.
Its output can only be between 0V and +110V. The AC component of the waveform will have positive and negative components.

The 465 circuit is very similar, and my service manual shows some waveforms (87, 88, 89). Perhaps those might help your understanding.



I was not keen to buy another tube for a scope I bought fairly cheaply for parts/repair. I first checked all the power supply voltages and they were within specification so I was dismayed not to get a spot or trace with the beam finder. I still don’t know why it appeared, but the X amplifier is flaky and I sometimes get a spot and sometimes a trace. I should probably re-seat some more transistors.
Don't forget that other traditional problem areas are dirty switches, "dry" electrolytics, and accumulated muck in the EHT circuits.


Bert Haskins
 

On 8/12/2020 11:10 AM, tenareze32@... wrote:
I have read both those sections, but my understanding is lacking. I carried out the procedure described in 5-26 and it checked out OK, although the trace was very bright all the time.
I will check the Z-axis amplifier as it does not appear to be producing enough negative potential.
I was not keen to buy another tube for a scope I bought fairly cheaply for parts/repair. I first checked all the power supply voltages and they were within specification so I was dismayed not to get a spot or trace with the beam finder. I still don’t know why it appeared, but the X amplifier is flaky and I sometimes get a spot and sometimes a trace. I should probably re-seat some more transistors.
Simon