TEKTRONIX 2211 INTENSITY MAX OUT

[beanhauler]

I have a bit of an issue with this scope with the intensity control not being able to turn down.
The scope has two intensity pots arranged  as a dual pot on the front. Turning the post has no effect in storage mode or non storage mode.
Board mounted bias pot has no effect although wiper changes volts from 101V to about 47V.
Between Grid and cathode the differential voltage changes from 8.7V to 15.26V which i assume is not enough to negatively bias the grid relative to the cathode.
I am unable to measure absolute voltages as yet as i need a HV probe but am building one.
One side of R842 changes from 5 to 15V the other side of CR853 changes from 14.94 to 40V.
If the wiper of R851 can only go down to 47V min then this voltage appears too low perhaps.
I have checked all diodes which seem ok and replaced C854 1000 C855 0.01  R860 6.2M and all focus resistors.
focus works fine from about centre pot position but beam intensity melts the screen.
Just cant work out why i cant get enough negative bias on the grid.

does anyone have any idea. Any input gratefully received as now starting to push my knowledge of glass tubes.
If the cathode volatge was too low would that have any effect on the differential i wonder or excessive ripple.
 

[George Edmonds]

Hi


You appear to have a problem with the cathode to first grid voltage, normally it is a negative voltage but I do not have the specifications for the Tektronix CRT used.

Is the waveform shown at 12B correct?

Absolute voltages for a CRT are usually not critical, voltages relative to the cathode ones are.  I don’t understand why you are changing resistors in the focus chain when the problem is with the CRT bias voltages.

G Edmonds

[beanhauler]

Thanks George
I changed the focus resistors purely due to the age of them and many people advising to do so.
The tube is part number 154-0907-00 which has an anode of around 12.5K but i have no idea on the rest of its spec.
I will try and get a picture of the waveform at 12B. I assume it has an unblanking pulse there or else it would not have any beam current?. Mind you i suppose if all was up the wall then the unblanking would be overridden.
I have measured grid to earth and its around 4.5 Meg ohms which would seem to be OK.
I really need to look at the absolute voltage on the cathode i think rather than relative.

Thanks again for your input

[George Edmonds]

Hi

Absolute voltages are not important, relative ones are.

Later I will post an explanation for you.

G Edmonds

[beanhauler]

here are some pictures i took of 12B. With my trusty TEK 2235 which i rescued from the grave with an inverter transformer from the states. Long story that one but i digress.
Picture of expected waveform at 12B from manual
Shows 3ms and a height of 20V
My pictures are with scope set 2 ms div and 20v per division y
Note that the waveform is not an absolute match and i can vary the intensity control from 0v to 40V
The scope mid position is ground.

I have also added a waveform of intensity in storage mode. Note the chars at top.
Intensity control has the same range but waveform is a little different at test point 842.

 i also took a snap at R853 sine wave 50 us 20v div which looks about right and sitting on a positive dc of about 20V
which from the manual would be about right i think

Looking at the display it does hava little bit of iintensity modulation on it when displaying its iternal waveform


As the waveform at 12B is able to be taken to zero volts with the intensity control via the pots then you would assume that the grid is now blanked.....ther is no unblanking signal. Yes the problem must be the bias around grid cathode and i probably need to measure absolute voltages for that. I do have ahigh voltage scope probe so maybe that needs to be used.

OK used my high voltage scope probe and pin 2 cathode sitting at about  minus 1080V. Schematic implies that it should be around 1.8KV. Tweaking intensity control has no real effect although seeing a slight ripple on it one end.
Pin 3 the grid appears as exactly the same voltage minus 1080V. Intensity pot has no effect on thye voltage on grid.
What is going on here...is the tube short internally i wonder.

i think i need to unplug the tube here to see what effect that has maybe unless someone has a better idea?

[George Edmonds]

Hi

As with all things, particularly Tektronix, are all the internal power supply output voltages and ripple level correct?

Now if as you say the cathode voltage is sitting at about -1kV and so is the control grid (G1)the CRT will be operating at maximum intensity, which I believe it is.  A word of warning, you can easily get screen burn or cathode stripping under these conditions.  What you need is for the control grid to be more negative by say -40V to say -75V relative to the cathode so as to bias the CRT off.

From years of experience I distrust high voltage probes unless they have been calibrated with the readout meter.  This may appear to be a strange comment in today’s digital world, but your best friend when trying to find a fault around the gun circuitry of a CRT is something like an AVO 8.  On its 2.5kV or 3kV range it has a very high input resistance and is very well insulated, just do not touch it or the probes when using it when measuring some 2kV.

The incorrect -1.8kV supply to the cathode is the prime problem to look at first.  There is an over wind on the main inverter transformer that appears at point 11 of the High Voltage and CRT schematic.  This AC voltage is then rectified by the first stage of the eight stage Cockcroft and Walton voltage multiplier contained in U975.  The output DC is then filtered by C975, C978, C979 and R976, R978, check these components, in particular the capacitors for breakdown/leakage.

Ignore 10.8kV the EHT/PDA output of U975 for now UNLESS there is evidence that loading on this output is effecting the 1.8kV supply (usually resulting in audible effects or visible sparking).  Regrettably the most common cause of problems with Tektronix scopes is the voltage multiplier U975, in your case, suffering from an internal high voltage breakdown.

An internal short in the CRT is highly unlikely, but not impossible.

Hope this helps

G Edmonds

[beanhauler]

Thanks for the pointers George.
Given me some more work to do.

I think the 1.8K supply is the issue either being pulled down or the output is short on the inverter transformer.
I had this before in my 2235 but it was so severe that teh inverter would not run even with all the loads disconnected apart from the HV circuit.

i'll work through your ideas

kr

chris

[George Edmonds]

Hi Chris

I doubt if you have problems with the inverter transformer, in your original posting you stated that the +100V supply to the bias pot was correct, this voltage is derived directly from the inverter stage without any form of regulation so it would appear that the inverter stage is operating correctly and is not overloaded.

The 1.8kV cathode supply also comes from the inverter transformer via U975 the X8 voltage multiplier which also provides the CRT PDA voltage.  To measure the 1.8kV at the inverter transformer is very difficult for the normal user as any voltage multiplier probe must also be frequency compensated, not even the infamous Fluke 80K-40 high voltage probe is, I would not trust it much over a few hundred Hz.

You need something like a Tektronix P6015A probe to measure the inverter transformer 1.8kV output and a second mortgage to buy one.

It is also hard to see that a load on the 1.8kV cathode supply is causing the problem.  The inverter stage is fully overload protected and would shut down (start ticking) with a 1.8kVoverload.

Sorry to have to say this, but are you certain that your measurements of the 1.8kV supply are accurate.  If they are the most likely cause is U975 the voltage multiplier, the most common part  to fail part of any CRT oscilloscope, at present I have a 2215 for repair with a faulty one which looks to be unobtainable.

G Edmonds

[Wallace Gasiewicz]

George,

Your Commonwealth buddies in Canada have the HV multiplier, very reasonable price:

https://www.sphere.bc.ca/test/stuffday.html#tekfans

There is one for the 2315
I don't know if this would work in the OP's Scope

Wally

[beanhauler]

Hi George

Took a litttle deviation while i built a HV probe to measure the cathode and grid voltages.
Actually i calibrated it on a 60V supply and when i potted it and measured it on the scopes 100v supply it read 25% too high, typical.
Anyway i thought iw would try some relative measurements any way.
Measured the cathode at pin 2 at switch on  - 2531 Volts and settling to -2480 which i know is about 25% too high so should be around 1.8KV.
Ripple seems high measured as 100V say 75V but maybe expected. Turning intensity control or grid bias has little or no effect as expected.
Pin 3 grid i measured as around -2514V and the grid bias pot changed it down to -2505.
Given only 34V difference between grid and cathode [less with my error probe]i can understand why i'm not getting any dimming of the trace.
 I think i'd better go back and check the level of ripple on all the inverter supplies before i go any further.
There is something not right here and i'm at a loss to understand what it is.  If ripple on the inverter supplies is causing this i will be very surprised. Still you never know. I am thinking that maybe i should block change all of the diodes cr855/851/853 and perhaps the c's for the cathode smoothing although i have not see these fail before.

[beanhauler]

Checked all voltages after the inverter and all within tolerance and ripple spec of tektronix.
Actually most are a few millivolts of ripple and nowhere near the 30mV allowed.
Waggling the intensity control up and down provides a waveform of up to 40V positive at TP 842.Beyond CR853 the waveform is fairly awful.
Ihave changed C855 and 854 and no change.
I think i will change out C975/6/9 to see if its excessive ripple on the cathode line. I have no idea if 75V or so of ripple would cause an issue on 1.8Kv.
I do have negative volts after U975 but wonder if the diode in the block is bad.

This is tough.

[George Edmonds]

Hi Chris

At a quick look I cannot find what the voltage at TP842 should be.

Ripple could be a major problem as the are a lot of inputs to the intensity circuits which are AC coupled.

I have a 2215 and as soon as I have time I will measure the CRT grid cut off voltage.

It is very limiting to not to be able to measure the voltages around the CRT gun accurately.

Normally any problems with U975, a X8 Cockcroft and Walton voltage multiplier would cause the main power supply to shutdown with the scope making a ticking sound.

C975, C978 and C979 could well be the cause of your problem, BUT you have no way of testing them as it would have to be done at say 2.5KV to be certain.  I can test them as I have a 2KV plus power supply limited to an output maximum current of 20mA

Will try to determine what the CRT grid cut off voltage should be in the next week.

G Edmonds

[beanhauler]

Thank you George for your continuing support, its much appreciated.
 have ordered three 10n caps from farnell rated at 3KV and will try those in circuit to see if any effect. I will also change R860 to a 1% 6.2M resistor and 487ohm i think R858.
I will also start changing some diodes but the original ones are hard to come by.
I think the the originals are 1n646 and have some BYW55 so may try them. They have low cap and rev V is greater than 225V of the originals.
All the components seem ok when measured but like you say unless you have high voltage cap checkers and a decent volt probe its a bit trial and error.
I had an old crotech scope years ago which would come on bright and then slowly fade away. This was caused by the cathode caps going low value and excessive ripple shutting off the grid. This seems to be the reverse. I'm beginning to wonder if there is a way to shut the beam current off by addition of a suitable resistor between cathode and grid /gnd to see if it is possible to shut the beam off
Another weird thing is that the focus seems to function ok but the beam width seems a little wide, probably due to the intensity maxed out.
The grid bias pot does change its value ok but i see no real change on the other side of CR851 R850. CR851 though does measure OK.
Just waiting for the Farnell order to arrive.

[George Edmonds]

Hi

Can you please give me the part number that is on the actual power supply transformer.  It should look like, but not the same as 154-0861-00

All the best

George

[beanhauler]

Hi George

Unfortunately i cant see a part number from above
I can try a endoscope later if you think it may be on the side of it

I take it you mean th eHF transformer and not the torroid

[beanhauler]

Changed HV caps on cathode smoothing circuit
No change to intensity

All measured 10n out of circuit.

Next to try R860/858 and then some diodes
This may beat me 

[George Edmonds]

Hi Chris

I asked you for the part number for the inverter transformer T902 because Tektronix got the CRT heater voltage wrong on a lot of 2000 series scopes.  The heater voltage was to high resulting in cathode poisoning with time.  All scopes should have been retrofitted, should is the operative word.

It also may be that the 2011 did not have the above problem, TekWiki does not have some of the usual information it normally provides for the 2011.

I have a grave yard collection of 2000 series scopes including a 2235 in an unknown condition, this scope has a CRT circuit very similar to your 2011.  Life at the moment is a little manic, but as soon as I can I will see if I can get the 2235 working and at least determine the CRT grid cut off voltage.

One way or another we will find out what is wrong with your 2011


G Edmonds

[beanhauler]

Thank you George

You are a true gent and thank you so much for your patience.
I changed R860 the 6.2M and the 487 ohm resistor in parallel through it via a diode well two actually.
The old ones seemed fine and measured OK.
Still no joy but i did some cold checks again around the tube.
Measuring between grid and cathode with all connected  i get around 200K on first connection which goes downward slowly as something charges. This is the same whatever way round the leads from the multimeter are.If this is in parallel with the 6.2M resistor then the voltage between cathode and grid would be nearly equal. Comp-lication are the c's around it though. I might just cut the track and see what the resistance is with grid to cathode open.
I think i need to think of a way to shut off the tube with a resistor combination to prove that it can at least be shut off. I do have a 2013 scope with a good tube but do not know if they are the same. Might open it up and take a look.

[George Edmonds]

Hi Chris

Did you mean 2213 not 2013, if 2213 I also have one in at present in unknown condition.  It looks like it may have a shot CRT as someone has disconnected it.

Apart from positively establishing the Cathode-Grid cut off voltage it will be of little help to you as it has a complex auto brightness provision which is not the same as your 2211.

I am now starting to understand just how the CRT grid voltage is generated.  Not a simple matter as it uses an AC to DC restorer synchronised the main inverter frequency.

As a vast over simplification , you have a CRT grid sitting at about -2kV upon which you need to impose a varying low DC voltage, this cannot be done with DC coupling so it is done with capacitive coupled AC which is then restored to DC on top of the -2kV DC CRT grid voltage.

You can help to stop screen burn by defocusing the beam.

We will get there.

G Edmonds

[beanhauler]

Yes sorry George i did mean 2213.
looking at the tube in this one 2211 it says part number 154-0907-00.
I cant find anything on the 2213 tube and  i wonder if its the same tube but i doubt it.
Funny thing is i cant find anything in the service manual for the 2211 that identifies the tube number. Did tektronix leave that info out strange.

Thanks for your explanation of the control of the grid/cathode. You are right as its using the ac voltage clamped by the diodes to shift the voltage across the caps and hence vary the grid cathode voltage.
The description of the operation in the manual is pretty thorough and component wise i have just about changed everything apart form the diodes which all look ok statically. Maybe one of them is breaking down with HV although the tube brightness is a constant and not really being modulated.
I think they are coming out. I am wondering if the z mod output voltage is high enough and i need to measure to be sure its from +3v to 75v

Continuing on at the moment. I will get there eventually and end up with  a better understanding of how this works. Its just a shame the voltages are so high and need to be somewhat careful.:-) I think if i had the right kit you could fix this in two minutes. Still all good fun.