Starting a new thread as although this is on the same scope, this is a different question and I want to avoid another long thread.

I looked in the 455 service manual and in the Tektronix 'Troubleshooting Scopes' manual without luck.


The situation is, that the trace and waveform doesn't really go out of the bottom half of the CRT display. Furthermore, the 'Position' control seems to work backwards.

Has anyone encountered this before and have any idea as to where to check first?



James

Check the deflection plate connections.

--
Bob Haas

After checking the connections:

1. Measure the voltage on each deflection plate for the two extreme settings of the vertical position control.

2. Check that Ch1 and Ch2 have exactly the same behaviour in terms of how far the beam moves and whether the vertical movement is in the 'wrong' sense.

3. Put the scope in XY mode. Can you move the spot across the full screen using the X position control.

4. Lots more tests but let's see the answer to the first three to narrow things down!

Regards,

Roger

Hi Roger, hi Bob.

Looking over the connections reveals nothing loose or disconnected, although I did discover another missing component, this time on the main board. C386, a 4.7µF 35v axial.

As for the bullet points;

1. Measuring the two vertical deflection wires (blue/wh & brown/wh) From Gnd, both channels read from +5.05v at the 'min' POSITION level, up to +22.33v at the 'max'. Across the two wires it measures from -17v @ 'min' up to +17 at 'max'

2. Channels one and two are equally as confused and in an identical manner.

3. Yes. Horizontally.


Hope that helps shine some light onto things.

Seems to me that this thing is in need of at least a basic calibration.


James

An interesting observation is that as the INTENSITY is reduced, that the trace rises and can be controlled by the POSITION knob. Once the intensity is increased the trace quickly drops to around the 2nd graticule line from the bottom.

Almost as if the CRT is drawing the current away from the vertical section...?

On 5/19/2022 9:50 PM, James55 wrote:

An interesting observation is that as the INTENSITY is reduced, that the trace rises and can be controlled by the POSITION knob. Once the intensity is increased the trace quickly drops to around the 2nd graticule line from the bottom.

Almost as if the CRT is drawing the current away from the vertical section...?

Or that you have some filter caps that are on their very last legs.

James,

I find this very puzzling! From your measurements on the vertical deflection plates it should be possible to set the vertical position control so that the voltages on the two plates are equal. This should give no vertical deflection of the beam even if the cathode voltage is changing with beam current. To be doubly sure you could try removing the wires from the vertical amplifier to the pins on the CRT, and adjust the brightness to see if the beam moves. You could also check the voltage on pin 7 of the CRT (should be 14.1V according to the manual) but I would be surprised if an error there moved the beam very much.

Given that the beam does move with changes in beam current can you check the low voltage supplies, particularly +32V, +5V and -5V to see if they change with changes in beam brightness.

Regards,

Roger

Hi Roger.

Don't have free time right now, however, with the wires connected, the trace only moves slightly upwards, and only when the brightness is raised from zero but then remains motionless.The voltages also remain stable as the intensity is adjusted.

Had a quick look but couldn't see which was pin 7 on the CRT so shall have a better look later .

@Bert

Interesting that you say that, as there is some ripple on the 18000µF cap. Measured on a different scope it reads around 1.5v
When measured with a component tester however, the cap reads 22000µF, 3.1% Vloss and 0.0 Ω ESR

This cap was installed 'as new' just a few weeks ago.

James,

what about the corresponding rectifiers for that cap? Wouldn't leaky diodes also cause ripple?

-- Jeff Dutky

High ripple doesn't require component failure, just too much current being drawn from the supply. I am using the phone at the moment so it is not easy to find the location of the suspect cap but it sounds like it could be +5V? + / - 5V and +32V drive the vertical pre-amp and the output to the CRT plates so they are potential problem areas. There should be a current sensing resistor in each regulator so you can measure voltage drop across that.

Roger

Hi Roger,
without wishing to sound thick, are you talking about the op-amps when you say, 'regulators'?


James

I will check the schematics in the morning, but typically the low voltage regulators have an op-amp or a differential pair to compare the regulated voltage with the reference. The comparator output goes to the base of a couple of emitter followers to get the necessary current gain and often the emitter of the power transistor has a series resistor of 1 R or so which senses the output current and as the voltage drop increases it feeds back into the comparator circuitry to lower the output voltage if there is excess current.

If you measure the voltage drop across this resistor it directly gives you the output current but you need a little guesswork to decide if this is more than the PSU can cope with.

My point was that you need to distinguish between high ripple caused by a fault in the diodes/bulk smoothing capacitor and high ripple caused by excessive current load.

My difficulty in interpreting what you measure is this, if the voltages on the vertical deflection plates are correct and the beam does not deflect upward then the most obvious explanation is internal damage in the CRT. On the other hand if the beam moves when you change the brightness then it could be poor supply regulation most likely of the CRT. You would expect the cathode voltage to drop with more beam current and that makes the beam easier to deflect (slower electrons take longer to travel past the plates and get a bigger sideways kick) so the beam should move away from the CRT centre as the brightness increases.

You probably need to make some measurements on the low and high voltage supplies to see what if anything changes with changes in brightness.

Best of luck with the search,

Roger

Looking through the schematic, I am not seeing any 1R resistors. The closest (that I can find) being 0.51Ω

As for trace movement and brightness, the trace drops down the screen as the brightness increases. I must say, I am hoping the CRT is ok however, as mentioned, this scope has no case whatsoever due to it having been smashed into many pieces.
It seems a miracle that it works at all!


James

As HV drops in value, the deflection sensitivity increases, so the symptoms you describe are entirely consistent with poor HV regulation (brighter beam --> reduced HV). I doubt very much that a CRT fault is to blame. (The CRT could still have problems, but this behavior is unlikely to be one of them.)

--Cheers,
Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

If you position the beam closer to the centerline, the amount of shift should go down; you should be able to find a position that is insensitive to intensity changes. If you move the beam to the upper part of the screen the shift should go the other way. That is, if it's an HV regulation problem.

Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

My earlier reply seems to be lost, apologies if we end up with two!

Agreed that deflection away from the CRT centre increases with a drop in cathode voltage. It should also show up as a change in X deflection if James can display any sort of waveform.

Earlier in the thread James says he can't position the beam in the upper half of the CRT but also measures the delta-V between the Y plates changing from +17V to -17V by moving the vertical position control. This is what seems to cast some suspicion on CRT damage but other possibilities also need to be chased.

The current sensing resistors are R768 on -5V,R748 on +5V and R734 on +32V. James says these voltages are stable with changes in beam brightness. James, can you also check whether the unregulated 32V changes with brightness? This supplies the current to the oscillator for the high voltages on the CRT.

I guess you should also check for connectivity from the Y amplifier to the metal pins on the CRT neck in case there is a break in the wires or a bad contact from the crimp connector to the glass to metal seal.

Regards,

Roger

Hi again,

thanks for pointing out the resistors.

R768 has a 347mV drop across it (-5.343 to -4.996v)
R734 - 248mV (+32.248 to +32.00v)
R748 - 303mV (+5.303 to +5.00v)

Whereas the other rails all hold stable, the +32v 'unregulated' does in fact drop with an increase in the brightness, . Measuring from L558, reads between 50.63v down to 50.34v from minimum to maximum brightness.

As for the tube connections, without actually removing the CRT, they all seem sound.

Hope this helps shine some light on things?


James

Is it just me, or does 50 V seem a little high for the +32 V rail (even if it's unregulated)?

-- Jeff Dutky

On Thu, May 26, 2022 at 10:30 PM, Jeff Dutky wrote:

Is it just me, or does 50 V seem a little high for the +32 V rail (even if
it's unregulated)?

-- Jeff Dutky

Schematic says nominal is 52.2V, measured voltage looks reasonable.
Ozan

+95V is generated from HV transformer. If you observe TP+95V in sheet <2> (CRT & Z Axis sheet), does it stay regulated or does it change with intensity?

Ozan