Got a Tek 454A the other day and did a quick test. It turns out that the only way to see a trace on the screen is to hold down the “Beam Finder” switch. As soon as it was released, the trace was gone. When I turned the "Intensity" knob all the way to the max, I can see some blurry shadow-like stuff on the screen. I shot a video to demo the problem:
https://youtu.be/7HERqOtBGOo?si=wpvnHFCTCqLt0IzT

So far, I have tried the following for troubleshooting:
1. Tried all vertical sensitivities and it didn’t make any changes. Also tried a x10 probe, still nothing. Switched trigger to GND, nothing. Adjusted the “position” knob and I could barely see some slight movement of the shadow - in beam finder mode, it moved the line up and down so the knob works fine.
2. Measured the PSU rails, including the -12V, +12V, and +75V and they were all good.
3. Measured the sawtooth waveform on pin L on the Sweep A board and the waveform looked fine and the Peak-to-Peak voltage was very close to 10V.
4. Cleaned the Beam Finder switch.

I’m curious, why in beam finder mode, the waveform and the trace were clearly visible - it’s just that the screen was vertically compresses, which I suppose it normal. That said, everything disappeared as soon as I let go off the momentary switch. I did some research and it seems that the beam finder mode bypasses the intensity control circuitry so does it mean this circuitry is faulty? How can I verify that and troubleshoot?

Any thoughts are appreciated!

To me it looks like the beam is being vertically deflected a lot.

You mentioned yoy adjusted "position", but in your video the channel 1 vertical position is all the way counter clockwise, so the beam is all the way at the bottom.

The slow sweep speeds definitely show a sweeping beam, it's just deflected off screen.

If this doesn't fix it, the vertical amplifier/deflection board may be of issue.

Benjamin

Þann mán., 23. des. 2024 kl. 03:39 skrifaði Jeffrey Wang via groups.io
<xwsunrise@...>:

I’m curious, why in beam finder mode, the waveform and the trace were
clearly visible - it’s just that the screen was vertically compresses,
which I suppose it normal. That said, everything disappeared as soon as I
let go off the momentary switch. I did some research and it seems that the
beam finder mode bypasses the intensity control circuitry so does it mean
this circuitry is faulty? How can I verify that and troubleshoot?

For most of the video your vertical sensitivity was set to 20mV/DIV, which
is way too sensitive. You also have the variable control out of the detent
as witnessed by the uncal light. This would lead to a trace that is mostly
off screen.
This is not to say that there isn’t a problem with the problem with the
scope, but at those settings the video doesn’t show a problem, it might
well be WAI.

Try setting the scope to 0.2V/DIV (VAR in detent) and e.g. 1ms/DIV and see
whether you can then get a trace on the CRT.

Thank you for the info. I shot another video to demonstrate the issue. I have virtually tested all vertical and horizontal sensitivity combinations to no avail. I just realized that the UNCAL light was on for Channel 1 but I also adjust the var knob after I shot the video so the light is off. That said, it didn't change anything. I hope this video helps in identifying the issues.
https://www.youtube.com/watch?v=W_pNMTIXrTc

Thanks!

I shot another video to demo the current situation:
https://www.youtube.com/watch?v=W_pNMTIXrTc

How do I troubleshoot the vertical amplifier board?

From your settings, it’s my impression that user error still plays a role here:
- Ch1 ampl. still shows “uncal”. Not a cause for no trace but you don’t seem to know it does not match what you say about your settings of the vertical knobs.
- Time base speed setting is still “uncal”. Also, as in remark above.
- Most importantly: The A trigger source is set to *external*. That means, no trace without beam finder unless external trigger.…

Raymond
Reading the basic instructions may help.

Center the vertical positioning control with the sensitivity at 1 volt/cm or greater (minimizes signal pickup).

ground vertical input.

at the vertical deflection plates, measure the voltages.  They should be equal.

vary the vertical centering to see if you can get the voltages on each plate to match.  At that point, the trace is centered.  If you can't do this, then the trace won't center.

You can either start from the end or the beginning.

Looking at the schematic, there's one input to the vertical amplifier, which goes through a differential pair.  This produces a positive going and a negative going signal.  Each stage should have the same DC voltage with any signal being of opposite phase. If one signal through the amplifier chain doesn't match, then investigate that stage.  Transistors can be swapped from plus to minus to enable checking bad transistors.

There should be a very good section on how the amplifier works in the manual.  You'll find that the horizontal and vertical amplifiers are quite similar.

One possibility, given that the beam finder switch forces the beam to be on screen by reducing amplifier gain, is that you check around that switch for possible bad contacts.

Harvey

You can begin troubleshooting the vertical amplifier by chacking voltages at specified test points. They will tell us a lot.
Go to https://w140.com/tekwiki/wiki/454 and download the 454A manual, not the operator manual. It will have schematics and board layout diagrams near the end of the manual.

Go to schematic #4 and measure voltages, referenced to chassis ground.

To answer your question from the video, I belive the beam finder works by disconnecting a direct -12V rail from the vertical amplofier, and instead putting it through a 750 ohm resistor. This will reduce the vertical amplifier's deflection range ability, thus you can see the beam when using beam finder. That is the whole point of the beamfinder function anyway, to get the beam in the display.

When the beamfinder is activated, it is in the lower half of the display, indicating the beam is being deflected down out of the display. There is no vertical scretching of the beam, the beam is simply hitting of the phosphors off of the display screen and reflecting light inside the tube.

Again, use a DMM to measure voltages.

Recommend spots:

Junction between C369 and L369

L343 (any side is fine for just sinple voltage measurements)

Junction between VR339 and R339

Junction between R311 and R312

Base of Q424

Base of Q324

Base of Q474 and Q347

Emitter of Q414 and Q314

Thise voltages will help us discover the problem.

Also I doubt it's user error. Cal lights are likely from bad switch contacts as it looks like the knobs are indeed in the dented position.

Benjamin

That means, no trace without beam finder unless external trigger.…

No it doesn’t because auto-trigger is on…

Did you try channel 2? same result?
Did you play with vertical positions? Both normal pos. and with beam finder (BF) on? Vert. pos. should influence with BF activated as well.
Of course, big offset error in (common) vertical amplifier is quite possible but try and narrow things down first.

Raymond

Also I doubt it's user error. Cal lights are likely from bad switch contacts as it looks like the knobs are indeed in the dented position.

Sure but since OP did not mention it…

Raymond

On Mon, Dec 23, 2024 at 11:42 AM Jeffrey Wang via groups.io <xwsunrise=
yahoo.com@groups.io> wrote:

How do I troubleshoot the vertical amplifier board?

If this is your first scope repair, you might want to read through this
wonderful document: https://w140.com/tekwiki/images/5/5d/068-0315-00.pdf.
This is from a time when oscilloscope users were more or less expected to
troubleshoot and fix their own equipment :).

Still trying to get a bit further without having to take measurements may yield more info.
If both channels show the same behavior: Since bad contacts are rather frequent with old equipment like this, wiggle the various transistors in the vertical amplifier; they’re all socketed. Also slightly move back and forth vertical position adjustment pot. Don’t know by heart if and where in this model. Preferably do this while power is off.

Should you do measurements at/near the vertical amplifier, be very careful not to short the outputs (or plates) to ground, which very likely would kill the output transistors.

Raymond

Jeffrey,

The uncal lights and x5 lights are still on. The uncal lights should turn off when you go to the detent position which you will feel. As others have said, the switch could be dirty making the light come on even though it is in teh detent position. Set triggering to a-c or d-c. Set the source switch, lower right corner, to int. Set the v/div switch to .2 or .5V/div and 1ms/div for horiz.. That should get the trace on the screen.

Having two A versions as well as the 453/A and 454, I know there are a number of resistors that are underrated in wattage that caused problems in mine. The 454A has ones that are lower power rated than is being dissipated, especially in the horizontal section. You could very well have resistors that are well out of tolerance or open. Clean the pins and sockets of the transistors. That will cause problems like you are seeing. If you want a list of parts I changed in mine, I will send it.

Mark

- Adjusted Ch1 var and the "uncal" light is off, as I mentioned in post #207706
- The var for A & B time base is in detent, as you can see clearly in my video. That said, I cleaned the pot and it measured smoothly from around 5K ohm to 0 while I turn the red "VAR" knob.
- It is now set to "internal".

Still nothing.

Thank you Ben for the detailed info - I really appreciate it! Here are my measurements for the reference points you listed:
Junction between C369 and L369: 12.04V
L343 (any side is fine for just sinple voltage measurements): -11.98V
Junction between VR339 and R339: -6.44V
Junction between R311 and R312: 0.897V
Base of Q424: 8.94V
Base of Q324: 9.7V
Base of Q474 and Q347: 11.86V and 10.68V (I assume it's Q374? Can't seem to find Q347)
Emitter of Q414 and Q314: 0.174V and 0.192V

I hope this helps to some extent. Also like I mentioned in another reply, the pot for the A & B VAR knob has been cleaned and it seems to work fine but the UNCAL light stays on no matter how I adjust the knob.

CH 2 is the same. Vertical Position knobs work in Beam Finder mode.

Yes I just downloaded it. Will spend some time reading it.

I switched to x1 and the MAG light tuned off. Only UNCAL is still on, even though I have already cleaned the pot. Switched the triggering back and forth between AC and DC. Set the Source switch to "INT". v/div is set to .2v/div and ms/div is set to 1ms/div. Trace is still invisible :(

Could you send the parts list via DM? Thank you, Mark!

I’ve been troubleshooting the oscilloscope further, focusing mainly on the Vertical Amplifier board. I’ve attached pictures of the boards and the schematic diagram from the service manual, with two connectors marked because disconnecting them impacted the behavior (you need to log in to view these pictures):
Main Board: https://groups.io/g/TekScopes/photo/299721/3870949/Verical%20Output%20-%20Circuit%20Board.jpg?p=Created%2C%2C%2C20%2C2%2C0%2C0
Thick Film Hybrid Board: https://groups.io/g/TekScopes/photo/299721/3870951/Verical%20Output%20-%20Thick%20Film%20Circuit%20Board.jpg?p=Created%2C%2C%2C20%2C2%2C0%2C0
Diagram: https://groups.io/g/TekScopes/photo/299721/3870950/Verical%20Output%20-%20Schematic%20Diagram.jpg?p=Created%2C%2C%2C20%2C2%2C0%2C0

Here are my findings:
1. Disconnecting P442 completely resulted in a horizontal line at the screen's center. Adjusting the Vertical Position knob had no effect, and pressing the Beam Finder switch only shortened the line without changing its position.
2. Connecting only Pin 5 of P442 (+75V) gave the same result as disconnecting it entirely.
3. Connecting Pins 3, 4, and 5 of P442 also had the same result.
4. The line disappeared if Pin 1 or Pin 5 of P442 was connected.
5. Disconnecting P385 while P442 was fully connected caused the line to reappear.
6. Connecting only Pin 1 of P385 moved the line slightly upward.
7. Connecting only Pin 2 of P385 kept the line at the center.
8. The line disappeared if both pins of P385 were connected.
9. When the line was centered, voltages on the upper and lower rails matched closely.

Looking at the schematic, the 'Thick Film Hybrid' board (btw, it's a PITA to solder on it) connects to the main vertical amplifier board via diodes VR363 and VR463 and through P385. Additional findings:
1. Feeding +12V to L369 and +75V to L385 simultaneously seemed to have caused the vertical deflection to malfunction.
2. Voltages at the emitters of Q314 (~6.27V) and Q414 (~3.67V) suggested the trace was off-screen.
3. When the horizontal line was centered, both emitter voltages were around -3.85V, a notable difference.

At this point, I feel close to identifying the fault but am unsure which direction to take next. Any insights or suggestions would be much appreciated!

Bonjour,

Have used and fixed Tektronix scopes back to 1968, the first solid state portable 454, 453. Amazing these 50 years old scopes can still work at all. Most need a complete examination for tant and lytic caps,dirty switches and controls, and basic PS, CRT and HV check and cal.
Even if you fix the immédiat issues, expect the HV transformer, HV mult to fail with time.

So, reading this thread, it's a very an ineffective process, I suspect OP needs the full service manual, follow procedure and diagrams for a logical troubleshooting plan.




Bon chance

Jon