Tektronix 465B Problem.

[davidgem1406]

My Tek. 465B went faulty while in use. The horizontal trace suddenly went to a large very bright spot (about 5 mm) at the right hand side of the screen, and since then had disappeared to the right.
The intensity, focus and shift no longer worked, then the smell of overheating arrived but no smoke could be seen.

I have now started to investigate the problem and assumed it would be in the horizontal output amplifier. That was my first check.
Measuring the voltages on the CRT plates, the left hand voltage was zero, the right hand at 90V
Other measurements as follows:

+5V at R4158 = 2.7V

V at VR4341 + = 107V (Should be 110V)
V at VR4341 - = 97V (Should be 103V)

+55V at R4344 = 48V (Should be 55V)

- 8V at R4260 = +0.76
- 8V at R4262 = +0.65V

As the 5V and -8V were a long way out I turned to the power supplies and checked each in turn having the following results:

+110V unregulated = 161V
+110V regulated = 103V

+55V = 48.7V

+15V unregulated = 24.2V (Should be 21.4V)
+15V regulated = 0V

+5V = 2.6V
-8V =0.6V

The values for the +5V and -8V can be ignored as they depend on the +15V to be working.

I found that the +15V rail had a direct short to ground.
Referring to the power distribution diagram I found that the service link W4440 isolated the +15V from the horizontal circuits, the link was lifted.

However, the SC remained, the fan assembly was lifted, that made no change. The circuit for the Calibration unit shows that a direct short would not be possible if that circuit failed, it was working OK at the time of the failure.

Next, at the +15V supply, I disconnected C4331 and CR4405, the SC remained.

Attention now has to turn to the CRT circuitry as that is the only item that has not been isolated according to the power distribution diagram, from the +15V rail, there seems to be no way to do that.

I can only find four point that go to the +15V rail at R4048 (18K), R4220 (390R), CR4321 (Base of Q4222) and R4059B (Intensity).
However, looking at the circuit I can't see any way a failure at one of these points could produce a direct SC to ground.

I'm now wondering if I may have missed something that is still connected to the +15V rail.

As I am writing this I just looked again at the power distribution diagram and found 2 more service jumpers on the +15V rail W7292, and W4481. I will have to disconnect these, I will post the result later.

Any help with this problem would be much appreciated.

Thanks
Regards Dave.

[tony brady]

very common for a blue tant to go short across the rail, only a case of which one!

cheers
Tony

[davidgem1406]

Hello Tony,

Thanks for your input.
I have checked all the tantalum capacitors, it was quite easy to do.
One was short circuit and located on the vertical amplifier board. I replaced it with an electrolytic of 10uf.
Put things back as they should be, powered up and it is all working now.
All power supply voltages are as they should be

110.2, 55.1, 14.9, 5.0, and -8.0

[ajgriff]

Glad another Tek 465B soldiers on. A fine example of methodical fault finding too.

Alan

[davidgem1406]

Thank you, Alan I'm happy it is back working now as it's waited several weeks for me to get round to doing it.

There is one thing I did notice though, when the sync switch is in the auto position it is not possible to obtain a stable waveform, although I do not normally use that.
I was wondering if that is normal a some problem that I should look at?

Now I have my Telequipment D83 to look at, that has poor sync making it difficult to obtain a stationary waveform.

Best regards,
Dave.

[Craig Sawyers]

The 465B I think (I'd have to check the schematics) uses tunnel diodes in the trigger circuit. These, and the associated resistors, can drift over the decades. Requires following the adjustment procedure for the trigger circuit.

Craig

[ajgriff]

Originally Posted by Craig Sawyers

The 465B I think (I'd have to check the schematics) uses tunnel diodes in the trigger circuit.

Your memory serves you well Craig. Two tunnel diodes for each circuit (A & B).

Alan

[ajgriff]

Originally Posted by davidgem1406

There is one thing I did notice though, when the sync switch is in the auto position it is not possible to obtain a stable waveform, although I do not normally use that.

These scopes are renowned for stable triggering. Problems are often associated with dirty switch contacts now that these machines have been around for a good few decades.

If you do have to investigate the sweep/trigger circuits remember that tunnel diodes are flighty little creatures and can very easily be dispatched to the great component warehouse in the sky especially if tested with a DMM in the conventional way. They are essentially very fast switching devices and have not been manufactured for many a long year. Replacements are hard to find although ex-Russian military equivalents are an option. Usually TDs either work or they don't so if the timebases are basically functional I wouldn't touch the TDs other than to ensure that the leads are making good contact in their tiny little sockets.

Timebases are not always the easiest things to troubleshoot. However, if one timebase is working ok there is the option to compare voltages and waveforms using a second scope. Most of the circuitry is readily accessible on the right hand side of the machine. I assume you have a copy of the service manual but do you also have the Tek 'Troubleshooting Your Oscilloscope' guide? It's an excellent document and I can post a PDF copy here to save you searching for it if needed.

Alan

[davidgem1406]

Thank you all for your input here.

I think for now, unless it is a problem, I will leave the trigger circuits alone as it is only apparent on Auto/Normal, but I will check the switches latter. I will remember about the 'Tunnel Diodes' for sure.
With channel 1 or channel 2 selected it all seems to works fine.

Alan,
Yes, I do have the full service manual. Thank you very much for your offer of the pdf. "Troubleshooting Your Oscilloscope" but I was lucky and found it on line at the first try.
Found here https://vintagetek.org

Regards to all,
Dave.

[RogerEvans]

Dave,

After your last post (#9) I am unclear as to the nature of the triggering problem. Initially I thought it was the 'Trig Mode' selector working OK in Normal (blank screen if no trigger) and not in Auto. The switching for this is very simple, it just grounds one pin of U7375 so the fault is likely to be either the switch as Alan suggests or a faulty U7375.

Post #9 implies it could be a problem with Trig Source selector with Ch1 and Ch2 working OK but not Norm (auto selection of Ch1 or Ch2 as appropriate). I believe this is a common problem with a variety of tek scopes and I am not sure which set of conditions are a fault and which are simply that the way it works is not what you expect. If you have unrelated signals on Ch1 and Ch2 and are using Alt vertical mode then I don't think any of my analogue Tek scopes give a stable display in the auto setting ('mode' on a 7000 frame).

When you have a stable trace the timebase should trigger on a signal amplitude of one minor division of the graticule (more at very high frequency - see the manual for details). If that works OK then you don't have much wrong with the triggering .

Roger

[davidgem1406]

Hello Roger,

If I take the calibration square wave and feed it to vertical channel 1 with channel 1 selected. Then select Channel 1 on the horizontal Source switch i will have a solidly locked trace when adjusting the level control. The same is true for channel 2.

However, if I move the source switch to Normal I have a continually moving waveform regardless of the level control setting. I expect it to lock automatically.

The Telequipment D83 has the same setup but using buttons. Whereas the Tek calls it Normal, the D83 calls it Auto.
My calling it Auto/Normal was maybe rather confusing.

Thanks,
Regards Dave.

[RogerEvans]

Dave,

That is fairly clearly a fault! If you have access to a service manual for the 465B and look at schematic <3> the trigger signal for 'Normal' is picked off the vertical amplifier after the Ch1, Ch2 switching. The relevant part is at the top right where Q1215 and Q1118 buffer the signal fed to J1101. There should be a miniature coax link from J1101 to J7215 on schematic <6> where the trigger selection switches are located.

There is a quick test to see if the fault lies in the trigger selection switches. Set the scope to display the B sweep, triggered after delay. And set B trigger first to Ch1, then to Normal. If you can get a stable display in both cases then the Q1215, Q1118 circuitry is OK and the fault lies in the switches for A Sweep trigger selection S7312A.

Since everything is DC coupled you can follow the signal path with a multimeter, the voltage at any point should change with the vertical position control and the input coupling set to ground. Or you could just trace the calibrator signal with your D83.

Regards,

Roger

[davidgem1406]

Hi Roger,

Thanks for that information, it will be a great help. I do have the full service manual.
I have the D93 on the bench at the moment, to check out what the triggering is doing on that. Although I'm not taking it apart just yet though.

Regards Dave.

[davidgem1406]

Hi Roger,

The check has now been done that you recommended above (#12) using B sweep and triggered after delay, I had a solidly locked waveform in all positions of the source switch, including external. I checked both input channels, both were fine.

I then turned to the A sweep, checked all positions of the source switch, including external, and those of the coupling switch. Then the trigger Mode in auto and normal.
Everything was good with a solidly locked waveform under all conditions.

That was a pleasing result as to get access to the switches would have been a bit of a strip down job to remove the board assembly.

How that comes about I know not. I can only think that operating the switches I don't normally use may have cleared something. Time will tell no doubt.

Thank you very much for your assistance it is much appreciated.

Regards Dave.

[RogerEvans]

Dave,

Glad to hear it is fixed, the push button switches are a regular problem, the lever operated ones are less of an issue but all mechanical connections including the push on connectors between boards and controls can give trouble. Using it regularly is the best solution!

Regards,

Roger