Author Topic: Tektronix 465 Oscilloscope Troubleshooting (Read 20751 times)

DanielSpokane · « **on:** March 31, 2018, 04:35:37 am »

Does anyone have any experience trouble shooting a Tektronix 465 Oscilloscope? I picked one up for a steal at a flea market... it looks pretty clean, but I'm having a couple issues getting it going. The two issues that stand out are: 1) the trace is funky (non linear) w/ no signal (see pictures), and 2) the scope does not seem to recognize a probe/signal when hooked up to either channel (IE signal trace does not change (maybe a little but not much), the appropriate 5 or 50mv light does not come on corresponding to the probe, etc.). IE the signal trace is messed up and the scope is not changing/receiving a signal.

Any ideas?

tggzzz · « **Reply #1 on:** March 31, 2018, 08:33:24 am »

Don't bother thinking of alternatives until you have checked each of the many PSU rails to ensure both the voltage and ripple are within limits.

If not, then don't bother thinking of alternatives until you have rectified (ho ho) that.

Then use standard divide-and-conquer techniques, in combination with all the info in the service manual.

DimitriP · « **Reply #2 on:** March 31, 2018, 10:30:39 am »

Two things.
In your pictures both iinput coupling switches are set to GND. Set the input coupling to either AC or DC and try again.
That's was the good news .

If the trace changes shape as you are moving the vertical position up and down ..the CRT is damaged - probably-most likely

DanielSpokane · « **Reply #3 on:** March 31, 2018, 04:22:56 pm »

Thanks for the suggestions. I changed the coupling and used the probe and get indication that the probes are working. Does seem like maybe a screen or power supply problem.
I took a video of what it's doing including a couple adjustments.

https://www.youtube.com/watch?v=gSGErmCHR4M&feature=youtu.be

What do you think? I'm thinking maybe screen?

bd139 · « **Reply #4 on:** March 31, 2018, 04:34:14 pm »

Check power supply first. There are test points on the main board. They go absolutely mental and nothing works properly if the supply rails are out and this is the number one problem with them. I think 90%+ of the broken scopes I've seen from Tek are power supply issues. It's usually, at this age down to one of three things in priority order: (1) shorted tantalum capacitors, (2) knackered bridge rectifier, (3) knackered filter capacitors.

I've fixed a veritable ton of these now. Couple of reference threads for same line of scopes:

https://www.eevblog.com/forum/repair/tektronix-475-repair-assistance/

https://www.eevblog.com/forum/repair/tektronix-465b-repair-97327/

I'm using a 465 at the moment as well which was repaired.

DanielSpokane · « **Reply #5 on:** March 31, 2018, 08:11:23 pm »

Thanks for the ideas. I feel like I might be in a little over my head on this one... but I'm going to see what i can do.
I pulled the cover and turned it over to the power supply board. i tested the following test points:

TP 1423-- shorted and sparked and a light on the board turned on... and I didn't give me a voltage. Very weird.
TP 1486-- 43.3V
TP 1590-- 9.87V
TP 1594-- 8.79V

Below are a couple pictures from the manual. One is the board i tested and the locations of the above voltages. Following that is the schematic of the power supply. It seems weird that the test points shown on the adjustment layout does not show up on the schematic.

Still puzzling on next step.

DimitriP · « **Reply #6 on:** March 31, 2018, 08:16:09 pm »

Quote

What do you think? I'm thinking maybe screen?

Unfortunately it looks that way.
Here is a link to what's going on: https://entertaininghacks.wordpress.com/2016/03/09/rescuing-a-broken-tektronix-465-crt/

One the bright side, if that's the only thing wrong with it, "all you need" is a replacement CRT.

The way iit behaves, the trace will look it's best at the very top or bottom of the screen, so you should be able to veify the inputs and triggering are working.

bd139 · « **Reply #7 on:** March 31, 2018, 08:26:33 pm »

I doubt it’s that. Symptoms say otherwise.

That wasn’t a very good test run and inconclusive. The TPs you poked were mostly not power supply ones. The light on the board and the spark was you nearly zapping yourself on the HT. don’t poke anything around anywhere in the scope. There’s enough volts to kill you instantly in a few places.

Check the following TPs and let us know the voltages.

TP1568
TP1558
TP1548
TP1536
TP1518

Do these relative to the one marked ground on the middle of the board.

Be careful. This is a dangerous bit of kit. Highest test point voltage there is 110 volts which will wake you up if you’re not careful. Do each one carefully and methodically.

tggzzz · « **Reply #8 on:** March 31, 2018, 08:39:14 pm »

Quote from: DanielSpokane on March 31, 2018, 08:11:23 pm

It seems weird that the test points shown on the adjustment layout does not show up on the schematic.

There are many schematics; you haven't found the right one.

With Tek, Rxxxx is physically and logically close to Cxxxx and TPxxxx etc. That should give you pointers.

DimitriP · « **Reply #9 on:** March 31, 2018, 08:42:35 pm »

Quote

I doubt it’s that. Symptoms say otherwise.

Doubting is free.
Having seen and replaced a crt with a bad mesh in real life ...priceless.

tggzzz · « **Reply #10 on:** March 31, 2018, 08:48:11 pm »

Quote from: DimitriP on March 31, 2018, 08:16:09 pm

Here is a link to what's going on: https://entertaininghacks.wordpress.com/2016/03/09/rescuing-a-broken-tektronix-465-crt/

The OP's scope isn't casting shadows on nearby walls, therefore the grid connection isn't broken.

The strange looping in the traces may be due to a bent mesh (as in those pictures), or even due to some crud on the mesh. In one CRT small loops suddenly appeared on the screen; shaking the CRT moved the crud on the mesh elsewhere in the CRT (for a while, at least).

bd139 · « **Reply #11 on:** March 31, 2018, 08:53:07 pm »

Yes definitely not grid. Those tubes are stupid bright when that goes.

I’ve seen these patterns when the front end FET gate resistor broke off a 475 before as well. It was so high impedance it went mental.

DanielSpokane · « **Reply #12 on:** March 31, 2018, 08:53:30 pm »

I was careful and took readings on the following test points:

TP1568 -- 8.09V
TP1548 -- 12.87V
TP1536 -- 55.9V
TP1558 -- 5.08V
TP1518 -- 110.07V

Thoughts?

bd139 · « **Reply #13 on:** March 31, 2018, 08:56:56 pm »

TP1548 is low. This should be around 15V. Check capacitor C1549. Should be a big blue tantalum one towards the middle of the unit.

Unplug the unit, give it a couple of minutes and measure the resistance across it. If it’s less than about 50 ohms, snip one lead of that capacitor off at the base of the capacitor and power it up again and see what happens. Measure the voltage at TP1548 again. If it is 15v and the scope works then desolder it and stuff a 100uF 25V electrolytic or better in there. If it isn’t, post back

DimitriP · « **Reply #14 on:** March 31, 2018, 09:05:30 pm »

Quote from: bd139 on March 31, 2018, 08:53:07 pm

Yes definitely not grid. Those tubes are stupid bright when that goes.

I’ve seen these patterns when the front end FET gate resistor broke off a 475 before as well. It was so high impedance it went mental.

I'll just wait..the bad mesh not going anywhere ....

bd139 · « **Reply #15 on:** March 31, 2018, 09:08:38 pm »

That’s completely different to the OP’s traces. And yes that’s a knackered tube.

DanielSpokane · « **Reply #16 on:** March 31, 2018, 09:13:02 pm »

The resistances across C1549 is 174ohms.

bd139 · « **Reply #17 on:** March 31, 2018, 09:14:09 pm »

Was it a blue tant (square one)?

DanielSpokane · « **Reply #18 on:** March 31, 2018, 09:18:45 pm »

Not sure how to identify a blue tant. See picture. It is the yellow/purple one located next to the test points.

DanielSpokane · « **Reply #19 on:** March 31, 2018, 09:43:05 pm »

At 174ohms that is not terribly far from 50ohms. Should I clip and measure?

bd139 · « **Reply #20 on:** March 31, 2018, 09:46:39 pm »

Sorry for delay was checking service manual. That is probably first step yes. It's the striped one in the middle. You can also desolder one leg as an alternative or if you don't have a replacement part handy.

You might have to do a few of these before you find it.

Yours is later than mine is and has the newer capacitors in it (also unreliable!). That cap is almost universally the first cap that goes wonky. There's another one under the HT shield but your CRT is displaying a trace so that's fine at the moment.

When you replace it later, remove the screw or it'll sink all the heat into the chassis and it'll be impossible to clean up the holes.

DanielSpokane · « **Reply #21 on:** March 31, 2018, 09:57:54 pm »

I desoldered one leg of the cap and turned on the scope and tested TP1548 and it is still 12.8V. I tried to measure capacitance across the capacitor and can't seem to get a reading. I measured resistance across it and it changes, etc. typical of a capacitor.

Appreciate the help!

bd139 · « **Reply #22 on:** March 31, 2018, 10:07:00 pm »

Gah this one is going to be difficult then.

Next step is a visual inspection of all capacitors on all the boards. Look for cracked or burned tantalum capacitors that look the same as that one. They may be present on any of the four boards in the scope. The timebase one is difficult to see but there's one on that you can observe if you look top down. I use a plastic ruler and move it across in inch steps then scan from left and right to find them all and inspect.

If you can't find anything obvious then the next steps are:

1. Check for an AC voltage on the TPs listed before. You can probably get away with a DMM on AC volts for this but another oscilloscope is better. This will identify any problems in the left half of the power supply (rectifiers, smoothing caps, voltage regulators). If that looks ok, i.e no AC voltage registered, proceed to...
2. Finding all tantalum capacitors on the 15V rail and testing them.

I have had to do a wholesale replacement process on the entire scope before. Takes a while but is worth it.

Once this is reading 15V, diagnostics may proceed. The problem as always with these is that everything depends on this being spot on.

DanielSpokane · « **Reply #23 on:** March 31, 2018, 10:35:15 pm »

I looked at all the tant caps and the only one that looked like it had a small hairline crack is C1113 on the timing circuit board (see picture). No burnt looking caps that I could see.

Also, I checked for AC on the test points. Found the following:

TP1568 -- DC 8.09V -- AC 115mV
TP1548 -- DC 12.87V -- 3.03V
TP1536 -- DC 55.9V -- 0.77V
TP1558 -- DC 5.08V -- 184mV
TP1518 -- DC 110.07V -- 0.72V

bd139 · « **Reply #24 on:** March 31, 2018, 11:30:25 pm »

Ok desolder one leg of that. That’s a blue tant of doom. Crack is suspicious and that board has 15v rail.

It’s 00:30 here so going to bed now. Will check back in morning.


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Author Topic: Tektronix 465 Oscilloscope Troubleshooting (Read 20751 times)

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