Wide Trace - Harris 3133 25mhz dual trace scope

[warf135]

This is a Harris 3133 25Mhz dual trace oscilloscope. it was working perfectly, then suddenly the trace went very wide (over an inch). all functions of the scope appear to still be working as normal apart from the trace being abnormally wide.

Turning the intensity control varies the wide trace a tiny bit and focus has no effect. (both controls have been tested and are working fine)

I believe this is possibly an issue with bad caps?

Any assistance is welcomed, Thanks!

Norm (aka Warf135)

 

[Resqueline]

Some oscilloscopes have an astigmatism control (or adjustment) that affects the height/width ratio of the spot. I have no idea what might have happened here though.

 

[warf135]

Thanks Res, I'll check if it does indeed have that control inside the scope and see if there is any clues...

I have read many forums where scopes have gone wide trace and the problem seems to be with capacitors going bad somewhere inside the scope.

I think I have found which board contains the problem (by unplugging different connections and such)

There are four 100nf 1000v caps on this board and wiggling two of them with a screw driver (to basically vary the capacitance) seems to effect the trace width.

I cannot identify what type these capacitors are so will need to upload a pic for assistance (not sure if i can do that yet due to not enough posts)

Thanks again Res!

Norm (aka warf135)

 

[Resqueline]

Ok. Sounds like it may be a disc shaped ceramic cap. I've never had to fix a 'scope before but I once found a bad one such in a TV.

 

[warf135]

scope caps

They are not ceramic disc caps... here is a pic of them:

its the 2 blue rectangle caps in the pic... they are 100nf and i think 1000 volts (by the 1000 on them)

Anyone know of suitable replacements for these?

Norm (aka warf135)

 

[(*steve*)]

If the problem is significantly affected by wiggling the caps, and especially if the problem happened suddenly, the fix may be as simple as resoldering the capacitor connections.

It is a possibility that one of the solder joints has failed. It is also possible that the joint in question is near the capacitors and wiggling the capacitors flexes the board ever so slightly. If you poke the board (with some sort of plastic probe) does this also affect the beam width?

 

[warf135]

Thanks for the reply steve, I have already re-soldered about 90% of the components on that board as well as testing resistors diodes and transistors.. the caps also test ok for capacitance but i fear they may be breaking down under load

I have tried the poke it technique too and the only time it made a difference was when i touched those 2 caps in the picture, so i guess the next thing to do is to swap them out and see if it cures it... (if i can locate some suitable replacements)

Norm (aka warf135)

 

[davenn]

you could look for 100n 1000V polyprop type ... they would be suitable.

can you tell if these caps are in the hi voltage section of the tube power supply
or in some other lower voltage section?
You may find they are in a lower voltage section and you could use caps rated at ~ 100 - 600V.
BE CAREFUL there are some serious voltages in scopes

Dave

 

[warf135]

Hi Dave thanks for the reply... I am always careful when going inside any electronic kit, especially old TV's and stuff.

here is a better picture of the board. The board has wires going to focus and intensity controls as well as a load of wires going to the tube:

I'll have a look for some poly. caps to see whats available, thanks for that info.

Norm (aka warf135)


UPDATE: Tested voltage at caps and it is 475v dc, so could probably get away with 600v caps
UPDATE 2: 100nf 1000v poly caps ordered... lets hope that they fix my scope!

 

[warf135]

OK, all 4 100n 1000v caps replaced and it has made no effect on the problem... (i'm 99% sure that the fault lies on the board in my pic above)

The yellow capacitor by the focus/intensity plug, connects directly to the focus circuit.. maybe that one is kaput and the focus circuit is not working, making the trace wide???

Its a 0.047uf 2kv cap, but again i cannot determine its type... UPDATE: its a Polyester Film capacitor unless I am mistaken, guess i'll get one and swap it out (i must get a decent cap tester one day)


Norm (aka warf135)

 

[MattyMatt]

I'm certainly not claiming to be an expert by any means, but what you are describing is kind of similar to what I am seeing in my scope... I'm working on mine still, but here's a link.

https://www.electronicspoint.com/tektronix-475a-display-problem-horizontal-t232993.html

BTW, I don't know if anyone else is having the same problem, but I cannot see your pictures you have posted.

Matty-

 

[warf135]

Hi Matty, Thanks for your input, but my scope problem is different to yours.. My scope works fine, its just that the trace is about 2 inches wide instead of being a tiny dot/line.

I dont know whats happened to my pics, i cant see them either, but nevermind as the reason i needed the pics has been sorted and the caps were identified and replaced.


Norm (aka warf135)

 

[MattyMatt]

Glad to hear its all good