Oscilloscope help

[BinaryMike]

If your DMM doesn't have sufficient DC voltage range, you could look up its input resistance and add resistance externally to extend the range. If it's 10M, for instance, then adding 10M in series with the probe will double the range. Just keep in mind that resistors have voltage ratings -- you might need to use multiple resistors in series to work around that constraint.

 

[Eastham]

I used my screwdriver clipped to chassis ground and was able to draw a arc, so I'm definitely getting high voltage.

 

[Eastham]

Given I was able to get a dot on the screen before that seemed to stretch vertically I'm wondering, could my issue be in the time base circuit? Or maybe I've lost horizontal hold.

 

[BinaryMike]

I used my screwdriver clipped to chassis ground and was able to draw a arc, so I'm definitely getting high voltage.

Maybe, if we can assume that your high voltage supply survived the diagnosis. Suggest you proceed by checking medium and low voltage DC supplies to the horizontal sweep circuits.

 

[Eastham]

Yup all voltages present and accounted for, gingerly used my meter to test the flyback voltage hoping it wouldn't explode in my hand and I got 1900v on the high voltage.

 

[Eastham]

Ooohh, I might be onto something with that, the flyback voltage is supposed to be 3Kv and I'm only getting 1.9

 

[BinaryMike]

This sounds awfully risky to me. Most DMMs are rated for 1KV or less. Unless you know for a fact that yours can handle it, keep it well away from such high voltages! The readings cannot be trusted, and the abuse could make it blow up in your face. I suggested an alternate solution in post #21.

 

[Eastham]

Yeah, I admit what I did was dangerous and stupid. But my meter isn't exactly a cheap £5 job from China, it's CAT II and CAT III rated hand has pretty good explosion protection, It's an Extech EX330 if you're curious, not exactly a Fluke but for the price it's a damn fine meter and not far off RMS calibrated meters.
Anyway, I just tested the high voltage diodes and guess what? They're all open circuit, well two of them are, one of them broke in half as I unsoldered and pulled it out! they're diodes D407, 408 and 409 on the PSU board. SCM30's, I'm thinking these would be good replacements? http://uk.rs-online.com/web/p/rectifier-schottky-diodes/6298679/

 

[Eastham]

Okay, I went and ordered some Vishay GP02-40 rectifier diodes, rated at 4Kv and 250mA they should work me thinks. Hopefully this gets it back up 'n' running again! I'll take some time tomorrow to test all the other components on the PSU board while I have the boards and CRT out. I also took the time to give every control a good shot of contact cleaner while it's apart, not that they needed it but hey, can't hurt.

 

[rave0035]

I used my screwdriver clipped to chassis ground and was able to draw a arc, so I'm definitely getting high voltage.

On a 3kv rail?

 

[BinaryMike]

It's an Extech EX330 if you're curious, not exactly a Fluke but for the price it's a damn fine meter...

I'm always curious. Here's what I found:





The second item relates to high-voltage rectifier testing. High-voltage diodes are made up of multiple lower-voltage diodes stacked inside the package, which means that forward voltage is many times the typical 600mV of a single silicon junction. The EX330's 1.5V test voltage prevents it from testing this class of rectifier diodes.

I don't mean to discourage you or anyone else from digging into this stuff with gusto and learning it any way you can, but please take care. High voltage risk management always demands top priority -- don't do the measurement if safety isn't assured first.

 

[Eastham]

On a 3kv rail?

It was a trick an old friend who has passed thought me, he used to work on CRT TV's before they became obsolete, "Keep your hands off the chassis, clip a screwdriver to ground and try to draw an arc with a insulated screwdriver, unless it's a hot chassis" to be fair it was more a spark than an arc.

I'm always curious. Here's what I found:

View attachment 875569

View attachment 875570

The second item relates to high-voltage rectifier testing. High-voltage diodes are made up of multiple lower-voltage diodes stacked inside the package, which means that forward voltage is many times the typical 600mV of a single silicon junction. The EX330's 1.5V test voltage prevents it from testing this class of rectifier diodes.

I don't mean to discourage you or anyone else from digging into this stuff with gusto and learning it any way you can, but please take care. High voltage risk management always demands top priority -- don't do the measurement if safety isn't assured first.

I realized that when I put the diodes in my component tester and they tested fine, still holding out hope that the one that fell apart as I removed it was the issue. I can get triggering if I feed in a signal so I'm fairly sure the problem has something to do with the CRT. I agree I need to be a bit safer when messing with high voltage but I have been making sure to discharge the CRT and have been wearing nitrile rubber gloves when the 'scope is live.

 

[ghazzer]

Nice try Mike, but I think the OP is more like a zener. It will probably take an exploding DMM or a near lethal shock before his resistance breaks down and he comes to grips with the danger.

 

[Eastham]

I'm sure natural selection will take me someday...

 

[Eastham]

Well I suppose this bulged/ vented reservoir cap on the 5V rail is helping anything so I've replaced it with a 10,000µF 50v cap I had in my parts bin, it's the only thing I had to hand that larger than the original value and still big enough to fit in the original bracket with the help of a spacer I made. Amazingly it still measures 4,700µF but the ESR it through the roof.

 

[for_p1]

Well I suppose this bulged/ vented reservoir cap on the 5V rail is helping anything so I've replaced it with a 10,000µF 50v cap I had in my parts bin, it's the only thing I had to hand that larger than the original value and still big enough to fit in the original bracket with the help of a spacer I made. Amazingly it still measures 4,700µF but the ESR it through the roof.

Capacitors like that one do NOT vent by themselves. This is sure sign that something else is fried and needs to be replaced first.

 

[Eastham]

Capacitors like that one do NOT vent by themselves. This is sure sign that something else is fried and needs to be replaced first.

Voltage across the replacement is fine almost spot on 5v, could just be because it's reached the end of it's life span.

 

[for_p1]

Voltage across the replacement is fine almost spot on 5v, could just be because it's reached the end of it's life span.

Usually no. Excessive ripple can cause this. That happens when one or more diodes in bridge rectifier fail open. To check - you need another oscilloscope.

 

[Eastham]

The 5v rectifier is a potted 4 pin package mounted directly to the transformer. Unfortunately I don't have access to another scope to check.