Leader LBO-508 Scope Repair

[petertronics]

Hello there.

My old and trusty Leader LBO-508 broke down. On Monday I left my Oscilloscope for 10 minutes alone while it was powered on. After I came back the CRT did not display any traces anymore  . So I cracked it open and looked around. I found in the power supply section some dodgy looking caps and replaced them. I also checked the CRT filament and as far as I can tell the CRT is ok. Then I tried to power on my Scope but still no trace. So I checked the HV-side of the power supply and only measured around 150V instead of the 1500V needed by the CRT. The voltage supply works just fine. I poked around a little bit more and found the Culprit in the voltage doubler. One of the 0.5uf 1500WV oil Capacitors went completely dead. So I went out to my local electronics store and tried to find a cheap replacement. The only cheap ones they had were 1uF. So i thought to myself that I could buy 2 of them and put them into series and get 0.5uF. Is this a good and advisable thing to do or could I just put one of 1uF in the same place as the old one?

If pictures are needed I can post them.

Edit: changed subject to the correct model.

Any help would be appreciated.

[poot36]

If you do that you will need to place balancing resistors on each of the capacitors to balance out the voltage across each capacitors so that you don't wind up over volting  them.

[petertronics]

I guess something like this? Are the large resistors fiting or should I use smaller ones?

Edit: also added some pictures of the scope.

[PA0PBZ]

It looks like some of the original caps are maybe already in series?
Also, what is the exact model, google has no idea what an LBO-580 is.

[petertronics]

Sorry mistyped the name it is a Leader LBO 508

I also found the correct service manual.

https://elektrotanya.com/leader_lbo-507_508_sm.pdf/download.html

Edit: As far as I can tell the big caps are in a villard cascade, to get to the desired 1,5kV

[PA0PBZ]

Look at page 37 (pdf numbering), you can see that C118+C119 and C120+C121 are in series already.
Makes sense since there's 2KV there. Also please keep in mind that C117 has to be non-polarized!

[petertronics]

The broken cap is c117. I tried to measure the cap with my chinese component tester and got only a broken or no component response. I also confirmed with my multimeter this result. Also a picture of the replacement capacitors I got from my local store. My original plan was to put them into series to get the desired 0.5uF.

[petertronics]

Third Picture

[PA0PBZ]

Putting them in series should be fine, and since there's now a single 1.5KV cap in there you don't risk too much voltage over it, so as far as I can see the balancing resistors are optional and might even interfere with the doubler function.

[petertronics]

It is alive. It has not entered the heaven for scopes ......yet

Thank you PA0PBZ.

[Chris Hoppe]

Hey I just fixed my Leader LBO-508A and thought I would just drop some part numbers here in case it helps anyone.

I replaced all the oil caps with poly caps. There's two pairs of 0.5µF/1600V caps in series, and I replaced with a single 0.22µF/3000V. These are physically larger so I mounted them behind the board, and jumpered out the second cap. I was a little worried about high-voltage isolation distances, so I floated the caps off the board about 10mm, and used some zip-ties and silicone tape to keep some distance between them. (Not sure this is necessary, I don't work with high-voltage much.)

BOM:
(2) 0.22µF/ 3000V 224PPA302KS Illinois Capacitor
(1) 0.47µF/2000V C4CAWUD3470AA1J Kemet
(2) 47µF/450V MAL204315479E3
(1) 47µF/160V MAL204211479E3
(1) 1000uF/50V Panasonic FC (I don't have the part number it was in my inventory)

[Chris Hoppe]

Top side

[Chris Hoppe]

Oh, one small detail, there's a 270K power resistor in series with the caps that got burnt because of the bad cap's electrical leakage. It burned the board a bit, and being high-voltage land, I thought I should cut a slot in the board like so... Probably doesn't matter, this resistor will have a relatively small voltage across it.