Need advice on an old oscilloscope

[pallee]

Hi!

This is my first post on this forum

I have acquired an old oscilloscope, an Advance instruments OS250 (<- link to manual). It starts and all functions appears to be working, however, I am unable to center the sweep with the the "x-shift dial" (top right on the image). In the linked photo the "x-shift" is positioned as far right as possible but the sweep only reaches 2 cm from the left. I am quite new to electronics and oscilloscopes... especially fixing them... If you have any idea what might be wrong I would really appreciate your help!

Best regards,
Paul

[retrolefty]

Problem is in the horizontal deflection circuit. Problem is it usually takes another scope to troubleshoot that one.

[simonmc]

Looks like you can download a service manual from here. First hit on google, when searching for "advance instruments os250 manual". There are others if thats no good.

http://www.opweb.de/en/model.php?id=7634

[Sarasir]

Hi,
check the horizontal output stage for symmetrie. It should be a differential design as the two plates of the CRT have to be steered in antiphase. This circuitry needs higher voltage supplied let's say about 150V and one side may have leakage or is just fully damaged. Check the transistors an diodes first.
Good luck

[TonyPh]

I had a similar issue with an Advance/Gould OS300 which was traced to a blown high voltage output transistor on the horizontal. I replaced both and its been fine since. Careful with them as the body is likely at a high potential.

In my case one of the transistors was tracking a voltage from 30v to 180v while the other sat at the same potential. By probing the body and turning the horizontal adjust I could see the problem.

I don't know why it blew initially but suspect it overheated or shorted to the bent case cover.

[pallee]

In my case one of the transistors was tracking a voltage from 30v to 180v while the other sat at the same potential. By probing the body and turning the horizontal adjust I could see the problem.

Looks like you got it spot on TonyPh and Sarasir . I opened the scope and found the two high voltage transistors. One is stuck on 170 volt (TR116) and the other one is tracking with the horizontal shift from 45-100 volt (TR115). Also I noticed that TR115 is developing much more heat than TR116.

EDIT: I found that TR116 is actually tracking ±0.5v depending on H-shift. But considering that it is suposed to track 50-100 volt i would say that there is something wrong

Looks like you can download a service manual from here. First hit on google, when searching for "advance instruments os250 manual". There are others if thats no good.

http://www.opweb.de/en/model.php?id=7634

Simonmc, the one you linked looks just like the one I linked in OP, but thanks anyway I really appreciate all help i can get!

Looks as if the broken transistor is a BF258. Do you know of any good suppliers who can ship to Sweden? or should I try ebay?

Thanks again!
Paul

[TonyPh]

I got my BF259 (went for the slightly higher voltage one) from eBay. My actual outputs were unobtanium but grabbed the BF259 and it worked just fine, but still ran hot.

[Sarasir]

Put some kind of heat sink to it if the thing works. It's normal for this transistors to get hot, in some cheap machines so much that one can burn it' fingers.

[Larry F]

Not only can you burn your fingers, but the collector can be hot with 150 volts or more.  Be careful checking temperatures.   Most likely candidate is one of the horizontal output transistors, but if there is a peaking coil in the collector circuit, those also tend to open up through the years.

[pallee]

Thanks everyone for helping me out! I have ordered a couple of BF259 (since the specs seeps to be identical to BF258 except the voltage rating, as TonyPh pointed out) and eagerly awaiting their arrival. I will be reporting back as soon as I am ready to repair the scope!

/Paul

[pallee]

Hi again!

I finally had time to get to work on the scope.

Just as TonyPh said, I replaced the transistor to a BF259 and it works brilliant!

Thanks a bunch!
/Paul

[TonyPh]

Hi Paul. Glad it worked out for you.

Tony.