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Oscilloscope Topwad 7021 repair

 
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evh



Joined: 06 Sep 2006
Posts: 12
Location: england

PostPosted: Sun Dec 12, 2010 4:27 pm    Post subject: Oscilloscope Topwad 7021 repair Reply with quote

The oscilloscope (Topwad 7021) does work but in the beginning of the beams there is bright dots and at the beam ends the image is shrinked.Intensity regulator is turned completely back but beams are bright and turning the regulator makes week influence.The same is with the focus regulator as well.
Here are the pictures:
http://www.picvalley.net/v.php?p=u/1776/19069740909772369061292075432SY1XQaYjIjpS0GLMwAR0.JPG

http://www.picvalley.net/v.php?p=u/2194/135517110811181386051292075449ViPhYAI285NwumZAQOdA.JPG

http://www.picvalley.net/v.php?p=u/1888/5630784074204399381292075825pRl4QeGlpfl1Mkm35LMj.JPG

I think the problem is in power supply of tube which is Toshiba 150BTB31A.
I couldn't find a schematic diagram of power supply ,service manual,or tube pin outs...
Any help will be appreciated.
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evh



Joined: 06 Sep 2006
Posts: 12
Location: england

PostPosted: Thu Dec 16, 2010 10:27 am    Post subject: Reply with quote

Any help for power supply diagrams?
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evh



Joined: 06 Sep 2006
Posts: 12
Location: england

PostPosted: Wed Dec 22, 2010 10:22 pm    Post subject: Reply with quote

I found the problem without schematic.Very hard to find - one resistor have been changed the value from 220 Ohm to 6 KOhm and a capacitor leakage...
Now everything works fine.
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evh



Joined: 06 Sep 2006
Posts: 12
Location: england

PostPosted: Thu Dec 23, 2010 10:12 am    Post subject: Reply with quote

It's not a joke,it's true.It happened for second time to me resistor to change the value and it was not power resistor.Really more often happened resistor to break off...
The voltage on the capacitor is 160V and its value was 2.2uF/350V.I putted 4.7uF/450V.
The oscilloscope is still about 20 years old...
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vtech



Joined: 08 May 2006
Posts: 1264
Location: USA

PostPosted: Thu Dec 23, 2010 10:35 pm    Post subject: Reply with quote

Disregard the previous moronic poster--- I deleted it

Good find..It is rather rare for low-value resistors to change so much in value.--probably more age related. More common for higher value resistors to change value.
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jts1957



Joined: 24 Nov 2008
Posts: 2475
Location: Far, Far Away

PostPosted: Fri Dec 24, 2010 3:31 am    Post subject: Reply with quote

Unlike the, for example, 1.8 ohm ones in some convergence circuits? Surprised
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evh



Joined: 06 Sep 2006
Posts: 12
Location: england

PostPosted: Fri Dec 24, 2010 10:44 am    Post subject: Reply with quote

The resistor value was changed but not constant,it sifted slowly or unsteady 3,4,6...KOhm
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