Oscilloscope C1- 79 problems

[teslapand]

Hello all,

I have got an old Russian oscilloscope C1-79 that wasn't working well.(http://www.izmer-tech.narod.ru/oscil/oscil.html)
With power on, everything looks like ok.
All controls on the front panel appear to function normally.
The two traces appear correctly, brightness and focus going well.

I tried with sinusoidal signal at the entrance A and B, signal was synchronized well.

The oscilloscope works very well until 20 minutes.
After 20 minutes R68(5,1 ohms) from power supply(?22.044.072 ?3) heats very strong.
At the same time happens next:

-transistor T1(2T808A) [?23.211.029 ?3] heats strong.
-current through fusibile ??(2A) [?23.211.029 ?3] rise from 0,34 A to 1,02 A.
-tension bar (27V) - collector T1 [?23.211.029 ?3] decreases from 32 V to 19 V
-brightness and focus decreases  very much so the traces disappear.
-fan rotation speed decreases(the pwer supply of the fan is 27 V)
After I power off the oscilloscope and it is getting cold, again the oscilloscope works well.
I checked fan and it is ok.
I checked T1(2T808A), T4(2T602b) C3, C4(1uF)[?23.211.029 ?3] and it is ok.
I untied the terminal 7 from transformer (T?) [?23.211.029 ?3] but still persisted the same defect.
Anyone have any ideas fixing this problem?!

Thank and sorry for my poor english.

[teslapand]

The russian figures does'nt appears in this topic!?!

[TAMHAN]

Do you have access to cold spray?

Maybe selectively spraying, thereby cooling, parts of the planar might help!

[teslapand]

So, you are saying to use cold spray to indentify the defective part.

It might be a bad idea, I might damage something else with spray.

Anyway, I do not have cool spray.
This malfunction of the oscilloscope might be common for russian oscilloscopes, because another friends of mine tell me that their oscilloscope  have the same  malfunction.

[MK14]

So, you are saying to use cold spray to indentify the defective part.

It might be a bad idea, I might damage something else with spray.

Anyway, I do not have cool spray.
This malfunction of the oscilloscope might be common for russian oscilloscopes, because another friends of mine tell me that their oscilloscope  have the same  malfunction.

You are suppose to use, special cooling/freezing "gas" cans. These are made out of inert gases, which are relatively safe to use on electronics. As long as you don't use it too much, and freeze up moisture in the air, and end up getting the electronics wet, that way.

[GEuser]

Seems like big drop on T1 collector , I'd follow the emitter to see where goes , I could not follow where 8----9 goes .

Just a thought ...

[wraper]

Quiet obviously problem is not in the PSU. You should look into the high voltage converter (Y15). The worst you might have is that voltage multiplayer is toast.