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heater voltage of scope picture tube ?
heater voltage of scope picture tube ?
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Old 4th September 2005, 08:06 PM   #1
Bernhard is offline Bernhard  Germany
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Join Date: Apr 2002
Location: Munich
Default heater voltage of scope picture tube ?


what is the heater voltage of a scope picture tube ?

I got a "sold as dead" spectrum analyzer from ebay and another one from a flea market.

Instead of using them only to replace the mechanical damaged PSU of my third analyzer and to have another spare parts, I like to repair one of them.

The one from the flea market worked for a few hours after removing and putting back some pcbs. ( Did the same with my third one when I got it, contacts clean this way )
There suddenly was a silent crackling sound and the tube went dark.
A fuse in a 15V line to the high voltage transformer was blown but I couldn't find a shorted transistor or cap.
Now a new fuse does not blow anymore but still get no picture.
And the heater filament does not glow.
I put the pcb from the ebay analyzer there and it works, but this pcb looks totaly different, high voltage parts are inside epoxy.
Also this pcb contains regulations for +/-15V and +/- 30V.
While the original pcb had LM regulators, the new one got modules that convert from 5V to the +/-15 and 30V.
Now hell , instead of +/-15V there are +/-20V and those directly feed op amps and maybe DACs and ADCs everywhere.
Also more than 30V, the 30V come from 2 module outputs connected in series.
On the pcb are test points that say 15V and 30V so something must be wrong.
Could there be some external connection missing on the modules ?
I have no idea how there can be 20V instead of 15, those are three modules and they can not be dead alltogether.
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Old 18th September 2005, 04:03 PM   #2
Bernhard is offline Bernhard  Germany
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Join Date: Apr 2002
Location: Munich
If the high voltage transformer for the picture tube got a short somewhere, which I suspect because I checked everything else,

could it help to put oil inside for isolation ?

It is a very open construction and oil would flow inside everywhere.

The transistor on the primary side gets hot and the circuit does not swing.

Very strange, the 5V to +/-15V modules on the other board work correct in the other analyzer where they come from.

The modules are UD5-15D45 I read somewhere "charge pump", do they need external output caps or other connections, anybody have data sheet ?
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