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VT320 repair - wobbly display

Alegend

Experienced Member
Joined
Jun 13, 2017
Messages
86
Location
Israel
Hi All,

A question to CRT circuitry gurus or anyone who might have seen this in the past and somehow got it repaired.

One of my VT320s developed a wobbly display, see video here: Wobbly VT320

The moment I switch it off, when no line AC voltage is present, the wobble disappears and the display stabilizes perfectly before vanishing.

This fault has developed while sitting on the shelf, so my suspicion is a bad electrolytic capacitor somewhere, but I really have no idea.

I have measured the voltages coming out of the power board into the main board (5V, 12V , 18.5V DC), they are all correct and quiet.

Something is likely broken in the horizontal section, perhaps the vertical as well ? I have replaced capacitor C207 (schematic says 6.2uf 35V bipolar, actual part was a 8.2uf 35V bipolar, replaced with a 10uf 35V bipolar) but the issue persists.

Any suggestion as to what is broken (or where to probe) will be greatly appreciated.

Thanks!
-Alon.
 
I would immediately suspect the power supply, either a dried-out filter capacitor or a shorted diode in the bridge rectifier.

-Dave
 
or where to prob

I would immediately suspect the power supply, either a dried-out filter capacitor or a shorted diode in the bridge rectifier.

-Dave
Correct ! thanks Dave on careful inspection the +18.5VDC line is not clean, there is a high frequency ripple - nearly 800mVp-p.
Capacitor C525 (330uf 25V) was completely dead, I have replaced it as well as C526 and now the display is nearly wobble-free.

Still investigating the cause of the remaining tremble, likely cause is that the supply ripple is not gone, still around 250mVp-p.

-Alon.
 
Both are present... the noise has a 50Hz line voltage component as well as a ~88KHz component.

Leaving the terminal powered for 10 minutes appears to make the tiny wobble go away completely, something inside heats up and apparently it behaves better somehow. The 18.5V line directly drives both the horizontal coil circuit and the TDA1170 vertical control chip.
-Alon.
 

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The regulator controller in a VT320 is a UC3843; 88kHz is a believable frequency for that chip. It's interesting that the wobble goes away as it warms up; that's not a failure mode that I've seen in something like this before. I'll be greatly interested to see if someone else knows something about that and chimes in.

-Dave
 
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