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PET 2001-N 32: inconsistent vsync

>>> Given that you actually saw the fault and now it has gone is not great.

>>> So it turns out to be the (blue) wire between the pcb and yoke.

Hugo. It seems as though the OP found a cause and a fix...

Dave
 
Yes - moving the wire restores the vert separation. Reflowing the both ends (though I suspect it was the end at the yoke) has stopped the now frequent vert collapse and the the occasional vertical flicker.

I suspect it was my initial removals of the crt pcb was what stressed the wire connection in the first place

I’ll give it further soak testing, but over an hour or so today - the flickering and vertical collapse is gone.

Cheers
Mike
 
On my PET VDU I fitted single connector pins to the pcb for the Yoke wires so that I could unplug the yoke at will. I used gold plated 0.9mm pins and single sockets (a Jaycar part). They are rated at 1A average. But one thing to note , with the peak current it can be as high a 2 to 2.5A in these small VDU's which run a relatively low B+, because for some screen size and EHT a certain deflection power is required, and the lower the operating voltages the higher the current required. This is also one other reason that a special super low ESR Coupling capacitor is required for the H yoke coils.
 
I left the monitor on for over an hour and it’s started to develop a flicker towards the end. I’ll let it cool and see if it’s a similar behavior - good until it gets really warm
 
So it seems a problem remains and it could be thermally sensitive. It is a matter of determining again if the signal to the VDU is stable or not and if the problem is in the VDU or outside of it.

There was a case where one of these PET VDU's had an issue with the 7812 voltage regulator IC. They pushed their luck with this design in that the dissipation in that part is high, the current is nearly 1A and the dissipation can get very high especially if the voltage feeding the VDU Is high in the case of relatively high line voltage. And the arrangement is just ok if the tab on the part is properly thermally bunded to the large heat sink with thermal paste and enough pressure. But two things went wrong, the paste dries out over time and in some cases they riveted the tab to the heatsink/pcb. This distorted the tab around the rivet (as its only soft copper) because the rivet expanded in the hole on the tab, so when the paste dries out it has a poor thermal contact. This might be the cause, but usually if the regulator is thermally shutting down the output voltage drops to around 7V and the scan collapses, so I'm not sure if that could be the cause of the problem in this case, but its worth checking. If it is the regulator should be replaced and a new one fitted with fresh paste and a screw/nut instead. Also there are two sorts of tabs on the TO-220 package now, many of the ones from major suppliers are thin as they were made for soldering to a pcb surface, but it needs a vintage one with the thick tab.
 
.......and of course one other explanation, there may have been two faults, one causing the total vertical collapse you found caused by the blue wire and the other the "flicker" problem, which has remained latent until it has now returned, which intermittent faults have a nasty habit of doing.
 
Thanks Hugo,

I did a bit more reflowing at the yoke and replaced the 7812 with a similar (but 1.5A) linear voltage regulator (thick tab, bolted, new thermal paste) and it’s going good now.

I really appreciate your (& Dave’s) help

Cheers
Mike
 
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