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Old 9th Aug 2020, 11:55 am   #1
birksholt
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Default Panasonic U1

Hi all,
The Panasonic's frame output circuit has me a bit puzzled. Initially, the fault it had was that the picture was squashed vertically. R411 had gone up in value to about 7k and replacing it sorted that out. It worked for a little bit and then the vertical deflection went altogether.

I had a look through Television magazine and it suggested that the fault was often caused by D403 causing the output transistor to turn on hard and blow the fusible resistor R442. D403 is an SV02 which is basically two diodes in series in a single package. I checked R442 and it had blown so I replaced it with a normal resistor and gave it a quick test and the vertical deflection was working again but sure enough within a few minutes, the voltage across D403 was getting up to around 2V and increasing.

Of courese, these SV02 diodes are no longer available so I had to improvise. I had happened to measure the voltage drop of D403 when I was looking for the original fault and it was around 1.2v so I had a look through my diodes and found some 1N4937s which together made up about the same. I tried these and it worked so I put a 1R 1/2W fusible resistor in R442. It worked until I turned the set off then upon turning it on again, the frame collapse was back and R442 open again.

I put a 1 watt resistor in R442 and started measuring some voltages. Across R442 there was 680mV so no wonder the 1/2W resistor was opening. I thought maybe I've over biased it. I tried replacing one of the makeshift D403 diodes with a Schottky to reduce the bias. This worked very well, so well in fact that it reduced the bias to the point where there was visible crossover distortion. I started measuring voltages again and this time got 660mV across R442. 100R resistors R422 and R423 with capacitor C410 form a bootstrap current source and across R423 I measured 8V so 80mA there. Across R424 which is .47R i measured 270mV so 574mA there and that accounts for almost all the current flowing in R442.

So this is what has me puzzled, if even with the output stage grossly under biased the circuit still draws 660mA, how are they getting away with a 1/2W 1R resistor? The original resistor I took out was a 1/2W one so it's not that the diagram is wrong. Am I missing something here or approaching it the wrong way? I would appreciate your thoughts.

Cheers
Jack
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Old 11th Aug 2020, 10:16 am   #2
Welsh Anorak
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Default Re: Panasonic U1

This may sound odd, but we always found that those Panasonic fusible resistors needed to be replaced by exact parts. A normal fusible would fail. I know, half a watt is half a watt, but I wonder...
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Old 11th Aug 2020, 11:27 pm   #3
birksholt
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Default Re: Panasonic U1

Thanks Glyn. So I might not be going mad then (at least on this). I was thinking it's got to be either something wrong with the load circuit, but I'd have thought that would show itself on the screen, or Panasonic's watts must be bigger than mine. Looking at the note in TV magazine again, it does say actually that the Panasonic part should be used and gives the type number. That should have probably told me something. There's another one of the same elsewhere in the circuit so I'll see what sort of current that's passing and whether I might be able to use that one and put a normal fusible in it's place as I can't find the Panasonic ones. They must have been using them for quite a while though as it's the same ones in the Euro 4 some 18 years or so later.

It does give a good picture. The magic tuning line doesn't seem to be working though, it just covers most of the screen and doesn't vary in width although the AFC seems to work. I thought I had a problem with the touch tuning as when selecting 1, it looked like 7 was being selected but it turned out it was actually selecting 1, just the wrong light was coming on. A new neon bulb fixed it.
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