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Old 2nd Nov 2019, 5:10 pm   #1
GreasedMonkey
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Default GEC Junior fineline picture problem.

After months of reliable service following the resto of this set, it has sat unused for a couple of weeks. I turned it on this afternoon to be greeted with a rather squat sguiggle of a picture.

Any suggestions before I pull the back off and get stuck into the schematic.

Thanks in advance.
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Old 2nd Nov 2019, 5:20 pm   #2
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Default Re: GEC Junior fineline picture problem.

looks like you maybe have trouble in the Sync separator area.Maybe aged capacitors or a failed transistor/ dry joint etc
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Old 2nd Nov 2019, 6:51 pm   #3
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Default Re: GEC Junior fineline picture problem.

It looks like the line speed is way off the correct frequency. I can't remember if these sets have a line oscillator coil but if it does I would check to see if the core is loose and has slipped.
Otherwise suspect any polystyrene capacitors in the line osc circuit in the first instance. Is there a line hold pot ? have you checked that?
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Old 2nd Nov 2019, 7:08 pm   #4
toshiba tony
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Default Re: GEC Junior fineline picture problem.

Coupled with lack of field amplitude? An odd one.
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Old 2nd Nov 2019, 7:19 pm   #5
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Default Re: GEC Junior fineline picture problem.

I seem to recall there being a line oscillator coil, but can’t be 100% certain until I either get the back off or get the book out, which will be later this evening.
Thanks for the suggestions so far.

One of the faults on the set when I got it was field collapse, but it was total collapse not partial.

Duff capacitor is a strong possibility as I did not do a blanket replacement during the resto and there is at least one not behaving itself entirely as the picture shifts slightly to the left as the set warms up.
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Old 2nd Nov 2019, 8:07 pm   #6
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Default Re: GEC Junior fineline picture problem.

Have you measured the dc supply voltage from the regulator? It’s usually around 11V in such transistor 12V battery/230Vac mains portable TVs.
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Old 2nd Nov 2019, 9:29 pm   #7
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Default Re: GEC Junior fineline picture problem.

The power supply to the horizontal output stage is normal and by implication the EHT is normal, if it was low the scanning width would be seriously reduced. So its not a low voltage power supply problem globally.

The two evident faults are loss of horizontal hold (the line scan frequency is off) and reduced vertical deflection. The question is why the two faults are linked, it may be evident on inspecting the schematic how this could happen.

The primary fault to tackle initially is the reduced vertical scan.The causes for this could be:

Defective connection or defective height control.
Reduced power supply rail to vertical deflection amplifier (seems unlikely with normal width).
Faulty component in either the vertical oscillator and sawtooth integrator/charging the capacitor there, or fault in the V amplifier itself lowing the gain. So either the sawtooth voltage signal is low or the gain of the amplifier has dropped. Chief suspects are electrolytic caps of course.

Once the cause of reduced raster height is rectified, then if the horizontal scan frequency offset is still there, and the H hold won't lock in, it should be fairly easy to determine why.

It may not be the case, but one theoretical explanation of how two faults like this could be caused by one problem, imagine if there is an R-C filtered low current power supply rail somewhere which is being used to charge the sawtooth capacitor in the vertical oscillator, and also used as part of a voltage feed to the horizontal AFC. And say the capacitor was leaky or the resistor gone high. You would end up with loss of H hold and reduced raster height, from one fault. This is why its important to see the schematic to see if such a problem was possible in the particular set, or if the faults are independent.

(Apologies if this reads like the answer to a City & Guilds examination question set in 1970)

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Old 3rd Nov 2019, 3:23 pm   #8
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Default Re: GEC Junior fineline picture problem.

I would suggest finding the reason the line speed is way off first as I suspect that once the line stage is operating correctly the picture height will be correct.
I have seen this before when the line is miles off the correct frequency.
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Old 3rd Nov 2019, 11:50 pm   #9
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Default Re: GEC Junior fineline picture problem.

Quote:
Originally Posted by slidertogrid View Post
I would suggest finding the reason the line speed is way off first.
The line speed in fact cannot be way off, even when in the time frame of a video field you see 10 or 20 bars.

Another thing is that the actual horizontal scan amplitude (picture width), if its close to normal indicates three things:

1) The line frequency must be close to correct . (recall that even when you adjust the H hold control on your set and get it looking "way off" the scanning width is fairly stable).

2) The power supply voltage to the horizontal output stage must be close to correct. The reason for this is that the width is determined by how long the H output transistor (which acts in switch-mode) is turned on by the driving waveform, as, after the transistor is switched on, the yoke current builds up at a rate of V/L amps per second, where V is the power supply voltage and L the yoke & output transformer inductance which are a constant.

3) the EHT is likely correct too as it is directly proportional to the supply voltage applied to the H scan stage.

Expanding on point 2 & 3 the relationship between the power supply voltage to the width of the raster on the screen is easy to work out, in other words to answer the question how does the power supply voltage affect the picture width in transistorized Horizontal deflection ?

Since for magnetic deflection the amount of deflection is inversely proportional to the square root of the EHT voltage (as you know if the EHT drops the picture gets bigger or the EHT increases the raster gets smaller all other things equal) and the EHT itself is proportional to the power supply voltage , simplifying you get the width is proportional to V/(root.V). or simply root.V.

So lets say you drop the power supply voltage V to the H output stage from 100% to 80%, the width of the raster doesn't drop to 80% & it drops to about 89%.

In summary though, for a transistor TV, you can be fairly confident, if the scanning width is ok, the power supply to the H output stage is fine, the scanning frequency is about right (even if not locked) and probably the EHT is also ok
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Old 4th Nov 2019, 3:20 pm   #10
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Default Re: GEC Junior fineline picture problem.

Thanks for the replies so far.

I have checked the set and their is a line oscillator coil (L201) I am confident the core has not moved as it seems to be glued in place.

So we seem to have 2 opinions, either the line speed is way of and correcting this will restore picture height, or field amplitude is way off and correcting this will restore horizontal lock. Now, I am NOT a TV expert, so I may be barking up entirely the wrong tree from here on in, but lets give it a shot.

From the schematic, (attached) The circuit around TR210 forms an oscillator, frequency controlled by changing voltage at the base of the transistor. TR209, Diodes 206 and 207 (hard to read as right in the page joint) and resistors 211 and 212 will create the sawtooth wave.

So I guess my first step would be to scope up the junction between the two diodes and have a look for a nice sawtooth wave there. Can anybody sugest what frequency this sawtooth should be? At least then I will know if it is, or is not massively out.

Please let me know if I have gotten this completely wrong.
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Old 4th Nov 2019, 5:59 pm   #11
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Default Re: GEC Junior fineline picture problem.

The line period is 64 microseconds in the 625 line system = 15.625kHz.

So that should be the period/frequency in the line oscillator stage.
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Old 4th Nov 2019, 8:50 pm   #12
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Default Re: GEC Junior fineline picture problem.

So the line frequency seems about right, healthy looking sawtooth wave with about the correct period.
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Old 4th Nov 2019, 9:01 pm   #13
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Default Re: GEC Junior fineline picture problem.

In which case, the flywheel sync circuit doesn't seem to be locking onto the incoming line sync pulses.

Have you checked that line sync pulses from the sync separator/phase splitter stages are getting to the phase detector diodes and that the reference pulses fed back from the line output stage (usually off the line output transformer) to the phase detector diodes are present?
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Old 4th Nov 2019, 9:04 pm   #14
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Default Re: GEC Junior fineline picture problem.

Is the wavy trace the scope or the line drive being modulated by another frequency?
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Old 4th Nov 2019, 9:32 pm   #15
GreasedMonkey
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Default Re: GEC Junior fineline picture problem.

Quote:
Originally Posted by dazzlevision View Post

In which case, the flywheel sync circuit doesn't seem to be locking onto the incoming line sync pulses.

Have you checked that line sync pulses from the sync separator/phase splitter stages are getting to the phase detector diodes and that the reference pulses fed back from the line output stage (usually off the line output transformer) to the phase detector diodes are present?
Would this cause the lack of vertical deflection?
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Old 4th Nov 2019, 9:33 pm   #16
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Default Re: GEC Junior fineline picture problem.

Quote:
Originally Posted by Nuvistor View Post
Is the wavy trace the scope or the line drive being modulated by another frequency?
Most likely the scope. It is not a great scope and has multiple issues, but my ‘good’ scope recently died on me.
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Old 4th Nov 2019, 10:55 pm   #17
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Default Re: GEC Junior fineline picture problem.

Quote:
Originally Posted by GreasedMonkey View Post
So the line frequency seems about right, healthy looking sawtooth wave with about the correct period.
I explained in both post #7 and post #9 that the H frequency will be about correct ( even if not locked), this was clearly evident from your picture.

So your fault anomaly is subtle in affecting the line oscillator and severe affecting the vertical scan amplitude. Therefore, in the absence of the schematic at least, as I advised before, the place to start looking is in the vertical scan oscillator and amplifier, not the line oscillator.

However, now you have the schematic it is possible to to suggest a single component which could be responsible for both faults simultaneously. Or a power supply rail interaction. See attached.

If the V sync coupling capacitor was leaking it could alter the DC conditions of the transistor feeding the flywheel circuit, thereby altering the H frequency a little and it could upset the vertical oscillator too altering the vertical scan amplitude & frequency. So that is the component I would check first.

If it wasn't that, I would concentrate on the V scan circuitry first correct the fault there and H osc/ line lock problem second.

Also, notice that the decoupled power supply rail that powers the vertical amplifier, also supplies bias & power to the transistor driving the flywheel circuit and sets some conditions to the H osc, so check that supply rail too, if it was reduced you will have the two faults.
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Old 5th Nov 2019, 12:16 am   #18
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Default Re: GEC Junior fineline picture problem.

Some notes again on how the vertical amplitude can be low, these are some:

# vertical amplitude pot problem.
# low level vertical sawtooth presented to the vertical amplifier.
# Low gain vertical amplifier.
# Low power supply voltage vertical amplifier.
# Wrong vertical frequency (check hold pot too).

The last one is the more interesting of the options. Normally, with the correct vertical oscillator frequency sawtooth, the flyback part of it is a rapid discharge of the sawtooth capacitor, coordinated by the frequency or period of the V oscillator, the sawtooth voltage a much slower charge rate in between. Therefore, lets say some fault doubles the vertical frequency the amplitude of the sawtooth voltage will about drop in half, as will the raster height. This is how a problem upsetting the vertical oscillator can also alter the raster height.

So its worth checking the vertical frequency on the scope initially. If that is ok, you know its a sawtooth amplitude issue or a gain issue in the vertical amplifier or power supply issue.
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Old 5th Nov 2019, 11:25 am   #19
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Default Re: GEC Junior fineline picture problem.

Since you have a 'scope and the faults are apparently interlinked, I'd be inclined to have a look at the sync separator stage. Looking at the picture it appears the frame speed is way out and the line speed slightly out. I'd check the PSU to the sync separator for any ripple, double check the HT voltage then have a close look at the transistor and coupling to the timebases.
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Old 5th Nov 2019, 12:45 pm   #20
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Default Re: GEC Junior fineline picture problem.

Assuming the capacitor that couples the V sync from the sync separator to the V osc is not leaking (as I suggested as a possible fault) then a fault in the sync separator transistor stage cannot likely alter the field rate or the vertical amplitude, because its AC coupling of the vertical sync pulse to the V oscillator.

A sync sep stage fault, or abnormal DC conditions there, could alter the H synchronization though, because the sync sep stage is DC coupled to the phase splitter and the flywheel circuit relies on pretty much two equal and opposite pulses being presented to it from the phase splitter. So if the phase splitter's bias was off it could affect the H synchronization.
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