Sync troubleshooting.

[delaitt]

Hello

Whilst troubleshooting an intermittent fault where the image sometimes gets scrambled (1st pic) and need to switch between 819/625 to clear the fault until is starts again, I appear to have caused another issue. The new issue is that the screen is now split into two images in half with the sync in between. The only thing I did was to replace a capacitor in the sync separating section. I put back the old cap but it is still the same. It may be there is a faulty component or contact in the area but cannot find it. It is a French Arphone TV (EL300, 110 degree tube) and I've included the schematic. The pattern generator is in 819 lines and the displayed images are in 625L. is there a way to troubleshoot this one ?

Thanks

Thierry

[FERNSEH]

Seems like the frame timebase is running at half speed. 25Hz instead of the correct 50Hz. Does the set have a flywheel line sync system?

DFWB.

[delaitt]

Thanks. There is a 6AL5 diode connected between a dedicated winding of the LOPT and the vertical oscillator ECL85. is this the flywheel line sync system?

Thierry

[FERNSEH]

Hi Thierry,
It's most likely the 6AL5 is the line sync phase discriminator and possibly the triode section of the ECL85 is the phase splitter which provides the diodes with anti phase line pulses.

DFWB.

[Nuvistor]

It appears the ECL85 is a triode frame blocking osc and pentode frame output. A 12AU7 is the line osc, appears to be a cathode coupled multivib but could be very wrong.
Not sorted out the 6AL5 but yes probably the discriminator.

[Nuvistor]

There is a ECF80 pentode section sync sep, a 12AT7 ‘comparator’ half of which is used in grounded grid for the frame sync shaper. The video signal is switched earlier to retain the correct phase to supply the sync separator.
Not sure about the triode in the ECF80 and half of the 12AT7. I will stick my neck out and say it appears to be direct line sync but the circuit is laid out in a manner I am not accustomed to and finding it a bit difficult. That my problem though not the circuit.

Edit. It appears to be direct sync on one line system, sync feed from anode of sync sep, but the other system comes from the comparator circuit so this could be flywheel unless it’s a noise gate.

Goodness knows what the 6AL5 is doing.

Anyone else suggestions I am getting a bit bogged down.

[Nuvistor]

The 6AL5, one diode produces a negative pulse for line blanking of the CRT, feeds G1 of the CRT. the other half produces a positive voltage to feed what I think is the set EHT control in the G1 circuit of the line output valve. It then goes on the feed the anode of the frame osc triode, cannot read the voltage on the diagram.
One half of the winding feeding the 6AL5 goes to the comparator circuit.
That’s about the best I can do, any corrections welcome.

[FERNSEH]

Hi Frank,
I should have read the circuit diagram first rather than jump to conclusions. As far as the frame timebase goes it would appear that one of the sections of the 12AT7 comparator is used as the frame sync pulse shaper, in fact the circuit resembles that of the KB VC2 series. The grid of the triode is to ground and the sync input supplied to the cathode.
Is that an inductor in the cathode circuit of the 12AU7 line oscillator? Also, the sync feed or control voltage to the line oscillator is fed from different sources according to which line standard is selected.

DFWB.

[Nuvistor]

It could be an inductor in that cathode circuit, too blurred to be sure.

[ms660]

Looks like an inductor to me as well, there's an extra capacitor that's switched in across it for the lower line speed so far as I can make out.

Lawrence.

[FERNSEH]

It might be what is called a "stabilising coil" . Something similar was used in certain Pye TVs, the VT4 for example. The strange thing about the coil is that it is adjusted to a frequency which different from the line timebase.
A similar line oscillator circuit was used in the Pye FV4. The coil L20 is adjusted to 8,000 c.p.s.

DFWB.

[delaitt]

thanks for your replies.

effectively the vertical timebase is running at 25 Hertz as shown on the scope with a period of 40ms. The top signal on the scope is taken at the grid of the ECL85 pentode.

The bottom signal on the scope is taken at the cathode of the triode of the ECF80 synchro and with a frame pulse of 20ms

I've highlighted the inductor in the horizontal timebase as a yellow rectangle. The red rectangle shows the transformer of the blocking oscillator connected to the ECL85

Thanks

Thierry

[FERNSEH]

The only frequency determining components are the resistor and capacitor marked in red on the circuit diagram. The sawtooth forming capacitor is "C".

DFWB.

[delaitt]

many thanks. the vertical sync potentiometer simply needed to be tuned quite significantly (I did not turn it enough for some reason). The vertical frame oscillator is now back at 50hertz

I have a few other anomalies with the set as follows. Hope it's ok to piggyback on this thread.

1. every few minutes or so the image gets scrambled as shown in the first picture. It requires to switch between 625 & 819 to restore the image. is this a horizontal sync issue?

2. The left border of the image on the CRT starts after a few centimetre.

3. the top & bottom of the vertical lines (as shown in the 4th picture) tend to go out of focus if I push the brightness a bit. not sure where to search? EL183, focus, or CRT has too many hours?

Thanks

Thierry.

[delaitt]

the scope images show what happens to the signal when the image gets "scrambled" with a horizontal sync issue. This only happens when in 625L.

The top signal is taken at the grid of the 12AU7 line oscillator (input from ECF80)
The bottom signal is taken at the grid of the EL300

The 1st image shows a quite noisy input signal to the line oscillator from the ECF80 when the horizontal line sync issue occurs. The 2nd image shows when the line oscillator is in sync and works correctly.

Thanks

T.

[1100 man]

Hi Thierry,
Looking at your first picture in post 14, I would say that the line speed is way too fast. I wonder if the fault is actually in the line oscillator rather than the flywheel sync circuit that controls it? Maybe there is a bit of leakage in the system switch which is upsetting the oscillator?

The flywheel circuit produces a dc control voltage which adjusts the frequency of the line oscillator. It would be interesting to monitor this and also the line frequency and see what happens when the 'scrambling' occurs.

I've never had the fun of playing with a dual standard 625/819 set so i'm rather envious!

EDIT
Looking at your 'scope traces, sure enough, when the fault occurs, the line frequency increases. When all is well it's 3 divisions and during the fault it's about 2.5 divisions.

I would be inclined to disconnect the line sync pulses to the flywheel circuit so that it free runs. adjust the line hold pot for an almost complete picture and see if the fault appears. If it still does, then it can't be the sync pulses/ sync separator causing the problem and must be in the line osc or flywheel part.

All the best
Nick

[FERNSEH]

My interpretation on how the line sync works is that on 819 lines the line oscillator operates in direct sync mode and on 625 lines the flywheel sync system comes into action. Is there any instructions in the service manual on how to adjust that coil in the line oscillator. In an earlier post I mentioned that some early Pye sets employed a resonant circuit in the oscillator and I remember the line synchronising would work in a erratic manner if the phasing coil wasn't correctly adjusted.

Might be a good idea to convert the circuit to direct sync for both line standards.

DFWB.

[FERNSEH]

The 1960 range of UK made Philco TVs employed a very simple and effective line flywheel sync system. The triode functions as a comparator, a positive going flyback pulse from the line output transformer is supplied to the anode and positive sync pulses are supplied to the grid. Any phase difference between the flyback and sync pulses produces a control voltage for the line multivibrator.
The Airphone TV uses a similar flywheel sync system.

DFWB.

[delaitt]

thank you. I've changed the valve for the line oscillator, comparator, 6al5, and ecf80 just and this makes no difference.

I can see the DC control voltage dropping by half at the grid of the line oscillator when the problem occurs.

I do not have the service manual for this set to see if there was a special procedure for adjusting the line oscillator.

If it try to disconnect the grid of the line oscillator from the comparator to let it run freely, do I connect the grid of the line oscillator to the ground during the testing?

Thanks

Thierry

[FERNSEH]

Further study of the circuit diagram reveals that the flywheel sync control voltage comes from the triode section of the ECF80 sync separator valve. It functions as a cathode follower.

"If it try to disconnect the grid of the line oscillator from the comparator to let it run freely, do I connect the grid of the line oscillator to the ground during the testing?"

The design of the line oscillator may not permit that. It might be a better idea to connect a resistor of say 100Kohms between the grid and ground.

DFWB.