Restoration of an Ekco TS46

[murphymad]

If the focus is creeping as the set is running for up to 2 hours before you reach the end of the focus pot, and, this can be repeated each time you run the TV from cold, I would suggest that the focus coil unit is heating up and the insulation within the coil may be the problem.

To test this, make the connections to the focus coil "quickly" detachable and then measure the DC resistance when completely cold before running the set until the focus pot reaches its end stop (approx. 2 hours) then switch off and re-measure the resistance again.

A small difference will be expected, but a big difference will point to the insulation inside the focus coil unit breaking down.
These coils do get hot!

You should also be measuring the EHT with an Electrostatic meter, rather than a resistive prod type.

Mike...

[Heatercathodeshort]

Does the EHT vary in sympathy with the focus drift? J.

[line sync]

Thanks both mike and john for your replys.
I didn't check the eht with an electrostatic volt meter so probably would not have had a reliable reading.
I used a rank eht meter made for colour sets of the early 1970`s.
I didn't do a before and after check john.

Robin

[Duke_Nukem]

A puzzle indeed.

I'm wondering if it is hum in the R.F. E.H.T. generator. Given the breathing effect and that it is causing defocusing suggests hum on the E.H.T in this set, I'm wondering if it is a mains pulse that is the root cause. Normally the ripple would look like a small triangle-ish shape, however if there is resistance in series then the voltage seen on the input to that resistor would see a pulse superimposed on the triangle due to the current surge when the cap is being topped up.

Bridging the HT cap wont reduce it since the cap is on the other side of this hypothetical resistance.

So, maybe this resistance is in the earth connection which is a possibility, especially as the chassis is painted, or maybe just a minor oops in the wiring / routing differently ? Wouldn't expect this pulse voltage to be very much at all, but if it gets into the oscillator circuit it could be multiplied up on the EHT. I know you increased the EHT cap but it would need a lot more capacitance added for 50Hz effects.

TTFN,
Jon

[line sync]

Thanks jon for the interesting reply.
I`ll check the wiring and earthing of the eht dept on monday .
Just a quick question , I gutted the two visconol eht caps and fitted a .001 disc inside each one rated at 8kv , is that good enough ?

Robin

[Freya]

You may not have seen this similar post ?
https://www.vintage-radio.net/forum/...ad.php?t=49152

lots of pointers

[line sync]

Thanks for the pointer Stephen , I'm now armed with a few more things to check.

Robin

[line sync]

Right ok next instalment.
No good news to report but I've done some checks which might narrow things down a bit.
The resistance of the focus coil increased by 15 ohms after the chassis had been run for two hours , when cold it read 134 ohms and when hot it read 149 ohm , is this acceptable?
Also the HT line to the EHT unit rose by 11v.
The focus did not drift so badly to day and the eht kept at 5kV.
One point I noticed was the EHT dropped to 2.5kV when the brightness control was turned up fully but bear in mind I'm not using an electrostatic volt meter.
I've not replaced C70 in the EHT dept which is a .003 silver mica because I don't know what the working voltage should be, will a 400V cap be OK?

Robin

[Jac]

If I calculate it correctly, that resistance rise equals a temperature rise of about 28°C, which is acceptable.

Jac

[Heatercathodeshort]

The R.F. EHT system in the Ekco TS46 is one of the more critical units. It has to be 100% or it simply will not work. Try the effect of removing the screening can that surrounds the self contained EHT unit and note any difference in the EHT supply. The result may be interesting. It is tightly screened due to oscillator harmonic interfearance and the proximity of the screen [ brass/mess/copper?] can have a significant effect on the Q of the coil. Mine reads 6kv with a resistive meter and the TS48 at the museum is just on 7kv. The value of R81, cathode bias resistor [P61] has a great effect on the EHT voltage but will not cause the EHT to drop. In my own opinion I don't think the choice of the P61 power triode is a good one. EHT units of this type tended to employ a beam tetrode such as the 6V6G or 185BT. The P61 gets very hot! John.

[line sync]

I`ve spent lots of time on this chassis over the last week or so and not really got any where.
John, removing the screening box from the eht unit makes the eht rise by approx 500v.
I have been unable to cure the hum on the picture , I`ve had the eht unit off the chassis and fully checked it over , also the chassis earths.
I had wrongly thought the effect was called breathing but I now know this means the picture is changing size and my chassis is not doing that.
Thank you Stephen for sending me the Practical television imfo.
I`m still very puzzled why the test card is in good focus except for the bottom.
I`ve tried adjusting the centring screws and moving the focus assembly backward and forward but nothing will allow the bottom of the test card to focus in.
Whilst restoring and painting the chassis , the focus unit was also dismantled ,sanded , de-rusted and painted.
I did take notes of how the unit came apart and was care full to re-assemble it the same way but two discs on the rear could go on one of four ways and I had forgot to mark them but didn't think it would matter because it was a coil assembly and not a magnet one, could this be where the problem lies ?
Below are pictures showing the focus problem and just out of interest the eht on the bright pictures is 4.75kv and the darker picture is 5kv.
The focusing problem looks like convergence errors on a colour tube.
The focus on the darker picture is near perfect.
Robin

[DangerMan]

What happens if you rotate the tube in the chassis, within the limits of the EHT cable?

If nothing changes at all, then at least you can rule out some misaligment of the gun... maybe.

I think I can see another screw which looks like it is not meant to be an adjuster; I presume this is part of the machanical assembly. Is it possible the focus assembly has been reassembled with a slight tilt on the coil or one of the pole pieces that might cause an astigamatic field ?

[Heatercathodeshort]

This looks very similar to my TV22 that has a slightly gassy MW22-18 fitted. The miss convergence effect is identical. Mine recovers to a greater extent after a period.
The screen phosphor on your CRM92 may have changed chemically producing the slight shading at low brightness. The third picture shows a very course structure. I have a CRM92 in a Ekco TS88 that produces a picture 'behind a veil' as if you were looking through the screen coating, most odd but watchable. Just thoughts. John.

[Freya]

This might be worth a look / try, im almost out of ideas now, John beat me to the gassy idea whilst I was looking through the Tv mags for this.

[ENGLISH VICTOR]

Because the width does not change with brightness given that a separate EHT system is being employed the eht must be fairly stable because for a given scan current the width is inversly proportional to the square of the EHT value, ie lower EHT more width and of course height. I would be very suprised if you could find a CRM91 or CRM92 that did not go out of focus at full beam current. Given the good focus at lower brightnes levels I might be inclined to leave all well alone. I presume that with an EHT of 2.4KV the picture was overscanned by an apreciable amount.
Victor

[PJL]

Although it could be an artefact of the camera there are distinct lines which suggests it is modulation and not a focus problem as such. However, the modulation effect seems to be both vertical and horizontal. Sort of thing you might get if there was a transformer too near.

The clue must be in the first picture where the fault appears only at the bottom of the screen. How is the focus coil current supplied? Any chance this is getting modulated?

[Freya]

this shows the focus arrangement

[Heatercathodeshort]

Check the 32uf if you have not already done so. If O/C can give some odd modulation effects. J.

[Freya]

What about if the 2K wire wound focus control track, if that was open circuit ?

[PJL]

It does seem a plausible explanation assuming the bottom of the scan corresponds to the peak current in the frame drive.

When the screen is showing the symptoms in picture 1, you should notice the focus control also changes the extent of modulation at the bottom of the picture.