GEC BT318 picture problems

[lloydwells]

Having now replaced every capacitor in the set I have a picture but with the following problems.
Focus not very good on any of the three settings it's a bit blurry.
With test card C on screen I can't shift the frame to the right enough to make it even, it's always about an inch over to the left
The Contrast and Brightness controls have a serious effect on the size of the picture as either control is turned up the picture gets larger.
After about 15 minutes the picture fades trying to adjust the contrast or brightness barely brings the picture back.

This is the first TV I have tackled and I struggling to find service data for it if anyone has the trader sheet it would be much appreciated.

[Lloyd 1985]

Hi

This sounds like something is going wrong with the line output stage, possibly a damp line output transformer. The usual cure for that is to remove the LOPT and put it somewhere warm like an airing cupboard for a week or 2. If it uses a valve EHT rectifier, it may be on its way out, and worth trying a replacement. have a look to see if its heater is glowing before and during the fault. Out of focus and the picture growing when the brightness is turned up point to low EHT. Might be worth checking your replacement capacitors, especially the boost capacitor, also worth checking resistors in the line output stage.

I don't have any service data for this set, but it will definitely be needed to help find such things as the boost capacitor.

For the picture shift, are there any shift controls on the tube neck?

Regards,
Lloyd.

[lloydwells]

She who I don't obey is complaining the set is screeching on disconnecting the speaker it's apparently the same ( I have absolutely ridiculous tinnitus) so I rather suspect something in the EHT dept. Discussions were had last week that it's possibly on the wrong EHT rectifier EY86? After swapping the Z749's about it's improved a bit as for neck controls I have one which seems to alter the vertical position of the picture can't for the life of e see how to do the horizontal.

[Freya]

There are usually two plates for adjustment of the position, one for horizontal, one for vertical.
The high pitch noise your wife can hear is almost certainly the 10.125 khz line frequency that 405 line sets emit, its unavoidable,

[Peter.N.]

I would second a damp line output transformer but they take a lot of drying out, it has probably been lying around for years absorbing dampness so it really needs removing and leaving in an airing cupboard or similar for a period - unless anyone knows of a quick way of drying them out, but I have never heard of one.

As the transformer warms up the moisture in the EHT winding in particular evapourates and permeates the whole winding, low EHT is the cause of the picture 'ballooning' when the brightness is increased.

Peter

[KeithsTV]

I've removed the moisture from a TV125 LOPT using a vacuum oven where I used to work. It was left under vacuum at around 40 to 50C for the best part of a day and when refitted didn't fade out.

Keith

[FERNSEH]

Some time ago was an alternative method to drive out moisture in the transformer windings was discussed. The transformer EHT overwind is connected to a bench power supply and adjusted to feed a current of say 20mA through the windings. This method will provide sufficient power to warm up the windings without doing any harm.

DFWB.

[lloydwells]

Ok as previously mentioned set was unused since 1974. I've replaced every Hunts capacitor in it. The 100-300uf cap, the 16-32 can is still the original. Vertical form, vertical height and video int presets have been replaced and pretty much every joint on the video board resoldered.

On switch on I get a strange hissing sound a bit like a wax capacitor frying as the audio comes up (connected to the Aurora ) the tone motor boats somewhat. The hissing reduces if the horizontal hold is twiddled about and eventually stops and the test card comes up. This is shifted too far to the left. I can't for the life of me find any way of shifting it over.

The picture snakes that is to say it ripples very slowly from top to bottom any ideas greatly appreciated.

[Freya]

Do you still get the hissing sound if you take the top cap off the line output valve.
Wavy verticals can be caused by too much signal from the aurora 15-18dB is needed for most sets.

[lloydwells]

Ive ordered an attenuator will see what that does a soggy lopt is a possibility it was being stored in a shed when rescued.