GEC 9 Inch TV

[Radio Tech]

A few more waxie caps now changed, I always check these as I replace just to make sure that they have gone out of spec, invariably the capacitors I have changed and replaced with modern types are leaky and out of spec. I replaced yet another electrolytic, C83 a 0.05uF, changing this brought the sound quality up. Below is a better test card picture, more to follow though folks.

Regards

Ken

[brianc]

Hi Ken.
A 0.05mfd capacitor will not be an electrolytic. The original will have been a paper type to be replaced by a modern polyester one.

[Radio Tech]

Hello Brian,

Yes your quite right, I forgot, I was obviously thinking of another part of the circuit, but I will check this just in case I have put an electrolytic in place by mistake. But now the set has been on a few times, the two large capacitors that are on the EHT line I think one of them as gone very leaky as R59 a 10K half watt resistor was starting to burn up and the picture has now disappeared, C45 and C46 are 0.1uF at 6000V capacitors and are in one can with a common ground, not sure what I can put in place of these at the moment though, looks like some more brain teasing stuff ahead.

Best wishes

Ken

[Freya]

What is everyone else's view on using readily available ceramic capacitors rated at 6kv in this position as a suggestion ? unsuitable ?

[Neil Purling]

Would the original capacitors be something oil insulated, like TCC Visconol X. I can't imagine a plain waxed paper capacitor being rated at 6KV.

[brianc]

Remember that this set has mains derived EHT which, of course, is 50Hz. This means that the caps will be much higher capacitance than for Flyback EHT. The caps in this set are 0.1mfd at 6kV so they will have to be polyester or mylar types. These are available from several sources (BVWS, Ask Jan and many Chinese sources). Visconals are also available but probably faulty by now.

[Radio Tech]

Hi Folks

Below is a photograph of the EHT smoothing capacitor I think is causing the 10K half Watt resistor to burn up, the can hold two capacitors that are 0.1uF (100,000pF) in value , and has a common negative, but one side according to my capacitance meter reads 10nF the other half reads 112nF but the strange thing is that putting it on an AVO LCR Bridge doesn't indicate any leakage, perhaps the bridge cant go up far enough to indicate if there is leakage. Anyone got ideas what to replace it with?

Regards

Ken

Below is a better test card picture

That looks like an improvement upon any of tonight's TV!

[brianc]

Originally Posted by Radio Tech

Anyone got ideas what to replace it with?

Please read my earlier post, Ken. I suggested a couple of sources. They're not hard to find and if I were you, I would open the original can and re-stuff it.
From what I make out, the picture has disappeared so one of the sections has probably gone leaky. Try running the set with one section disconnected starting with the low reading one. If it is the fault of a leaky section the picture should appear but with positional mains hum on it.

[Radio Tech]

Thanks Brian for the information on the troublesome capacitor, I put a post on the wanted section and Michael has kindly offered to get some 50nF at 6kV, 4 of them from somewhere in Germany. In the meantime I have found three more waxie's that required replacement, these were C103, C100, and C71, all in a very awkward place but with a little patients I managed to get the old ones out and new in.

Ken

[Radio Tech]

Hi everyone who is following this thread on my 9 inch TV, I am still bothered about why after the set has been producing a good picture for about 10 mins or so suddenly starts to fade and then the series resistor R59 a 10K from the cathode of the U33 starts to burn up. I tried a replacement of the capacitors C45 C46 but this made no difference, I am starting to wonder if its something to do with the tube after warming up, I am not quite sure how to finally trace this odd fault.

Best wishes

Ken

[Freya]

Could be excessive beam current due to the tube being slightly gassy, or the overwind on the lopt could be breaking down.
John may have a better idea.

[Station X]

Excuse my ignorance, but do sets with mains derived EHT have a LOPTX?

[peter_scott]

They do but not with an overwind.

Peter

[Duke_Nukem]

Does the picture bloom as it disappears ? Just want to confirm the EHT really is responsible (there may be an unrelated issue regarding the 10K resistor).

TTFN,
Jon

[Radio Tech]

Hi Jon

The picture seems to start fading and as it does so, the picture gets badly out of focus and in the process of the picture fading away so does the brilliance then the picture is lost altogether, but as I say beyond this point if left the 10k resistor will start to burn up but of course to prevent damage I cut the power off.
Regards

Ken

[Hunts smoothing bomb]

Ken,

I know you have replaced the EHT caps but have you actually run the set without them connected at all to see if the 10k resistor still overheats?

Are you able to measure the resistance of the 10k resistor and also the voltage dropped across it? (May need an EHT probe)
With this you will be able to work out what current is being drawn by the tube.

Cheers

[Heatercathodeshort]

First of all you are dealing with a LETHAL EHT system with this receiver. Just another warning.
Mains derived EHT chassis do have a line output transformer minus it's EHT overwind as would be found in conventional flyback systems. The transformer is similar to an audio output transformer but is of course insulated to a high standard and capable of handling 10k/cs spiky peaks easily. The line output stage resembles an audio output stage with the scan coils connected in place of the speaker.
If the 10k resistor is getting hot and the EHT reducing it can only be down to a couple of components. The bleed resistors could be faulty in some strange way reducing in value as the chassis warms up. Very unlikely but easily proved by disconnecting. Are you positive the two high voltage capacitors have 100% perfect insulation? The bleeder network tag board may have become conductive but this usually bursts into flames if faulty. Worth a check. If they are blameless the only item left is the CRT itself that is probably gassy. You may be able to see a purple glow in the neck as the picture fades. [My KB EV30 thread will show the very strange effects I encountered with a slightly gassy Brimar C12B CRT,] Disconnecting the EHT connector will again prove this returning the EHT to normal.
CAUTION again!! You need a good EHT meter to diagnose faults on mains EHT systems. Guess work and sloppy methods will not do. With the bleeder disconnected the EHT capacitors will hold a charge for many hours unless drained by a partial short on the high voltage line. You really need to take the greatest of care when working on this chassis. Remove the mains plug before any test and short the capacitors to earth and add a shorting link to chassis but remember to remove it before powering up or you will blow the fuses and the U33. Tedious I know but it's so easy to make a fatal mistake. Mains EHT is a silent killer.
My money is on the CRT. The 9" GEC tubes had all sorts of odd faults that were cleared up by 1950.
Take care Ken. John.

[Heatercathodeshort]

Snap! Just seen your reply Lee. J.

[Radio Tech]

Thanks John and Lee for info, and yes I am extremely careful with this knowing how lethal this set can be, I haven't got a meter capable of measuring EHT but will press on, its such a good picture when the set is functioning.

Best wishes

Ken