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Old 25th Jul 2013, 7:10 pm   #1
Murphy Maniac
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Default Ekco trc139

I have been working on my TRC139 for ages now and have reached a dead end, I would be grateful for any advice as where to look next.
I have a clear, bright and crisp picture which locks perfectly but in the wrong place. The picture is divided in two by a black line across the centre. There is also a pronounced flicker which does not respond to anything. The width control is set for minimum width and the picture is still slightly too wide. I have not checked R48 and C38 ( from Trader Sheet 1007) so have not yet ruled these out.
I would welcome any suggestions as to how to proceed, and any explanation as to why this is happening.
Many thanks,
Geoff.
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Old 25th Jul 2013, 7:38 pm   #2
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Default Re: Ekco trc139

Hi Geoff.

Your excellent description of the symptoms coupled with the picture leads to the correct diagnosis. The frame timebase is running at exactly half speed hence the flicker. If you study the photo you'll see that it is actually nearly two pictures. Sort the frame frequency out and all will be well. Try from one end of the frame hold to the other for starters...

Good luck.
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Old 25th Jul 2013, 7:55 pm   #3
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Default Re: Ekco trc139

Not far out as Tas suggests. OOO! You have one of my favourite receivers. First thing to replace is the Thyratron [gas relay] frame time base generator valve, Mazda 6K25. These fail in many strange ways often locking the picture but bouncing violently in a vertical direction. There are very few components in the frame generator stage. Make sure you have fitted the correct value components especially capacitors providing of course you have actually replaced them, a must.
C19 .5uf and C18 .1uf [TS88 circuit which is the same frame wise] is highly suspect if the valve is good. You cannot test Thyratrons and you may have to try a couple for reliable operation. The timebase should lock solid as rock in the centre of the frame hold control. Hope this helps. John.
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Old 25th Jul 2013, 8:18 pm   #4
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Default Re: Ekco trc139

The frame oscillator is running at exactly half speed, that is 25 cycles per second.
I have not got the circuit diagram to hand but I seem to remember that the set employs a gas thyratron as the frame oscillator. I think it is a 6K25?
The frequency is determined by the value of the ramp forming capacitor which connected across the thyratron and of course the values of the resistors also associated with the 6K25.
I'll look up the circuit when I return to the shop.

DFWB.
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Old 25th Jul 2013, 8:21 pm   #5
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Default Re: Ekco trc139

Hi john,
You beat me to the first reply. The phone range at the same time I was writing my response.

DFWB.
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Old 25th Jul 2013, 8:30 pm   #6
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Default Re: Ekco trc139

Thanks for your diagnosis, that makes sense to me. I have changed everything around the thyratron to no avail, but I have not tried a replacement valve assuming that, as I had a decent stable picture, it must be doing its job. Which of the components are responsible for setting the operating frequency of the time-base?
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Old 25th Jul 2013, 8:36 pm   #7
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Default Re: Ekco trc139

Thanks Fernseh, I was just typing my reply as you were sending yours! It is indeed a 6K25 and all the resistors around it were way out of spec. so have been replaced, as have the associated caps.
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Old 25th Jul 2013, 9:31 pm   #8
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Default Re: Ekco trc139

Thanks for the help everyone. I am an idiot, Tas you were spot on, scanning across the frame hold sorted the time-base speed and all is well. I have a slight amount of sound on vision which I think is sorted with C19 and the volume is not good. These are relatively minor problems which I feel more confident about. I can rest easy tonight now. Thanks again.
Best Wishes,
Geoff.
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Old 25th Jul 2013, 10:37 pm   #9
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Default Re: Ekco trc139

Hi Geoff,
Here's the circuit diagram of the timebase of the Ekco TS88. I think it's a similar circuit employed in your TRC139. Positive going frame sync pulses initiate the switch on time of the 6K25, thus discharging C19.

The other circuit is of the simplist possible timebase generator. No use whatsoever for TV work because it cannot be syncronised. The capacitor charges though the series resistor from the HT supply. When the neon reaches the critical firing point the capacitor is discharged, the neon switches off and the whole process starts all over again.

DFWB.
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Old 26th Jul 2013, 10:30 am   #10
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Default Re: Ekco trc139

Hi Fernseh,
Thanks for that, at last I feel that I have mastered this. I am still on a very steep learning curve when it comes to TV which at times feels more like a saw-tooth; I just begin to think I have got something beaten and it all comes crashing around my ears again!
Time to get busy dotting the i's and crossing the t's now so that the set can be pressed into normal service.
Thanks again,
Geoff.
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Old 26th Jul 2013, 6:45 pm   #11
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Default Re: Ekco trc139

Quote:
Originally Posted by Murphy Maniac View Post
... Tas you were spot on, scanning across the frame hold sorted the time-base speed and all is well...
Glad to be of help. Looking good now. Some top advice from two top engineers too namely John and David above.
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Old 26th Jul 2013, 8:47 pm   #12
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Default Re: Ekco trc139

It is worth mentioning that the resistor values should not be taken as correct from the diagram Fernseh provided, in the TRC139 a couple of the values are little different.

Stephen

In late versions of the sets R43 is changed to 100k, R49 changed to 150k
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Old 26th Jul 2013, 10:06 pm   #13
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Default Re: Ekco trc139

Forgot to say that those component locations and values are from the Ekco service data.

regards

Stephen
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Old 27th Jul 2013, 12:52 pm   #14
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Default Re: Ekco trc139

The TRC139 is a superhet and may require a slight adjustment to the local oscillator. I think the trimmer is accessible through the top of the large screening can adjacent to the 6C9 frequency changer. Do not twiddle any other cores in the I.F. transformers! If this does not cure the sound fault you will need to replace all the decoupling capacitors from the screen grids of the 6F13's to chassis. They are usually .003uf and need to be ceramic types if possible. The sound should fill the room with these models. The sound output valve is a 6P25 in most chassis or a 10P13 in the radio versions. I have been using a TC112, the TRF version of this chassis as my main receiver since last October..
John.
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Old 27th Jul 2013, 8:22 pm   #15
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Default Re: Ekco trc139

Thanks for the advice, something is seriously at fault in the sound circuitry but time pressures mean I will have to 'put it to bed' for a while; I may be some time but I am sure I will be back! I will definitely not be doing any twiddling with the if's, the thoughts of going near a TV set with a trimming tool is the stuff of nightmares to me.
Best Wishes,
Geoff.
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