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Old 29th May 2023, 12:49 am   #1
Philips210
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Join Date: May 2007
Location: Redruth, Cornwall, UK.
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Default Unaohm EP 686D TV Pattern Generator

Hello again.

I obtained this ex BBC Unaohm EP 686D PAL colour TV pattern generator over ten years ago. It's an Italian made unit and dates back to 1977. I think it's a nicely enginereed pattern generator with good accessibility to the PCBs for servicing.
At the time, I started to do some servicing work on it. There were a number of electrically leaky electrolytic caps in the power supply that I replaced. I discovered a problem with the mains transformer. The wiring's insulation had become very brittle and reminds me of 1950's Bush radios with perished rubber wiring. The transformer has a number of primary winding taps for the correct mains voltage for your country. There seems to be an open circuit on one of the leads. I decided to put it back together and not bother.

A few days ago, I brought it down from the loft and decided to have another look as I am currently working on the Television magazine test card generator. I thought it might be good to see how the circles are generated in the Unaohm.
I noticed the mains transformer was left disconnected and a note inside detailing which electros were replaced. It was dated 27/05/2013. I can't believe that was ten years ago! I am slightly annoyed in that I can't find any servicing notes. I always take photos and make sketches with notes but for some reason I don't know what happened to them.
Finding the correct circuit diagram isn't easy but I downloaded service information for the model EP 686E which is in Italian. It appears to be largely the same as mine. The sound is 5.5MHz as opposed to 6MHz for the UK but other than that it's essentially the same.
The EP 686E appears to have just a 220V primary winding for the mains Tx.

I don't suppose anyone has the correct service manual for the EP 686D by any chance?

Please see a few pics attached:

In the first pic, the paper label legend has dropped off due to dried out adhesive.

The second pic with the label placed in its correct position. Note how faded it is. That will be a good task to reproduce a new label using say MS Paint.

In the third pic is the PSU and Circle Generator PCB.

Pic 4 shows the Sync and Blanking Generator PCB with standard TTL ICs

Pic shows the PAL Colour Encoder and Sound Generator PCB.

It's an interesting design and I'd really like to get this working again at some point.

Regards,
Symon
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Old 29th May 2023, 4:14 pm   #2
Chris55000
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Default Re: Unaohm EP 686D TV Pattern Generator

Afternoon Symon!

There's plenty of space inside your EP686 and I recommend the following transformer :–

https://a.aliexpress.com/_mNUOjG4

You want an "R30–TS10534," with the following specs :–

Primary 0–115–230 V, Connect black to mains "N", red and grey together and blue to mains "L".

Sec 1 12V 0.83 A, Sec 2 12V 0.83 A, Sec 3 20V 0.4A Sec 4 20V 0.4A

Connect the two 12V sections in series, one 12V outer to one a.c. side of the bridge, the second 12V outer to the other a.c. side of the bridge, join one side of each 12V together and connect to earth.

Connect the two 20 V windings in series, insulate the centre–point and tie it up so it can't short anywhere, then connect one end to 0V and the other (40V) end to the rectifier diode for the varicap tuning voltage.

The very small loading of the varicap tuning supply.means you'll get 60–70 V across the reservoir cap after half–wave rectification, which should suit the TAA550 and it's series feed resistor nicely!

These transformers mount on a flat panel and have an interwinding screen!

I think you'll find a black "Sifam" type cap fits the middle of your tuning control knob!

Chris Williams
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It's an enigma, that's what it is! This thing's not fixed because it doesn't want to be fixed!

Last edited by Chris55000; 29th May 2023 at 4:33 pm.
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Old 29th May 2023, 10:08 pm   #3
Philips210
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Default Re: Unaohm EP 686D TV Pattern Generator

Hello Chris.

Thanks for taking the time to work out a suitable mains transformer and also for the link to the replacement.

I haven't yet confirmed whether the original transformer is unserviceable. There is no continuity from one of the primary taps and haven't checked whether it's the 230V one. The rubber insulation of the wiring is really grotty though and is literally crumbling away. It seems the pattern generator may have been exposed to water ingress. There appears to be some corrosion around where the transformer is mounted on the chassis. I probably won't trust the insulation resistance of the transformer and will opt for a replacement. In the meantime I can test the pattern generator by powering it from my bench power supplies.

The service manual I recently downloaded is reasonably clear but is in Italian and wondered if the pdf can be readily translated into English.

Regards,
Symon
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Old 31st May 2023, 11:45 am   #4
Chris55000
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Default Re: Unaohm EP 686D TV Pattern Generator

Morning Symon!

The likeliest way you'll get your manual translated is to upload it to one of the "free online OCR" services that can carry out OCR, then load it into Word, (2016 and later can translate it "on the fly" but you are liable to get plenty of nonsense in the ensuing translation unfortunately!

I use a very old copy of pre–subscription Acrobat DC that will OCR it and also permit it to be edited, so I retype over the original text in English!

Chris Williams
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