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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only.

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Old 24th Feb 2012, 4:41 pm   #1
Marconi_MPT4
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Default Roberts R200

Hi, This set appeared on my bench with the fault description as 'dead'. Apart from a slight crackle from the volume control, there was no sound at all. Measuring the dc conditions around the notorious AF117 transistors revealed all were within a few percent of those marked on the schematic. In fact the two IF transistors had previously been changed to a modern germanium type, hopefully without dendrite problems.

Armed with an oscilloscope probe, checked the oscillator was producing an ample signal level and found the converted signals very healthy right up to the second IF TR3 collector. Good

A cold check with an AVO on ohms range to see if the detector diode was OK read about 4.5k in both directions. Faulty diode? Desoldered and removed what is probably the most difficult to access IFT in the set containing L3 an L4 plus OA90 etc. Diode measured OK, IFT secondary winding open circuit !!

Examining the core assembly to see if it was possible to rewind the secondary looks to be a problem due to the sealant used in its construction. See attached picture.

Has anyone successfully managed to disassemble the core and if so what solvent worked?

Perhaps it may be possible to use a transformer from another completely different set or obtain an IFT from a scrap Roberts.

Cheers

Rich
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Old 24th Feb 2012, 6:57 pm   #2
John_BS
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Default Re: Roberts R200

It seems odd that a coil with no current through it would go O/C. Have you tried measuring the coil across the green exposed sections (razor off a bit) prior to the terminals?

John
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Old 25th Feb 2012, 12:44 am   #3
Steve_P
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Default Re: Roberts R200

Odd, but not impossible. Green Spot can strike any coil, regardless of whether it carries current!

See if this guy has one: http://www.pasttimesradio.co.uk/spares/spares.html

Cheers,

Steve P.
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Old 4th Mar 2012, 9:02 pm   #4
Marconi_MPT4
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Default Re: Roberts R200

I seem to be forever delayed this week since posting, but following on from John's comment to scrape the enamel unfortunately still resulted in an open circuit reading.

Out of curiosity and armed with a strong light and magnifying lens I eventually discovered there was a problem where the green wires go under the varnish at the top of the ferrite. After judicious prodding a connection could be made. Ah! Retrieved the end and resoldered the green wire to what was a very short stub. The dc resistance measured at the base pins is now approx 1 ohm.

To finish the repair put a dab of PCB varnish to insulate exposed connection.

Steve thanks for the link, it will be useful to follow up if this repair fails.

Cheers

Rich
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