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Decca DM45.

 
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Re: Decca DM45.

Post by Derren » Fri May 20, 2016 5:48 pm

Thanks Till,

Hope to be calling in soon to see how you're getting on and discuss some other sets I have issues with. Shame the top of the cabinet is not so good on this one, have had a go at them before but the results aren't always great. Might end up being covered with some suitable ornament from the period just to get it usable/presentable :)

 
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Re: Decca DM45.

Post by Till Eulenspiegel » Fri May 20, 2016 8:52 pm

Hi Derren,
After the electrical restoration is done I'll tidy up the cabinet top surface.
After half an hour the width of the picture remains unchanged. There's plenty spare width to permit adjustment by a pulse feedback circuit or a tapped resistor network in the anode supply to the PY81 booster diode.
After the line output stage is tamed there are a few minor jobs to do to the set before it can be fully signed off. For example particular attention to the interlace which is at present not good.

Till Eulenspiegel.

 
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Re: Decca DM45.

Post by Till Eulenspiegel » Sat May 21, 2016 12:13 pm

The original Decca scan coils have been reconnected which has resulted in even more picture width. The EHT is 15KV which is the correct final anode voltage for the AW43-88 CRT.

Till Eulenspiegel.

 
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Re: Decca DM45.

Post by Till Eulenspiegel » Sat May 21, 2016 5:56 pm

The set has been operating for more than three hours and the picture width remains unchanged. Now it's time to decide which width control system is to be used.
The PL81 seems to be coping well with the jellypot, no sign of overheating. The height is now considerably reduced so it's most likely with the original LOPTx the EHT was a bit on the low side. It is now proven to be reason for the width problem was the old transformer. Now there is too much.

Till Eulenspiegel.

 
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Re: Decca DM45.

Post by Bobhowe » Sat May 21, 2016 6:37 pm

Hi David why don't you start a poll to keep the jelly pot regards Bob

 
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Re: Decca DM45.

Post by Till Eulenspiegel » Sat May 21, 2016 8:14 pm

Hi Bob,
the jellypot gets my vote as one the most useful line output transformers ever.
The first BRC jellypot line output transformer was used in the 900 series dual standard sets. Very early production sets employed a conventional transformer similar to the one used in the 850 series. Jellypot will be found in all 1966 models.
I believe the "jelly" potting compound was developed by Dow Corning and is still available.

Till Eulenspiegel.

 
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Re: Decca DM45.

Post by Cathovisor » Sat May 21, 2016 10:17 pm

Forgive me for being dumb, but... if "Jellypots" were so reliable, how come there are so many spares available of them (well, to those ex-trade at least)? :aab

 
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Re: Decca DM45.

Post by Till Eulenspiegel » Sat May 21, 2016 10:46 pm

The jellypots were removed from various 950 and 1400 series TVs that were chopped up in the 80s and 90s.
Seems that I had the foresight that these old transformers might be of some use in the future for vintage TV restoration projects. In fact the first jellypot conversion was done in 1985 and the set which was modified at that time was a Bush TV22. The job was done one summer Sunday afternoon.
A special EHT doubler was made using three BY182 silicon diodes and 0.001mfd HV disc capacitors. An additional valve a PY800 was employed as the boost diode instead of one section of the PZ30. A separate line timebase blocking oscillator was used to drive the PL38. The blocking transformer was removed from a 1400 series receiver.


Till Eulenspiegel.

 
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Re: Decca DM45.

Post by Till Eulenspiegel » Thu Jun 09, 2016 1:35 pm

At present the set has no means of controlling the width and as a result the picture is too wide. Some method of adjustment is required.
I'm considering reintroducing the adjustable series inductor between the line transformer and the scan coils.
I'll report what the results are like.

Till Eulenspiegel.

 
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Re: Decca DM45.

Post by Till Eulenspiegel » Thu Jun 09, 2016 6:28 pm

The width control inductor has been rewired into the line timebase circuit and it certainly functions in a manner but not as I would like it to do. When the control is adjusted the picture does come in from the sides but in fact what happens the picture becomes compressed about the middle of the scan. It is operating like a linearity control, or to be exact more like an unlinearity control. Need to think up other width control ideas.

Till Eulenspiegel.

 
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Re: Decca DM45.

Post by Till Eulenspiegel » Fri Jun 10, 2016 4:18 pm

I'm starting to make sense with the Decca DM45. We now have an adjustable width control, salvaged from the assembly which was mounted on the line output transformer. The width control is now located on the right side of the chassis and is accessible without swinging out the chassis.
I've corrected the line drive to the PL81, the mark to space ratio is 45:55 now, just right.
The jellypot is working fine. The original Thorn set from which the transformer came from had a screen grid decoupling capacitor for the line output valve. Fitting one will improve performance.
Picture quality is excellent. The Mullard AW43-88 CRT is now fifty-six years old!

Till Eulenspiegel.





 
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Re: Decca DM45.

Post by Derren » Fri Jun 10, 2016 5:01 pm

Excellent, thank heavens for that, maybe we will have to stop assuming things are 'an easy fix' in future :)

 
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Re: Decca DM45.

Post by Till Eulenspiegel » Sat Jun 11, 2016 11:55 am

Hi Derek,
Fifty years ago the DM45 would have been an easy fix, one would have simply ordered a replacement line output transformer from the Decca service department.
But that's how I'm finding restoration jobs now, the days of finding something with a light layer dust seem to be over, just change a few capacitors and job done. Some of the sets I'm doing nowadays look as if they had been dragged out of a canal.

Till Eulenspiegel

 
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Re: Decca DM45.

Post by Focus 2 » Mon Jun 13, 2016 9:08 pm

I was at David's earlier on and saw the set in action. It's working very well with the Thorn Jellypot Lopt. Another restoration job that's beyond first class. They were taken at an angle so a little distorted. These photos do no justice for the set.
A couple of pics taken on my phone. I was impressed by the vertical lift up chassis allowing excellent access for servicing. This idea returned for the 80 series solid state colour sets!
Cheers
Brian.
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Re: Decca DM45.

Post by Till Eulenspiegel » Tue Jun 14, 2016 1:40 pm

Hi Brian,
thanks for posting up the pictures for me.
It's now just a matter of tidying up the set. The cabinet top has received attention, the old lacquer has been sanded off and the surface will be French polished.
An electrolytic capacitor (C50) in the line flywheel discriminator has been replaced. The original 2mfd elco was replaced some time in the seventies with a 2.2mfd 450 volt component, that working voltage is too high. In fact the voltage across the component is only 60V,
I reckon the original was a 100volt electrolytic. It's been replaced by a 2.2mfd 100volt polyester capacitor.
Intermittent sound was caused by dry soldering joints on the right side loudspeaker.

Till Eulenspiegel.

 
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Re: Decca DM45.

Post by Till Eulenspiegel » Tue Jun 14, 2016 5:24 pm

Replacing C50 has brought about an improvement of the stability of line timebase synchronising.
The Decca DM45 has FM radio. The turret tuner selects three stations, BBC Light, Third and Home, marked as HTL on the control panel.
The 10.7Mhz IF transformers needed tweeking up to improve performance. Sound quality is excellent.
The 38.15Mhz TV sound IF transformers also needed adjustment, the sound trap also needed readjustment, this action has improved picture quality.

Till Eulenspiegel.

 
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Re: Decca DM45.

Post by Doz » Tue Jun 14, 2016 6:54 pm

Great work Till. :aad

 
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Re: Decca DM45.

Post by Till Eulenspiegel » Tue Jun 14, 2016 9:00 pm

Hi Andy,
I'm pleasantly surprised at the quality of the pictures this set now displays. The slight fuzziness turned out to be a mistuned 38.15Mhz sound trap in the vision IF amplifier. The receiver can display the 3Mhz bars on the test card.
I'm particularly pleased with FM radio performance, the only drawback being the limited choice of radio stations, the tuning range is only 88 to 96Mhz.
There's just the cabinet top to attend to now and then it's job done.

Till Eulenspiegel.

 
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Re: Decca DM45.

Post by Derren » Thu Jun 16, 2016 4:38 pm

Excellent. Well it must indeed be a damn good tube assuming it's the original, it's had a long service life already bearing in mind it appears to have been repaired in the 70s!

 
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Re: Decca DM45.

Post by Till Eulenspiegel » Thu Jun 16, 2016 5:30 pm

Hi Derren,
the horizontal linearity has improved thus removing the slight right side compression of the picture.
This set just gets better.
5:25pm: I've just finished spiriting off the cabinet, that's the scary part of French polishing where spirit is applied to the rubber to achieve the final finish. If you don't get it right the whole surface can be dragged off and then you have to start all over again.

Till Eulenspiegel.

 
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Re: Decca DM45.

Post by Till Eulenspiegel » Fri Jun 17, 2016 4:59 pm

The re-polishing job has been done and top of the cabinet looks a lot better now.
The position of the width control coil in the circuit has been now changed. Before it was in series with the line scanning coils. It is now in series with the anode supply of the boost diode. The reason for doing this is to reduce the excess width on the left side of the picture.
As is well known the boost diode circuit contributes about 40% of the line scan by using reclaimed energy from the scan coils. The reposititioned width control still adjusts the width, however, by means of it's position in the circuit it also reduces amount reclaimed energy.

I understand the AW43-88 CRT has an aspect ratio of 5:4 so the picture has to be over scanned by 7% or so. This early 110 degree tube appeared in the UK and Europe early 1959 but the equivalent 17CVP4 was introduced in America in 1957.
first UK made TV to use a 110 degree CRT was the Pye PV110 in late 1958.

Till Eulenspiegel.

 
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Re: Decca DM45.

Post by Terrykc » Fri Jun 17, 2016 6:28 pm

nuvistor wrote:In the 60's with the customers approval I used to blanket change those Wimas in Grundig tape recorders, never found a good one. Very few electrical faults in those machines were caused by any other component.

Don't forget the WW resistor feeding the record/replay relay in the TK20!

When I was at college, they bought an NTSC flying spot scanner from STC - allegedly fire damaged after a fire at their Footscray factory - but there was no signs of any damage so I think STC just used it to as an excuse to get rid of an old bit of kit (although, around 1965, I doubt it was that old!)

The row of 19" racks ran down the centre of a lab, effectively splitting it in two. As it was entirely valved, it kept the place warm!

Anyway, something that I noticed in one of the racks were those horrible Wima caps - except that these were branded Plessey!

 
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Re: Decca DM45.

Post by nuvistor » Fri Jun 17, 2016 7:06 pm

Terrykc wrote:
Anyway, something that I noticed in one of the racks were those horrible Wima caps - except that these were branded Plessey!

To give Wima there due, they turned around their bad reputation and in the 70's produced some very reliable capacitors. The names, long forgotten but came in different colours for I think different dielectric material. We came to use them for most jobs and very rarely let me down.
Took some persuading but once we put them to use were very happy with them.

Now did Hunts do the same? I don't recall they did but you may remember.

Frank

 
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Re: Decca DM45.

Post by Cathovisor » Fri Jun 17, 2016 8:32 pm

Wima caps were used in Link cameras to decouple the tube HV supplies: they never let us down.

 
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Re: Decca DM45.

Post by Refugee » Sat Jun 18, 2016 12:54 pm

The X and Y rated ones are just as bad as Rifa ones but I agree that some of the other types are just fine.

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