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Decca DM45.

 
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Decca DM45.

Post by Till Eulenspiegel » Sat Apr 23, 2016 5:50 pm

This set has the potential to be a great performer. The CRT is nice and bright and the sound is OK as well. The first fault to solve is the line oscillator which is running too slow, that's most likely to be a resistor or resistors in the oscillator having gone high value. The first suspects are R48 and R51, let's find out if that is the case.
The Decca employs a flywheel sync system centred around V8, an ECL80. V8A is the line sync inverter and V8B is the coincidence detector. Decca first used this circuit in the 1958 model DM3C and continued to use it up to 1969 in the companies' first single standard receiver.

Till Eulenspiegel.
Attachments
DM45_0086.jpg
DM45_0088.jpg

 
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Re: Decca DM45.

Post by Till Eulenspiegel » Sat Apr 23, 2016 6:28 pm

After replacing R48 (1.0Mohms) the line timebase frequency is now correct but another problem has manifested itself, bent verticals.
Most likely to be a sync separator fault.
R48 had risen in value to 1.55Mohms.

Till Eulenspiegel.
Attachments
DM45_0089.jpg

 
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Re: Decca DM45.

Post by Colourstar » Sat Apr 23, 2016 7:34 pm

Hey that looks good Till. I don't think I've ever seen one of these Deccas in action, so I'm keen to see how it shapes up. Were they a good seller / well liked in their day?

And most importantly.... do you have the unique wooden tripod stand?

Steve

 
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Re: Decca DM45.

Post by Till Eulenspiegel » Sat Apr 23, 2016 8:31 pm

Hi Steve,
The DM45 was a good set and well liked by it's owners, but only after all the Wima "Tropadur" capacitors were replaced. These capacitors were terrible, Rediffusion used them in Mark 6 and 7 TV receivers. Also possibly used in some Peto-Scott TV receivers.

The set doesn't belong to me and is owned by a Forum member.

Decca made a variant of the DM45, the DM35 which did not have FM radio facilities. The DM35 was equipped with a turret tuner which used an unusual tuner valve, a PCF84. This set along with the 21" version are the only TV receivers known to employ the PCF84.
Model 666 is a TV radiogram combi which used a modified DM45 chassis. The chassis was equipped with an additional audio amplifier for the stereo record player.

Till Eulenspiegel.

 
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Re: Decca DM45.

Post by Till Eulenspiegel » Sat Apr 23, 2016 10:45 pm

Sync separator performance has been improved by replacing C41, the coupling capacitor between the video amplifier and the grid of the sync separator valve. Another benefit from replacing C41 is now the contrast control works as it should. Simple mean level vision AGC derived from the negative volts present at the grid of the sync separator.
The Frame linearity needs attending to, there is still a number of Wima capacitors present, they have to be replaced.

Till Eulenspiegel.
Attachments
DM45_0090.jpg

 
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Re: Decca DM45.

Post by Derren » Sat Apr 23, 2016 11:19 pm

Looking good Till, well done. Have got high hopes for this set it seemed a good example electronically at least. Yes Steve the tripod stand/legs are here with me, I don't remember this particular set in action but I do remember seeing similar Decca and KB models with the 'tambour' doors which this one also has that close over the screen front still working in the 70s. I can remember aunts and uncles who couldn't yet afford colour still having this type of set from that 1959 era of style as late as the mid-late 70s. This one turned up at a local auction a few weeks ago, only disappointments it doesn't have its original two pin press in mains connector lead, and the veneer is stained/damaged on the top in front of the unusual but handy lift up top control panel. This one powered up on a Variac with good initial results and no work at all! Could have had a go myself but was happy to hand it over to Till who seemed to remember them fondly and has more experience of them.

 
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Re: Decca DM45.

Post by Till Eulenspiegel » Sun Apr 24, 2016 12:00 am

Hi Derren,
I'm making good progress with this set. The Frame linearity has been considerably improved by replacing C99 in the linearity feedback circuit. The capacitor was a Wima 0.05mfd 500V component. Replaced by a Philips 0.047mfd polyester capacitor.
After fifteen minutes or so the width does come in from the sides. It's getting a bit late now so I'll return to the set tomorrow.

Till Eulenspiegel.
Attachments
000_0091.jpg

 
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Re: Decca DM45.

Post by PYE625 » Sun Apr 24, 2016 12:10 am

A very nice set indeed.
Are the Wima's you mention the brown ones?
Have had some in a Philips reel to reel and are dreadfully leaky.
Attachments
rsz_005.jpg

 
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Re: Decca DM45.

Post by Till Eulenspiegel » Sun Apr 24, 2016 1:25 am

That's exactly the type of Wima capacitor used in the Decca. Those Wima capacitors were failing only a few years after the sets were made.
Way back in the early sixties the mindset was to locate a faulty capacitor but that was all, there was no blanket jobs in which all the suspect capacitors were changed. That was changed by the early seventies when it was realised all those rubbishy capacitors should be replaced.
I remember one radio and TV shop in Newcastle in which the owner used to buy Mullard polyester capacitors by the sack full. He'd sell them to other TV repair firms including mine.

Till Eulenspiegel.

 
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Re: Decca DM45.

Post by colourmaster » Sun Apr 24, 2016 6:54 am

Hi David
This style of set must have carried on for a long time as I have a version which has a 23" tube fitted , I think the model is DM39 . It also has it's tripod stand .
The interlace on mine is very good .
Regards.
Gary.

 
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Re: Decca DM45.

Post by Colourstar » Sun Apr 24, 2016 9:03 am

What a great auction find, Derren. You must have been delighted with that. It looks like it will be a terrific set once Till has worked his magic on it. Let's hope the decreasing width isn't anything too sinister. A bit of residual damp is pretty much par for the course on a set that hasn't been used in years it seems.

Those WIMA caps also turn up in Grundig reel to reels like the TK20. First time I've known of them being used in a television chassis.

Steve

 
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Re: Decca DM45.

Post by Briancuff » Sun Apr 24, 2016 10:19 am

Back in the 1960s, there were two EMI telecine machines at lime grove that became very unreliable. We traced this to exactly that type of Wima cap giving up. Being a in a broadcast situation, this could not be tolerated and we did a blanket change of all Wima caps, changing them for Mullard mustard types. a few years later, we rebuilt the whole of the signal chain, copying the electronics, including chassis layouts, from the later Rank Cintel machines - I'm sure that that was illegal at the time but I don't think that Cintel would have complained as the BBC ended up with more than 30 very expensive machines!

 
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Re: Decca DM45.

Post by Till Eulenspiegel » Sun Apr 24, 2016 10:47 am

The frame output valve cathode bias capacitor is dated 1972 which tells us that this set was in use well into the seventies. Who knows whether the set was still in use with the first owner or as a trade in against something like an early colour set?

The 23" version DR39 was introduced in early 1961. Strangely, the Mullard "square" corner 23" CRT appeared many months before the 19". With regard to Derren's set, my principle concerns are finding out why the picture width comes in after 15 minutes and also to improve the interlace.

Till Eulenspiegel.

 
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Re: Decca DM45.

Post by nuvistor » Sun Apr 24, 2016 12:37 pm

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. It saved the machine coming back after a couple of months with another fault.
What a difference the later 70's Wima capacitors were, very reliable, never had them go in service life, what they are like after 40 years I don't know.
Frank

 
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Re: Decca DM45.

Post by Derren » Sun Apr 24, 2016 2:56 pm

Till Eulenspiegel wrote:The frame output valve cathode bias capacitor is dated 1972 which tells us that this set was in use well into the seventies. Who knows whether the set was still in use with the first owner or as a trade in against something like an early colour set?



Till Eulenspiegel.



Oh that's interesting, it tips a nod to what I was saying about some of these types of set being used well into the 70s. When I found it it was switched to the radio position, so I did wonder if it could have been used as a radio only, either once the TV section had ceased to work well, or even after the 405 transmissions ceased! Hopefully as Steve suggested it may just be suffering from some damp re. the width. There was quite a thick blanket of dust/fluff on it all which I removed before powering it up.

 
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Re: Decca DM45.

Post by Till Eulenspiegel » Sun Apr 24, 2016 3:13 pm

Hi Derren,
Perhaps the Decca was used as a second set? although strangely such a practise was rare in the 60s and 70s. I'd often sell a new customer a new colour set and would suggest that the old 405 set could be kept as a spare or installed in another room. Hardly any customers took up the suggestion. The usual response was " just take the thing away". My shop opened in 1972 and by those times 405 line TVs were just regarded as junk.
A few firms were still renting 405 sets out well into seventies. Sets like the Murphy V310 and V410.

Till Eulenspiegel

 
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Re: Decca DM45.

Post by Derren » Sun Apr 24, 2016 3:26 pm

Or what about another member of the household being given the old set? That's what happened with me, I can remember having a dual standard 1960s B & W Cossor set in my bedroom when I was at school in the 70s, it was given to me when we got a colour set downstairs I reckon as it was our main set before that! Wouldn't know the model number but I would recognise it anywhere, it had lots of white plastic on the front but with veneered sides, a push button motorised tuner and a metal stand.

 
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Re: Decca DM45.

Post by Till Eulenspiegel » Sun Apr 24, 2016 4:25 pm

That'll be the Philips made Cossor CT1910 etc, these were absolutely superb sets. The last real Cossor TVs were the models 950 and 951. From 1960 on all Cossor TVs were really rebadged Philips except about 1964 when a number of Cossor and Peto-Scott sets were made by Plessey. The chassis that used two ECH84 triode-heptodes.

Till Eulenspiegel.

 
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Re: Decca DM45.

Post by PYE625 » Sun Apr 24, 2016 4:53 pm

Till Eulenspiegel wrote:Hi Derren,
I'm making good progress with this set. The Frame linearity has been considerably improved by replacing C99 in the linearity feedback circuit. The capacitor was a Wima 0.05mfd 500V component. Replaced by a Philips 0.047mfd polyester capacitor.
After fifteen minutes or so the width does come in from the sides. It's getting a bit late now so I'll return to the set tomorrow.

Till Eulenspiegel.


Complete guess here, but if the LOPT is of similar construction to the Bush types........ :aaq

 
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Re: Decca DM45.

Post by Till Eulenspiegel » Mon Apr 25, 2016 12:37 am

The line output transformer resembles the one fitted in the Thorn 600, 700 and 850 series. Possibly made by Plessey.
Today, (Sunday) the boost capacitor was replaced, it was getting quite warm. A replacement fitted, no improvement, the width still comes in. Lets hope the fault is not caused by the failure of the line output transformer. Another Jellypot conversion coming up?

Till Eulenspiegel.

 
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Re: Decca DM45.

Post by Till Eulenspiegel » Mon Apr 25, 2016 4:29 pm

Today the scan coil "S" correction capacitor C57 (0.35microfarads) was replaced. Doing this did result in a slight increase of width and improved line linearity, the original capacitor was found to be leaky.
The negative going line drive waveform to the control grid of the PL81 is correct @ 150 P - P.
Just after the warm up time the picture has full width, in fact a slight overscan which is correct for this type of tube. Shown in the first picture. The second picture shows a slight reduction of width after five minutes.

Till Eulenspiegel.
Attachments
DM45_0092.jpg
DM45_0093.jpg

 
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Re: Decca DM45.

Post by Derren » Mon Apr 25, 2016 4:58 pm

Looking good! Might the width not settle down eventually? I've known some sets take a very long time to settle down to the right proportions once back in use. By the way what is a "jellypot", think I've got a rough idea what you mean :)

 
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Re: Decca DM45.

Post by Till Eulenspiegel » Mon Apr 25, 2016 5:15 pm

Hi Derren,
There was a topic about reviving line output transformers on the UKVVR Forum.
In fact I was thinking about just leaving the set on for as long as possible, the idea being to "sweat it out" so to speak.
The Jellypot? The type of line output transformer used by Thorn from 1965 on in various mono and colour receivers. Starting with the 950 mono and ending with the 8800 series CTVs.

Till Eulenspiegel.

 
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Re: Decca DM45.

Post by Lloyd » Mon Apr 25, 2016 6:54 pm

Hi

That's looking pretty good so far, despite the width decreasing! Just thought I'd throw this little spanner into the works, is the width decreasing definitely being caused by something in the line stage? Only reason I ask is because I had an Ekco set (think it might have featured on here somewhere) which had decreasing width over the space of 5 minutes or so after switch on, and it turned out to be an electrolytic capacitor hidden away in the IF stages dragging down the HT, and getting very hot in the process! I'm surprised it didn't go bang, looked very close to it! Apologies if you have already looked into this :)

Regards,
Lloyd.

 
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Re: Decca DM45.

Post by PYE625 » Mon Apr 25, 2016 7:09 pm

Forgive my ignorance of the circuit, but I was wondering if a thermistor is used in the line scan coils for temperature compensation as there often is for the frame scan coils.

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