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Old 21st Oct 2018, 9:08 pm   #1
Colourstar
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Default Decca DM4/C

Thanks are due to Mikey for letting me have this set- actually he said it was going to be dumped if no one took it, although I'm sure Mike's much too nice a chap to really do that...

Who knows when it last saw any power, but it was certainly full of dust and spider webs. The cabinet has been enjoyed by a few woodworm, so is currently in quarantine.

I noticed that the crt is a rebuild courtesy of Midland Tubes - anyone remember them? This set incorportates VHF radio facilities with positions for Home, Light and Third on the tuner knob.

With the chassis free from it's cabinet, a paintbrush saw off the worst of the dust and grime. I noticed that the voltage selector had been left in the 225v position, which didn't bode well, so I moved it over to 245v. Mains was applied (gently and gradually) with an eye on the HT, which began to rise to the point where there were signs of life from the speaker. With a bit more juice the line whistle started up although there was no EHT. The boost cap on this chassis is the size of an old HP2 battery (nearly) and was reading more like a resistor. With that replaced, the EY86 glowed merrily but still no raster. Hmm. The ion trap magnet? Yes- a little twist and a lovely bright image appeared.

From there on in the faults were pretty straightforward, with low gain down to nothing more than a corroded aerial socket. The line hold control was at the end of it's travel, so the picture would only just lock. The 220k series resistor had risen to about 290k. The cogging effect on the test card was down to the usual coupling cap between the video amp and sync separator valves. Only a couple more caps were needed to resolve the poor linearity.

The picture now is really good, bright and linear. There are still plenty of black Hunts caps to change and I've not changed anything at all on the signals side, so I'm surprised it works as well as it does.

All good fun!

Steve
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Old 21st Oct 2018, 9:09 pm   #2
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Default Re: Decca DM4/C

And an under-chassis view. Nice and easy to work on!
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Old 21st Oct 2018, 9:22 pm   #3
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Default Re: Decca DM4/C

Hi Steve,
Now why have you been inside on such a glorious day playing with old TV's? Looks a nice design of chassis to work on- I've never had the pleasure of a Decca from this era! Hopefully no LOPT issues on this one!
Like me, I see you don't have a croc clip for your meter lead and just poke it in a convenient hole!

Interesting to see the other things in your workroom!

Cheers
Nick
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Old 21st Oct 2018, 9:37 pm   #4
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Default Re: Decca DM4/C

Thanks Nick. I work outside in a garden all week, so being indoors is quite a novelty!

I wasn't expecting to make a start on this set so quickly, having only acquired it yesterday, but I knew if I didn't it would end up on the 'to-do' pile of eternity....

Unlike later slimline sets where everything is crammed in, often on a vertical chassis with the rising heat toasting the components, 50s sets have plenty of room for the air to circulate so nothing has a really hard life and access is generally pretty good (as long as you don't mind the crt coming along for the ride when you remove the chassis...)

Steve
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Old 21st Oct 2018, 10:16 pm   #5
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Default Re: Decca DM4/C

We had the DM2/C which I think was basically the same chassis minus the radio and only 14". Good sets but the tube on ours was as flat as the proverbial witches t*t! The CRT heater was wired to a boost transformer that was eventually put on maximum boost and when the set was switched on from cold you could briefly see the pattern of the heat slots in the back panel reflected off the wall behind, the heater was that bright......!

Looks like you have a good one there, excellent result with very little work.
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Old 22nd Oct 2018, 8:44 am   #6
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Default Re: Decca DM4/C

There are two versions of the DM4/C. The early one uses a MW43-64/69 70 degree tube and the later the AW43-80 90 degree. They are very good receivers and enjoyed a long life.

These were one of the first chassis manufactured by Decca themselves. Early Decca models were by courtesy of Plessey. John.
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Old 22nd Oct 2018, 9:09 am   #7
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Default Re: Decca DM4/C

So pleased to see this went to a good home ... nice one, Mikey/Steve
Guy

(ref. https://www.vintage-radio.net/forum/...05&postcount=1 )
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Old 22nd Oct 2018, 1:38 pm   #8
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Default Re: Decca DM4/C

Hi Steve,
That's a great result for a scrapper. Nice 3MHz bars.
Sometimes the free sets are the most rewarding, which is I guess because you can never loose out really.

Are the FM stations on the turret tuner, if so what do the push buttons do?

Cheers
Andy
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Old 23rd Oct 2018, 6:41 pm   #9
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Default Re: Decca DM4/C

Quote:
Originally Posted by Sideband View Post
We had the DM2/C which I think was basically the same chassis minus the radio and only 14". Good sets but the tube on ours was as flat as the proverbial witches t*t! The CRT heater was wired to a boost transformer that was eventually put on maximum boost and when the set was switched on from cold you could briefly see the pattern of the heat slots in the back panel reflected off the wall behind, the heater was that bright......!

Looks like you have a good one there, excellent result with very little work.
In the early '90s I had a DM2/C passed onto me which did have VHF radio facilities. I was told it had a new CRT fitted but the Lopt was duff, but given a couple of "Unknown" ex equipment spares.

It was interesting to note the Lopt fitted had an EY51 in a "bathtub" where one of the replacements utilised an EY86 with valveholder in its place. This was the one I fitted which turned out to be a good one.

There was sound and a raster, but no picture. Replacing the video output valve, EF80 I think it was, sorted that out resulting in a very good working set. The new CRT had the date 1967(!). It turned out to be in first class order.

There was an unusual intermittent problem however, intermittent live aerial sockets! You could tell when the fault was present as the metal trim would have that "feel" you get when a 3-wired appliance wasn't earthed.

Cutting the story short I unfortunately parted with the set for a swap. Tragically I understand it was destroyed with other sets but let's not dwell on that.

Great to see a set from this series again Steve, brought back a lot of memories. The layout for servicing is indeed excellent.

Cheers,
Brian
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Old 23rd Oct 2018, 7:23 pm   #10
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Default Re: Decca DM4/C

Hi Guy

It's not the same set as the one in the list! This one came from my old pal Mike Bennett in Solihull

Hi Andy

Yes, a down-at-heel scrapper is always a good fix and bulky mid 50s wooden boxes don't have much of a following. As for the radio, there are (H)ome, (L)ight and (T)hird positions marked on the turret tuner. No pushbuttons in sight!

Hi Brian

Yes I remember your Decca set and it's habit of 'going live'. Was it as long ago as the early 90s? Did you know that one of these sets appears in the pop video to the Boomtown Rats 'I Don't Like Mondays' ?

It would be good to find a set of legs for the Decca as technically it's a consolette and not a table set. Unusually they have wooden threaded ends that screw directly into the cabinet. No metal fixings or plates whatsoever!


Steve
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Old 23rd Oct 2018, 9:20 pm   #11
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Default Re: Decca DM4/C

Quote:
Originally Posted by Colourstar View Post
It's not the same set as the one in the list! This one came from my old pal Mike Bennett in Solihull
I'll put a fiver on it being the same one that Mikey & I collected from my mate's gaff at Cookhill a few weeks ago ...
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Old 23rd Oct 2018, 10:49 pm   #12
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Default Re: Decca DM4/C

You're right Guy- I've looked at the photos in the 'for disposal' thread and it is the same set. I never made the connection! The marks on the card back are the giveaway.

Steve
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Old 24th Oct 2018, 12:59 am   #13
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Default Re: Decca DM4/C

Quote:
Originally Posted by Colourstar View Post
No pushbuttons in sight!
Hi Steve,
Looking again at your photos, I realize that what I thought were push buttons are actually the preset controls. I must get my eyes tested!

Cheers
Andy
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Old 24th Oct 2018, 10:50 am   #14
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Default Re: Decca DM4/C

Quote:
Originally Posted by Colourstar View Post
You're right Guy- I've looked at the photos in the 'for disposal' thread and it is the same set. I never made the connection! The marks on the card back are the giveaway.
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Old 24th Oct 2018, 6:15 pm   #15
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Default Re: Decca DM4/C

Hi Steve

In the late 50's I worked at Decca when they were in Brixton Road Kennington, we used to churn out anywhere between 225 and 250 every day, it was mainly the DM4/c that were churned out

Near the frame output valve, we used to put our initials is there anything visible in that area.

I managed to get a chassis that was completely empty and over a period of time I built up a DM4/C, they had cracking pictures, I would say almost unbeatable, I seem to remember that later sets used an ECF80 or PCF80 and the triode was used as cathode follower, this improved the final video response.

I got rid of the set years ago but it was good set

Bill
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Old 24th Oct 2018, 6:51 pm   #16
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Default Re: Decca DM4/C

Hi Bill

How wonderful to hear from someone who was at Decca when these sets were being assembled! I've looked at the chassis near the frame valve and it could either be an 'F' or an 'X' ;maybe just a cross. Dates on capacitors are mainly late 1957 to early 1958.

Interestingly in the red 'Radio & TV Servicing' book it lists the sync separator/part frame oscillator and line oscillator valves as ECL80, but on my set they are both PCL83. Mine also uses an EY86 EHT rectifier, rather than the earlier EY51 type.

Do you remember whether the cabinets were made in-house or brought in from an external supplier?

Steve
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Old 24th Oct 2018, 8:36 pm   #17
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Default Re: Decca DM4/C

Hi Colourstar

From memory the cabinets were brought in but I could not say where from.

Again from memory I thought both of the line osc and frame osc were ECL80
and I think the audio output was PCL83 but I am going 60 odd years

I also seem to remember that the older sets , DM14-17 used the EY51 and the DM4/C used the EY86 but both slots of TV's were very similar

Bill
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Old 24th Oct 2018, 9:34 pm   #18
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Default Re: Decca DM4/C

These are excellent sets. Information for the DM4C can be found in the 1957/58 Radio and Television servicing book. Apart from the type of CRT, the DM4C is electrically identical to the 14" model, the DM2C.

The line oscillator is an ECL80 triode-pentode and is a cross coupled multi-vibrator. The screen grid of the pentode section serves as the anode of one half of the multi-vibrator. The sawtooth drive for the line output valve is developed across pentode anode load.

A later model employing the same chassis metalwork and a similar cabinet is the DM3C This new in 1958 receiver employs a Mullard AW43-80 CRT and was the first Decca TV to have flywheel line synchronising. An ECL80 is used as a coincidence detector. This circuit was used all later Decca TVs including the last "real Decca" model the MS2000.

DFWB.
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Old 24th Oct 2018, 9:50 pm   #19
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Default Re: Decca DM4/C

The DM2C and I guess it is the same for the DM4C has a line drive trimmer TC6.
From the 1957/58 R & T servicing book: Line drive- Mounted at the rear of the chassis is a compression trimmer capacitor (TC6) This controls the input to the line output valve; in turn, determining the conduction period of the circuit. Correct "timing" between the operation of the boost diode and line output valve is made by adjusting TC6 to a point where the boost voltage, measured on a high-resistance voltmeter at pin 10 of the picture tube base (A1) is 500 volts.

DFWB.
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Old 1st Nov 2018, 8:08 pm   #20
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Default Re: Decca DM4/C

The downfall of Decca, in my opinion, was the WIMA capacitors and the awful turret tuner they started to use, I cannot remember what make it was, but the valve holders used to break up almost as you looked at it let alone touch the valves, and then they went over to the Thorn 850 chassis that had its problems but was not to bad and then I seem to remember they went over to a Tatung chassis' who then when it was to late Tatung pulled the plug and Decca was no more.
Such a shame, Decca made good quality equipment, both TV wise and Radio/gram

Bill
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