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Old 4th Oct 2015, 2:18 pm   #1
Heatercathodeshort
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Default 'Beau Decca 131 projection receiver 1950'

I am at the present time bringing back to life a Beau Decca 131 projection receiver dating from around 1951 for the museum at Dulwich.

It has been a bit drawn out due to the passing of Gerry with all the changes that inevitably take place. I would add that the museum is in good shape and open to visitors [by appointment of course] and it can only get better.

The Power supply, a very heavy mains isolated affair with a mains transformer to supply the valve heaters and approximately 350v for the H.T. line, has been overhauled with new electrolytics and should be OK.

The main IF and time base chassis has been completely overhauled [therapy needed] and looks good. Only an actual powering up will prove this to be true.

The receiver was produced for Decca by Plessey as were all Decca television receivers prior to the 'DM' model numbers. [Decca Manufacture]

I thought you might like to see the 25kv EHT unit manufactured by the Philips company in the Netherlands. All post war projection receivers in this country employed a folded version of the SCHMIDT optical system manufactured by Philips together with the two and a half inch projection tube [MW6-2] and the 25kv EHT unit.

The sealed oil filled unit contains the ringing coil and three special quality EY51 rectifiers in a tripler arrangement. They are not replaceable but have a very long life. An EBC33 connected as a blocking oscillator provides the drive to the EL38 in a similar arrangement to a line output stage. A feedback winding in association with the two strapped diodes in the EBC33 provides a degree of stabilization.

The unit is mounted on the right side of the cabinet looking in from the back and is self contained other than it's HT and heater supplies that are drawn from the main chassis.

This example was very rusty and required removal of the sealed ringing choke to allow a 'deep clean' and de rust. This took a while this morning as you can imagine but access is very good and it only took a short time to replace the 60 year old black tar capacitors. They actually read almost useable but these units tended to suffer from poor regulation [No PD500s back then..] and everything needs to be 100% for satisfactory operation so no chances were taken.

On completion the unit was connected to my Griffin power supply set for 350v HT and 6.3v heaters. It soon warmed up and the whine of the blocking oscillator could be clearly heard. Soon that nice EHT rustle you used to experience just before the picture appeared on those early colour receivers was sensed and 25KV was registered on my stand by EHT meter. Success!

Now...I wonder if the main chassis will behave itself when it comes to powering up?

If all goes well I hope to do this next Friday and will take some more pictures so that you can see the beautiful beast in full glory..

I'll keep you posted. Regards, John.
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Old 4th Oct 2015, 2:52 pm   #2
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Default Re: Philips 25kv EHT unit.

Hi John,
It's over ten years since I fired up my Decca 131. The screen has gone yellow and a replacement for it has been found, taken from a scrap Samsung projection TV. The Decca 131 I have here has been fitted with a Cyldon turret tuner.
The Griffin power supplier? Is that the same Accrington Lancashire firm that made TV sets in the early fifties?

DFWB.
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Old 4th Oct 2015, 3:32 pm   #3
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Default Re: Philips 25kv EHT unit.

In case you are wondering what's inside the can, please see the attached pictures.
You can clearly see the three EY51 rectifier valves and some capacitors of the tripler circuit.
It is more or less a ringing choke configuration with a feedback winding.
If the circuit works as intended, the output voltage is fairly constant @ c. 25kV.

Jac
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Old 4th Oct 2015, 3:56 pm   #4
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Default Re: Philips 25kv EHT unit.

Hi Jac,
The optical unit was marketed in the United States under the Norelco name. Did Philips send the EHT unit to the States as well?

An earlier optical assembly was fitted in the Philips 799. The CRT was four inch type similar to the one used in the pre-war TEL61 model. Can that be confirmed?
I don't think any other UK manufacturer employed it in a projection receiver.

DFWB.
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Old 4th Oct 2015, 4:18 pm   #5
Jac
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Default Re: Philips 25kv EHT unit.

That's right David.
The Norelco units were made in the Netherlands.

As far as I know the 799 also used a Schmidt optics with an MW6 CRT.
The optical box was a bit different from the later cast units.

The pre-war Philips did not have the Schmidt optical system.
Is was a projection tube with lenses in front of it.
That indeed seems to have used a 4" tube @ 25kV.

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Jac

EDIT: at some time (probably 1936) Philips made a hollow front projection tube as well.
Never seen one in the flesh.
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Old 4th Oct 2015, 4:57 pm   #6
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Default Re: Philips 25kv EHT unit.

From Jonz valvepage website, the magnificent Philips TEL61:

http://www.thevalvepage.com/tvmanu/p...61/philips.htm

Jon has offered up an excellent write up about this set.

One wonders how many were actually sold:

DFWB.
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Old 4th Oct 2015, 5:03 pm   #7
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Default Re: Philips 25kv EHT unit.

Hello Jac,
Thanks for those internal pictures of the ringing choke/tripler. Absolutely fascinating! I knew what was inside but never opened one up. I knew the EY51s were smaller than the standard type and of high grade manufacture.

270m/a of heater power required for those. I would imagine that fitting some high voltage silicon rectifiers would cure the uncertain regulation that struggled to cope with fast changes of brightness.

I did fit a colour tripler to a Philips 1800 about 20 years ago. The picture was superb.
Regards, John
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Old 10th Oct 2015, 9:51 am   #8
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Default Re: Philips 25kv EHT unit.

Yesterday afternoon a little progress was made on the 131. The EHT unit was refitted together with the main chassis and the very heavy power supply.

Switching on was quite boring, no bangs as such with the H.T. rail rising to it's correct potential. No line whistle could be heard, just the drone of the EHT oscillator.

Increasing the brightness produced nothing on the screen suggesting that the tube protection circuit had come into operation due to the failed timebase/timebases. The line output valve had H.T. on its anode and was quite warm so I guess it is not being driven.
No familiar buzz could be detected from the frame department. It may be a common cause. [So is snow in summer]
Switching off produced the classic fading 'blob' confirming my thoughts.

It was decided to remove the chassis and the CRT unit minus the optical box to my familiar workshop where detailed fault finding can continue with the aid of a scope.

The pictures show the units. On the left the heavy power supply. Left front, control panel with on/off switch. Centre, main chassis underside. To the right, EHT unit and CRT assembly including scan coils and focus coil.

I nice quiet evening should sort it out. Probably......Regards, John.
PS That workbench looks cluttered..It is 8' X 4' I need a bigger one.
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Old 10th Oct 2015, 1:16 pm   #9
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Default Re: Philips 25kv EHT unit.

You'll be speaking with a squeeky voice if you run that setup for too long, John!
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Old 11th Oct 2015, 5:20 pm   #10
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Default Re: Philips 25kv EHT unit.

[Moderators, can you please change this thread to 'Beau Decca 131 projection receiver 1950' as it has now drifted into a restoration. Thanks]

It took a while to sort out the various plugs and sockets interconnecting the various units. The EHT was now present but only the fading blob presented itself on switch off.

It was time to scope the timebases to see which one had packed up bringing in the CRT protection circuit. The PL38 lacked its usual blue glow that can be seen on the internals of the glass bulb when operating correctly. It was also getting warm rather quickly and a check at the control grid pin 5 revealed just a small amount of mush.

The line generator is a Mazda T41 Thyratron with very few components to worry about. The cathode bypass capacitor a 50uf 12v tubular was S/C and soon replaced. A check from cathode to earth now showed an O/C pointing to the line hold control. In fact every wirewound preset was O/C! Frame hold, Frame lin, Line hold, Line width, Line linearity, RF gain, Contrast the lot!

I spent over an hour looking for a large box of presets only to find it under the bench.. Suitable replacements were laboriously fitted and power applied.

The EL38 glowed a welcome blue and the line whistle could be heard faintly. A scope check showed full drive and a small spark could be drawn from the top cap. Full of confidence I applied the power but still the screen refused to illiminate. So on to the frame timebase.

A scope check at the anode of the T41 frame generator showed no output and yes you have guessed it..The 50uf cathode bypass capacitor was S/C!
Replacing this produced a scrambled mess on the tiny 2 1/2" screen and to be honest it was very difficult to see what was actually going on.

There were no signals. The 131 had been converted to ITA with the very popular Cyldon 10mc/s tuner. This only involved removing the EF42 RF amp and frequency changer valves and replacing them with the tuner plugs.
The tuner was still in the cabinet at the museum so I decided to fit a couple of EF42s into the blank holes and work with it on Band 1 as it was originally.

There were still no signals so it was decided to sort out the hum bars, poor frame lin and other weird displayed symptoms.

The HT was up to scratch but a heavy 100c/s waveform could be detected on the 350v line. After a few checks it was discovered that the 100uf 500v smoother in the separate power supply was completely O/C despite reading innocent on the capacitor tester. I have been caught by that one before.

Replacing this gave me a very good shaped raster, brilliant but not brilliant enough for a projection receiver operating at 25KV. The EHT unit also lacked the change in note of the EHT blocking oscillator when the brightness was increased to full calling for an increase in beam current. More of this to come.

There was still no sound or vision signals. Feeding the signal generator into the aerial socket at 45mc/s showed zero even at full output but winding it up to around 50mc/s produced the classic bars of the modulated carrier.

The receiver had obviously been operating in foreign parts, probably channel 3as it originated south of the Thames. The oscillator adjustment on these Plessey IF panels only allows for a limited variation in frequency so no go there. I messed about with low value capacitors across the coil, all to no avail so it was decided to rewind the coil itself.

The original consisted of five turns of heavy gauge tinned copper wire mounted on a ceramic former. A large dust iron core mounted on a brass adjuster thread allowed for final adjustment.
I wound 7 turns of similar gauge wire around a marker pen and soldered it in place of the original coil. After a bit of 'adustment' with a plastic knitting needle [opening out the turns] it tuned to 45mc/s exactly!

The results are as you see by the pictures. The MW6-2 CRT is low emission. This is very unusual with these small projection tubes as most of them died of screen exhaustion [turning a light brown colour] before the gun died. The test card turns negative when you increase the brightness level hence the lack of beam current explained earlier. I have a couple of CRTs packed away so with a bit of luck, all is not lost.

There is still no sound. [I always have frustrating sound problems with these old receivers!] The audio amp works fine but there is a breakdown between the detector and preamp, a single 6L18 triode. The output valve is a 6V6GT.

The picture of the test card does not show it anything like as good as it actually is viewed directly of course. It should be stunning and capable of lighting a room.

Care has to be taken due to the possibility of X rays radiation from the front of the tube but should be ok if the brilliance level is maintained at a low level.

A lot more to do and I will keep you informed. Regards, John.
PD Tremendous fun!
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Old 12th Oct 2015, 5:04 pm   #11
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Default Re: 'Beau Decca 131 projection receiver 1950'

Right then. It's time to make a start on my Decca 131. The first job is to replace that tatty screen.
Standby for the pictures.

DFWB.
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Old 12th Oct 2015, 5:46 pm   #12
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Default Re: 'Beau Decca 131 projection receiver 1950'

First pictures of my Decca projection set. It's actually a model 121.

Three Mazda 6F13 pentodes are missing in the receiver unit. Apart from inter-electrode capacitances the Mullard EF42 is similar.
The Philips sourced EHT unit can be seen in the rear view on the right side of the cabinet.
The screen is 16 X 12 inches. That would be considered a large screen TV in 1950. Twenty inches diagonal.

DFWB.
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Old 12th Oct 2015, 6:28 pm   #13
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Default Re: 'Beau Decca 131 projection receiver 1950'

Hi all,
According to a Mullard book I had when I were a sprog service engineer, even if you worked on the production line 24/7/365 you would not receive the then recommended minimum dose of X rays from the bare Schmidt optics.
I appreciate you should not take chances with these things but in general servicing I don't think there is anything to concern yourself about.
I am sure if you live in Cornwall you are more likely to get irradiated from the Radon gas emitted there.

John

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Old 12th Oct 2015, 6:44 pm   #14
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Default Re: 'Beau Decca 131 projection receiver 1950'

The discoloured screen. The ex-Samsung screen I intend to use as a replacement has a fine ribbed effect on one side. Now this begs the question, should the ribs be vertical or horizontal? If it is the latter, would this cause an interference effect with the picture line structure?

DFWB.
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Old 12th Oct 2015, 9:21 pm   #15
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Default Re: 'Beau Decca 131 projection receiver 1950'

The pictures show the final local oscillator coil for channel 1. The switch off spot showing the dark areas of the poisoned cathode giving low tube emission.
The original oscillator coil and the complicated sync separator necessary for the excellent interlace required by a high magnification projection system.

Just the tube to replace and sorting out the lack of sound. Regards, John.
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Old 12th Oct 2015, 11:26 pm   #16
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Default Re: 'Beau Decca 131 projection receiver 1950'

The Cyldon Type "C" tuner as fitted in my Decca 121. I believe John's 131 has a similar tuner. I've seen other Decca projection sets which have had this type of tuner installed. Perhaps it was a factory sponsored multi channel conversion?
Valves are ECC84 and ECF80. Note the two B8A plugs to facilitate easy conversion.

DFWB.
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Old 13th Oct 2015, 8:39 am   #17
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Default Re: 'Beau Decca 131 projection receiver 1950'

Exactly the same tuner in mine David, probably with coils for channels 2 and 10, a Cyldon 10L.
It was simplicity in itself to convert to ITA with these tuners. Simply remove the RF amplifier and frequency changer EF42 valves and replace them with the two adaptor plugs picking up HT and heaters from the RF plug and feeding the tuner IF in with the other.

In fringe areas the IF plug could be removed and the output fed directly into the grid of the frequency changer converting it to an extra IF stage. It was necessary of course to stop the local oscillator by shorting the coil. J.
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Old 13th Oct 2015, 1:38 pm   #18
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Default Re: 'Beau Decca 131 projection receiver 1950'

Summoned up the courage today and connected the Decca 121 to the mains.
The sound is OK but all that can be seen on the screen is a defocussed raster, the focus control has no effect whatsoever. The focus control system does need explanation and the official Decca service devotes a whole page column about this. Other makers of projection TVs such as Ferranti and the Philips 1400A employ a focus stabiliser valve, a power pentode like the EL41 or PL82.
The Decca does have a valve in the focus circuit, one half of V19, a 6SN7GT. This valve receives a controlling voltage from the cathode of the EHT generator valve. I'd imagine variations of the cathode volts of the EHT valve relate to the demands on the EHT current taken by the CRT.
I'm sure it will all become clear as time is spent on study of the set.

DFWB.
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Old 13th Oct 2015, 3:43 pm   #19
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Default Re: 'Beau Decca 131 projection receiver 1950'

A simplified circuit diagram of the focus control system employed in the Decca 121 and 131. The later model 141 omits the stabiliser valve V19.
The focus coil serves as the loss element to supply the 268 volt HT supply to the earlier stages which demand a much lower voltage.
The focus control itself supplies the RF and IF amplifier valves V1,2,3 & 4.
It doesn't appear to a very nice circuit but in fact I'm told it works very well indeed.

DFWB.
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Old 13th Oct 2015, 5:17 pm   #20
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Default Re: 'Beau Decca 131 projection receiver 1950'

Now we know why the focus is bad and the image on the CRT screen is dim.
The EHT measures less than 10KV.
It's likely one of the EY51 tripler diodes has gone faulty.

DFWB.
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