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Old 13th Oct 2015, 5:24 pm   #21
Heatercathodeshort
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Default Re: 'Beau Decca 131 projection receiver 1950'

May not be David. Check all the resistors in the EHT unit particularly the ones in the regulation circuit. Early versions were carbon composition but the ones in mine were all Philips high stability types. The capacitors are leaky after all these years and have odd values. Check the frequency of the oscillator, the blocking transformer could be faulty.

I may have a spare ringing can hidden away.. John.
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Old 13th Oct 2015, 5:57 pm   #22
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Default Re: 'Beau Decca 131 projection receiver 1950'

Hi John,
A good number of components have been replaced. All the tar (goudron) capacitors have been changed with ones with generous voltage ratings.
No resistors have been replaced, at least so far.
According to the Decca service data the blocking oscillator frequency should 1000c/s and the ringing frequency 25Kc/s.

DFWB.
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Old 13th Oct 2015, 6:30 pm   #23
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Default Re: 'Beau Decca 131 projection receiver 1950'

Circuit diagram of the Philips 25KV EHT generator.
No values given for the tripler capacitors C62, C63 and C64. "Not accessible".

It is possible to draw a 3 to 4mm spark from the anode connector of the EL38.
Maybe that's not enough.

DFWB.
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Old 13th Oct 2015, 6:42 pm   #24
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Default Re: 'Beau Decca 131 projection receiver 1950'

That spark is surprisingly large. Mine is quite tiny when producing 25KV and has to be drawn from the bottom of the top cap where it meets the glass. It has that classic 'groan' that should alter with varying beam current.J.
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Old 13th Oct 2015, 7:03 pm   #25
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Default Re: 'Beau Decca 131 projection receiver 1950'

Quote:
Originally Posted by FERNSEH View Post
Circuit diagram of the Philips 25KV EHT generator.
No values given for the tripler capacitors C62, C63 and C64. "Not accessible".
David,

These 3 capacitors are in the can.
They are each made up of several capacitors in series.
The values indeed are not publicized, but if I am not mistaken they are (ref. to the numbers in your schematic diagram):
C62 = 2.5nF-20kV
C63 = 5nF-10kV
C64 = 2.5nF-20kV

The 5nF capacitor is made of 4 capacitors in series; the 5nF has 8 capacitors in series. (All these capacitors are identical.)

I'm afraid this doesn't help you much further.
Jac
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Old 13th Oct 2015, 8:29 pm   #26
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Default Re: 'Beau Decca 131 projection receiver 1950'

Good evening Jac,
If in the event one of the EY51 rectifiers is faulty, would it be a good idea to replace all of them with more modern silicon EHT diodes? I still have the BY182 and also a good number of 10KV diodes.
The oil, benign or is it of a possible harmful nature? But, before opening the transformer can, the components in blocking oscillator will be checked. Also confirm that the oscillator is operating at the correct frequency. 1KHz

DFWB.
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Old 13th Oct 2015, 8:42 pm   #27
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Default Re: 'Beau Decca 131 projection receiver 1950'

Good evening David,

It certainly is possible to replace the EY51s by silicon diodes.
Or by new EY51s

I had the oil examined and it didn't contain PCBs.

The transformer coil is vacuum impregnated with the oil, so do not keep it dry for too long. It would be best to keep the coil in oil.
The capacitors are suspect as well, but there could also be a fault in the coil itself.

The repeat frequency of the ringing choke is c. 1kHz, and the ringing frequency itself c. 21-23 kHz.

Hope this helps a bit.

Jac
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Old 13th Oct 2015, 9:00 pm   #28
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Default Re: 'Beau Decca 131 projection receiver 1950'

Hello Jac,
I'll check the oscillator frequency first.
Projection TV sets were on sale in 1961 made by Valradio and maybe others. John reminds me that the NERA firm made one as late as 1968. In those times if you wanted a 3 X 4 foot screen then a projection TV was the only answer.
Was Philips making the EHT units up to the late sixties? Or perhaps the remaining projection TV manufactures were making their own EHT assemblies.

DFWB.
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Old 13th Oct 2015, 10:21 pm   #29
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Default Re: 'Beau Decca 131 projection receiver 1950'

The waveform present at the control grid of the EL38. The repetition rate is slightly under 1mS = 1KHz. The drive is -200 volts p-p. What matters is in order to initiate the ring in the transformer the transition from peak volts to zero is rapid.

DFWB.
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Old 14th Oct 2015, 9:08 am   #30
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Default Re: 'Beau Decca 131 projection receiver 1950'

Hello David,
Mmm, looks like that baked bean tin is lacking tomato sauce.. The waveform looks spot on.

I have located another one in Fingal's cave and it is only a simple matter to give it a real test in my 131.
Philips certainly supplied the unit well into the 60's. I guess they had hundreds left over when the projection receiver production stopped abruptly with the introduction of the 21" direct view tube in 1954.
I will have a go over the weekend. If it's any good I can send it via carrier Pidgeon. We have some real hefty ones down here so keep the lofts open. Regards, John.
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Old 14th Oct 2015, 11:30 am   #31
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Default Re: 'Beau Decca 131 projection receiver 1950'

Oh no!
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Old 22nd Oct 2015, 9:25 am   #32
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Default Re: 'Beau Decca 131 projection receiver 1950'

Hello David,
I have tested my spare ringing choke in the 131. Unfortunately it gave just enough EHT [?] to produce a very feint switch off spot and even that vanished after a short while. Shorting the EHT lead to chassis produced no spark at all!

I guess the high voltage capacitors in the tripler circuit are leaky.
It may be worth a go at removing the oil via the EHT lead out screw and gently taking a hacksaw to the top folded rim. With care this could be removed allowing access to the contents. It could easily be soldered up and refilled with oil after boiling off any moisture content.

The alternative would be to remove the tripler components including the 3 X EY51s and fit an external colour type tripler connected via a lead accessed via the original output port. This would also remove 270m/a of heater current and would probably result in better regulation.

Just thoughts at the moment. The faulty unit is on the right in the picture. It appears to have been removed from a Philips by the look of the yellow EL38 lead and connector.

Hope to fit a good tube in mine this weekend.
You can't win them all! Regards, John.
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Old 22nd Oct 2015, 10:18 am   #33
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Default Re: 'Beau Decca 131 projection receiver 1950'

Hi John,
Those capacitors in the tripler network can be reason for low EHT.
The EY51 diodes are often blameless. Jac has informed me that the capacitors have a value of 2.5nF (2500pF) apiece.
The nearest value I have is 2nF in the form of disc capacitors, these were bought from Sendz Components many years ago. It's unlikely the capacitors could be used inside the transformer can, they are too big.

An external tripler might solve the problem.

DFWB.
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