View Single Post
Old 11th Oct 2015, 5:20 pm   #10
Heatercathodeshort
Dekatron
 
Heatercathodeshort's Avatar
 
Join Date: Jun 2005
Location: Warnham, West Sussex. 10 miles south of DORKING.
Posts: 9,145
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!
Attached Thumbnails
Click image for larger version

Name:	131projection.jpg
Views:	462
Size:	75.9 KB
ID:	114160   Click image for larger version

Name:	131projection (1).jpg
Views:	426
Size:	84.6 KB
ID:	114161   Click image for larger version

Name:	131projection (2).jpg
Views:	421
Size:	133.5 KB
ID:	114162  
Heatercathodeshort is offline