Hi,
Steve Mcvoy has found a television picture tube for my Marconi set, a 7BP4. So, I could replace the radar tube with the 7BP4. Thanks to Steve for finding and sending it to me.
The 7BP4 is of the same design as the 7BP7, but with a different phosphor:
http://bs.cyty.com/menschen/e-etzold...ATestcardC.jpg
http://bs.cyty.com/menschen/e-etzold...ctureDVB-t.jpg
Hugo Holden has made some suggestions for changes. I did
- short out N-81, but connected focus coil with 81.
- Attenuator added (R-C at the cathode of the CRT) due to the higher gain of the CRT.
- two 2M5 resistors were added parallel to R64 (bleeder chain) and CRT screen grid was connected with the middle point of both 2M5 resistors. I get 200V at screen grid, and when I switch off the set, the brightness on the screen disappears before the picture become a small spot which could damage the phosphor.
- I have added an internal vertical blanking with 10nF at the plate of the vertical output valve and 250K in series at the cathode of the picture tube. With this modification, I can lower the contrast and increase brightness without retrace lines on the screen.
- The X41C was weak. I have replaced the valve with a X41, one with a bakelite socket. This new valve provides more amplification than the valve before, I have to lower the R.F. input to -10 dB to get a clean picture. Otherwise I have moire and strong sound hum bars in the picture when contrast is fully turned on.
- I have replaced C64 with a non-electrolytic high grade film capacitor. I have kept the original cap on the chassis. Now I have no cracks and white spots in the picture. The original one has shot my U52.
Here are some recent photos of the chassis:
http://bs.cyty.com/menschen/e-etzold...th7BP4A_04.jpg
http://bs.cyty.com/menschen/e-etzold...th7BP4A_03.jpg
http://bs.cyty.com/menschen/e-etzold...th7BP4A_05.jpg
Kind regards,
Eckhard