07-06-2007, 08:47 AM
Hello,
the 640 code 121 is working since last night at 3 A.M. !!!
I've made the bias with cathode resistor and cap for the two 78 and the 6A7
The RF transformer was out of order. The long wave winding was unsolder and the primary was broken at 8 turns from the end (hopefully it was then end of the winding)
I've recap everything and re-wire as the original (except one thing, the push-pull.... because the interstage transformer was not here. The 42's are connected plate to plate, screen to screen, cathode to cathode and the link with the 85 is made with 2 capacitors).
I plug the radio and it works.
now, I've to wind a interstage transformer and an output transformer to use the amplifier in push-pull mode.
The 640 is great. a very good sound. The shortwave are marvellous, the vernier is a great piece of gear.
There's only one problem and I've to work on it. On longwave, I've got some king of oscillation and motor-boating stage under 1000M.
I don't know if it's between oscillator stage and Antenna stage, or RF stage and antenna stage... I the grid of the 78 RF stage is grounded, the oscillation stop. SO I think the problem is between antenna stage and something else in the radio. Do you experience problem like that on long-wave ?
After the holidays, I will write an article on these restoration. I've done all the measurement of the inductance in µH of all the winding, if it can help....
Best regards
Frederic ABIGNOLI
the 640 code 121 is working since last night at 3 A.M. !!!
I've made the bias with cathode resistor and cap for the two 78 and the 6A7
The RF transformer was out of order. The long wave winding was unsolder and the primary was broken at 8 turns from the end (hopefully it was then end of the winding)
I've recap everything and re-wire as the original (except one thing, the push-pull.... because the interstage transformer was not here. The 42's are connected plate to plate, screen to screen, cathode to cathode and the link with the 85 is made with 2 capacitors).
I plug the radio and it works.
now, I've to wind a interstage transformer and an output transformer to use the amplifier in push-pull mode.
The 640 is great. a very good sound. The shortwave are marvellous, the vernier is a great piece of gear.
There's only one problem and I've to work on it. On longwave, I've got some king of oscillation and motor-boating stage under 1000M.
I don't know if it's between oscillator stage and Antenna stage, or RF stage and antenna stage... I the grid of the 78 RF stage is grounded, the oscillation stop. SO I think the problem is between antenna stage and something else in the radio. Do you experience problem like that on long-wave ?
After the holidays, I will write an article on these restoration. I've done all the measurement of the inductance in µH of all the winding, if it can help....
Best regards
Frederic ABIGNOLI
