30-07-2018, 03:36 PM
A little more progress, more capacator chaanges, the dodgy looking audio coupling cap was very leaky, the volume control isnt quite as bad as I thought now DC isnt getting onto the output statge grid.
The video detector diode was very leaky but just about worked ok but I changed it anyway. One of the diode valves was making poor contact in its socket. I'd better clean them all. I have a EL81 on order as Ive nothing to swap it with for testing.
I measured the free running line frequency and found it to be only about 6 kHz maximum, Ive bridged the seris resistor to raise it up a bit and now its 9 kHz, the boost voltage is higher now and the screen looks brighter as well.
I cant see a screen grid decoupling capacator at all on the line output stage, there is clearly one on the schematic. Could it have been removed from the design? I'll try adding one to see what differene it makes.
The whole timbase circuits couold probably benefit from a total rebuild, it has lots of high value resistors which may need replacing, access is a bit poor for changing individual parts.
I had a setback as well, my raspberry pi frequency generator gor zapped somehow and Ive had to transfer it to another one. The GPIO test now fails on some of the ports - oh well.
I started to make an EHT probe a while back, I'll give it go on the CRT and see what its getting.
Bye for now, David.
The video detector diode was very leaky but just about worked ok but I changed it anyway. One of the diode valves was making poor contact in its socket. I'd better clean them all. I have a EL81 on order as Ive nothing to swap it with for testing.
I measured the free running line frequency and found it to be only about 6 kHz maximum, Ive bridged the seris resistor to raise it up a bit and now its 9 kHz, the boost voltage is higher now and the screen looks brighter as well.
I cant see a screen grid decoupling capacator at all on the line output stage, there is clearly one on the schematic. Could it have been removed from the design? I'll try adding one to see what differene it makes.
The whole timbase circuits couold probably benefit from a total rebuild, it has lots of high value resistors which may need replacing, access is a bit poor for changing individual parts.
I had a setback as well, my raspberry pi frequency generator gor zapped somehow and Ive had to transfer it to another one. The GPIO test now fails on some of the ports - oh well.
I started to make an EHT probe a while back, I'll give it go on the CRT and see what its getting.
Bye for now, David.
