Linearity issues and LOPT Proximity. - UK Vintage Radio Repair and Restoration Discussion Forum

ElectroRelics · 15th Oct 2021, 8:59 pm

Hi All,

I've posted a thread in the Vintage Test Equipment section as I am working through a Marconi 2955R that I bought faulty and addressing each issue bit by bit, but I'm currently held up with a linearity issue on the CRT, the CRT on this is rotated by 90 degrees so what is seen as the vertical scan on the CRT is actually driven by the Horizontal Deflection and LOPT Drive circuitry.

I cannot find out exactly what is causing this to have a linearity issue causing shrunken text, it is always in exactly the same part of the CRT.

I'm posting this in here as I have had a bit of a eureka moment and finally something is showing a change within this area. And hopefully someone will have come across this issue with an LOPT or H Drive circuitry.

I found out by accident if I held the back of a plastic brush on or near to the LOPT the linearity/shrunken text issue would get worse. It is not a dry joint it is affected by holding anything near the LOPT.

I'm trying to work out if it is the LOPT at fault or whether something else is happening here, I've never come across a fault like this with a CRT before.

Here is a video on YouTube I made, please ignore the noises it is not me tapping something it is just difficult to try and get the brush near it to show this issue and film at the same time.

The LOPT is unobtainium for this device, so I'd really like to know if it is faulty, what is wrong with it or if it is not faulty is it something else that is feeding back in through leaky diodes or something?

https://youtu.be/TWW2ebBtDIY

Also this is the original thread if anyone is interested at what I've tried so far etc.
https://www.vintage-radio.net/forum/...d.php?t=184553

15th Oct 2021, 8:59 pm	#1
ElectroRelics Triode Join Date: Oct 2021 Location: Newport, Monmouthshire, Wales, UK. Posts: 34	Linearity issues and LOPT Proximity. Hi All, I've posted a thread in the Vintage Test Equipment section as I am working through a Marconi 2955R that I bought faulty and addressing each issue bit by bit, but I'm currently held up with a linearity issue on the CRT, the CRT on this is rotated by 90 degrees so what is seen as the vertical scan on the CRT is actually driven by the Horizontal Deflection and LOPT Drive circuitry. I cannot find out exactly what is causing this to have a linearity issue causing shrunken text, it is always in exactly the same part of the CRT. I'm posting this in here as I have had a bit of a eureka moment and finally something is showing a change within this area. And hopefully someone will have come across this issue with an LOPT or H Drive circuitry. I found out by accident if I held the back of a plastic brush on or near to the LOPT the linearity/shrunken text issue would get worse. It is not a dry joint it is affected by holding anything near the LOPT. I'm trying to work out if it is the LOPT at fault or whether something else is happening here, I've never come across a fault like this with a CRT before. Here is a video on YouTube I made, please ignore the noises it is not me tapping something it is just difficult to try and get the brush near it to show this issue and film at the same time. The LOPT is unobtainium for this device, so I'd really like to know if it is faulty, what is wrong with it or if it is not faulty is it something else that is feeding back in through leaky diodes or something? https://youtu.be/TWW2ebBtDIY Also this is the original thread if anyone is interested at what I've tried so far etc. https://www.vintage-radio.net/forum/...d.php?t=184553