Width & Focus in DuMont RA-103

[old_coot88]

Originally Posted by cbenham

This note and your early comment about the focus coming in momentarily then getting blurry again as well as the higher voltage than shown in the schematic has me wondering if by chance an electrolytic you installed is wired wrong?

This has happened to me. I mis-wired a cap in the power supply and while the set worked, the picture was too wide and the focus control would only get 'close' to correct at the end of rotation of the control.

Good point! Hum (ripple) in the focus coil could definitely cause the problem. Phil, have you measured for ripple across the coil?

On the other (excess width) problem, there's two 22K, 2W resistors in parallel feeding the screen grid of the HO tube. Have you measured those? Carbon comp 2W resistors that run hot have a habit of going low in value. That's because the phenolic casing carbonizes, throwing low resistance across the actual element. Bill(oc)