Gradual failure of Scopex 4D 25 - UK Vintage Radio Repair and Restoration Discussion Forum

Goosegrass153 · 5th Oct 2022, 3:57 pm

Earlier this year it was a fully functional cathode ray oscilloscope in daily use. One day the x time base ceased but I discovered that pressing the "Locate" button restored the x-scan. Whilst the vertical, y-direction, scale was distorted I could still make some measurements. Then that too ceased working and all I can get is bright spot in the centre. Can anyone suggest what to start looking at to effect even a partial repair?

frsimen · 5th Oct 2022, 7:38 pm

Hello and welcome to the forum.

The best thing to start with is to check that the +110V, +20V and -20V supplies are correct. Be aware that the -20V supply tracks +20V supply, so a fault on the +20V will affect the -20V output too.

One further test is to see whether any of the shift controls can move the spot when adjusted.

Be very careful when working on this oscilloscope, the mains derived EHT can kill, so keep well clear of the high voltage areas.

Paula

G6Tanuki · 5th Oct 2022, 8:05 pm

How has it been stored? I've had all sorts of odd issues with gear that uses high voltages when it's spent time in damp environments.

[I consider anything greater than 60% RH as unacceptably-damp for old electronics].

Can you put it in a room heated to at least 20C along with a dehumidifier for a week or so before investigating further?

5th Oct 2022, 3:57 pm	#1
Goosegrass153 New Member Join Date: Oct 2022 Location: Camberley, Surrey, UK. Posts: 1	Gradual failure of Scopex 4D 25 Earlier this year it was a fully functional cathode ray oscilloscope in daily use. One day the x time base ceased but I discovered that pressing the "Locate" button restored the x-scan. Whilst the vertical, y-direction, scale was distorted I could still make some measurements. Then that too ceased working and all I can get is bright spot in the centre. Can anyone suggest what to start looking at to effect even a partial repair?

5th Oct 2022, 7:38 pm	#2
frsimen Heptode Join Date: Jan 2011 Location: Croydon, London, UK. Posts: 773	Re: Gradual failure of Scopex 4D 25 Hello and welcome to the forum. The best thing to start with is to check that the +110V, +20V and -20V supplies are correct. Be aware that the -20V supply tracks +20V supply, so a fault on the +20V will affect the -20V output too. One further test is to see whether any of the shift controls can move the spot when adjusted. Be very careful when working on this oscilloscope, the mains derived EHT can kill, so keep well clear of the high voltage areas. Paula

5th Oct 2022, 8:05 pm	#3
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 14,005	Re: Gradual failure of Scopex 4D 25 How has it been stored? I've had all sorts of odd issues with gear that uses high voltages when it's spent time in damp environments. [I consider anything greater than 60% RH as unacceptably-damp for old electronics]. Can you put it in a room heated to at least 20C along with a dehumidifier for a week or so before investigating further? __________________ I'm the Operator of my Pocket Calculator. -Kraftwerk.