Gould 1421 digital storage oscilloscope - UK Vintage Radio Repair and Restoration Discussion Forum

high_vacuum_house · 5th Nov 2010, 10:22 pm

Hello there,
This oscilloscope was advertised on here a few weeks ago free because it was faulty. I like a bit of a challenge especially when it comes to old test equipment or old tellies!

The oscilloscope arrived on Thursday in the post. Powering it up showed that the CRT beam was not being cut off (blanking) when it should leaving a mess of lines on the screen.
I spent this evening going through the circuit diagram seeing how it worked and I found the 210 volt HT rail down at 160 volts. Removing the HT fuse did not bring up the HT rail.

I had a look with another oscilloscope I have at the 210 volt DC HT rail and found that rather than DC the rail was predominantly AC going down to 10 volts at the lowest point of each sine wave!!.

There was only one electrolytic capacitor that was in this part of the circuit (a 33uF 350 volt LCR made cap) and on removing it I found that it had been leaking. On replacement with another 33uF 450 volt cap (that was destined for a 405 line telly!) the HT line sprang back to life with the correct 210 volts HT and almost no AC ripple.

Having a play with the controls proves that this capacitor was at fault and now the CRT blanking is working and its replacement means the oscilloscope is now in working order.
Many thanks,

Christopher Capener

Guitarist28 · 6th Nov 2010, 3:36 pm

Hi Chrostopher,
Well done and sounds like you have bagged a bargain. I hope that you get years of good use from your new 'scope.

Regards

Rob

5th Nov 2010, 10:22 pm	#1
high_vacuum_house Octode Join Date: May 2003 Location: Belper Derbyshire Posts: 1,936	Gould 1421 digital storage oscilloscope Hello there, This oscilloscope was advertised on here a few weeks ago free because it was faulty. I like a bit of a challenge especially when it comes to old test equipment or old tellies! The oscilloscope arrived on Thursday in the post. Powering it up showed that the CRT beam was not being cut off (blanking) when it should leaving a mess of lines on the screen. I spent this evening going through the circuit diagram seeing how it worked and I found the 210 volt HT rail down at 160 volts. Removing the HT fuse did not bring up the HT rail. I had a look with another oscilloscope I have at the 210 volt DC HT rail and found that rather than DC the rail was predominantly AC going down to 10 volts at the lowest point of each sine wave!!. There was only one electrolytic capacitor that was in this part of the circuit (a 33uF 350 volt LCR made cap) and on removing it I found that it had been leaking. On replacement with another 33uF 450 volt cap (that was destined for a 405 line telly!) the HT line sprang back to life with the correct 210 volts HT and almost no AC ripple. Having a play with the controls proves that this capacitor was at fault and now the CRT blanking is working and its replacement means the oscilloscope is now in working order. Many thanks, Christopher Capener __________________ Interests in the collection and restoration of Tefifon players and 405 line television

6th Nov 2010, 3:36 pm	#2
Guitarist28 Heptode Join Date: Dec 2009 Location: Hockley, Essex, UK. Posts: 575	Re: Gould 1421 digital storage oscilloscope Hi Chrostopher, Well done and sounds like you have bagged a bargain. I hope that you get years of good use from your new 'scope. Regards Rob