Cossor 1039 Oscilloscope - Page 2 - UK Vintage Radio Repair and Restoration Discussion Forum

WaveyDipole · 24th May 2021, 2:19 pm

Bill,

Thank you for the detailed write-up.

1. To confirm, voltage was measured between p1 and p4., between GND and RV7 (full EHT), across the heater and across R29+R30_R31 and GND. All readings appear stable over 10 mins.

2. Point noted.

3. To measure between p1 and p4, I used a high voltage probe connected to a DMM to reduce any loading as much as possible. Its also safer on the meter and the operator!. I did not use the AVO for any of these readings.

Regarding C7, this is a rather large and somewhat unusual looking chassis mounted 2kV rated ceramic type of capacitor. Ceramics don't often fail, but they can be prone to temperature effects so i guess its possible that it may have a bearing. I might try temporarily wiring a substitute as you suggest to see what happens. The trace is indeed dimmer towards the right.

The 470k resistor between the CRT grid and BRILL pot wiper has been replaced as it was out of tolerance and reading somewhare around 550k. The 27k one was fine.

5. The focus is stable and does not appear to change as the trace fades.

The Mk1 uses a different tube which means that the surrounding circuitry may be different although I have not compared the diagrams.

WME_bill · 24th May 2021, 3:31 pm

Cossor 1039.
1. If the trace fades towards the right, then it suggests the bright-up is failing in sosme way. Try disconnecting C7 completely. you will now not get a bright-up pulse fed through from the Transitron screen grid, which you can compensate by advancing the brilliance control(s). The bright -up pulse will be about 50v +ve.
2. So the focus stays unaltered during the "fade". This suggests the EHT voltage chain is stable, so brilliance setting will be stable.
Change of brightness during trace can only be caused by variation of grid voltage (bright-up pulse). Change of accelerating voltage (=brilliance/focus) will not be quick enough to have effect during a trace.
Change of all the trace length over a period is change of accelerating voltage or tube emission.
wme_bill

WaveyDipole · 26th May 2021, 9:31 am

Bill, I experimented with disconnecting C7 from the circuit as suggested. This does even out the brightness of the trace but a second curved trace line which I presume to be the flyback is visible when a trace is displayed. I tried substituting C7 with 2 x 4.7nF in parallel but the same fade to the right is evident. I will experiment a bit further when I have a bit more time.

24th May 2021, 2:19 pm	#21
WaveyDipole Nonode Join Date: Mar 2013 Location: Leicester, UK Posts: 2,535	Re: Cossor 1039 Oscilloscope Bill, Thank you for the detailed write-up. 1. To confirm, voltage was measured between p1 and p4., between GND and RV7 (full EHT), across the heater and across R29+R30_R31 and GND. All readings appear stable over 10 mins. 2. Point noted. 3. To measure between p1 and p4, I used a high voltage probe connected to a DMM to reduce any loading as much as possible. Its also safer on the meter and the operator!. I did not use the AVO for any of these readings. Regarding C7, this is a rather large and somewhat unusual looking chassis mounted 2kV rated ceramic type of capacitor. Ceramics don't often fail, but they can be prone to temperature effects so i guess its possible that it may have a bearing. I might try temporarily wiring a substitute as you suggest to see what happens. The trace is indeed dimmer towards the right. The 470k resistor between the CRT grid and BRILL pot wiper has been replaced as it was out of tolerance and reading somewhare around 550k. The 27k one was fine. 5. The focus is stable and does not appear to change as the trace fades. The Mk1 uses a different tube which means that the surrounding circuitry may be different although I have not compared the diagrams.

24th May 2021, 3:31 pm	#22
WME_bill Octode Join Date: Oct 2007 Location: Exeter, Devon, UK. Posts: 1,554	Re: Cossor 1039 Oscilloscope Cossor 1039. 1. If the trace fades towards the right, then it suggests the bright-up is failing in sosme way. Try disconnecting C7 completely. you will now not get a bright-up pulse fed through from the Transitron screen grid, which you can compensate by advancing the brilliance control(s). The bright -up pulse will be about 50v +ve. 2. So the focus stays unaltered during the "fade". This suggests the EHT voltage chain is stable, so brilliance setting will be stable. Change of brightness during trace can only be caused by variation of grid voltage (bright-up pulse). Change of accelerating voltage (=brilliance/focus) will not be quick enough to have effect during a trace. Change of all the trace length over a period is change of accelerating voltage or tube emission. wme_bill

26th May 2021, 9:31 am	#23
WaveyDipole Nonode Join Date: Mar 2013 Location: Leicester, UK Posts: 2,535	Re: Cossor 1039 Oscilloscope Bill, I experimented with disconnecting C7 from the circuit as suggested. This does even out the brightness of the trace but a second curved trace line which I presume to be the flyback is visible when a trace is displayed. I tried substituting C7 with 2 x 4.7nF in parallel but the same fade to the right is evident. I will experiment a bit further when I have a bit more time.