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WME_bill · 23rd May 2021, 11:42 am

Cossor 1039 Mk2.
May I summarise what you have found. It is important when fault finding in something as complex as a scope to concentrate on one thing at a time.
The trace fades a little after a few minutes use. Not during the trace. This suggests a heat or voltage stress effect around the CRT feeds.
1. EHT voltage dropping. Between the accelerator anode p4 and the cathode p1. Have you checked the voltage actually between these pins. This will confirm the brilliance pots and slider settings, RV7 and RV8.
If you have a digital (10Mohm) meter , use it as preferable to your Avo as you want as low a current as possible. Connect up the Avo (on 100v range, or better 250v range) with the unit switched off. Then switch on. Preferable not to touch the meter, as it is all floating at 800v or so. Watch the readings as the trace fades. You are looking for perhaps 10 to 20v negative change.
2. Tube emission going. I would not expect this to drop in use and then come back later. If it drops, then it will stay dropped. So unlikely.
3. Grid p4 and Cathode p1 voltage changing. This is difficult to test, as it is all high impedance, and with a tube current of 1uamp, the 50 ua of your Avo will pull the grid-cathode voltage all over the place. If you are trying to measure grid to chassis voltage, then grid will swing vastly positive, and probably the ensuing current destroy the tube.
Try connecting the Avo between pins p4 and p1, when switched off. Set at 250v range to keep the current drawn down, and expect 10-20V variation as it fades.
You have changed the diode MR1, so that probably exonerates that.
What about C7, which carries the positive moving bright-up pulse from the screen of the transitron to the tube.
If that is leaking slightly, then the CRT grid will decline negative during the trace, making the trace get dimmer towards the right (does it?). And after a time under voltage stress (it apparently does). Change it. Voltage rating must be 1000v or greater. Value not critical; anything over 1nf (0.001) enough for this test. If you operate the sweep, keep the trace speed high, as a smaller capacitance will result in the trace fading towards the end of each sweep as the capacitance discharges the square pulse of sweep duration produced by the screen of the transitron timebase.
4. The two resistors around the CRT grid should be alright, as they are under little voltage stress: R13 - 27k, R24 -470k. Might be worth changing the 470k. Also cleaning around the pins and CRT base.
5. You do not say if the Focus changes as the trace fades. That could suggest the EHT voltage chain is varying. Though you have apparently checked that.

I have a 1039 Mk1, so cannot fully cross check what results you should expect.
Let us know the resuts of these extra tests.
wme_bill.

23rd May 2021, 11:42 am	#20
WME_bill Octode Join Date: Oct 2007 Location: Exeter, Devon, UK. Posts: 1,554	Re: Cossor 1039 Oscilloscope Cossor 1039 Mk2. May I summarise what you have found. It is important when fault finding in something as complex as a scope to concentrate on one thing at a time. The trace fades a little after a few minutes use. Not during the trace. This suggests a heat or voltage stress effect around the CRT feeds. 1. EHT voltage dropping. Between the accelerator anode p4 and the cathode p1. Have you checked the voltage actually between these pins. This will confirm the brilliance pots and slider settings, RV7 and RV8. If you have a digital (10Mohm) meter , use it as preferable to your Avo as you want as low a current as possible. Connect up the Avo (on 100v range, or better 250v range) with the unit switched off. Then switch on. Preferable not to touch the meter, as it is all floating at 800v or so. Watch the readings as the trace fades. You are looking for perhaps 10 to 20v negative change. 2. Tube emission going. I would not expect this to drop in use and then come back later. If it drops, then it will stay dropped. So unlikely. 3. Grid p4 and Cathode p1 voltage changing. This is difficult to test, as it is all high impedance, and with a tube current of 1uamp, the 50 ua of your Avo will pull the grid-cathode voltage all over the place. If you are trying to measure grid to chassis voltage, then grid will swing vastly positive, and probably the ensuing current destroy the tube. Try connecting the Avo between pins p4 and p1, when switched off. Set at 250v range to keep the current drawn down, and expect 10-20V variation as it fades. You have changed the diode MR1, so that probably exonerates that. What about C7, which carries the positive moving bright-up pulse from the screen of the transitron to the tube. If that is leaking slightly, then the CRT grid will decline negative during the trace, making the trace get dimmer towards the right (does it?). And after a time under voltage stress (it apparently does). Change it. Voltage rating must be 1000v or greater. Value not critical; anything over 1nf (0.001) enough for this test. If you operate the sweep, keep the trace speed high, as a smaller capacitance will result in the trace fading towards the end of each sweep as the capacitance discharges the square pulse of sweep duration produced by the screen of the transitron timebase. 4. The two resistors around the CRT grid should be alright, as they are under little voltage stress: R13 - 27k, R24 -470k. Might be worth changing the 470k. Also cleaning around the pins and CRT base. 5. You do not say if the Focus changes as the trace fades. That could suggest the EHT voltage chain is varying. Though you have apparently checked that. I have a 1039 Mk1, so cannot fully cross check what results you should expect. Let us know the resuts of these extra tests. wme_bill.