blanking problems.? You might want to check the CRT bias adjustments and similar voltages.? Turning down the intensity should make it vanish.? If not, then definitely blanking problems.
You might want to see if the unblanking pulses are doing what they're supposed to.? (or blanking pulses).? In some older scopes, there are diodes that have gone bad.? Do be careful, since they may run the cathode and grid at elevated voltages (like 2KV).
Grid - cathode voltage *should* change with the intensity settings.?? Hmmm......
Look at the crt circuit in the schematic. The diodes can/should be replaced with 1N4937 and I strongly suggest replacing R888 with a Vishay VR37 series 22meg resistor. That carbon will be out of tolerance. I would also suggest replacing the 1mfd condensers for the 70V supplies with a Nichicon ULD type and can be higher in capacitance, e.g. 2.2mfd, and the 1mfd decoupling from the crt bias with a 250V film type. Check the crt bias pot to make sure it is still good. The pot could be open. I have found these and a few others open in the 7000 series then replace them with a 10 turn 3/4W type. Yours does not need to be this type, unless you want it and the pot is bad.
I am thinking the Bias pot is R873, its labeled Beam. Last week I found that pot had come apart. No trace. If I pressed down on that pot the trace came alive. I ran the scope with a 20K pot and it functioned. Then I wired the circuit with fixed resistors. 25k, 25K with a center tap until the replacement pots arrive.
I checked the 1uf capacitors with my Sencore meter and do not suspect them to be the issue. I will order replacements. The ones in there are axial and the ULD you recommended is radial. Is that what you had in mind?
I have ordered the 1N4937 diodes. It was previously recommended to replace the diodes in the DC Restorer circuit. I am not sure what the DC Restorer circuit is. Can you point that out.
I am thinking the Bias pot is R873, its labeled Beam. Last week I found that pot had come apart. No trace. If I pressed down on that pot the trace came alive. I ran the scope with a 20K pot and it functioned. Then I wired the circuit with fixed resistors. 25k, 25K with a center tap until the replacement pots arrive.
I checked the 1uf capacitors with my Sencore meter and do not suspect them to be the issue. I will order replacements. The ones in there are axial and the ULD you recommended is radial. Is that what you had in mind?
I have ordered the 1N4937 diodes. It was previously recommended to replace the diodes in the DC Restorer circuit. I am not sure what the DC Restorer circuit is. Can you point that out.
Thanks!
Barry
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The DC Restorer caps are C881, 2, 3, 5 and 7. They go between the main (A1) board and the HV auxiliary (A5) board. In my experience, they are more likely the problem than the diodes on the A5 board. But replacing the caps is a bear. By coincidence, I am doing the final edits of a video on replacing two tantalums on the main board which are under the A5 board and require removing the A5 board. I will add this intensity problem to the video, which I hope to get out today.
The d-c restorer diodes are on the high voltage board, A5 page <8> in the manual. The 1mfd condensers are on boards A1 and A2. All of this is on page 156-157 when looking at the PDF of the manual. The ULD types are radial and can replace axial types. Bend the leads at the bottom so they are a a right angle to the condenser body then bend where the leads will go in the holes easily, solder then trim any extra lead off. The ULD types are designed for the high frequency filtering these see, high temp and low ESR with a long life. I see Mouser has the ULD in 400V for this replacement. This is the Mouser p/n for the 22meg resistor (R888): 594-5053DM22M00F. The bad pot can be replaced with a fixed resistor from ground to the low side and the top to the 70V with the wiper to the circuit as usual. This is when you know the bias you want and have a lower value such as 20,000 ohms. A 1/2W or so 50,000 ohm pot as the replacement is fine. It likely will be physically bigger than original. That is set and left alone being inside the unit. If you want fine control of this, a 10/20 turn type can be used. I doubt that is necessary.
Thank you for the informative video Bob. Efforts like that make it more practical to keep these wonderful old instruments going for a while longer. The magic smoke came out of my SC503, going to have to investigate soon.
Bob Haas
radeng
Jean-Paul