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| Beckman Industrial 9102 scope problem |
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| floobydust:
The scope seems to be a mix of 20MHz and 40MHz parts. I don't think the 20MHz CRT even has a 2nd anode, unlike the 40MHz CRT. I can only guess at the sections to look at with the tools you have. It's a bit tough to find the section that is misbehaving. IC2001 looked like a good voltage controlling the HV DC-DC converter. The 140V/160V rail seems a bit high for current, it powers the H and V deflection and beam intensity control circuits. Assuming it stays up at 158V, then the trace fading problem is somewhere else. Another place to look is the beam current circuit. It runs off the +140V (+160V) rails and could draw excessive current from that rail. Q3001-Q3006. That section is to modulate the -2kV (using only 300V transistors) with CRT grid bias. You could take some measurements after the intensity control VR3002 on the transistors and look for drifting voltages as the trace dims. Note the Z-axis input is a duplicate circuit. For measuring the scope's HV power supply, you can make a poor man's HV probe with a string of 10MEG through-hole resistors inside a straw or pen. I take 1/4W parts past 500V without problems, so use 4-5 in series. With power off, I put it in place and power on to take a reading on a DMM. If the (DMM) ground/alligator clip lifts doing this, there will be drama. Knowing the DMM input resistance, you can calculate the divider ratio and work backward to the HV value. This would tell us if the -2kV supply is steady. If the focus also goes way fuzzy with a dimming trace, that I think points to the HV fading. If instead it was the beam current (control), the trace could get dim but remain largely in focus. |
| kf7pcl:
--- Quote from: floobydust on June 07, 2018, 08:14:35 pm ---The scope seems to be a mix of 20MHz and 40MHz parts. I don't think the 20MHz CRT even has a 2nd anode, unlike the 40MHz CRT. --- End quote --- It doesn't --- Quote ---The 140V/160V rail seems a bit high for current, it powers the H and V deflection and beam intensity control circuits. Assuming it stays up at 158V, then the trace fading problem is somewhere else. --- End quote --- I try not to run it too long but the 160V rail sags slightly over time (down about 1V after 10 minutes) but I don't think that's enough to cause a problem. --- Quote ---For measuring the scope's HV power supply, you can make a poor man's HV probe with a string of 10MEG through-hole resistors inside a straw or pen. I take 1/4W parts past 500V without problems, so use 4-5 in series. With power off, I put it in place and power on to take a reading on a DMM. If the (DMM) ground/alligator clip lifts doing this, there will be drama. Knowing the DMM input resistance, you can calculate the divider ratio and work backward to the HV value. This would tell us if the -2kV supply is steady. --- End quote --- I managed to find some 10M resistors and strung 6 in series. The actual sum came out to be 59.6M. The meter input impedance is 10M I measured -292V which if my math is correct means that it's -2.03kV. That seems fine to me. --- Quote ---If the focus also goes way fuzzy with a dimming trace, that I think points to the HV fading. If instead it was the beam current (control), the trace could get dim but remain largely in focus. --- End quote --- I havn't reproduced the problem with the disappearing trace lately. Maybe because I'm not running it long enough though. I found another hot part: Q812 which seems to be a horizontal CRT driver is getting up to ~90C. It's neighboring Q809, Q810 and Q810 are in the 60s/70s I attached a picture which shows the trace brightness. This is as bright as it gets |
| floobydust:
It sounds like all the power supply voltages are good. Kind of need another clue. What you need is a faded dimmed trace, in X-Y mode measure some voltages to compare to when the trace has good brightness. If it's temperature related, hot air or freeze-spray (not on the HV section OK, frost conducts!) might be worth trying. For the horizontal deflection transistor running hot, with the (H) POS ADJ control centered, I would make sure VR807 is reasonable and VR806 if you like to adjust for x10MAG centering. In X-Y mode you can measure the X and Y plates voltages. Each +,- amp is symmetrical. 150BTB31A X-axis 19-21.6 VDC/cm, Y-axis 10.8-13.2 VDC/cm and a 14cm screen. I think if a V or H deflection is not balanced but the front panel control is cranked, it might cause the (defl amp) bias current to be upset. Or if trying to cover for a lazy deflection transistor or DC offset in the sweep oscillator with the front panel control. I thought if the deflection plates (both in the pair) had low voltage, the acceleration would be a bit less, but the astigmatism would go off too. Maybe look at C2025 (CRT pin 12) I think that's the 1st acceleration anode. Cap should be over 100V rating Otherwise, it might be the beam blanking is drifting and upsetting the intensity (grid bias), and not easy to troubleshoot. Or try inject a signal on the Z-axis and see how it does. The CRT bias tells a lot, but measuring it is too difficult as -2kV common-mode exists and a DMM would be dangerous. If instead you measure the (HV) cathode (CRT pin2) and then control grid (CRT pin3), the difference is the CRT bias, beam cutoff is typically -25 to -65V for the 150BT131. It might tell, with a maxxed out intensity control if the CRT is biased weak. |
| kf7pcl:
--- Quote from: floobydust on June 10, 2018, 05:09:51 am --- For the horizontal deflection transistor running hot, with the (H) POS ADJ control centered, I would make sure VR807 is reasonable and VR806 if you like to adjust for x10MAG centering. In X-Y mode you can measure the X and Y plates voltages. Each +,- amp is symmetrical. 150BTB31A X-axis 19-21.6 VDC/cm, Y-axis 10.8-13.2 VDC/cm and a 14cm screen. --- End quote --- The front panel horizontal is almost centered to bring the spot to center. Adjusting VR807 has absolutely no effect on spot position. Q912 exceeded 100C according to my probe! When I put the scope in XY mode, the transistor cools down by about 20C --- Quote ---I think if a V or H deflection is not balanced but the front panel control is cranked, it might cause the (defl amp) bias current to be upset. Or if trying to cover for a lazy deflection transistor or DC offset in the sweep oscillator with the front panel control. --- End quote --- The front panel control is almost centered. --- Quote ---I thought if the deflection plates (both in the pair) had low voltage, the acceleration would be a bit less, but the astigmatism would go off too. --- End quote --- TBH, I'm not quite sure what this means. --- Quote ---Maybe look at C2025 (CRT pin 12) I think that's the 1st acceleration anode. Cap should be over 100V rating --- End quote --- It has a 50V rating --- Quote ---Otherwise, it might be the beam blanking is drifting and upsetting the intensity (grid bias), and not easy to troubleshoot. Or try inject a signal on the Z-axis and see how it does. --- End quote --- What is the normal amplitude range for Z then? --- Quote ---The CRT bias tells a lot, but measuring it is too difficult as -2kV common-mode exists and a DMM would be dangerous. If instead you measure the (HV) cathode (CRT pin2) and then control grid (CRT pin3), the difference is the CRT bias, beam cutoff is typically -25 to -65V for the 150BT131. It might tell, with a maxxed out intensity control if the CRT is biased weak. --- End quote --- I measured the voltage on pin 2 and 3 of the CRT. But every time I put the probe on the grid, it causes a Heisenberg issue: The CRT becomes unusually bright and the intensity control has no effect at all. I got 292.9 on the cathode and 289.2 on the grid. That's a difference of 3.7V or 26V when you consider the divider. |
| Allen49:
I ran across a website at userrmanual.com that has 100,000 manuals and will sell you a manual for the Beckman 9102 for about $10.00. |
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