Hi guys,
I have been beating my head against the wall for 2 days and am out of ideas. The problem is I can't get the trace to center vertically without the position control being maxed out in one direction, and then it's still not quite centered.
It's a nice Tektronix 575 curve tracer and I am having a problem with just one section, the vertical input amp. I have attached the schematic and indicated the voltages I measured in red. The horizontal amp is identical and it works fine, all the voltages are what they should be.
The problem is in the first section, V454, the top tube in the diff pair. The voltage is low on the plate, and no matter what I have done, it doesn't come back to 50V, it's stuck at about 14.
What I have done so far....
1. all PS voltages are fine
1. new tubes
2. swapped all R's in the plate section, and checked most of the rest of them.
3. pulled all tubes after the first section, no change which is why I think it is just the first stage.
4. the position control works, and varies the voltage, but the range in the top tube is way off only 10-14V rather than 45-52. I made all the tests with the input grounded, although it makes no difference either way.
I've been staring at this too long and just need a little external input. It's a pretty simple circuit...what am I missing.
Cheers
I have been beating my head against the wall for 2 days and am out of ideas. The problem is I can't get the trace to center vertically without the position control being maxed out in one direction, and then it's still not quite centered.
It's a nice Tektronix 575 curve tracer and I am having a problem with just one section, the vertical input amp. I have attached the schematic and indicated the voltages I measured in red. The horizontal amp is identical and it works fine, all the voltages are what they should be.
The problem is in the first section, V454, the top tube in the diff pair. The voltage is low on the plate, and no matter what I have done, it doesn't come back to 50V, it's stuck at about 14.
What I have done so far....
1. all PS voltages are fine
1. new tubes
2. swapped all R's in the plate section, and checked most of the rest of them.
3. pulled all tubes after the first section, no change which is why I think it is just the first stage.
4. the position control works, and varies the voltage, but the range in the top tube is way off only 10-14V rather than 45-52. I made all the tests with the input grounded, although it makes no difference either way.
I've been staring at this too long and just need a little external input. It's a pretty simple circuit...what am I missing.
Cheers
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Hi kilohertz,
Disclaimer: I have no familiarity with this equipment, but I do enjoy troubleshooting.
Some random questions first, in no particular order:
You indicate the trace is nearly centered with controls at extreme settings; do the the deflection plate voltages at the CRT correspond to indicated voltages in schematic? Do the horizontal and vertical deflection plates behave similarly re measured voltages? When centered, do H & V plates have similar voltages? Does behavior "feel" like the CRT is OK with driving stages at fault?
Would you measure corresponding voltages on the Horizontal section for comparison, especially cathode and screen voltages? Your indicated voltages suggest the V444/V454 tubes are passing more than nominal current and I'm trying to puzzle why. I believe there's negative feedback around these stages ( e.g. via R490 & R495). So would you include plate voltages V484A/B (and V384A/B) in your measurements? Thanks.
Disclaimer: I have no familiarity with this equipment, but I do enjoy troubleshooting.
Some random questions first, in no particular order:
You indicate the trace is nearly centered with controls at extreme settings; do the the deflection plate voltages at the CRT correspond to indicated voltages in schematic? Do the horizontal and vertical deflection plates behave similarly re measured voltages? When centered, do H & V plates have similar voltages? Does behavior "feel" like the CRT is OK with driving stages at fault?
Would you measure corresponding voltages on the Horizontal section for comparison, especially cathode and screen voltages? Your indicated voltages suggest the V444/V454 tubes are passing more than nominal current and I'm trying to puzzle why. I believe there's negative feedback around these stages ( e.g. via R490 & R495). So would you include plate voltages V484A/B (and V384A/B) in your measurements? Thanks.
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A wild guess: make sure C461 is not shorted or leaky. There should be very little DC voltage drop across R461.
Good luck.
Good luck.
BSST,
Thank you so much for the suggestions...some really good remote troubleshooting, but I finally found the trouble this afternoon, about a half hour after I posted...wouldn't it figure. 🙄
You were right on when you suggested the feedback loop, you are obviously familiar with tube ccts and your questions were spot on and ones I had asked myself, and at first I didn't think that was the problem as both cathode voltages were correct on the first tube...but it turns out one of the 800K R's was open. Yeah!
Changed that (actually both of them) and it's back up and running. I went thru the calibration and now I am on to the next problem, the stepping cct, ...but that's for tomorrow. Brain fried, must eat and sleep.
Thanks again for the excellent suggestions, hang in for part 2 tomorrow. 😀
Cheers
Thank you so much for the suggestions...some really good remote troubleshooting, but I finally found the trouble this afternoon, about a half hour after I posted...wouldn't it figure. 🙄
You were right on when you suggested the feedback loop, you are obviously familiar with tube ccts and your questions were spot on and ones I had asked myself, and at first I didn't think that was the problem as both cathode voltages were correct on the first tube...but it turns out one of the 800K R's was open. Yeah!
Changed that (actually both of them) and it's back up and running. I went thru the calibration and now I am on to the next problem, the stepping cct, ...but that's for tomorrow. Brain fried, must eat and sleep.Thanks again for the excellent suggestions, hang in for part 2 tomorrow. 😀
Cheers
Yup pulled that out as well, no change, it looks like our posts crossed, so you will see, that the vertical stage is fixed. 😉
Cheers
BTW, the horizontal section wasn't quite perfect so I checked the same 800K R's on that section and found one of them up to 1M, from 800K, and these look like they are top quality 1% R's...changed those as well and now both sections are perfect.
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