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| Tektronix 2235 repair thread |
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| floobydust:
--- Quote from: bd139 on November 27, 2018, 05:24:30 pm ---Ok did some analysis on the flickering while it's open and safely connected to DC and I found that voltage at TP842 drops whenever it flickers. This is not on the driver side of things after hooking a scope to the collector of Q840, setting persistence to infinite and leaving it there for a few minutes so I assume the flickering is an HT issue. I will replace the known hooky HT resistors first and see if that solves it. If not, I will do all the capacitors. If not that I will cry into it and find a donor unit for multipler and transformer. I will win if it kills me :-DD --- End quote --- Never give up :-/O The HV 0.01uF caps are pretty loud when they arc over inside. Looking at the pcb in the complete dark you can see any arcing. But the multiplier parts are encapsulated and hard to see/hear anything. It's possible to test the HV multiplier with a car ignition coil and HV probe, and CCFL tubes as a load. Good thread on remaking a Tek HV multiplier: https://www.eevblog.com/forum/repair/tektronix-sc504-hv-multiplier/ I think the 2235 is well worth the trouble to repair. The scope is a great drive. So easy to see subtle signal qualities in the analog world, 2mV/DIV sensitivity you can actually use instead of SMPS hash all over the place like you see with modern DSO's. The 2235A I have is a total dog in comparison. I hate that scope, it never triggers properly and seeing the plethora of Tektronix ECO's, it looks like the scope was a rough change when Tek went to CADD. I have to spend time and see if it needs repairs or ECO's done. |
| bd139:
Yeah I’m really liking this scope. When it works :-DD The multiplier doesn’t worry me too much. I built and played with lots of multipliers (usually resulting in swearing and zaps) as a kid. The transformer does worry me though! My TDS210 is amazingly quiet. Another keeper that one :) |
| David Hess:
--- Quote from: floobydust on November 28, 2018, 06:24:34 pm ---Thanks David for pointing out the two different grounds on C908. I'm not sure how T906 is wound but this extra winding surely is to lower EMI. When I see care and attention like this, I wonder how using a 48VDC replacement SMPS can work without wrecking the scope's noise floor. --- End quote --- If the TL594 preregulator is not being used, then there is no interference from it to cancel. |
| David Hess:
--- Quote from: bd139 on November 27, 2018, 05:24:30 pm ---Ok did some analysis on the flickering while it's open and safely connected to DC and I found that voltage at TP842 drops whenever it flickers. This is not on the driver side of things after hooking a scope to the collector of Q840, setting persistence to infinite and leaving it there for a few minutes so I assume the flickering is an HT issue. --- End quote --- Those points are the output of the z-axis amplifier which controls the trace brightness. Are you saying that TP842 glitches but the collector of Q840 does not? An intermittent failure on the high voltage side of the z-axis amplifier is possible but I would expect it to have the same effect on both side of R842 which is just there it isolate the capacitance from the amplifier output. Before getting too involved with replacing parts, I would fix the problem with the focus resistor chain that you identified. The original Allen-Bradley carbon composition resistors R888 to R892 were replaced with Vishay VR37 resistors and R886 was replaced with a Vishay VR25 resistor. These high voltage film resistors are inexpensively available. |
| bd139:
Yes that is correct. TP842 glitches but Q840 collector doesn’t. Looks like whatever is happening is only pulling the voltage down there by about 4 volts. It is weird as I’d expect Q840 to drop a bit there too but it doesn’t. I stuck the scope across both sides of it and did a differential measurement which matched. I have my suspicions the resistor isn’t healthy. It will be replaced with a nice fresh Vishay MRS25 when I do the dividers. I have already ordered some fresh VR37 510k resistors. 50 of the things as it was cheaper than buying 6 from RS :palm: ... unfortunately back ordered due on 6th December. Fuse is now sorted. I got some slow blow ones (arrived in a massive cardboard box big enough to get 2500 sheets of paper in covered in fragile tape. More :palm: ). Power up doesn’t pop the fuse now so the power supply is repaired and working. Runs pretty cool. After 30 minutes the bracket was only warm to the touch and nothing to worry about. Can’t progress further yet so I am shelving this and fixing my 465 and 475A until the resistors arrive. Got a collection going now :-DD |
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