tektronix D755 CRT problem?
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From: nesesu on 7 Sep 2009 15:24 On Sep 7, 7:23 am, IZ8DWF <francesco.messi...(a)gmail.com> wrote: > Hello all, > > I hope this newsgroup is still alive, I see too much spam lately :( > > I'm repairing an old telequipment/tektronix D755 scope for a friend. > So far it started with no power supply voltages, turned out to be > some cold solder joints in the power supply PCB. > Then it was badly out of focus, tracked down to open high value > resistors on the EHT PCB. After mounting back the EHT pcb it developed > a short on the +105V rail (which wasn't there before). It turned out > to be two shorted transistors on the Z amplifier PCB. > One transistor is a BC207 and the other is reported on the manual as > FRB749/SPS5286 (which is unknown in the whole internet it seems). I'm > going to substitute it with an high voltage NPN (it sits right > between the 105V supply and ground). > What seems strange to me is these two transistors dying after > repairing the EHT board. However the EHT is now fully functional and I > have a clean focused track, however with full brightness until I > substitute the two transistors in the Z board. > I also set the -2500V cathode bias correctly in the mean time. > Now I have a full scan track on the CRT, time base seems to work > correctly and also track vertical position works, but I see a very > strange effect on the track: > with vertical inputs grounded I can move the horizontal sweep from the > bottom to the top of the CRT, however the track is perfectly > horizontal only on the half part of the CRT, when moving up it > develops an arc of circle near the right end of the sweep, the circle > become larger the more it goes up until it reach about the anode > connection where the track shows a couple of sinusoidal periods; going > further up the circular arc changes direction (track goes down instead > of up vertically) and behaves in a specular manner. > Now I don't know if there's something that needs tuning in the X or Y > drivers, > unfortunately this scope requires specialized calibration plugins to > go through all calibration steps, so I won't be able to follow the > exact calibration procedure. > However it seems to me that this kind of fault could be due to a bad > CRT, I'm not sure the Y voltage on the plaques should be variable > during a bare horizontal sweep, but also I'm not a great expert of > scopes. > Any hint is welcome. > > Best regards > Francesco IZ8DWF On the 'no signal' trace distortion, check the position of the trace vertical position pot for centre of rotation and see if the trace is in approximately the centre of the screen [vertically]. It is possible that the DC balance of the vertical amplifier is way off centre due to misadjustment or a component failure in the vertical amplifier or failure of one of the V+ or V- supply lines into the amp. I have seen what I think you are describing if the trace is shifted off screen due to a large DC component on the measured signal and trying to bring it on screen with the vertical position control. The Telequipment 'scopes of that period [I have a D83] have very poor resistors. I eventually changed out all resistors in the power supply and several in the trigger circuits. The high value ones [10meg] in the feedback divider mostly went open circuit causing the EHT to skyrocket, and the metal film ones in the trigger circuits became very temperature sensitive and would drift like crazy. Otherwise a nice 60MHz 'scope. Still going strong. Neil S.
From: frank on 7 Sep 2009 15:25 Michael A. Terrell <mike.terrell(a)earthlink.net> wrote: > > That's because you're posting through Google, where the spam comes > from. Get a real NNTP news serve and you will see very little. ok, I did it and you're right (fortunately the old "tin" newsreader still exists). However is a bit a p.i.t.a. to remember (or find out) what news server to use whenever I change ISP (I travel lot). Best regards and sorry for the OT. Frank IZ8DWF
From: frank on 7 Sep 2009 15:30 N_Cook <diverse(a)tcp.co.uk> wrote: > > Telequipment was taken over by Tektronix about that time, > maybe this repair tip is useful Well, yes, it at least gives me an idea on the SPS5286 substitution. I'm still very curious to understand if the fault I'm observing can be caused by an electronic problem or if it's definitely a bad CRT, that changes very much the time I'm going to spend fixing this scope. Thanks and best regards. Frank IZ8DWF
From: IZ8DWF on 7 Sep 2009 10:23 Hello all, I hope this newsgroup is still alive, I see too much spam lately :( I'm repairing an old telequipment/tektronix D755 scope for a friend. So far it started with no power supply voltages, turned out to be some cold solder joints in the power supply PCB. Then it was badly out of focus, tracked down to open high value resistors on the EHT PCB. After mounting back the EHT pcb it developed a short on the +105V rail (which wasn't there before). It turned out to be two shorted transistors on the Z amplifier PCB. One transistor is a BC207 and the other is reported on the manual as FRB749/SPS5286 (which is unknown in the whole internet it seems). I'm going to substitute it with an high voltage NPN (it sits right between the 105V supply and ground). What seems strange to me is these two transistors dying after repairing the EHT board. However the EHT is now fully functional and I have a clean focused track, however with full brightness until I substitute the two transistors in the Z board. I also set the -2500V cathode bias correctly in the mean time. Now I have a full scan track on the CRT, time base seems to work correctly and also track vertical position works, but I see a very strange effect on the track: with vertical inputs grounded I can move the horizontal sweep from the bottom to the top of the CRT, however the track is perfectly horizontal only on the half part of the CRT, when moving up it develops an arc of circle near the right end of the sweep, the circle become larger the more it goes up until it reach about the anode connection where the track shows a couple of sinusoidal periods; going further up the circular arc changes direction (track goes down instead of up vertically) and behaves in a specular manner. Now I don't know if there's something that needs tuning in the X or Y drivers, unfortunately this scope requires specialized calibration plugins to go through all calibration steps, so I won't be able to follow the exact calibration procedure. However it seems to me that this kind of fault could be due to a bad CRT, I'm not sure the Y voltage on the plaques should be variable during a bare horizontal sweep, but also I'm not a great expert of scopes. Any hint is welcome. Best regards Francesco IZ8DWF
From: N_Cook on 7 Sep 2009 12:53 IZ8DWF <francesco.messineo(a)gmail.com> wrote in message news:09a6b09e-4b5b-4715-a795-34c5e0863fe9(a)w10g2000yqf.googlegroups.com... > Hello all, > > I hope this newsgroup is still alive, I see too much spam lately :( > > I'm repairing an old telequipment/tektronix D755 scope for a friend. > So far it started with no power supply voltages, turned out to be > some cold solder joints in the power supply PCB. > Then it was badly out of focus, tracked down to open high value > resistors on the EHT PCB. After mounting back the EHT pcb it developed > a short on the +105V rail (which wasn't there before). It turned out > to be two shorted transistors on the Z amplifier PCB. > One transistor is a BC207 and the other is reported on the manual as > FRB749/SPS5286 (which is unknown in the whole internet it seems). I'm > going to substitute it with an high voltage NPN (it sits right > between the 105V supply and ground). > What seems strange to me is these two transistors dying after > repairing the EHT board. However the EHT is now fully functional and I > have a clean focused track, however with full brightness until I > substitute the two transistors in the Z board. > I also set the -2500V cathode bias correctly in the mean time. > Now I have a full scan track on the CRT, time base seems to work > correctly and also track vertical position works, but I see a very > strange effect on the track: > with vertical inputs grounded I can move the horizontal sweep from the > bottom to the top of the CRT, however the track is perfectly > horizontal only on the half part of the CRT, when moving up it > develops an arc of circle near the right end of the sweep, the circle > become larger the more it goes up until it reach about the anode > connection where the track shows a couple of sinusoidal periods; going > further up the circular arc changes direction (track goes down instead > of up vertically) and behaves in a specular manner. > Now I don't know if there's something that needs tuning in the X or Y > drivers, > unfortunately this scope requires specialized calibration plugins to > go through all calibration steps, so I won't be able to follow the > exact calibration procedure. > However it seems to me that this kind of fault could be due to a bad > CRT, I'm not sure the Y voltage on the plaques should be variable > during a bare horizontal sweep, but also I'm not a great expert of > scopes. > Any hint is welcome. > > Best regards > Francesco IZ8DWF Telequipment was taken over by Tektronix about that time, maybe this repair tip is useful Telequipment D63 scope. 3 faults 1 High voltage discharge crackling noise. Even in the dark it was not possible to see where the discharge was so probably in the step-up transformer or the quintupler. The display was too bright and post deflection accelerator voltage was too high so much reduced trace size in X and Y sense.The CRT 2cnd electrode should have been -1400V but was -2200V so EHT probably 19KV rather than 12KV. The EHT oscillator transistor had been replaced by a BD131. Changing the biasing pot to the preliminary drive transistor made no effect. The primary of the oscillator o/p transformer was 56V pk-pk not the correct 38V. Although the DC voltage varied from 60V to 100V altering this pot. One of the DC divider chain of 3x 5.1M was o/c so no -ve feedback component to the oscillator. 2 Traces in X sense squashed up to left of screen. One of the SPS5286 final X plate drivers was o/c replaced with BF338. 3 The third trace (Right hand PI) was static bright line with no Y displacement. A trace to 330 ohm,1.5W dropper from 105V to one of the Y2 driver transistors was loose and touching another trace. With this 330 removed and no 105V then no 3rd trace, only appearing on beam finding activation. -- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on http://home.graffiti.net/diverse:graffiti.net/
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