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Tektronix 7633 Scope, Can anyone please help Old64goat!!!
finom1:
--- Quote from: james_s on May 02, 2017, 07:48:17 pm ---Does this use a voltage multiplier to generate the high voltage? I've seen shorted capacitors in those cause a lack of HV while not greatly affecting lower voltages coming off the same transformer.
--- End quote ---
Great question, could you please see his latest video and ask that same question again.
Thanks again!!!
finom1:
Question,
What type of High Voltage Probe does the old64goat need to troubleshoot his scope?
I think we need some urgent help with this one to keep him safe:)
tautech:
--- Quote from: finom1 on May 02, 2017, 09:15:54 pm ---Question,
What type of High Voltage Probe does the old64goat need to troubleshoot his scope?
I think we need some urgent help with this one to keep him safe:)
--- End quote ---
Only a DMM capable of measuring the EHT, the voltage will be listed in the SM and you want a little V headroom.
Be aware the EHT voltage is for the CRT cathode and it is negative and commonly in the 1.5 - 2.5kV range. Check the SM.
I use old AVO's for this as they have a 2.5 or 3kV range, depending on the model.
Assume the PDA is working until proven otherwise, it can be +10kV and tricky to measure without a UHV probe.
PDA multipliers do fail but it is rare, for now assume it works.
finom1:
Can someone please provide screen shots or pdf scans of what the old64goat is asking for?
Here is what he has written:
"This PDF is driving me nuts, I can't find anything on it.
David posted this answer on his latest post:
"1. If the -1475 cathode voltage is missing, then check that Q1195 is good, capacitor C1198 is good, and that unregulated +15 volts is present across C1198; these are all shown in the upper left corner of schematic 7. Fuse F814 located on the rectifier board and shown on the left side of schematic 8 supplies the unregulated +15 volts to the high voltage board but I think Old64goat checked it.
"Where is schematic 7?
I can't find schematic 8, ALL fuses test good.
As I had mentioned in the newest video I am getting all the low voltages on that connector
(disconnected) from the Z-AXIS board.
If there are other low voltage test points I can't find them.
What may help is a print out of all the schematics & voltage test points for this no HV problem.
Maybe you can post this for me.
Thanks,
Bill"
Please help provide these documents to help solve his problem if you can. He is very thankful for all the help he is getting!!!
David Hess:
Going to roughly respond to the video in order:
1. It looks like the triggering is working. The internal trigger on these oscilloscopes is pretty complicated because the mainframe selects from the left or right trigger sources and the dual trace plug-ins select from their channel 1 or channel 2 sources; that makes 4 internal trigger sources so it is easy to have it configured in a way that it seems like it is not working.
2. It looks like the power lamp is burned out. It runs on the +5 volt regulated output but since the triggering apparently works, we know the +5 volts is working so the lamp is probably just burned out.
3. The transistors on the horizontal CRT amplifier board run hot; I cannot hold my finger on them when they are operating. I verified this on my 7603 which is a very similar design. The load resistors run hot also; that is why they are so large.
3. I figured out the mystery of the CRT heater being active while the high voltage is apparently missing.
The heater for the CRT electron gun is toward the back of the CRT and not easily visible. What Old64goat saw is the flood gun heater which is used on storage CRTs to "flood" the screen with electrons as part of the storage capability. You can tell because the flood gun heater that you see is below the horizontal deflection plates and the electron CRT gun is toward the back of the CRT before any of the deflection plates. The flood gun heater runs off of the -15 volt supply through an 8 ohm as shown on schematic 10 and will be powered whether the high voltage inverter is operating or not.
4. So that means that the high voltage inverter which supplies the electron gun heater, -1475 cathode voltage, and PDA voltage is not operating. That leads back to checking Q1195, capacitor C1198, and that the unregulated +15 volt supply is present across C1198. If those are all good, then the high voltage regulation loop needs to be checked.
--- Quote from: dave_k on May 02, 2017, 07:36:20 pm ---In his first video, he says the pilot light for the main power light is not working. This might indicate a short to ground on the 5v supply rail. I had a similar fault in a 7603 scope, which was traced to a shorted cap on the main interface PCB.
--- End quote ---
The triggering works and Old64goat verified the presence of the +5 volt regulated output at the test point.
--- Quote from: james_s on May 02, 2017, 07:48:17 pm ---Does this use a voltage multiplier to generate the high voltage? I've seen shorted capacitors in those cause a lack of HV while not greatly affecting lower voltages coming off the same transformer.
--- End quote ---
The high voltage multiplier for the PDA is separate from the cathode voltage rectifier but they share a winding. It is possible that the high voltage multiplier is shorted but I am not sure what result that would produce. I suspect it would blow fuse F814. If fuse F814 is good, then we should still see *something* at the negative cathode voltage test point if the inverter is operating.
--- Quote from: finom1 on May 02, 2017, 09:15:54 pm ---Question, what type of High Voltage Probe does the old64goat need to troubleshoot his scope?
I think we need some urgent help with this one to keep him safe:)
--- End quote ---
His VTV works to 1500 volts which is good enough for checking the cathode voltage and that is all that is needed at this point. I have a Fluke 40k-6 which I use to measure cathode voltages but it is only good to 6 kilovolts. It is usually not necessary to check the very high PDA voltage; we should be able to get *something* on the CRT without dealing with the PDA.
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