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Tektronix 7633 Scope, Can anyone please help Old64goat!!!
finom1:
Hello,
Old64goat just sent this message:
"As I said I can not find Q1195.
I can not find C1198.
The way that PDF is set uo I have to scroll through all 320 pages to try to find anything, I hate it."
Does anyone know what page Q115 & C118 are located on?
Thank you for helping!!!
finom1:
I just got another update from Old4goat:
"This is getting OLD...LOL
I did find Q1185 & C1198 on the HV schematic which I did PRINT OUT, but when I look at the HV board "picture" they are NOT shown.
I spent two hours looking at the pictorials & schematics and printed them all out.
If Q1195 & C1198 are INSIDE the HV box they must be hidden and are not serviceable.
Another thing is how the heck am I going to see if I got +15 volts across C1198 if it is INSIDE the HV box?"
Can someone please advice on next steps to take?
Thank you for helping!!!
tautech:
--- Quote from: finom1 on May 03, 2017, 06:03:24 pm ---I just got another update from Old4goat:
"This is getting OLD...LOL
I did find Q1185 & C1198 on the HV schematic which I did PRINT OUT, but when I look at the HV board "picture" they are NOT shown.
I spent two hours looking at the pictorials & schematics and printed them all out.
If Q1195 & C1198 are INSIDE the HV box they must be hidden and are not serviceable.
Another thing is how the heck am I going to see if I got +15 volts across C1198 if it is INSIDE the HV box?"
Can someone please advice on next steps to take?
Thank you for helping!!!
--- End quote ---
This is getting to be hard work with Old4goat. I suspect the transistor he seeks is the main switcher for the HV SMPS, they are normally a TO-220 or similar and bolted to the frame or some heatsink. When this is the case it is often identified in the schematic with a dotted line indicating the device is NOT on the PCB.
Service manuals are written for technicians and without knowledge of the conventions used some considerable time studying and comparing with the physical layout MUST be invested for it all to make sense.
There are NO shortcuts, he must spend the time to understand the schematics, principles of operation, physical layout and reasons for it all.
All we that are self taught have had to follow this path and it need be seen as an investment of time for your future.
Good luck.
David Hess:
--- Quote from: finom1 on May 03, 2017, 05:48:39 pm ---Hello,
Old64goat just sent this message:
"As I said I can not find Q1195.
I can not find C1198.
The way that PDF is set uo I have to scroll through all 320 pages to try to find anything, I hate it."
Does anyone know what page Q115 & C118 are located on?
--- End quote ---
All of the high voltage inverter circuit including C1198 which decouples the unregulated +15 volt supply to the high voltage inverter and Q1195 which is the TO-3 power transistor for the inverter is shown in the upper left corner of schematic 7 which is PDF page 296.
The fuse for the unregulated +15 volt supply going to the high voltage inverter is shown on the left side of schematic 8 which is PDF page 300.
The 7633 is one of the increasingly few Tektronix oscilloscopes which I have *not* made a compilation of schematics for because the existing freely available service manual scan is pretty good. I have actually done about half of the work to make a set of dedicated schematics for the 7633 so maybe I should complete that. Do not count on anything quickly though as this takes time and the scans would not really look any better than the existing service manual.
Q1195 itself is a TO-3 packaged transistor and is exposed where it is mounted to the aluminum cover over the high voltage board. Look under the high voltage test point toward the back of the oscilloscope. (1) The case is the collector so it can be probed with a voltmeter to see if the +15 volt unregulated supply to the inverter is present. Be careful of course not to short it out to the chassis because this will blow fuse F814. When the inverter is operating, the exposed collector of Q1195 is where the primary side oscillating waveform of roughly 40kHz can be measured.
Usually the failure in this circuit is Q1195 drawing too much current because of some other failure which blows fuse F814. For this reason if the fuse is blown or if Q1195 is bad, the cover should be removed to gain access to the high voltage circuit board and the parts in the high voltage inverter should be tested. I am not sure how to get the cover off.
Sometimes Q1195 is blown and if that is the case, then it can be replaced with a modern On Semiconductor 2N3771G or 2N3772G. The original 2N3055 is a 0.3 MHz part so modern 2N3055s with their 2.0MHz and higher Ft tend to spuriously oscillate but an old 2N3055 could work.
For testing purposes, I would remove fuse F814 and use a current limited power supply to power the high voltage inverter through the fuse connection. Alternatively an LM317 configured as a current source with a single 2 watt 1.0 ohm resistor could replace the fuse temporarily.
(1) PDF pages 316 and 318 show the location of Q1195 as part number 110 toward the center top of the drawings but I agree with Old4goat, these drawings are difficult to read.
David Hess:
--- Quote from: tautech on May 03, 2017, 06:23:00 pm ---This is getting to be hard work with Old4goat. I suspect the transistor he seeks is the main switcher for the HV SMPS, they are normally a TO-220 or similar and bolted to the frame or some heatsink. When this is the case it is often identified in the schematic with a dotted line indicating the device is NOT on the PCB.
--- End quote ---
In this case it is an old slow 0.3MHz 2N3055 operating in a power blocking oscillator. The case (collector) is exposed and visible below the high voltage test point. Earlier oscilloscopes in this series used a push-pull configuration with two 2N3055s and I do not know why Tektronix changed it, maybe the single transistor version was less expensive?
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