Electronics > Repair

HP 1727A oscilloscope with multiple issues.

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OK, so it seems that the gate output is intermittent, most of the time when I was testing it today it was just sitting at 4V on the scope. The two input signals comp gate and delayed gate are fine, so its being lost somewhere on A13. I am not very good at visualizing circuits so I don't know how the signal path works, and it is very hard to get a scope probe onto most of A13 to trace things.


--- Quote ---I don't think there is a problem is the LVPS.
--- End quote ---
I don't think either. Perhaps I wasn't clear enough: I think the LVPS is going into protection mode after the "snap" event but the cause is somewhere. Perhaps the LVPS going in protection mode causes the "snap" you hear. Sometime capacitors especially tantalums fail intermittently and unpredictably and becoming basically a quite low resistor, like 50Ohm, which trips overcurrent protection in the power supply making it hard to detect because they don't continue cooking off and releasing smoke.

--- Quote ---One of the output transistors gets pretty hot while the other does not.
--- End quote ---
there you go... the one which is not getting hot is the suspicious one, but check all four Q13-16, the resistors and caps as well once the transistors are out of circuit.

--- Quote --- I am not very good at visualizing circuits so I don't know how the signal path works
--- End quote ---
reverse engineering is a crucial skill but before you get efficient at it you must learn forward engineering and the basic building blocks. It'll come with time and it's a muscle you have to excercise. I suggest you watch as many "back to the basics" videos (also the other are worth) as possible from w2aew and Mateo Aboy if you want to learn the basic building blocks of circuits and how they couple together. Taking a  proper course would also help  ;)

In the meantime, just ask... we can go through a section you don't understand. I can't promise accurate analysis: I'm just a hobbist but I have a degree in engineering and I learned some theory some years ago.

--- Quote ---it is very hard to get a scope probe onto most of A13 to trace things.
--- End quote ---
That can be hard. Take your time to set up probing eg. solder a lead wire to make a signal accessible. Probing and measuring is the only way to understand what's going on so don't dismiss it as a quick and easy thing to do. Also record the results, maybe take screenshots of the measurements, and take notes / record yourself (may seem silly but I do it all the time).

All the transistors I checked on A13 have been fine.  :-\ I checked Q13-16 and several others but found no bad ones. I also did a quick test with my multimeter on all the other transistors but none had less than several Kohms between their pins, so I don't think there are any more bad ones.

I would like to do some more testing today but I am quite busy, its my birthday,  :)

I am now lucky enough to own a Flir thermal camera, I have wanted one for a while as a troubleshooting tool  ^-^. Needless to say I pointed it at my scope and especially A13, but it didn't really show me much that I didn't already know. Q15 gets hot, as well as a couple of resistors next to it, but that is about it.

Something on A13 is unhappy, I just need to figure out what! |O

I have new information! I got back to probing after a short break from this scope. I found that the gate signal on TP1 is fine (although half the amplitude it says in the manual) as long as the HV oscillator transistor is NOT plugged in.  ??? When I do plug it in the gate signal stays there for about 4 seconds, and then fades down to constant low level in about 2 seconds. 

So does this mean that the problem could be on the HVPS? of course I wont discount other issues on A13, like the output amplitude.

I might see if there is a way I can measure the gate output current.


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