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HP 1741A oscilloscope - restoration project
tekfan:
So first thing I would do wether the scope works or not is to measure the power supply voltages. If one of them is missing or low it may be causing problems in the storage system. Both my HP scopes had problems with the power supply.
So take off the covers and start probing. The mains cabling is shielded away under a small plastic cover (near the voltage selector) so the risk of shock is very small. The HV power supply for the CRT is also under an aluminium cover. The test points for the supply voltages are clearly labeled on the power supply board. Check +15, -15, +5, +48, +120, +156 and -100 volt supplies with a multimeter (the case of the scope is ground). If they are there and stable then move onward.
I strongly suggest reading the Theory of operation section of the manual. Try to follow the circuit diagram. If you look closely there are also important voltages written on some circuit diagrams.
I wouldn't say the tube is dead just yet. If it behaves this strangely even in normal mode it's probably another fault.
Besides HP never made a scope with the same CRT as the 1741 so replacement can only be done with the same tube.
mayeri:
Hi TekFan,
You were right.
The supply voltages are not ok. It seems at first glance that may be some problems with the power supply.
The burned diode is almost certainly failed, the resistance may be not.
Now I have to check in quickly in Theory of Operation section of the manual relevant parts of the schematic of the power supply.
tekfan:
Great work so far
The resistor and diode are used as a primitive voltage regulator. That's actually a 100V zener diode in series with the 3.3k resistor. The -100V supply is used exclusively by the storage circuit so that may very well be the problem. The diode would probably have a too high voltage drop to be tested by a multimeter so the only hope is actual in circuit operation. The orange Sprague 10uF / 200V capacitor looks like it overheated from the nearby resistor. It will probably work if you leave still leave it in the circuit. They are very tough capacitors, but be sure to replace it in the long term.
Try to unplug the small board that interconnects all the boards in the scope. That way you can unload the power supply and see if the supply voltages rise to their nominal values. That way you can determine if there is a problem somewhere else in the scope. You can also slightly (or rather considerably) bend the boards out of the way so that you can connect the power supply to only one board at a time. That way you can isolate the problem at least down to board level.
Be sure to check the +48, +15, -15 and +5 volt supples. It's also used by the storage board. The unregulated +15V supply looks fine. It can actually be as high as 25V. The high voltage inverter for the CRT uses this supply. The other voltages also seem to be within spec.
I might add that both of my scopes had several faulty bridge rectifiers. When the scope is turned on you could hear the transformer humming loudly since the rectifiers had shorted. I also had problems with several shorted series pass transistors mounted on the back heatsink. Worth checking those with the diode test function. They are on connectors so no desoldering is needed.
Just keep measuring the thing. You'll eventually find what's wrong. :-/O
mazurov:
Tekfan: How do you clean the controls? I have a Tek 7934 which works very well but some knobs and buttons are stiff and/or "sticky".
mayeri:
Hi TekFan,
I replaced the diode, the resistor ok, 3,3k. Really, this resistor and diode very primitive voltage regulator solution. There was not 100v Zener at home, so I soldered together three diodes temporary (56 v + 33 v + 12 v). All three diodes are heating little bit, so it may need to add a series-pass transistor. Anyway, tomorrow I will buy a 100v zener.
I checked all the voltage again. It seems everything is ok (48, +15, +15, +5). I think so that each graetz works properly.
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