[bobfburns]

Hi All
I am in the process of repairing a second HP1741A scope belonging to a local ham radio colleague. The first one was my own scope which now appears to be fine. The faults include non-working beam finder and a 50Hz non-sinusoidal ripple on the displayed trace with no input signal. Further investigation showed the +5v line was +3v and U2 and CR2 on the LV Power Supply PCB were much too hot to touch. This is a coincidence because U2 had failed on my own scope some months back and had to be replaced along with several of the bridge rectifiers - in this case a past owner had run the scope on 230v mains but with the mains tap set to 120V!
Does anyone know if this U2 issue is a known failure problem please and what may have caused it?
Once I get the PSU back into operation I will look at further tests and calibration.

 

[shrtrnd]

Can't say I've worked a lot of them, but I haven't seen a problem with U2 by itself. Most likely caused by
problems with the power supply (that then affect U2).
Maybe somebody else here knows something about this that I don't(?)

 

[bobfburns]

Some more information following tests:
1. The +15v line is low at 10v due to the current limit being activated so some other lines are low as a result of using +15v as a reference.
2. CR2 bridge rectifier has been replaced with a new item fixing that overheating problem.
3. U2 (723 metal can regulator IC) overheats despite being replaced - cannot see why and the current limit is not activated. Also changed the associated power transistor with no difference.
4. Disconnecting the other PCBs by removing the link assembly and moving the LV PSU board so that the remaining PCB connectors may be disconnected brings all lines back to normal so something is dragging down the +15v line.
5. No lines show evidence of shorts.

 

[shrtrnd]

Sounds good replacing the bridge and the LM723, ... but I never replace those without replacing the connected capacitors. If your bridge went, it probably over-stressed the caps. Just replacing the semiconductors is going to
result in damaged replacement semiconductors.