HP 1745A scope busted

[aaronh]

All,

 I recently aquired a HP 1745A scope. It was working when I got it (at least to some degree) but now it will not power up. I'm a hack when it comes to these things (which is two notches below an amateur), so please bear with me on this. When I got it the unit fired up, two traces appeared, and for the most part it seemed that the major functions at least did something when I monkeyed with them.  I was able to display both traces using the calibration signal. Then I connected microphones to the two channels and was showing my kids what sound looks like in wave form. It's been a long time since I've had my hands on a scope, so I was learning all over again as I went. At any rate, last night I got brave and dug out an old RCA WR-50B RF singal generator I have had kicking around. It took some time to get a decent signal. I don't know that the signal generator is in the best of shape. At any rate, I left the two connected and turned on. I went to do some other things and forgot about it. This morning I noticed that the signal generator was still on, but the scope was not. It seems something happened to the scope where it won't even power up. The power led doesn't illuminate. The main fuse checks good. I pulled the top cover off and didn't find anything obvious. The fuse inside on the high voltage side also checks good. I have not pulled the bottom cover off to see what I find there. I am hoping someone will have some ideas where to look first. I appreciate any help you guys can offer.

Thanks,
Aaron

[tekfan]

Hi!
Welcome to the forum. I hope you'll find plenty of useful info here.

Remove the bottom and first look at the power supply board. You can start by just conneciting a multimeter to the plug and measure the resistance between line and neutral (the scope disconnected from the mains of course). It should be infinity when the scope is on and then drop to a few 10-100 ohms when you press the power switch. If you don't get a reading (always shows infinity) again check the fuse for continuity (it's best to measure right at the fuse holder terminals), then check if the power switch actually shorts its contacts or are they open even if the switch is pressed in the ON position, lastly check the transformer primary winding for continuity.

If so far everything checks out look at the bridge rectifiers. There are about 5 on the power supply board. Usually they short and thus blow the fuse but could also go open. If you have a multimeter with a diode check you can also measure if the diodes are actually conducting. Not just check if they are shorted or not. Most HP scopes of the same era have problems with these rectifiers. Also check for discoloration around them and burn marks.

Also check this thread:
https://www.eevblog.com/forum/testgear/hp-1741a-oscilloscope-restoration-project/

Your 1745a uses nearly the same boards as the 1740a and 1741a. Only the CRT is bigger.

 

[tautech]

The Tek pdf in this post was made for you.
https://www.eevblog.com/forum/repair/repair-documents-and-links-sticky-me-please-mods/

[aaronh]

Thanks for the help guys. I took the bottom cover off this evening. I found an obviously blown cap. One of the rectifiers has a small amount of discoloration near it. Not sure if it's toast or not. Unfortunately I don't have a meter with diode check. See pictures.

I also found a couple of other blue/grey do-dads which I am guessing are zener diodes? They have some black crud on their legs. The crud easily scrapes off. Not sure what that stuff is or what it means. In one picture the item in question has the Motorola M, a black band, and 02 printed on it. It's next to a couple resistors.

Here is what I could find out in the few minutes I was able to spare to look at it today. The fuse is good, I'm quite certain of that. The resistance on the each of the mains to ground is infinity regardless of power switch on or off. According to my quick check for continuity the power switch seems to be working.

Unfortunately I have to leave town unexpectedly, so it will be until the weekend before I can dig further.

I did manage to download a pdf of the operators manual as well as the service manual. Yay internet!

[aaronh]

Two more pics.

[aaronh]

Ok, I finally found some time to get back to this. I discovered that the transformer connector must have got very hot. It managed to desolder two pins and almost melt itself together. Things are not looking good.

[tautech]

Look for bulged E-caps and Fried Tantalums. The tants may not show any signs of shorting but if on the PCB, they are the first things I check. Check the manual cap parts list for tantalums, then verify each one on the PCB.
Look for any unusual signs of heat on the PCB for clues as to excessive current.

[DmitryL]

Ok, I finally found some time to get back to this. I discovered that the transformer connector must have got very hot. It managed to desolder two pins and almost melt itself together. Things are not looking good.

Take a _very_ close look at the connectors that hold boards together. I repaired a similar scope and a huge amount of connectors had circular cracks on solder joints, I had to re-solder a lot of them.
Cracks in solder joints -> bad contact-> heating (where high currents flow).
Also, check bridge rectifiers in PSU, they may be dead or half-dead.  Check PSU voltages and ripple, you should be able to find this information in service manual