HP 1740A Oscilloscope Repair (-15V Problem)

[rubiconjosh]

Hello,

  I recently obtained a non functioning HP 1740A that looks to be in great physical condition. I picked up the scope with two probes, two operators manuals and the service manual for $40. A PDF of the service manual can be found online: http://www.ko4bb.com/manuals/index.php?dir=HP_Agilent/HP_1740_Oscilloscope







   I purchased the Oscilloscope with the understanding that it did not work so the first thing I did was take the top and bottom covers off and do a visual inspection. The inside looks great, I do not see any evidence that it has been worked on.

  I checked the low voltage power supply voltages:

Voltage	Measured Voltage	Test Points
15 VDC	14.991 VDC	TP4 - TP3
43 VDC	42.97 VDC	TP5 - TP3
5 VDC	4.986 VDC	TP2 - TP3
-15 VDC	1.079 VDC	TP1 - TP3
120 VDC	121.29 VDC	TP6 - TP3

There is a problem with the -15 VDC. The technical manual indicated the DC voltage across C14 should be 20.6V. I measured 0.9387 VDC.



Next I unloaded the power supply by removing the Interface Assembly A14. My -15 VDC rail then measured 1.423 VDC.

I then disconnected P1 which connects the transformer to the power supply. I also disconnected Q6. I performed a diode check on the bridge rectifier. The following crudely marked up schematic shows what I believe are bad readings.



Here is a table of all of the checks I did on the bridge rectifier.

Forward Checks

+ Lead	- Lead	Reading
TP1	P1-5	0.516 VDC
TP1	P1-6	OL
P1-5	C14(+ side)	0.523 VDC
P1-6	C14(+ side)	OL

Reverse Checks all measure OL.

The plastic connector that connects the transformer to the power supply has some discoloration. This connector is physically near the bridge rectifiers. Does this discoloration look like evidence of a past component failure?





I believe I have found the problem, CR6 is bad. I do have a soldering iron, but I am new to repair work.
Have I full isolated the problem to CR6?

Thank you for your time.

[SeanB]

Dave did have a similar fault where the bridge rectifier was faulty, but also the connector and bridge had poor solder joints.  You probably will want to replace the bridge rectifier and all the other ones of the same type with ones with a higher current rating, so that they will run cooler. The discoloration on the connector is from the bridge rectifier running both, so using one which runs cooler ( higher current rating that fits the footprint) and resoldering the pins on the connector base ( you can see at least one is loose as it is crooked) will work. Just be careful, those traces will be brittle with age and might lift if you apply excess heat or force.

[rubiconjosh]

Thank you. I watched both of the 1740A Repair Videos.

I removed the low voltage power supply board. It appears that someone has worked on this board before. The solder on the backside of CR4 (bridge rectifier for the +15V) looks different from the rest of the board. I am new to this, but it looks like it was replaced and the pads were lifted.



Higher resolution of the above picture:

All of the soldering joints for the board interconnects have flux on them, I am assuming this is from the factory since it is consistent throughout the whole unit.


Tonight I will research appropriately higher current rated replacement bridge rectifiers and order them. Comparing photos on Digikey I think it is a WOG or WOM package device. At work I have access to a pre heater and desoldering station. I will bring the board to work on Monday to remove the bridge rectifiers and start cleaning the board.

[tggzzz]

Is it worth trying to assess what caused the rectifier to fail? It could be an internal fault, of course - but could an external component have caused excess current which then damaged the rectifier?

[rubiconjosh]

I had trouble finding the specifications of the bridge rectifier. I believe it is a 1.5 Amp 400V rectifier. Following SeanB's advice I ordered a higher current rated rectifier, 2 Amp 400V: https://www.digikey.com/product-detail/en/2W04G-E4%2F51/2W04G-E4%2F51GI-ND/604798. I ordered five of them so I can replace all of them. The tracking information shows they should arrive this Friday.

I took the board to work and removed the failed rectifier.


I used the vacuum extractor for the first time and I think I put a little too much heat on the board. I did not fully lift a pad, but one of them is starting to come up.


I had my iron set for 660 degrees. I am confident I will do better removing the rest of the rectifiers. Here is the setup I used.


Following tggzzz's advice I decided to see if I can determine if something caused the rectifier to fail. The first thing I did was measure the AC from the transformer going to the rectifier. I measured it with my Extech 430 at 18.21 VAC.

I then decided to use my Leader LPS-152 power supply to supply the 20.6 VDC that the failed rectifier should output. First I disconnected the power supply from the rest of the oscilloscope and set my current limit to around 200 mA. I measured -14.977 VDC on the -15 VDC rail and my power supply showed less than 40 mA of current.

I then reconnected everything in the Oscilloscope and powered it on, again supplying 20.6 VDC from my bench supply. I measured -14.969 VDC on the -15 VDC rail and my power supply showed about 330 mA of current.

Best of all, the display came on!


I do not know of any other test I can do to make sure nothing else could be causing the rectifier to fail. My plan is to clean up the power supply board and replace all of the rectifiers when my parts get here on Friday then test the full functionality of the Oscilloscope this weekend.

Thank you.

[tautech]

All of the soldering joints for the board interconnects have flux on them, I am assuming this is from the factory since it is consistent throughout the whole unit.

Not necessarily, the interconnects do have some stress on them and MORE when they are removed/reconnected. It would be quite normal for a technician to rework the solder joints if they knew anything about these 1740's.
Even a competent hobbyist with some experience with interconnects might examine the joints carefully and find they need rework. Very common with these HP's.

[tautech]

I then decided to use my Leader LPS-152 power supply to supply the 20.6 VDC that the failed rectifier should output. First I disconnected the power supply from the rest of the oscilloscope and set my current limit to around 200 mA. I measured -14.977 VDC on the -15 VDC rail and my power supply showed less than 40 mA of current.

I then reconnected everything in the Oscilloscope and powered it on, again supplying 20.6 VDC from my bench supply. I measured -14.969 VDC on the -15 VDC rail and my power supply showed about 330 mA of current.

Sounds like you are good to go. 
Easy fix, well done.