Damaged (?) HP 1741A (Here's looking at tekfan)

[MrAureliusR]

Gotta love it when parts show up and they're not the ones you ordered or turn out to be fakes/cheapies... 

Please do post a full documentation of the repair!! Quite excited to see what happens, and if you get it working!

[The_PCB_Guy]

Good evening folks--

The capacitor arrived today, this one the correct size. I soldered it in and finished up with the trace repair and popped the board back in its slot. I went to put the knobs back on and realized I had moved the contacts during disassembly. 

Before I can finish this repair, I'll need to know one thing: For the time/div knob (both inner and outer), what are the extremes? In other words, if both knobs are fully clockwise, what position should they be on? This should help me reinstall the rod properly, as I can adjust the inner contacts to their clockwise-most position, then position the knobs where they should be. I would assume the "delay" (outer) ring should be "off" in its fully-clockwise position, and main (inner knob) should be at 0.5uS?

I can't help but feel I should know this, but it's been a while 

Thanks folks!
Matt

EDIT: I figured it out, no need to reply to this post. 

[The_PCB_Guy]

Alright folks, as promised--A brief writeup of the repair process! 

Okay, so let's start from the beginning. A few weeks ago I purchased a HP 1741A digital storage oscilloscope on ebay for about $50 (came to $76 including shipping). My purchase was, of course, inspired by Dave's video about finding a perfectly decent scope on ebay for $50 or less. The 1741A is a 100MHz, dual channel scope, so I figured I'd jump on the opportunity to get a "new" scope (I donated my old one to a school, and have regretted it since  ).

So about a week later I received the package. The box was quite large, and the scope was well-packed. I took it out and plugged it in eagerly. Unfortunately all that turned up on the screen was a large green splotch (as shown earlier in this thread).



No amount of fiddling would change the screen at all. I eventually decided to do what I should have done in the first place, and followed Dave's advice--"Don't turn it on, take it apart!".

I took off the cover and caught a strong whiff of burnt electronics. A brief scan of the timebase board revealed this:



An electrolytic capacitor had blown out one end and severely corroded the surrounding board and traces. I contacted the seller and let him know that the scope did not work, despite what was claimed in the ebay ad. He was very kind and agreed to refund me the cost of the repairs.

I'm not sure how many photos I can add per post, so I'll stop here and continue in another one.

Matt

[The_PCB_Guy]

The first part of the repair process was to remove the damaged capacitor and clean up the board. I desoldered the one remaining capacitor lead and removed it from the board. I then used an exacto knife to carefully scrape off the corrosion so I could get a better view of the traces. As expected they were practically non-existent, so I also made a cut through each trace just outside the corroded area. This allowed me to peel off the damaged traces and have a clean, solid trace to solder to. Once cleaned, the board looked like this:



The board is still a bit discolored, but all of the corrosion was removed and was good to go. You'll notice in the photo that I also scraped off about 2-3mm of the solder mask from the end of each trace. This would allow me to solder to it more easily.

The next step was to acquire some copper foil tape. Ideally one should use heat-activated tape, as it works best for soldering applications, but the merchant I bought some from shipped it way too slowly. At the time of this writing, I still have not received it, and it's been about 3 weeks. I ended up buying some tape intended for trimming stained glass pieces. Not ideal, but it will work.



I cut out a piece just a little bit longer than the gap between the ends of the traces, where it would eventually be soldered. It's very important to cut it a little bit too long, otherwise you will not get a good connection later on when you solder it in.



And after some trimming, it looks about ready to be stuck down:



It's usually a good idea to tin the ends of the traces that you'll be soldering the tape to, just to make sure you have a good connection.



After the ends are soldered in, I like to put a very thin layer of solder over the entire trace, just to ensure there's a solid connection all the way across. The process is repeated for the second long trace that was damaged.

Here they both are--stuck down on the board and soldered:



In my next post, I'll show how to handle a trace that terminates in a solder pad.

[The_PCB_Guy]

Before I continue, I'd just like to apologize for the bad photography. My camera is about 10 years old, and the camera I bought to replace it was even worse. 

Okay, so in some of the previous photos you'll see the short trace coming out from the contact traces. Just beyond that, you'll see a hole in the board. This is where one lead of the capacitor should be, but there's nothing to solder it to (except for a tiny pad on the underside). In this type of setup it's often best to solder both sides, once again to ensure a good connection. Repairing this trace and providing a surface to solder the capacitor to is very simple. First things first, cut the copper tape to size. I also trimmed the corners to round out the end over the hole. This ensures that solder won't "leak" over the edges as much and accidentally make contact with another trace.



(Don't worry, I got rid of the scrap on the contacts before finishing it up)

After the tape was stuck down, I used the tip of the exacto knife to cut a tiny hole in it so the capacitor lead can fit through.



And we're ready for the capacitor!
It was very difficult to read the label on the blown capacitor, as it was burnt and discolored. Thankfully the service manual (http://cp.literature.agilent.com/litweb/pdf/01741-90911.pdf) is in public domain and I was able to find it easily with a quick google search. I looked at the schematic and found that the capacitor I needed was a 100uF one, but there was no mention of the voltage. The capacitors around the damaged one were rated for 35v, but from what I COULD read on the label of the blown one, it looked like it said 50v. I decided to buy a 63v one so I'd be covered either way.

I forgot to snap a photo of the cap before installation, but I'm sure you all know what a capacitor looks like. I soldered it to the board and finished the tin coating of the short trace. The cap is in!





In the second photo, please note the 1-2mm space between the capacitor and the board. This allows for air circulation and better cooling. If the capacitor gets too warm, the electrolyte will dry up and it will fail, possibly in the same way as the old one did.

That's pretty much the end of the repair process. Time to put it all back together....

After trimming the capacitor leads and reinstalling the contact hardware (front and back):




I had a spot of bother putting the knob back in, as mentioned a few posts ago, but I was eventually able to figure it out. I reinstalled the board to its socket on the main PCB and tightened the set-screws to make sure the knob doesn't slide out.




And we're set! Moment of truth.....

et, voila!



Beautiful! Needs a bit of tweaking here and there, but it all seems to be working as expected!

So that's it, a brief explanation of how to repair corroded traces on a PCB, as well as the replacement of a blown capacitor 

I wish I was a better writer and that I could have made this more interesting, but hopefully all the necessary information was provided and that someone else can learn from my experience

If anyone's got questions or comments, I'm happy to answer them!
Thanks for the interest!

Best wishes and kind regards,
Matt

[robrenz]

Very nice job ! but resizing your pictures to 1024 x 768 or smaller would be much nicer to view and you could put more pictures per post that way.

[The_PCB_Guy]

Very nice job ! but resizing your pictures to 1024 x 768 or smaller would be much nicer to view and you could put more pictures per post that way.

That probably would have been a good idea. I'll see about resizing and re-uploading them.

Thanks,
Matt

[edpalmer42]

Congratulations!

Enjoy your new toy.   

Ed

[The_PCB_Guy]

Congratulations!

Enjoy your new toy.   

Ed

Thank you kindly, Ed. Will do!