Oscilliscope Solutions

[dirknerkle]

Last night I was messing around with another experimental wireless Renard-type controller and it *kind-of* worked, and then it didn't.... and then it did... and then it didn't at all and wouldn't come back to life. I thought the chips maybe lost their firmware, so I reprogrammed them.... then maybe the XBee radio pooched-out, so I reprogrammed that, but no, no blinky.

So I turned to my oscilliscope for some answers. The nice square wave I got out of pin 2 of the Xbee radio confirmed it was receiving the signal and processing it, so I knew the XBee was okay. Testing pin 5 on the PIC16F688 showed the same square wave, so I knew the data was getting to the PIC and there were no breaks in the circuit from the XBee to the PIC. Testing pin 1 of the PIC confirmed there was 5vdc there so I knew the board's power section was working; testing pin 2 showed the 18.432mhz clock signal was there, so I knew the crystal was working fine. Testing pin 4 on the PIC I found there was no ZC signal. Hmmmm....

So I took a look at the H11AA1 chip... The A/C side of the chip showed a nice 60hz wave, but nothing on the DC side at the PIC. This smacked of either a bad chip, a short or a no-connect. I pulled the H11AA1 chip and made sure there was nothing inside the socket that would cause a short, flipped the board over and didn't see anything obvious, so I replaced it with a new H11AA1. Powered it up and... no change.... still no ZC.

Hmmm... time to go looking for no-connect issues. Flipping the board over and this time with the lighted magnifier, I found that one of the leads of a resistor adjacent to the H11AA1 chip looked like it was free-floating in the solder pad. AHA! Maybe I missed that one! I reflowed some solder there, powered the board up and ZC was back on pin 4 of the PIC. Problem solved.

The point of this diatribe is that without the oscilliscope, it would have been a lot harder to trace the problem down to a specific issue because a DVM isn't generally quick or sensitive enough to catch it.

For those of you who plan to build a lot of your own gear or who want to experiment with designing your own, I heartily encourage that you consider adding an oscilliscope to your workbench. It doesn't have to be super-expensive because if all you're looking for is the existence of a signal and don't intend to measure the specific timing, most cheap 'scopes would suffice. I purchased a Rigol DS1052E for about $400 last year and while it's only a 50mhz 'scope, it does everything I need it to do and has more capability than I'll probably ever need. Others whose jobs require faster sampling speeds and more functions will likely chime in on this thread, but for the electronics hobbiest, almost any 'scope would be a terrific addition to your diagnostic tool set.

[Mactayl]

I agree an O'scope is like having an other set of eyes for seeing things that can't be seen by the human eye or a meter, I use mine during development most of the time and it is worth it's weight in Gold (Tektronix 2440).;)

[Materdaddy]

I have a non-working Tektronix 5441. Got it from a friend, and the first time I plugged it in I saw magic smoke leave the scope. If anybody can help me get it back in, I'm all ears.

I'm thinking a cap blew or something. I haven't pulled it apart to see if it's something easily replaced yet because I don't know how to use the thing, and it hasn't been a high priority.

Would either of you also recommend a logic analyzer? The other reason I haven't rushed to fix the oscope is because I'm very close to picking up this guy: http://www.saleae.com/logic/

[underdog]

I suppose you couldn't call tech support...[thick accent] "Hi Dave, my name is [inaudible], first, have you checked to make sure it is plugged in?"

[Mactayl]

I have a non-working Tektronix 5441. Got it from a friend, and the first time I plugged it in I saw magic smoke leave the scope. If anybody can help me get it back in, I'm all ears.

I'm thinking a cap blew or something. I haven't pulled it apart to see if it's something easily replaced yet because I don't know how to use the thing, and it hasn't been a high priority.

Would either of you also recommend a logic analyzer? The other reason I haven't rushed to fix the oscope is because I'm very close to picking up this guy: http://www.saleae.com/logic/

I don't know if it would be worth fixing because the repair cost are usually very expensive. I would think seriously about a logic analyzer because it connects up to your PC and you can store a lot of data on your PC, also most of the signals you will be looking at are not very fast so you really don't need a high end Logic Analyzer as for the one from saleae, I couldn't tell you if it was good or bad, someone else may want to chime in on that.

[budude]

Nothing like a scope when you need one - I have the Hantek DSO-5200 USB scope and really like it but the Rigols can be had for only about $100 more and lots of folks like them. I have an ancient Radio Shack pulse detector that can tell you if there is a pulse either <1MHz, >1MHz or solid on or off. A DIY version of this might be handy to have around for basic troubleshooting and would probably be cheap to make.

[Materdaddy]

I don't know if it would be worth fixing because the repair cost are usually very expensive. I would think seriously about a logic analyzer because it connects up to your PC and you can store a lot of data on your PC, also most of the signals you will be looking at are not very fast so you really don't need a high end Logic Analyzer as for the one from saleae, I couldn't tell you if it was good or bad, someone else may want to chime in on that.

Thanks for the reply. I was first planning on seeing if I could tell what fried and hadn't really planned on sending it off to be repaired. Especially now, hearing you say the costs are prohibitive.

I'd love it if somebody out there who has the saleae logic analyzer can chime in because that thing looks slick, and I hope to hear it functions well also.

[Greg in Canby]

Fair to say "nothing to lose"?

Crack it open and see what you can find.

I have a neighbor whos company fried 6 or 7 flat screen TVs by over volting them. They were going to chuck them but he brought 5 over to me.

The deal was fix two or more I could have one. Nothing to lose! Ended up fixing all 5, two of which need new caps, two needed various diodes and one just needed an internal fuse.

I say go for it!

Greg

[Mactayl]

Yes by all means open it up and see if there is something evident and you can fix it. Sending it out for repair is not good idea because most of the time the repair cost starts about $200.00 and it goes up from there depending on what bad parts are replaced and there is about a 300% mark on parts at a repair center and I don't even want to tell you about labor costs.

[dirknerkle]

Just be careful what you touch -- that model has a CRT and like a television, there can be some serious voltage around that thing -- upwards of 20,000 volts... just enough to knock you into the next county...