Wavetek 141 generator explosion - Page 1
Forum section: electronics / testgear

author: joshhunsaker
author_rank: Contributor
timestamp: July 14, 2014, 11:44:35 pm
content: I'm hoping someone can explain a little bit of this to me, I'm not super familiar with these types of function generators. I picked this up on ebay recently, just got it in today... hooked it up and it appeared to be operating fine. Left it plugged in for about 3 hours being scoped by my Rigol DS1102E to verify that it was outputting ballpark frequencies and voltage levels.Randomly, while it was sitting there just apparently doing quite peachy when two capacitors exploded:I know as a general rule that I should probably go through and replace all these guys but I was wondering if the voltage on capacitors derates as a result of them drying out? I'm curious as to what might have caused two different capacitors to have bit the dust at the exact same time after a number of hours of working relatively nominally with no really crazy stuff happening on the scope.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 15, 2014, 12:33:04 am
content: Yes, they probably needed to be reformed. Which means, taking the voltage up slowly and monitoring the temperature of everything. Heat is what makes them explode, and reforming generates heat. Even without a variac, you can do a really shitty job by turning it on for short periods -- this doesn't prevent gas buildup (another possible failure mode), but keeps the average temperature down.Needless to say, you have to replace them now!Tim

author: edavid
author_rank: Super Contributor
timestamp: July 15, 2014, 04:15:48 am
content: I doubt that the capacitors in that unit are old enough to need reforming. I would (cautiously) check the power supply rails those caps were connected to, to see if the voltages are too high.

author: joshhunsaker
author_rank: Contributor
timestamp: July 15, 2014, 08:39:09 pm
content: Quote from: T3sl4co1l on July 15, 2014, 12:33:04 amAh, right. Good excuse to pick up a variac finally. This is really my first time getting into testing and potentially repairing gear this old so I figured there may be some serious caveats. Good call.Quote from: edavid on July 15, 2014, 04:15:48 amAlso good suggestion, although wouldn't I potentially risk getting nailed by another piece of exploding capacitor if I hook it back up and start checking voltages?? I mean, I've got some extra plexiglass I can use to protect my face but I'm not sure what the best course of action typically is in this scenario.

author: edavid
author_rank: Super Contributor
timestamp: July 15, 2014, 09:15:12 pm
content: Quote from: joshhunsaker on July 15, 2014, 08:39:09 pmThey probably would have blown up already, but wear safety glasses if you are worried, or hook up long test leads.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 16, 2014, 12:13:03 am
content: Yeah, probably shouldn't need too much... that transistor I guess is 1986 dated, but the design dates back to the 60s. Was it replaced? Was it original? I'd have to see more inside. Those style caps are sure ancient, I haven't seen anything made since the 60s or so that's got molded black plastic caps like that. It's possible they were still available much later and Wavetek simply never updated their design to buy anything newer. So, I don't know. Can't go wrong replacing them thoughTim

author: joshhunsaker
author_rank: Contributor
timestamp: July 16, 2014, 06:14:02 am
content: Quote from: T3sl4co1l on July 16, 2014, 12:13:03 amI don't know much about Wavetek, but the distortion characteristics on the generator were very good, it can go down to 0.05 hz and has a sync input to allow sweeps. The full schematic with even manufacturer part numbers was also online...!https://archive.org/details/bitsavers_wavetek141_5115460So I figured if something went wrong I'd at least have a ton of help to try a repair. I'll take the individual boards out and try to get some pictures up of it. Looks like a really nice piece of kit though from the layout.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 16, 2014, 10:08:23 pm
content: Yeah, I perused the manual, looks like a great example of '60s state-of-the-art high technology: discrete amplifiers everywhere, 100% silicon, hardly an IC in sight (and those mostly just where needed: now-obscure parts like matched diode arrays and the like).My Wavetek 193 is of similar development, but to implement the pulse logic and frequency response (20MHz), they used a mess of 10k series ECL! Lots of op-amps, but still no shortage of discretes.Tim