newbie question - Page 1
Forum section: electronics / beginners

author: joejoe
author_rank: Newbie
timestamp: August 28, 2017, 01:06:48 am
content: Hi folks,I am retired and just started back into electronics after 30+ years. A lot has changed.I have been doing a ton of reading, following this forum and viewing many of Dave's videos.MY question is this:I built the xr2206 function generator, and it does not work.During basic measurements, when I measure across the input filter caps(electrolytics) I get a +Vcc. However, when I measure from thepositive terminals to circuit ground, I get –Vcc. Negative voltage isapplied at some 2206 pins. Is this normal? I do remember Dave's tutorial onop-amps where the input can be a virtual ground, but not sure that applies.Appreciate any help.Joe

author: rstofer
author_rank: Super Contributor
timestamp: August 28, 2017, 02:05:35 am
content: Without seeing a schematic, it is impossible to give very precise help.This schematic is probably similar:https://www.sparkfun.com/datasheets/DevTools/functiongenschv2.jpgThe virtual ground is only used at the output to bias the signal. I wouldn't bet on this working if the incoming power is grounded on one side and you hook a scope ground lead to the output ground. Basically, relative to the virtual ground at the output, the signals can run above and below that value. Not somehow shorting it to circuit ground is the problem.You couldn't, for example, tie the TTL ground to the Wave Ground - that would take out the voltage divider setup.Again, that's for this schematic, not necessarily for yours.

author: Brumby
author_rank: Unknown
timestamp: August 28, 2017, 02:20:44 am
content: As has been said, it's a bit hard without a schematic - or a high quality photo of the lash-up.It's like asking for directions to McDonalds - but not telling anyone which city you're in.

author: joejoe
author_rank: Newbie
timestamp: August 28, 2017, 04:58:08 pm
content: Hi Guys,Thanks for the reply. I have attached the schematic, though hard to read (crappy copy with kit).C1 and C2 are the capacitors in question. +Vcc across their pins, -Vcc from plus to circuit ground.Still learning and this project is helpful that way.Thanks for any help.Joe

author: floobydust
author_rank: Super Contributor
timestamp: August 28, 2017, 05:09:00 pm
content: I am a bit confused what you call Vcc, if that is the input terminal block, or IC etc.I use (circuit) GND as the (-) reference point for my multimeter measurements, and I roam around using the (+) probe to take measurements.Unless you have an open GND connection between C1/C2 to circuit ground. I always forget that wire/jumper.With power off, I suggest to use an ohmeter to verify the ground connections. You should get continuity between each one.It's just that they can be really confusing sometimes if there is an open circuit there.

author: joejoe
author_rank: Newbie
timestamp: August 28, 2017, 05:57:41 pm
content: Hi floobyI am calling jk1 pin3 +vcc. (upper left).Thanks for suggestion, will do that and more experimenting.I will keep you all posted and try to keep the smoke to a minimum.Joe

author: joejoe
author_rank: Newbie
timestamp: August 28, 2017, 08:07:37 pm
content: Well, just goes to show you.I used one of my old plug in transformers to fit the jack on the kit.Turns out it was wired so it was feeding negative voltage.Switched to another one, and it is working (waveforms suck, but am on the road).Thanks for all the help.Joe