Y'all may have noticed that the ol' TinkerDwagon hasn't been in here rousing the rabble all day...
That's because he done wents on a road trip.
I met with fellow TEA member cyclin_al to trade off some of those Dell server PSUs I've been buggin' y'all with for some TE, of which I'll post pics of tomorrow, I think...
As threatened... some actual TEA today!!! ![Laughing :-DD](https://www.eevblog.com/forum/Smileys/default/smiley_laughing.gif.pagespeed.ce._hfWAz_QHO.gif)
This all started aboot a month ago, when I spotted this beauty on Kijiji for a mere CAD$50, advertised as "working, but output distorted when set above approx 1V", but pick-up only in Kanata, just South of Ottowa. I figure maybe a toasted diff amp or short pass transistor on the output, (really hoping NOT a fault in the DAC) so I PMed cyclin_al, and after some discussion we struck a deal for him to pick it up for me. A really long and ahem... amusing bit of "real life" later, this turned into a deal to trade 3 of my Dell PSUs for the 4070A.
![Wink ;)](https://www.eevblog.com/forum/Smileys/default/xwink.gif.pagespeed.ic.cldandycH0.png)
al watched my adventures in 3DP with some amusement; when the PSUs were ready we decided to meet up in Madoc to do our trade, and this was my half of the deal.
![](https://www.eevblog.com/forum/testgear/test-equipment-anonymous-(tea)-group-therapy-thread/?action=dlattach;attach=1096800;image)
Initial testing... 500mV looks pretty good on all standard generator functions.
![](https://www.eevblog.com/forum/testgear/test-equipment-anonymous-(tea)-group-therapy-thread/?action=dlattach;attach=1096804;image)
Uh-oh... now not so good-looking. Similar distortion on all other functions as well.
![](https://www.eevblog.com/forum/testgear/test-equipment-anonymous-(tea)-group-therapy-thread/?action=dlattach;attach=1096808;image)
And really bad, as promised at 1000mV. Similar distortion on all other functions as well. Twiddling the knobs on my scope, I notice the waveform disappears offscreen downwards... suggesting a negative DC offset.
![](https://www.eevblog.com/forum/testgear/test-equipment-anonymous-(tea)-group-therapy-thread/?action=dlattach;attach=1096812;image)
In hindsight, I really do wish I'd thought it through, considering how close I might have come to frying the frontend on my
![Broken! :-BROKE](https://www.eevblog.com/forum/Smileys/default/smiliey_scope_broke.gif.pagespeed.ce.z9RuKVWKjY.gif)
with this connected directly... in my defense, I DID check for voltage difference between the BNC GNDs BEFORE I connected the two; I ALWAYS do this before testing or probing anything. But I did it; it's done and this was the result after I popped off the 50Ω terminator to probe with my meter. Yup... as suspected... -12VDC.
FuckFuckFuckFuckFuckFuckFuck! -12V DC on the output, and no terminator acting as a shunt to protect my 54645A!!!
![Banging Head |O](https://www.eevblog.com/forum/Smileys/default/bangheadonwall.gif.pagespeed.ce.8iLbFYV4Bc.gif)
A quick check, and yes... I'm still in standard waveform mode, and there is no DC offset dialed in. It really is just dumping -12VDC across the output.
![Face Palm :palm:](https://www.eevblog.com/forum/Smileys/default/xfacepalm.gif.pagespeed.ic.EBDwh1hCfo.png)
Time to take it apart.
![](https://www.eevblog.com/forum/testgear/test-equipment-anonymous-(tea)-group-therapy-thread/?action=dlattach;attach=1096816;image)
So... cracked rubber bumper ring fixed with tape... some grrilla haz been in this thing.
![Roll Eyes ::)](https://www.eevblog.com/forum/Smileys/default/rolleyes.gif.pagespeed.ce.XcUK202Yqt.gif)
![](https://www.eevblog.com/forum/testgear/test-equipment-anonymous-(tea)-group-therapy-thread/?action=dlattach;attach=1096820;image)
You have got to be fucking kidding me...
![Face Palm :palm:](https://www.eevblog.com/forum/Smileys/default/xfacepalm.gif.pagespeed.ic.EBDwh1hCfo.png)
![](https://www.eevblog.com/forum/testgear/test-equipment-anonymous-(tea)-group-therapy-thread/?action=dlattach;attach=1096824;image)
Oh, this just keeps getting better. But hey... at least the grrilla knows what heat-shrink tubing is.
![](https://www.eevblog.com/forum/testgear/test-equipment-anonymous-(tea)-group-therapy-thread/?action=dlattach;attach=938592;image)
![](https://www.eevblog.com/forum/testgear/test-equipment-anonymous-(tea)-group-therapy-thread/?action=dlattach;attach=1096828;image)
Wow... really? WTF is with this ground wire...?
Time to do some testing. Okay... MCP on the output tests okay with junction test... no obviously shorted relays... no shorted tants in the output quadrant of the mainboard. Remove the PSU.... and actually, not that bad. That weird soldered GND wire is actually soldered to a eyelet to go under a screw on the PSU.
So I hop on the intardnet to look for some tech ref... and I stumble across this teardown video. It looks like the damaged rubber bumper is endemic to the design... it happened to his just like mine. Also, upon reviewing the video, I can see the GND wires soldered onto the AC Line Filter appears to be the way they were made... as are many of the DIP packages hand-soldered... and that DAC being sanded off too.
I'm thinking maybe... all that horrible stuff (aside from the obvious fan hack) may have been the way it was made, and this unit was just produced during the death throes of B&K as a manufacturing entity in the US... before they became
entirely a paper company having everything made by other people. And maybe
"Give a fuck" was in short supply.
![Confused :-//](https://www.eevblog.com/forum/Smileys/default/confused0024.gif.pagespeed.ce.5xOqKkq0Co.gif)
*sigh*
Okies... I have every shred of support off the B&K site... even have the software for generating waveforms... but cannot seem to find a service manual or schematic. The vid above states this is a repackaged
BNC Berkeley Nucleonics 625 or
Telelux SG-100; I'm going to continue to seek out any available tech ref I can find on those, but I think now I'm going to need to actually start scoping this thing out live to see where the DCV is being injected and the distortion starts.
Pray for me guys!
EDIT: Okies... So far, the only resource I've found is this schematic for the Telelux SG-100. Not very good, but enough to grok how the amplifier chain and attens all work. I think. I've attached it here for posterity's sake. ![Face Palm :palm:](https://www.eevblog.com/forum/Smileys/default/xfacepalm.gif.pagespeed.ic.EBDwh1hCfo.png)
![](https://www.eevblog.com/forum/testgear/bnc-625a-firmware/?action=dlattach;attach=1097744;image)
Here are some cleaned up versions of that schematic I made for my own use and posted in the originating thread. For future generations.
![Wink ;)](https://www.eevblog.com/forum/Smileys/default/xwink.gif.pagespeed.ic.cldandycH0.png)
mnem
![Tweak :-/O](https://www.eevblog.com/forum/Smileys/default/smiliey_cal.gif.pagespeed.ce.yYMcAvxBJk.gif)