gripes and more on cheap sampling

Entropy Struct entropy at thecia.net
Wed Jan 28 23:47:58 CET 1998


On Wed, 28 Jan 1998, Stopp,Gene wrote:
> Do you have a scope? It's really really really really useful for
> situations like this. If not, do try to get one, borrow, trade, buy,
> steal. Okay don't steal. Or if you do, don't say I told you to.

Welp, the main problem is that the electronics lab here at Brandeis is
small and inaccessable due to no EE or CE programs here; only physics uses
the lab. (and no avid electronics hobbyists around here that i'm aware of)  
Aggravating, yes, and since I just got a new DMM last week I really can't
afford a scope right now... maybe when the MIT flea market hits here next
spring, they sometimes have nice ones for between 100 and 200 bucks. 

> The cap thing sounds weird. Could have been bad caps, I guess, any
> chance they were in backwards? Tantalums *hate* that.

Nah, it wasn't as obvious as that. I'm leaving it as it is right now. I
would hazard a guess that it's just a bad cap, or cheap power supply
(mouser). Incidentally, the random out is pretty dead (or at least really
soft) as well, but I think that just means that I should have another look
at the last TL082 in the circuit. 

> A big warning about 3140's - I know it's in the ASM documentation, but
> it bears repeating: they blow up real easy! They are easily the most
> static-sensitive parts I've ever dealt with. Use excessive ESD
> precautions and keep the cats away. It's the price we pay for that teeny
> input current. It's not much of a stretch to believe that they're also
> kind of intolerant to abuse when installed, as well.

On the nose again, actually. I just whipped out the schematic and looked
at it seriously again; the problem i'm having was when i (being idiotic at
about 2am) replaced the 3080 I thought I zapped with a 3140... now all the
3140's in the module have an unhealthy white sheen on them, as would be
expected from doing something so boneheaded. (hey, that smoke *wasn't*
coming from my soldering iron!) But end result is I'm down $5 in parts and
nothing more.. not too bad, considering. And what doesn't kill you... 

> The 30 pf cap being needed depends on the dual op-amp that you use in
> the VCA. A 1458 wouldn't need it, for example, but the TL082 does. It
> squelches a supersonic oscillation (another place where a scope would
> lead to the solution quickly).

Yep, sure enough I'm using all TL082's for the opamps. 

> Good luck!

Thanks Gene, I'm always amazed that you're able to respond to people
personally. I realize a lot of these questions are rather "basic" for the
list, but I do appreciate it. 

Andrew




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