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DMM for life? - Page 3

post #31 of 35
Originally Posted by NeilR

I included persistent because you deserved it! I included lucky because there are so many snipers that time of day doesn't make that much of a difference anymore. It does, however, remove a large pool of potential less sophisticated bidders with day jobs.

No question, my Wavetek meters are not very saleable, but I figure the 10A large fuses in them are worth as much as the meters themselves cost me. I bought spare fuses that came with a free meter

And they are hard to come by. Which is why I am surprised that they do not sell well when they come up. My hat is off to Fluke's marketing. I bought two within one week. Actually the 2nd one was by accident because I forgot to set up a snipe group. But for 15 bucks I won't complain. It is very handy having multiple meters monitoring different things when an amp gets brought up the first time.

I only posted what I did because I think there are a lot of starving students here that are lead to believe that only a Fluke is good enough to get their Cmoy up and running, and that simply is not the case. I myself would like to own a Fluke, but the only ones that make sense for me (given my existing meters) would be the 187/189 series and I just haven't had the need to justify all the bidding it will take to bring one in at a decent price. I would like to find out what all the mystique is about. I have used them on occasion but never owned one.
Yep - those are all great points ... didn't think about the CMoy crowd (I am still one!). Up until this Fluke, I used a tiny IBM-throwaway analog-Triplett, with borrowed leads.
post #32 of 35
I actually mentioned Cmoy more or less sarcastically

There isn't anything on these DIY sites (M3/PPA/Gilmore) that requires a meter designed for 1000V readings with 0.01% accuracy in extreme environmental conditions where it might be run over by a truck. These things are designed to survive use on the Alaska Pipeline in mid-winter, I guess.

Our meters are actually very "overspec'd" physically and electrically for anything we build while it is sitting on our work benches in our environmentally controlled homes measuring the output from our 24V supplies, or even our power mains. (With the exception of matching, as I mentioned earlier)

Like I said, my hat's off to Fluke. I wish I had that marketing ability.
post #33 of 35

Bridge matched R's

The point of the thread: matching resistors on the cheap.

You don't need a super-cool DMM or LCR meter to match resistors to within .1% , all you need is a BRIDGE. It will be slower than a $5K meter, but if you start with a box of 50 nice RNC-series 1%, how wrong can you get?

Using your DMM set to millivolts/microvolts DC (you do have a 3.5 or 4.5 digit DMM, right?) on the external detector terminals of your BRIDGE (like an AN/URM-90), and the Unit Under Test (UUT, that would be the first resistor), adjust the dials for a null. You can pre-set the dials to the nominal value of R, then it will only be a tiny nudge to balance. A balanced bridge will be indicated when your DMM shows zero. Nudge it past this to see how much it takes to start indicating the other way, then go back.

This is just a DC match. A good bridge can do AC, too, with an internal or external oscillator. A good one for the AN/URM-90 is an HP 200CD (tooooobzzzz & transformers) because it can crank out the voltage, compared to most SS generators. HP 205AG has a super-bad output stage that can go well over 100V at audio frequencies.

Should I make a video?

Anyway, you won't be matching absolute values, you will be matching relative values, which is more important for opamp ratios and balanced signal noise rejection. If you have a Government Contract, you may need to offer matched absolute values, in which case you will be using fancy and expensive DMM's that have periodic calibration schedules. I miss having access to multiple opt-001 HP 3458A's and Fluke 57xx's that Unca Sam pays for! But, that lab stuff is not important AT ALL for good audio. Four 3.5 digit DMM's, a function generator, a 652A, a 334A and two 60MHZ analog scopes ($1000 used, maximum) will do for most everything, and if it won't, you can rent an Audio Precision analyzer (if Uncle Sam is demanding it).

Wavetek- ugh. Beckman-argh. Vintage General Radio -neat and repairable! 1960's HP & Tek are great, 1970's Fluke is fine, ESI is real-nice. The best thing about poor gear is that it goes away almost by itself, while the old stuff keeps working along with the the old tech's who handle it. Lab gear lasts, while field test equipment is trashed.

I'm not opposed to owning cheap analog CRT 'scopes (student grade, made in Taiwan) because I can toss them in the trunk and not worry. Many of them are copies of old Tek anyway. If I see them as surplus for $50, I buy even if not needed. Same with an old Fluke/HP/Philips/Tek DMM, buy it, and on-average, you will do well.

This stuff sure takes up a lot of space in my office!


r13....USAF 2P051/32450...Between the Sandbox Wars.
post #34 of 35
2P0X1? Rather appropriate AFSC for this thread
post #35 of 35
I just picked up a Sears brand Fluke 81438 at the pawn shop for $25. About this time last year I picked up an older Fluke 75 like this one for $20 at the same local pawn shop. I really like the 81438 as the display shows readings faster and updates more often than the older 75. The bar graph moves quicker too.
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