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Post by roly on Sept 10, 2020 23:44:47 GMT -5
I am modding a friends Oktava 012 small diaphragm condensers using a kit he bought on line.
Once I got to a point in the instructions where it says "reassemble the mic and test it, it should work perfectly", it passed signal but was very noisy and didn't have much gain.
I reviewed my work and concluded that I had miswired a 3 leg transistor that replaced a 4 leg transistor. I misunderstood the instructions and left the wrong hole vacant.
I did not test the resistors before installing them because it seemed to me that everything was packaged by a professional and felt there was no need to mistrust their judgment.
After noting my error, I striped all my work and started over.
This time I measured the resistors before reinstalling them and found that my Fluke 87V says OL.
Checked the other resistors intended for the other mic(unused)....same result.
Rewired it and the mic is still hooped.....capsule is fine, checked it on the unadulterated mic.
Colours (spot the Canadian or Brit) are brown, black, black, blue, gold......so.....100MEG.....yes?
Result is, I can't test the resistors to conclude weather or not my error has damaged them.
I hope I just cooked the transistor.
What would you do at this point?
cheers
Roly
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Post by Peegoo 🏁 on Sept 11, 2020 0:34:54 GMT -5
You might have bad resistors. Do you have others you can test to confirm your meter is working properly (or not)? If your meter isn't auto-ranging, make sure the dial is on the highest Ohms setting.
Once you know your meter is working, test the transistor with it to see if it's cooked.
This works for BJT/NPN/PNP type. It's pretty simple...here's a tutorial:
More good info here:
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Post by roly on Sept 11, 2020 2:42:26 GMT -5
Hi Geno
Thanks for your advice.
Didn't know that 100 meg resistors existed, 10 meg is the highest value I have ever encountered.
Guess it makes sense when dealing with the miniscule voltages mic capsules are intended to see.
I have no reason to doubt my two Fluke meters, although a tad old, I think they are the industry standard for the work I do.....fixing and building tube amps.
If you have a 1G resistor, please stick it on your meter and let me know if the meter sees it.
cheers
Roly
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Post by roly on Sept 11, 2020 22:06:29 GMT -5
Success...:>)
While moving a cap out of the way to get at the pads of the transistor I figured I had cooked, I noticed a cold solder joint on one leg of the cap.....corrected my work and boof....mic works.
My wiring error damaged nothing. Now, on to the second half of the mods on the first mic.
I am the typical result of an unsporting father....low self esteem haunts me to this day, despite the fact that I have accomplished many things I am proud of....guess that's another topic.
The mystery remains....why won't my meter see 1G resistors? I thought the fluke stuff was pretty good.
Cheers
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Post by kito75 on Sept 11, 2020 22:59:41 GMT -5
Typical digital multi-meter only reads up to about 20Meg to 50Meg ohms directly. Fluke DMM will display "OL" if you try to read a higher resistance than the meter is rated for so your meters are probably working fine. You can use a DC voltage source and ohms law if you want to measure a high value resistor.
1G = 1000 Meg ohms so a typical DMM won't measure directly. You need a meg ohm meter (megger) for that.
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Post by roly on Sept 11, 2020 23:49:46 GMT -5
kito75
Thanks for your advice.
I think I got lucky......my error damaged nothing.
One would think that the vendor would state that conventional meters won't see the 1G resistors.
Now I have an excuse to buy a new meter....:>)
cheers
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Post by roly on Sept 17, 2020 15:18:35 GMT -5
Success...:>) While moving a cap out of the way to get at the pads of the transistor I figured I had cooked, I noticed a cold solder joint on one leg of the cap.....corrected my work and boof....mic works. My wiring error damaged nothing. Now, on to the second half of the mods on the first mic. The mystery remains....why won't my meter see 1G resistors? I thought the fluke stuff was pretty good. Cheers |
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pdf64
Wholenote
Posts: 556
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Post by pdf64 on Sept 17, 2020 16:35:34 GMT -5
Any test instrument will have limitations and constraints, eg a 20ft tape measure can’t be expected to also deal equally well with miles and microns. Digital multimeters should be considered to be general purpose, rather than specialist, test instruments. Each of their measurement modes will limitations and constraints, eg any DMM should measure the Vdc of a typical battery or the Vac at a wall outlet pretty well; but accuracy (or even at the ballpark level) outside such typical use, eg below a few mV (dc or ac), Vac above1kHz, Vac with a Vdc offset, should not be taken for granted.
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Post by roly on Sept 17, 2020 21:39:42 GMT -5
Thanks for your advice Peter.
Do you own a meter that "sees" such high value resistors?
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pdf64
Wholenote
Posts: 556
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Post by pdf64 on Sept 18, 2020 10:34:16 GMT -5
I’ve never had occasion to try, but I’m pretty sure my best DMM, a Fluke 189, won’t cope with resistor values that high. If I had to check such a resistor, I think I’d use the procedure kito75 describes above - take it out of circuit, connect a highish voltage to one end of it and a current meter to the other. There’s only so far you can get with a 9V battery  |
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