Author Topic: Reliable Source for LM308s?  (Read 1513 times)

Philtre

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Re: Reliable Source for LM308s?
« Reply #30 on: June 11, 2018, 01:47:08 AM »
If you ordered one from ebay (or anywhere that's not trustworthy infact) how would you know if its real or not?

Measure resistance between pins 1 and 5 (the offset null pins). Single op-amps like TL071 or LM741 will have a resistance (around ~2k for the 741, higher for the TL071), the 308 will show no connection.

And if its a dual-opamp re-painted, the pinout won't match and it won't actually work in the circuit.

Good call, thanks. Do I measure this with voltage applied to the chip?

(Just had a despatch notice from eBay. Soon....)

somnif

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Re: Reliable Source for LM308s?
« Reply #31 on: June 11, 2018, 01:50:01 AM »
No just carefully touch your DMM leads to the chip.



Thats the 741, but the 071 has a similar topology with some different resistor values. You can measure resistance between those pins straight away.
« Last Edit: June 11, 2018, 01:57:00 AM by somnif »

chromesphere

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Re: Reliable Source for LM308s?
« Reply #32 on: June 11, 2018, 01:53:14 AM »
Excellent information Somnif!  To be honest i wasnt sure there was an easy way to check, i hope this info helps folks source genuine parts (tonnes of fakes out there...)
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somnif

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Re: Reliable Source for LM308s?
« Reply #33 on: June 11, 2018, 02:01:16 AM »
Note this trick doesn't work for externally compensated chips like the TL070, since it uses a similar compensation scheme to the '308. There is a diode between pins 1 and 8 in the 070, though, so that's something you could look for.

Philtre

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Re: Reliable Source for LM308s?
« Reply #34 on: June 13, 2018, 04:06:10 AM »
It arrived.

I measure no resistance between pins 1 and 5, and about 17k between pins 1 and 8.

somnif

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Re: Reliable Source for LM308s?
« Reply #35 on: June 13, 2018, 04:33:30 AM »
It arrived.

I measure no resistance between pins 1 and 5, and about 17k between pins 1 and 8.

Does it work in circuit? I'm not sure you should be seeing a resistance that low between 1 and 8, but I don't have one on hand to check. In the 308, 1 and 8 are connected via a 5.6k resistor in series with a bunch of transistor junctions. I have no idea what the resistance of that would be off hand I'm afraid.

An OP07 just has a couple resistors between 1 and 8, but their value is determined on-die, and isn't a published stated value, so no easy way to check there. .

Real way to check is just chuck it in a Rat. If you can, leave the compensation cap out. If it doesn't work, its a 308, if it does work, its something else.

ALL THAT SAID, there isn't really a huge profit margin in faking 308's. Rat aficionados may love the things, but pedal builders are a drop in the bucket, and they aren't really super special niche market chips like old BBDs. Chances are its probably real. And hell, if it sounds good, who cares?
« Last Edit: June 13, 2018, 04:35:04 AM by somnif »

Philtre

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Re: Reliable Source for LM308s?
« Reply #36 on: June 13, 2018, 04:40:15 AM »
It works in the Giant Hogweed circuit - http://www.davidrolo.com/wp-content/uploads/2017/07/GiantHogweedSchematic_zps445f5a36.png

I have an OP07 here and there is 16k between 1 and 8.

So if I snip the 30pf Cap should it not work if it's real?

Philtre

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Re: Reliable Source for LM308s?
« Reply #37 on: June 13, 2018, 04:47:09 AM »
It works the same in that circuit with no compensation cap.

chromesphere

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Re: Reliable Source for LM308s?
« Reply #38 on: June 13, 2018, 04:51:26 AM »
ALL THAT SAID, there isn't really a huge profit margin in faking 308's. Rat aficionados may love the things, but pedal builders are a drop in the bucket, and they aren't really super special niche market chips like old BBDs. Chances are its probably real. And hell, if it sounds good, who cares?

Scammers have faked chips and transistors of far less value so i wouldn't be so sure of that. 

I think it would be very handy to have in this thread the values measured on a confirmed lm308 so people can check there chips against it.  Amazingly i dont have one that i would trust as being 100% genuine.
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somnif

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Re: Reliable Source for LM308s?
« Reply #39 on: June 13, 2018, 04:52:34 AM »
Ok, yeah, sounding like you got a re-branded op07, that sucks  >:(

Rat's without a compensation cap can pass through sound, but they end up sounding sputtery and broken. If its behaving the same without a cap, and showing a similar resistance, I'd say its probably the same chip. Boo.

I dug out a metal-can 308 (ok, technically its a 108, but that just mean it works in cold weather) I had and see a 1-8 resistance of about 40 Megaohm. Not sure how that would compare to the DIP8 version, unfortunately.

Philtre

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Re: Reliable Source for LM308s?
« Reply #40 on: June 13, 2018, 04:59:43 AM »
A/B testing between the "LM308" and an OP-07 and it sounds almost exactly the same. 30pf cap makes no difference in or out.

I'll claim a refund, on principle.

somnif

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Re: Reliable Source for LM308s?
« Reply #41 on: June 13, 2018, 05:08:42 AM »
Well, if you want some vintage mil-spec mojo I found a UK-ebay auction for a pair of metal-can LM108's, 8 pounds for 2 of them. They actually include pictures, so that something.

Philtre

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Re: Reliable Source for LM308s?
« Reply #42 on: June 13, 2018, 06:47:47 AM »
I requested a return from the eBay seller. He just refunded, no questions, quibbles or even wanting the chip back. Perhaps he knows they're fake?

I think I'll stick with my OP07 in the Giant Hogweed circuit.

chromesphere

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Re: Reliable Source for LM308s?
« Reply #43 on: June 13, 2018, 08:29:36 PM »
If its a small amount the eBay seller probably just refunded to avoid any headaches.  eBay favors the seller in most cases and if you disputed that the chips were fake through the dispute resolution process chances are eBay would favor you over the seller.

For those reading this thread and are new to the hobby and electronics I should just reiterate here, ebay is not recommended for electronic parts.
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Philtre

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Re: Reliable Source for LM308s?
« Reply #44 on: June 13, 2018, 10:36:35 PM »
If its a small amount the eBay seller probably just refunded to avoid any headaches.  eBay favors the seller in most cases and if you disputed that the chips were fake through the dispute resolution process chances are eBay would favor you over the seller.

For those reading this thread and are new to the hobby and electronics I should just reiterate here, ebay is not recommended for electronic parts.

True, but I have bought AC128s from eBay and they've been fine. Resistors and caps can be OK.

Buying this "LM308" was an experiment. It was a case of "I gots to know". No loss financially, but experience gained.
« Last Edit: June 13, 2018, 11:31:19 PM by Philtre »