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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 29th Nov 2008, 8:18 pm   #1
Racalfanatic
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Default OC171 Equivalent?

Hi All,
I've just obtained an Eddystone EC10 and all 3 OC171s on the RF board are duff.
Interestingly, the ones on the IF board are all fine.
I've fitted AF117s and an OC71 for now but it has left the receiver a bit deaf and I'd like to find a reliable substitute.
Any suggestions?
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Old 29th Nov 2008, 8:29 pm   #2
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Default Re: OC171 Equivalent?

hi.
If possible try the AF12* (4-7) range if you are using an OC71 in the IF there will be no gain as this is an audio transistor. the AF117 isn't good on higher rf frequencies either.
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Old 29th Nov 2008, 8:39 pm   #3
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Default Re: OC171 Equivalent?

Hi Trevor,
Thanks for the info.
I was wondering about the AF106?
What are your thoughts?
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Old 29th Nov 2008, 9:48 pm   #4
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Default Re: OC171 Equivalent?

These are basically AF11x transistors - OC171 is the old designation. Try AF12x transistors as has been suggested. There's not a lot of point in using AF11x types as they're likely to develop internal shorts eventually, just like the OC171. The OC71 will be hopeless and I'm surprised it works at all.

People have had success subbing these transistors in RF and IF stages with silicon BC214s. Send me your address in a PM if you want a few to try out. The well known BC450 would also be worth trying but I don't have any of those to spare.

Paul
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Old 30th Nov 2008, 1:51 am   #5
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Default Re: OC171 Equivalent?

Cricklewood electronics list them as available, they are thoroughly reliable.

http://www.cricklewoodelectronics.co...=search&page=1
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Old 1st Dec 2008, 12:00 pm   #6
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Default Re: OC171 Equivalent?

You can "cure" your OC171s, at least temporarily, by shorting a 9V battery between the screen lead and the base,collector and emitter twisted together (or one at a time). The trouble will probably recur but possibly not for a long time, when you can repeat the process.
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Old 1st Dec 2008, 6:17 pm   #7
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Default Re: OC171 Equivalent?

The latest "Radio Bygones" (116) has an interesting peice by Gerry O'Hara covering just this,(including a picture of actual whiskers inside a transistor,) in his series on components.
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Old 1st Dec 2008, 7:58 pm   #8
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Default Re: OC171 Equivalent?

Quote:
Originally Posted by davidw View Post
The latest "Radio Bygones" (116) has an interesting peice by Gerry O'Hara covering just this,(including a picture of actual whiskers inside a transistor,) in his series on components.
For anyone that doesn't know of it, there's a lengthy thread in the archives exploring tin whiskers in some detail. It includes links to an analysis of the problem by NASA with some fascinating pictures.

Paul
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Old 1st Dec 2008, 8:31 pm   #9
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Default Re: OC171 Equivalent?

Fascinating link.
Now I have yet another job to do... dismantle some old transistors and examine them under the microscope.
Mike.
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Old 6th Dec 2008, 9:18 pm   #10
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Default Re: OC171 Equivalent?

Well,
I bought some NOS OC171s and they all have shorts to the screen connection.
They do work so long as I don't connect the screen connection though.
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Old 7th Dec 2008, 12:23 pm   #11
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Default Re: OC171 Equivalent?

I bought some of these too and "cured" them by twisting the b,c & e leads together and then connecting them to one terminal of a PP3 and the other terminal to the screen. They now work perfectly although the problem will probably come back some day.Incidentally when testing these for Hfe on a digital meter I find that about a third of OC170,171,AF11*s give a reading which starts to increase exponentially so they seem to be VERY susceptible to thermal runaway for some reason. They still work well in properly stabilised circuits though (presumably the meters just use a single resistor between battery + and base to set the base current),
Jim
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Old 7th Dec 2008, 12:40 pm   #12
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Default Re: OC171 Equivalent?

Quote:
Originally Posted by Racalfanatic View Post
I bought some NOS OC171s and they all have shorts to the screen connection.
I did warn you

The problem with bodges like disconnecting the screen wire is that the tin whiskers will continue to grow, and you will get another short sooner or later. An AF124 or AF125 is a direct replacement electrically and won't suffer from tin whiskers, but it has a different case.

As I said above, I'm happy to send you a few BC214s FOC for you to try out.

Paul
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Old 17th Dec 2008, 6:40 pm   #13
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Default Re: OC171 Equivalent?

Will the BC214 also replace the OC170?
Pat G3IKR
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Old 17th Dec 2008, 6:56 pm   #14
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Default Re: OC171 Equivalent?

Quote:
Originally Posted by Oldtimer74 View Post
Will the BC214 also replace the OC170?
Pat G3IKR
Generally, yes, but there are no guarantees when subbing a completely different transistor like this. It all depends on the circuit. Sometimes performance will be improved if the bias components are adjusted.

Paul
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Old 17th Dec 2008, 8:27 pm   #15
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Default Re: OC171 Equivalent?

See www.qsl.net/vk3jeg/edd_ec10.html for a OC171 replacement
John.
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Old 4th Mar 2018, 11:48 pm   #16
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Default OC171 Equivalent? Continued

I have come across a 1-Transistor FM Receiver at this site:

http://www.americanradiohistory.com/...ed-1961-01.pdf

The article for the radio I am referring to begins on page 82. As mentioned in the article, the radio could be built with either an OC171 or 2N384 transistor. So, it seems that the 2N384 is an equivalent for the OC171. Also, the article shows one of the common ways to avoid the 'tin whisker' problem, just don't use the shield wire, and bend it up, so it does not contact any of the other wires! This trick works most of the time, though sometimes it can cause the radio to become unstable.

Last edited by Station X; 4th Mar 2018 at 11:53 pm. Reason: Threads merged.
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Old 5th Mar 2018, 2:07 am   #17
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Default Re: OC171 Equivalent? Continued

2N384 transistors may be ordered from this site:

http://semiconductormuseum.com/Museu...N384_Index.htm

Transistors sold at this site are usually tested and good.
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Old 5th Mar 2018, 4:43 am   #18
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Default Re: OC171 Equivalent?

Replace AF11X (except AF118) and OC170/171 with AF178. You will be guaranteed no problems. Replace AF118 with 2SA358. Another option, that make good AF11x replacements are 2N2084.
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Old 5th Mar 2018, 9:37 am   #19
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Default Re: OC171 Equivalent?

As I and others have mentioned elsewhere on this site, the Russian GT322 Transistor is a good substitute for OC17x and AF11x series transistors. These are usually available from various eBay sellers for around £1 each in packs of 10, though I haven't checked lately. I don't have data for the GT322 to hand, but they are suitable for most circuits in which the OC170/171 were used.
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Old 5th Mar 2018, 10:00 am   #20
Argus25
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Default Re: OC171 Equivalent?

If you are considering a replacement type, you can in fact get away with many in RF amplifier and oscillator stages.

However, if you are replacing an AF11x or OC171 in an IF stage, be aware that the replacement type you select, must have a similarly low (or lower) base to collector feedback capacitance, so that is the main spec you need to compare, if not the IF may be unstable.

AF178 has a very low feedback capacitance so it will not give you any trouble. If you cannot get this spec for any other suggested substitutes, I would caution using them in the intermediate frequency amplifier stages for that reason.

Also, don't forget you are dealing with an EC10 here, So even in the RF and osc stages you require a superior transistor as the radio goes to 30MHz, so a 100MHz capable device is good. Some suggested substitutes might be fine for a MW waveband radio, but not the EC10, that is if you want an excellent result.

Last edited by Argus25; 5th Mar 2018 at 10:07 am.
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