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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 11th Dec 2017, 6:12 pm   #1
bill knox
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Default Replacement of OC171 in an Eddystone EC10

Hi

I have a Eddystone EC10 that I am going over, but I have found that several for the OC171 transistors are either intermittent or have a partial S/C from screen to the other pins.

I have seen on this forum about replacing the AFXXX range with other transistor's but although I have looked have not seen anything regarding the OC171.

Can anyone suggest a replacement for the OC171, the main problem seems to be the gain of so-called replacements, their hfe seems to be around the 50 mark where as the gain of an OC171 is rated at hfe 150.

Could I have some views on this please.

Regards

Bill
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Old 11th Dec 2017, 6:19 pm   #2
Herald1360
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Default Re: Replacement of OC171 in an Eddystone EC10

I would expect that the "usual suspects"- AF12x, GT322B etc would work just fine.
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Old 11th Dec 2017, 6:41 pm   #3
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Default Re: Replacement of OC171 in an Eddystone EC10

Agreed. The OC171 is essentially the same transistor as the AF11x, and the same replacement guidelines apply.

There is no one hfe value for a transistor, it all depends on the operating conditions.
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Old 11th Dec 2017, 6:58 pm   #4
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Default Re: Replacement of OC171 in an Eddystone EC10

Some people in the past have suggested using transistors like the AF139/AF239 in such applications - thinking "That's a UHF transistor - higher Ft so it's got to be better" - truth is, the Hfe of them is generally lower than the Hfe of a boring transistor like an AF114/115/124/125 "general band-II VHF-broadcast" one.
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Old 11th Dec 2017, 7:33 pm   #5
David G4EBT
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Default Re: Replacement of OC171 in an Eddystone EC10

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Originally Posted by bill knox View Post
Hi

Can anyone suggest a replacement for the OC171, the main problem seems to be the gain of so-called replacements, their hfe seems to be around the 50 mark where as the gain of an OC171 is rated at hfe 150.

Could I have some views on this please.

Regards

Bill
Billinsghurst (reputable long-established UK valve & transistor firm) have NOS ex WD CV7089 (OC71 equivalents) for 2.80 each.

Don't know if they're of any interest - they might be worth a punt:

https://www.ebay.co.uk/itm/CV7089-OC...-/191990595205

(Hfe isn't the be all and end all and isn't that important in some roles - ie, an oscillator needs to oscillate up to the desired frequency).

Good luck in your endeavours Bill.
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Old 11th Dec 2017, 7:39 pm   #6
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Default Re: Replacement of OC171 in an Eddystone EC10

I'd be worried that a "NOS" OC171 equivalent from the same era and heritage as the original OC171 would have the same tin-whiskerism issue that's probably the reason for the OP to need his original OC171 replacing...
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Old 11th Dec 2017, 8:04 pm   #7
paulsherwin
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Default Re: Replacement of OC171 in an Eddystone EC10

Agreed, best to steer clear. AF12xs can be bought for less than that.
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Old 11th Dec 2017, 8:12 pm   #8
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Default Re: Replacement of OC171 in an Eddystone EC10

The EC10 has 6 OC171 transistors but only 4 of them are in the RF and IF stages where high gain is important.

It may be possible to use a lower gain device for the Local Oscillator and the BFO.

Another factor is that they are not connected to the AGC line and so it may be possible to change the biasing and use silicon PNP devices.
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Old 12th Dec 2017, 12:15 am   #9
Argus25
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Default Re: Replacement of OC171 in an Eddystone EC10

I have replaced the OC171's in my EC-10 radios with AF178's with outstanding results. These have near identical and a little superior specs to the OC171, they look the part because of their larger size than an AF127's which don't look good in these radios and have short awkward lead wires.

There are also the Tesla resin filled OC169 & OC170 if you are lucky enough to get them, but they probably sold out like hot cakes.

The other excellent transistor for the application is the 2N2084, these have very long lead wires which is helpful. The last lot of these I got came from Halbleiter.

Have a look at post 5 and post 8 in this thread:

https://www.vintage-radio.net/forum/...d.php?t=139413

Don't bother replacing the OC171 with any transistor of the similar grease filled design, Tin whiskers will kill them in the end.

Since you are an EC-10 owner needing to replace transistors, you might be interested in this thread:

https://www.vintage-radio.net/forum/...d.php?t=139849

Last edited by Argus25; 12th Dec 2017 at 12:20 am.
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Old 12th Dec 2017, 5:19 pm   #10
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Default Re: Replacement of OC171 in an Eddystone EC10

GT322Bs have long leadouts (about 25mm) and are easier to fit than AF12x types.
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Old 12th Dec 2017, 6:00 pm   #11
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Default Re: Replacement of OC171 in an Eddystone EC10

Just beware that the leadout is different from AF12x!
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Old 12th Dec 2017, 6:55 pm   #12
David G4EBT
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Default Re: Replacement of OC171 in an Eddystone EC10

Quote:
Originally Posted by Argus25 View Post
I have replaced the OC171's in my EC-10 radios with AF178's with outstanding results. These have near identical and a little superior specs to the OC171, they look the part because of their larger size than an AF127's which don't look good in these radios and have short awkward lead wires.
Over 300 in stock here at 1.25 each, which might be of interest:

http://markhindes.easywebstore.co.uk...tor_A27YN.aspx
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Old 12th Dec 2017, 7:03 pm   #13
paulsherwin
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Default Re: Replacement of OC171 in an Eddystone EC10

Really, subbing these transistors isn't at all difficult. I have yet to hear from someone who has subbed a plausible alternative and had problems. Even going to silicon is usually trouble free.
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