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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 7th Feb 2018, 12:40 am   #1
Oldelectronics
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Default AF117 screen lead

Hi Folks
I was wondering how essential this lead is considering many advocate cutting it in the event of the dreaded tin whisker and I have worked on sets that have had the screen lead cut sometime in the past? The reason I ask is that I have seen Some P403/П403 pnp Ge Soviet transistors for sale At a very reasonable price and advertised as an equivalent of the 2N2084 but they seem to have only 3 leads but they are of a longish length. I assume Hacker would not have used screened transistors in the form of the AF117 if they thought it was not necessary but if its not I have a couple of radios with the horrible posts so it would also be one less to clean out as well as much cheaper if the difference between them is marginal. Thoughts/advise on this much appreciated.
Regards

Kev
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Old 7th Feb 2018, 12:45 am   #2
paulsherwin
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Default Re: AF117 screen lead

The screen lead was a bit of a gimmick and makes next to no difference in typical domestic radio circuits. The screening may be of some use at the top of the frequency range but even that is unlikely. I wouldn't worry about it.
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Old 7th Feb 2018, 12:52 am   #3
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Default Re: AF117 screen lead

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Originally Posted by paulsherwin View Post
The screen lead was a bit of a gimmick and makes next to no difference in typical domestic radio circuits. The screening may be of some use at the top of the frequency range but even that is unlikely. I wouldn't worry about it.
That's certainly been my experience ......
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Old 7th Feb 2018, 12:54 am   #4
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Default Re: AF117 screen lead

Thank you Paul much appreciated' Do you think the P403s would be suitable?

Kev
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Old 7th Feb 2018, 2:00 am   #5
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Default Re: AF117 screen lead

Judging by the number of odd transistor types, even silicon, that work in replacing AF1xx I would reckon that if it's Ge it stands more than an even chance of being just fine.
I have found the odd radio which is fussy about what the RF/mixer transistor is, particularly when using silicon, and that's about as bad as it gets in my experience.
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Old 7th Feb 2018, 2:36 am   #6
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Default Re: AF117 screen lead

Quote:
Originally Posted by paulsherwin View Post
The screen lead was a bit of a gimmick and makes next to no difference in typical domestic radio circuits. The screening may be of some use at the top of the frequency range but even that is unlikely. I wouldn't worry about it.
I agree with this especially for standard AM radios tuning 530 to 1700 kHz.

I think there is some benefit in the RF & mixer stages that go to 30MHz, such as those in an Eddystone EC-10 radio. But in standard transistor AM radios, and in the approx 455kHz IF stages, the screen lead-connection is probably neither here nor there.

There is one big thing to watch out for though. The AF11x series transistors have a very low collector to base feedback capacitance. It was this feature that allowed them to be used in IF amplifiers without neutralization, unlike the OC45 that requires it because its base collector capacitance might be 5 or 10 times higher.

Some supposed "germanium transistor equivalents" passed off as RF types have too high a collector to base capacitance by comparison to AF11x, so they could result in instability if used as substitutes in the IF stages for AF11x.

Likewise, another challenge in repairing early vintage transistor radios from the 1950's, is that the IF transistors can have a very high base to collector capacitance (with neutralization components to suit them) much higher than an OC45. So a replacement transistor must either match the original transistor for this parameter, or insert a low base-collector capacitance part and disconnect the neutralization components.
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Old 7th Feb 2018, 10:36 am   #7
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Default Re: AF117 screen lead

Thanks folks much appreciated I think I will order a small batch and see how they work out! The Cc Argus 25 is pf:10 according to the advert?

Kev

Last edited by Oldelectronics; 7th Feb 2018 at 10:48 am.
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Old 7th Feb 2018, 11:05 am   #8
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Default Re: AF117 screen lead

The AF11x series was used by radio manufacturers because it was the standard RF transistor of the period, not because it had any exotic special qualities (though Philips/Mullard were very happy for people to believe that). The standard RF/IF designs used in domestic radios are very forgiving and just about any Ge type can be used if the Ft is high enough for the circuit. I've even subbed OC45s for AF117s without problems in a MW/LW set, though I wouldn't suggest those as a routine replacement.
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Old 7th Feb 2018, 11:17 am   #9
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Default Re: AF117 screen lead

Thanks again Paul

Regards
Kev
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Old 8th Feb 2018, 1:25 am   #10
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Default Re: AF117 screen lead

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Originally Posted by paulsherwin View Post
I've even subbed OC45s for AF117s without problems in a MW/LW set, though I wouldn't suggest those as a routine replacement.
I'm surprised that you could get away with this in the IF stages, since the IF stage, if designed for an AF117, would have no neutralization and any IF stage with an OC45 would definitely need it. Though it wouldn't matter in the osc, RF and mixer stages the extra feedback capacitance of the OC45, but the stages would require re-tuning I think.
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