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Old 16th Jun 2019, 3:02 pm   #1
music-centre
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Default Problem with Hacker Herald RP35

Hello All,
I am repairing a later model of the Herald with the silicon transistors, it was originally silent apart from a very faint station on MW but replacing TR3 on the RF Board brought it back to working well. As I had a bag of these transistors I thought I would change the others on the RF board, all went well until I changed the last one on the board TR1 ( BF194) then it didn't work at all! - on putting back the BF194 it works perfectly.

Shouldn't an MPSH10 be OK in this position? this is the first time I have used any of the batch I bought but the others I have swapped are fine, I did try another in case it was a faulty one.
Cheers,
Steve.
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Old 16th Jun 2019, 3:10 pm   #2
vinrads
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Default Re: Problem with Hacker Herald RP35

I take it this is in the mixer /osc stage , according to the data sheet it should work , Mick.
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Old 16th Jun 2019, 3:16 pm   #3
'LIVEWIRE?'
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Default Re: Problem with Hacker Herald RP35

Surely, vinrads, the Mixer/Oscillator in this model is silicon (BF194)? If so, why replace it with a germanium type. For a start the BF194 is NPN, the AF12x series are PNP. I have no data on the MPSH10, but the phrase 'If it isn't broke, don't fix it' comes to mind, i.e. why replace a working transistor in the first place. The BF194/95 series, AFAIK, don't suffer from any kind of reliability problem. If this model did use the AF11x series of germanium transistors, I'd change them all on sight!
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Old 16th Jun 2019, 3:21 pm   #4
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Default Re: Problem with Hacker Herald RP35

Yes, it is - its only in that position that one wont work, all the others on the RF Board have been changed to these without any problem. I wondered if it is a poor spec batch of transistors I have bought and just not any good as oscillator, the radio is silent with one fitted in this position - I hope not, I bought rather a lot of them...
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Old 16th Jun 2019, 3:27 pm   #5
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Default Re: Problem with Hacker Herald RP35

Quote:
Originally Posted by 'LIVEWIRE?' View Post
Surely, vinrads, the Mixer/Oscillator in this model is silicon (BF194)? If so, why replace it with a germanium type. For a start the BF194 is NPN, the AF12x series are PNP. I have no data on the MPSH10, but the phrase 'If it isn't broke, don't fix it' comes to mind, i.e. why replace a working transistor in the first place. The BF194/95 series, AFAIK, don't suffer from any kind of reliability problem. If this model did use the AF11x series of germanium transistors, I'd change them all on sight!
I wouldn't usually change the lot like that have found before this that I changed one on another MW/LW hacker only for another of the BF194/5 to have failed a year or so later and this is a set I am fixing for a friend so didn't really want it back.
Steve.
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Old 16th Jun 2019, 3:55 pm   #6
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Default Re: Problem with Hacker Herald RP35

An MPSH10 should be fine as a replacement. My guess is that something is marginal in the circuit.

Obviously you should check and double check the leadouts.

I haven't found BF194 mixers to be at all critical either at AM or FM frequencies, and indeed have replaced them with BC548/2N3904 general purpose types in the past, though I do usually use an MPSH10 or 2SC2999. It might be worth trying whatever Si transistors you have to hand.

The BF194/5s are Lockfits and do suffer from the usual Lockfit issues, so swapping them out is normally a good idea.
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Old 16th Jun 2019, 4:17 pm   #7
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Default Re: Problem with Hacker Herald RP35

Quote:
Originally Posted by 'LIVEWIRE?' View Post
Surely, vinrads, the Mixer/Oscillator in this model is silicon (BF194)? If so, why replace it with a germanium type. For a start the BF194 is NPN, the AF12x series are PNP. I have no data on the MPSH10, but the phrase 'If it isn't broke, don't fix it' comes to mind, i.e. why replace a working transistor in the first place. The BF194/95 series, AFAIK, don't suffer from any kind of reliability problem. If this model did use the AF11x series of germanium transistors, I'd change them all on sight!
Where did I say use a Germanium type , I got confused and deleted it ! so sorry keep your shirt on , as for not replacing it it is well known that these transistors give trouble , so I would replace it , I think It's called future proofing . Mick.

Last edited by vinrads; 16th Jun 2019 at 4:28 pm.
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Old 16th Jun 2019, 4:42 pm   #8
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Default Re: Problem with Hacker Herald RP35

Quote:
Originally Posted by paulsherwin View Post
An MPSH10 should be fine as a replacement. My guess is that something is marginal in the circuit.

Obviously you should check and double check the leadouts.

I haven't found BF194 mixers to be at all critical either at AM or FM frequencies, and indeed have replaced them with BC548/2N3904 general purpose types in the past, though I do usually use an MPSH10 or 2SC2999. It might be worth trying whatever Si transistors you have to hand.

The BF194/5s are Lockfits and do suffer from the usual Lockfit issues, so swapping them out is normally a good idea.
Thanks Paul, I will see what else I have around to try and check the rest of the components in the area.
Steve.
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Old 17th Jun 2019, 12:36 am   #9
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Default Re: Problem with Hacker Herald RP35

Apologies, Mick. I was confusing the BF194/5 with BF184/5 series. The latter are metal can Si equivalents and don't, AFAIK, suffer from problems. That having been said I've not had many lockfit failures-maybe I've just been lucky. I now know you deleted your reference to AF12x transitors, too!
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Old 17th Jun 2019, 3:52 pm   #10
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Default Re: Problem with Hacker Herald RP35

Hi No problem there ,you must have seen my slip up in the few seconds , before I deleted it . Mick.
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