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Forum 141

Hacker Herald - AF117 replacement

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Westcliff
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I have a Hacker Herald that crackles faintly, but does nothing else. Now I know I should replace the electrolytics, but there are also three AF117s sitting there. Can I replace them with AF181? I'm asking because AF181s are all I have in the way of AF transistors.
Thanks.
Gus.

 
Posted : 14/11/2012 12:02 am
Anonymous
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I've replaced AF117 with Silicon PNP BC557 and 2N3906. Also with 60MHz Russian ex Military stock Germanium PNP. For AM Mixer/Osc and AM IF, as long as there is an emitter resistor it won't make much difference as long as its Germanium RF Type or almost ANY small signal Silicon PNP.

See my TR130 and Hacker RP18 threads.

The electrolytics can go dodgy.

 
Posted : 14/11/2012 12:23 am
Anonymous
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Hi Gus
The AF181 is a VHF low power PNP germanium transistor so it should work. The gain is about half of the AF117, but if the gain was quoted at VHF frequencies maybe it will be a bit better at lower frequencies.
I say give it a try.
or
Have you considered doing what I did in this thread, viewtopic.php?f=3&t=3331
Take the AF117 out of circuit and solder the E, B, and C wires together and then connect to a power supply of around 30 volts between the shield and the other 3 soldered wires.
It worked for me on 3 transistors and there is nothing to loose. It wont necessarily effect a long lasting cure as the whiskers may grow back in time.
Mike

 
Posted : 14/11/2012 8:54 am
Terrykc
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... It wont necessarily effect a long lasting cure as the whiskers may grow back in time ...

This problem has been discussed at length here and on the other forum. The problem is that there a lots of the damned things!

You don't know when you successfully zap one if there aren't several more that are not quite fully grown, just waiting to cause a repeat performance!

Some interesting pictures here http://nepp.nasa.gov/whisker/anecdote/a ... index.html

I've sometimes wondered if it might be possible to melt all of them at once. Tin a spot on the side of the can, then stand the transistor on its head against some kind of insulating material - a piece of wood with a location notch to stop it moving, perhaps.

Clamp a heat shunt* on the lead out wires to conduct heat away from the transistor itself and apply the iron to the can and hold it there for a reasonable time - the intention being for the entire can to reach a temperature higher than the melting point of tin.

An alternative, and possibly easier method would be to use a miniature gas blow torch, if you have one.

A possible fly in the ointment - literally - is the silcon gunk these transistors are filled with. It probably has the ability to store up a lot more heat than the can itself, so dunking the transistor in cold water for a couple of minutes afterwards, whilst keeping it upside down, might be a good idea ...

Whaddya fink?

* If you can find some short pieces of thick copper wire or suitable bar, you can make a simple heat shunt by soldering the copper pieces into the jaws of a crocodile clip, keeping the copper faces parallel.

This can be clipped onto the wires, effectively giving you a third hand.

When all else fails, read the instructions

 
Posted : 14/11/2012 11:11 am
Anonymous
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Hi
Terry I agree, zapping the whiskers is not likely to make them all disappear they are just as likely to fuse within a few thou of the case or junction, and could easily grow back.

A while ago now I heated up one of these AF117 transistors with a narrow flame precision blow torch, the transistor case popped apart in a few seconds. I think it was the silicon grease in side vaporising.
I cleaned all of the inside of the can and tried to clean as much of the gunk of from around the junction.
I put it back together with some fresh silicon grease.

It never worked again :O :O :O

I think Terry's idea sounds more feasible

Mike

 
Posted : 14/11/2012 12:03 pm
turretslug
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Ha, ha, that's one way to open them up...

 
Posted : 14/11/2012 12:54 pm
Anonymous
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Put a PNP Silicon SOT inside the can :)

 
Posted : 14/11/2012 1:28 pm
Westcliff
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I think I'll sort out the electrolytics first. I don't have a 30 volt power supply, or a blowtorch. :-o

Then, if the radio still doesn't work, I will try the AF181s. I do have loads of other Ge transistors so there may be some that will work if the AF181s don't.

Gus.

 
Posted : 18/11/2012 3:14 pm
Westcliff
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Now I have looked at some of the electrolytics. There are a number of large yellow ones, rated at 250v. Is such a rating really necessary? Unfortunately, the manufacturer's service instructions do not have voltage ratings for capacitors.

Gus.

 
Posted : 23/11/2012 3:53 pm
sideband
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They can't be electrolytics.....sound more like 250V polyester types. No manufacturer would use 250V electrolytic caps in a transistor radio that has a 9 or 18V supply.

Pictures please!

Rich.

 
Posted : 23/11/2012 4:12 pm
Westcliff
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Hi Rich, I don't think they are original. There are a few in the amplifier section and one in the other bit, where the AF117s lurk. Having said that, they probably are plain old capacitors, not electrolytic. I'll try to get a picture up later on.

Talking about AF117s, you can sometimes get them working if you cut one lead. Is it E or S that you cut?

Gus.

 
Posted : 23/11/2012 5:46 pm
Anonymous
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Hi Gus
It is the shield (S) you cut and sometimes get it working.
Mike

 
Posted : 23/11/2012 6:06 pm
Westcliff
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Thanks Mike. For some reason, I had in mind that you cut the E lead. The PCB in this set has everything nicely labelled so I'll cut the correct lead.

Gus.

 
Posted : 23/11/2012 6:57 pm
Mark Hennessy
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Don't cut the screen lead - pull it out of the PCB and fold it up out of the way. You'll be glad you did this in years to come, when a tin whisker comes into contact with another electrode, forming a short circuit between, say, collector and emitter.

Alternatively, just remove it and zap it properly. Solder E, B and C together and to one end of a capacitor. Charge said capacitor to as high as you can, then briefly touch the other end of the capacitor to the S lead. If you're using more than about 20-30V, be careful not to touch both ends of the capacitor at the same time as it might bite! I use just over 60V, for what that's worth. BTW, even a couple of PP9s in series will do if you don't have a bench PSU.

Having cleared away the obvious shorts, solder the capacitor to the screen and charge it up again. Then, very carefully, hit the transistor case from all angles with something metallic. This will catch any whiskers that are nearly reaching. To my knowledge, no-one else performs this step, but I reckon it's worth doing. I do it again if the transistor shows any sign of crackling when hit when it's back in the set.

Yes, of course they will fail at some point. But my customers are perfectly happy to accept that, and so-far I've not had a comeback - I reckon I've easily done a few hundred by now... The success rate is nearly 100%, and actually, those 2 or 3 transistors that didn't work afterwards probably didn't before (but of course there is no way to test before you've removed the whiskers).

I regard this operation a minor inconvenience that's well worth it IMHO - ripping them out and throwing them away just seems very wasteful. Especially on sets where they are highly visible and very easy to access. Of course, if it were a Mullard module - especially one that's damned hard to get at - where the AF11X's have very short leads, and they can't be seen - then replacement with AF12X's is perfectly acceptable. Pragmatism and all that...

 
Posted : 24/11/2012 6:46 pm
Anonymous
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Don't cut the screen lead - pull it out of the PCB and fold it up out of the way. You'll be glad you did this in years to come, when a tin whisker comes into contact with another electrode, forming a short circuit between, say, collector and emitter.

Hello Mark
I dont understand this statement. What use would the screen be if there was a short between collector and emitter.
Thanks
Miks

 
Posted : 24/11/2012 10:12 pm
Mark Hennessy
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At this point, you'll need to blast away the shorts as the transistor will stop working.

It's a lot easier to do if the screen lead is present. Otherwise, you have to use the metal can.

Actually, you occasionally have to do that with AC128/176/187/188 et al. It's a pain. The screen lead makes it easier. And, you can solder it back in place once done, restoring the set to its original condition. Not many sets actually require the screen, but some do.

 
Posted : 25/11/2012 12:52 am
Westcliff
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Thanks for the info, Mark. How does one charge a capacitor? And how is it done when the capacitor is attached at both ends to the various leads of a transistor? I'm not stupid, I just haven't had any training in or experience of electronics. I also don't understand how two PP9s can give you 30 or 60 volts.

Here is a picture of the big yellow capacitors I mentioned earlier, which was requested by Sideband.

Gus.

Forum 143

 
Posted : 25/11/2012 1:31 am
Mark Hennessy
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Agreed. But those blue electrolytics are almost certainly guaranteed to be leaking heavily.

Gus, charging a capacitor just involves connecting it to a voltage source. But I'm very aware that it's easy for me to say that, but if you're at the very beginning, such a statement might not make a lot of sense.

There are a lot of excellent books and web-sites out there for people starting out in electronics. Our school library had Mastering Electronics by John Watson, and I found that pretty OK at the time (back in the '80s) - I see that the current edition has mixed reviews from people who were in your position. Perhaps a basic GCSE-style book or website might be a good start?

We all started at the beginning :=D

 
Posted : 25/11/2012 10:30 am
Anonymous
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i am not shore i understand what you are trying to do .why would you want to charge a cap while in circuit?
to check the blue electrolytics remove them from the board first .if you have no way of testing them it may be better to change them all,most will be leaky.
rob t

 
Posted : 25/11/2012 12:09 pm
Terrykc
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Ah! Thread drift!

Rob, the blue electrolytics are a side issue and nobody is suggesting charging them, in or out of circuit!.

The capacitor charging bit refers to charging a capacitor so that it can then be connected to an AF11x transistor to zap the tin whiskers ...

When all else fails, read the instructions

 
Posted : 25/11/2012 2:04 pm
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