Hacker AF117 best alternatives?

[nebogipfel]

Good evening all,

I have been reading older posts and it looks like the recommended best replacement for AF 117's is the AF12* series.

I thought before I spend my limited hard earned cash I'd just seek to clarify that this is the case, and that as a beginner I am going for the best option. I note that AF125 are readily available at around £3 each but I see that AF121 are a bit cheaper, will they also work OK?

I am specifically looking at parts suited to direct replacements in a Sovereign and Sovereign II without any modifications to the supporting circuitry.

I appreciate this topic has been discussed but thought it would be worth asking because someone may be aware of a new source of transistors or indeed another recently discovered alternative which works well.

Thanks

Edit. My most recent Sovereign (an early black one) has a white grille on the rear cover, I assume this is an incorrect replacement?

[mhennessy]

Hi John,

The AF127 contains the same piece of germanium as the AF117 - it's just that it's in a different package, one that doesn't suffer from the tin whisker problem. Same applies to the AF124, 125, 126.

You might find it a bit tricky because the AF12X devices have shorter leads that might not reach the holes on the PCB.

Make sure you're happy about the lead-outs. They are not in a row like they are in the AF11X series.

And yes, many of the RP18s had white grilles

All the best,

Mark

[Nickthedentist]

Hello Mark,

What's the most reasonable source of AF124/5/6/7 that you know of at the moment?

Nick.

[nebogipfel]

Thanks for the quick reply Mark and the info about the white grille.

I have seen Littlediode as a source for new AF125's. It is they who also offer 121's at about £2.40 each. Would they be OK?

I'll be careful of the connections thanks

Edit: For some reason AF124's 127's are very expensive from the source mentioned above

[mhennessy]

To be honest, I never fit them - I just "treat" the original AF11X's. So far, nearly 100% success rate with my method over the last few years. Because Hacker sets are so easy to access, I honestly don't mind the prospect of having to re-treat them every few years. Not that I've had to repeat yet.

All of my AF12X stock have come from Hacker sets where a previous repairer has replaced them. I fit the correct AF12X in place, and save up the AF12X's for sets that are more difficult to work on - like the Roberts R707 that uses 3 of them in that impossible module.

The current prices of the AF12X range is too high for me to consider.

[Nickthedentist]

Thanks, Mark!

[paulsherwin]

If you've been reading other threads, you'll know that there are many cheaper alternatives to AF12x transistors, though they're not as foolproof. If cost is a significant factor, try some general purpose PNP silicon transistors such as BC214s. You can probably buy 20 of these for the price of a single AF125. Ebay sellers always have them.

Littlediode is a good supplier but not a particularly cheap source.

As Mark says, there are a number of techniques to repair AF11x transistors, but unless you remove the case and cover the exposed junction and wiring in hot melt glue or Araldite, the tin whiskers will grow back eventually.

[nebogipfel]

[Hello Paul,

Yes I have been reviewing the subject but was really wondering if anyone was aware of alternative (to AF12*) drop in replacements. I'm very much a novice so if I caused more problems than I cured I could get myself into a bit of a pickle I think I'll probably go for AF125's but may have a go at zapping too. It's a shame that these older transistors are becoming expensive but looking at the Hacker spares list they were quite expensive when they were current technology.

[paulsherwin]

All transistors were expensive back then.

The only direct equivalents are the AF12x series. I don't know the AF121 but I'll look it up in my Towers reference and get back to you.

Don't be afraid to try alternative transistors if you have some to hand. You're unlikely to damage anything, and they'll either work or they won't.

[mhennessy]

In the AM sections, the IF amplifiers should be fairly non-critical, but the oscillator might be fussy.

Less sure about the FM IF parts of the RP18. I'd only want to make recommendations having tried it and carefully measured the results. Things are generally more critical in there.

Apparently PNP germanium transistors made in Russia can work well, and are pretty cheap to come by. I have no direct experience of this, but have read several posts from a chap on another forum about this. Does stretch the bounds of originality if the transistors are on view - it's a personal thing of course, but an RP18 could be 50 years old by now, so I feel that it's nice to have working AF11X's on display if possible...

I did try to open up an AF117 once. I gave up after a few minutes, but will have another go one day. There is an old thread on here discussing this, and I think it was Kat who managed it? Or perhaps she had a go, and like me failed, but found that the heat destroyed the whiskers? Memory is slow and my ability to search is faltering, so it must be time for bed

[paulsherwin]

Towers says that the AF121 isn't the same transistor as the AF124/5/6/7 and gives the AF239 as an equivalent. It will probably work though.

You can remove the case from an AF11x by heating it up with a soldering iron while pulling on the wires. Personally I wouldn't try it other than as an experiment.

[Herald1360]

AF121 has Ft 270MHz which is about three times higher than AF11X series. There might be instability problems at VHF with a simple substitution. Or it may work perfectly well.....

[nebogipfel]

Originally Posted by paulsherwin

Towers says that the AF121 isn't the same transistor as the AF124/5/6/7 and gives the AF239 as an equivalent. It will probably work though.

Looks like I'll use AF125's then. Thanks for the info' Paul

[yesnaby]

Hello,

In a recent thread it was stated that J. Birkett of Lincoln was selling AF125 at 75p each. Might be worth a check.

Michael

[Keith]

Hi John,

I assume you've tried the simple fix of just cutting the screen lead. It worked for me - nothing to lose, after all!

[Philips210]

Hi.

I have often used the AF12x range if I had them available and personally wouldn't pay the rediculous prices for the AF11x series. AFIK, there is no advantage in using NOS AF11x transistors as the tin whisker problem is still going to be an issue.

I would agree with Mark, use the AF12x where access is poor as on the Roberts R707. Replacing the IF transistors in the R707 isn't a very pleasant experience!
Also try clearing the whiskers or cutting the screen lead close to the transistor as Keith points out.

I have in the past used AF239S transistors on a R707 but was told off for doing this by a member on another forum many years ago. The AF239S has a decent enough transition frequency rating but it's gain is quite low compared to the AF11x/AF12x series. Some transistor equivalents books do however suggest the AF239S as an alternative to the AF11x/AF12x series.

I haven't tried Paul's suggestion of using silicon transistors but that looks a good economic alternative. Tweaking of the biassing may be required in some circuits.

I've also bought early 1970's write-off transistor radios from car boot sales for a source of AF12x transistors. I wouldn't break up a good radio but using the expensive parts to repair a more derirable radio makes more sense.

Regards
Symon

[nebogipfel]

Originally Posted by Keith

Hi John,

I assume you've tried the simple fix of just cutting the screen lead. It worked for me - nothing to lose, after all!

Hi Keith,

One set in particular has been extensively "got at" with screen leads cut and different transistors all over the place, so my plan was to restore everything as close as possible to original spec' and then see what I've got.

I like the idea of retaining AF117's for total originality so zapping is a possibility but it would be nice to restore the sets and know that they should then be reliable. As I understand it zapping gets rid of existing shorts but does nothing for the whiskers still growing.

[mhennessy]

The trouble with cutting the screen lead:

1. It's OK until another whisker reaches another electrode, so now you have a short between case and collector, say, and case and emitter. In other words, a short between emitter and collector.

2. You have nothing to attach the charged capacitor.

OK, you can use a large croc-clip to the case, but I like to solder firmly when zapping...

If you want to lift the screen connection, pull it out of the PCB and fold it up alongside the case of the transistor. Fit an insulation sleeve if you like. This assumes that having the case moving up and down at 10.7 million times a second doesn't cause problems (stray capacitance, etc). Honestly, with practice, zapping one of these is a five minute job.

If you zap with a high voltage, the arc will take out a lot of the nearby whiskers. Yes, of course they will return, but I'm confident to say that we're talking years, not months.

Just my opinion

Mark

[Keith]

Hi Mark,

I'm sure you're right. "Zapping" the dendrites must be a longer term fix. My screen lead cutting fix is based on a sample size of just one (although it has lasted for ~10 years now!). I'd be interested in a description of your zapping technique.

[nebogipfel]

Originally Posted by Keith

Hi Mark,

I'd be interested in a description of your zapping technique.

Yes, might it be possible to make up a simple zapper so that a transistors could simply be plugged in then a switch thrown?