AF102 question

[westcliff]

I'm working on a Hacker Mini-Herald (RP17A), which has three AF117 transistors.  Can they be replaced with AF102?

Gus.

[ppppenguin]

I see no reason why not but a couple of warnings.

1: The AF102 has a higher Ft (260MHz vs 75Mz) so it may decide to oscillate in unpredictable ways
2: It's in the same package as AF117 so may be equally prone to tin whiskers. If it works as a transistor on a DC test it should be OK but there's no guarantee it won't grow whiskers in the future.
3: Capacitances will be slightly different so alignment may need tweaking

People have tried all sort of substitutes. The humble BC557/8/9 (or just about any small signal PNP silicon device) is plausible but may need the bias tweaked.

[Mark Hennessy]

Because the AF117s are so easy to access on an RP17A, I'd zap the existing whiskers. While it can be argued that this is only a short-term cure, I haven't had any comeback in over a decade of doing it. Must have done hundreds, possibly thousands, in that time.

It would be a different matter if we were discussing a Roberts R707, with the transistors buried in that Mullard module.

What I do is detailed here: https://www.markhennessy.co.uk/articles/....htm#treat

[Crackle]

Hi Gus
Hope you and your family are all keeping well.
I have used AF125 transistors to replace AF11x ones.
But you could try replacing them with AF102 ones if you have them already, one at a time, and test after each one.
Mike

[boater sam]

If you have any problems using AF102 it will likely be in the mixer stage, the IF stages are less fussy. One at a time is the way!

[Mike Watterson]

I've used cheap Russian Military Ge transistors in a Hacker and a Bush.
The problem was excess tin inside the can on some production runs, so AF117 or similar OCxxx parts still working are likely to keep working.

[westcliff]

Thank you all for your comments.  This is the first transistor radio I have done any transistor replacement on so I have taken on board the advice given.

Sadly one of the AF117s did not survive my attempt to remove it, so it will be replaced by an AF102.  If the other two 117s make it out alive, I'll zap them using the instructions supplied in Mark's link.

I'll make sure an AF117 is used in the mixer stage.

The good news is that my AF102s are all Dutch, so should not suffer from tin whiskers.

Gus.

[television1936]

Have you checked to confirm the transistors do have the"whisker" short to the can? Usually the transistor will function OK in circuit if you just remove the shield connection lead from connection to the PCB.

[Mark Hennessy]

When it comes to this, here's my standard advice:

Removing the screen connection is OK for a quick test, but be aware it's not conclusive, as the whiskers might have made contact with two electrodes. So don't give up hope if it doesn't work - after a proper "zapping", it's highly likely the transistor will be OK for another few decades.

Avoid cutting the screen wire - take the time to de-solder it. You'll need it later, after the zapping. OK, the screen might not be essential in many cases, but it is nice to put the set back to its original condition after the repair.

As I say, just my standard advice

[skyclad]

Good old AF117 think I may still have some somewhere. Cutting the screen was a good quick fix..