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Old 16th Jun 2005, 12:10 am   #21
howard
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Default Re: Roberts R300 AF117 transplant ...

Hello Darren,

Thankyou Darren, that all makes sense.

All my TR82s are in my own collection so I would like to keep them original so I shall pursue the AF117s but if I can't find any then I'll go for one of the substitutes.

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Old 16th Jun 2005, 10:07 am   #22
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Default Re: Roberts R300 AF117 transplant ...

Howard.


As you are keeping the sets for yourself at the moment.

Have you tried rescuing the faulty transistors ?.

Those that have internal shorts can in most cases be "repaired" by removing from the set, tying all the leads except the screen together, and then blowing the short with a 100uF cap charged to a high voltage, say 100V.
Using the higher voltage will also deal with the low resistance (not quite short) conditions as well.

Then reinstall the transistor with the screen lead removed.

I have done this in the past to a number of sets, R200's, VTR103, Hacker Sovereign etc, and they are all still working beautifully.

Not for the faint hearted, but worth a try, you have nothing to lose, if all else fails you can still use the silicon transplant method discussed.

Pete
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Old 16th Jun 2005, 12:31 pm   #23
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Default Re: Roberts R300 AF117 transplant ...

Hello Pete,

Gosh, that sounds errmmm ........

I've heard from Chris Tredwell at Mode Components now ... he had a job finding some original A117s after all, seems they're vanishing fast. I've ordered 6 @ £2.95 each for the best of my TR82s and will probably replace any others with the substitute BF450s.

Thankyou all for your help.

Howard
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Old 16th Jun 2005, 5:01 pm   #24
Darren-UK
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Default Re: Roberts R300 AF117 transplant ...

I can't say anything re Chris Tredwell as I've never dealt with him, but you need to be careful with some of these component supplies where obsolete components are concerned. Some - and I've found out the hard way - list certain items, take your order and your money and then attempt to source the components you've ordered.

Frankly I find it easier and cheaper to look out for tatty old trannies at car boots for a pound or two from which I can remove things like AFxxx's. Truth is, these AF117s (etc) seem to deteriorate with age as well as usage so you can't guarantee the integrity of NOS components. I once obtained some NOS AF117's and these were tested as ok. However, a couple of them failed after only a couple of hours use so there you go. That's my own experiences and views.
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Old 19th Jun 2005, 7:28 pm   #25
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Default Re: Roberts R300 AF117 transplant ...

The only replacement for the AF117 is AF280,the BF 450 etc are silicon.
There is another point,cutting the screen on the AF117 could lead to instability,I fail to see why people go out of there way to modify the design without the knowledge of circuit design.You can clear the screen to collector S/c by zapping with a few volts and a few amps,and if the transistor survives this,the screen should be reconnected,I have done this on many occasions with complete success.
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Old 20th Jun 2005, 1:06 am   #26
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Default Re: Roberts R300 AF117 transplant ...

By coincidence I have very recently placed some comments about this screen lead snipping in another forum section.

Regarding blowing out the s/c with some nice volts and amps, I use for this purpose an old 'Trix' model railway transformer which has various tappings for up to 18v at up to 3amps. Most of the time using this transformer is successful but I have to be careful to remember that the transformer output is constant - unlike a discharging capacitor - so have to tap the transformer leads onto the transistor leads quickly several times until the arcing ceases.

Now I bet that'll make some of you cringe eh ? but the method works !
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Old 6th Nov 2005, 10:34 pm   #27
Geoff G4ICD
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Default Re: Roberts R300 AF117 transplant ...

Quote:
Originally Posted by ppppenguin
I'm slightly surprised that you can get away with replacing germanium transistors with silicon and not altering the bias. The Vbe drop is about 0.3V for Ge and 0.7V for Si. While a few circuit configurations will be inherently pretty stable with respect to this, most are not. Have you measured the before and after collector currents? Always assuming you can find a good AF117

The other worry is that the high fT of modern transistors, even very modest ones like the BF450, may lead to parasitic oscillations. Even the very humble BC108 makes a pretty good RF oscillator. Maybe the likely reduction in bias current caused by the higher Vbe tends to starve the transistor which will reduce any tendency to oscillate.
.........................
I have just checked the Ic on TR1 on the Roberts R200, this is around .65m/a with the AF117, then I put the BF450 in and it measured .55m/a. Although I have done this replacement months ago i had never checked Ic. 1 set has been running continuous for 2 months without problems as a test. It shows no instability and no problems have been encountered, so it looks like a good replacement, BTW, I then use a masonary drill to "dig" into the AF117 and then stick the case on top of the BF450........
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Old 7th Nov 2005, 12:42 am   #28
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Default Re: Roberts R300 AF117 transplant ...

Hello All. I think it is worth replacing old components with more modern ones for reliability and peace of mind.and you don't have the worry of the radio not working when you come to use it this happened to me only two weeks ago so I say out with the AF117's and in with the BF450's lets face it who displays the inside of thier radios anyway I can understand the need to keep the radios in original condition but one day all old transistors will need to be replaced evetualy anyawy

Andy

Last edited by Andrew c; 7th Nov 2005 at 12:56 am.
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Old 12th Oct 2007, 10:08 pm   #29
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Default Re: Roberts R300 AF117 transplant ...

Even if you 'clear' the short by whatever means on the AF117, they will eventually go the same way in time...and the same if you snip the screen. The next tin whisker that grows may touch another electrode and give you another short. Hasnt this been covered before ?
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Old 28th Mar 2008, 9:18 pm   #30
John Jones
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Default Re: Roberts R300 AF117 transplant ...

I was given an R300 as a birthday present, and as all three AF117s had one or more electrodes s/c to the can, I replaced them all with BF450s. Having read this thread, I was confident that the radio would work. Unfortunately, not so; it was just about possible to hear one or two of the stronger stations very faintly with the volume fully up.

I was unable to find any faulty components, or voltages which looked wrong, so I cleared the shorts on the AF117s, and started refitting them, starting with TR1. This produced a slight improvement, but not much. I then refitted TR2 and this did the trick, plenty of stations at good volume. I was unable to replace TR3 because the third AF117 was faulty, so I don't know how much of an improvement, if any, this would give.

I've checked all the components, and they're all within tolerance. Can anybody shed some light on this, please. Are BF450s very variable, or something?

Thanks in advance,

John Jones
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Old 28th Mar 2008, 10:09 pm   #31
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Default Re: Roberts R300 AF117 transplant ...

No, but in some cases the set needs to be re-aligned afterwards.

Cheers,

Steve P.
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Old 28th Mar 2008, 10:34 pm   #32
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Default Re: Roberts R300 AF117 transplant ...

I have used the BF450 in a few different sets and they worked perfectly every time.
There is a different version with a suffix letter, which may have a different pin out.
I always use AF124/5 in modules, just in case the BF450 doesn't work and I have to go through it all again!
Looking at the data the BF 450 is one of the few silicon transistors to have an internal capacitance close to that of the AF11x/AF12x. I wonder if that could go some way towards explaining how it behaves?
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Old 28th Mar 2008, 10:42 pm   #33
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Default Re: Roberts R300 AF117 transplant ...

Quote:
Originally Posted by John Jones View Post
I've checked all the components, and they're all within tolerance. Can anybody shed some light on this, please. Are BF450s very variable, or something?
The BF450s don't vary much, but the radios do. I've usually found a large number of Si PNP transistors will work at least as well as the AF117s they've replaced, not just BF450s, but it depends on the circuit and indeed component tolerances. I keep a stock of different types to hand - they cost pennies on eBay - and experiment to find the best sub. Once I found only an AF124 would work (this is electrically identical to an AF11x) but this was an exception. I've found BC214s to work pretty well as a rule.

As Steve says, some realignment will usually be necessary for best performance because the internal capacitance will be different.

Paul
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Old 28th Mar 2008, 11:04 pm   #34
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Default Re: Roberts R300 AF117 transplant ...

The BF450s I bought were a bit of a fiddle to fit as the pin out is different from the AF transistor. It required twisting a leg and fitting some heatshrink.
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Old 28th Mar 2008, 11:44 pm   #35
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Default Re: Roberts R300 AF117 transplant ...

I should have said, the result I got was after re-alignment. That was my first thought, too.

I'll check the pinout; I went by a data sheet I'd downloaded. Maybe it wasn't the right one for the device. Although, having said that, one of them (TR3) seems to be working okay, so presumably it's fitted correctly, and I fitted the other two in the same way.

And, lastly, I used red and white sleeving on the emitter and collector respectively, so a s/c seems unlikely.

Thank you, gentlemen, for your suggestions; I'd welcome more, although I don't suppose that there's much more that can be said about this problem.

Thanks again,

John
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