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Old 14th Jun 2013, 4:50 pm   #1
johncsparks
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Default BC557B Cheap Replacement for AF117

Just finished changing the AF117's in a Hacker Sovereign II AM IF panel with BC557B's bought on eBay for 12p (less in quantity) and they work well.

I have an older Sovereign RP18 (which had already been fitted with AF127,s) and AM was good whereas my Sovereign II RP25 had barely average AM sensitivity and weak stations sounded distorted (but good on FM)

The RP25 had no obvious faults and some BC557B's were in the spares box so replaced all 3 AF117's on the IF panel (the separate osc. had previously been replaced with an AF12x) Result: equally good/bad, though potentially more reliable!
Compensated for the higher BE voltage for TR2 & 3 by adding 110K across the 56K AGC bias R7. Result: little improvement.

Had been resisting tweaking the IF cores, but had run out of options. After 2mins tweaking had a vast improvement, someone had got at it!

AM sensitivity is now excellent and slightly better than the good RP18, possibly due to the higher gain of the BC557B.

I have not tried this device as a replacement in FM areas. Possibly its higher internal capacitance than an AF11x could be an issue there?

John
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Old 14th Jun 2013, 6:53 pm   #2
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Default Re: BC557B Cheap Replacement for AF117

Hi John,

There are two issues when replacing transistors with different types.
One is the bias. Germanium transistors have about 0.2 - 0.3V Vbe bias voltage while silicon transistors have about 0.5 - 0.6V. Replacing a Ge transistor with a Si transistor will change the operating current of the transistor (it will be smaller). Depending on the circuit, one might want to change the bias resistors to achieve similar operating current.
Different devices have different internal capacitances. Again, depending the circuit, it might require touching up the alignment.

Peter
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Old 14th Jun 2013, 7:24 pm   #3
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Default Re: BC557B Cheap Replacement for AF117

What Peter says is true, but as has often been discussed in the past, it's possible to use a huge range of transistors as drop in replacements for AF11x's, including silicon types. It obviously depends on the circuit, but 1960s Ge transistors were very variable and consumer circuits were devised with this in mind.

I'd always encourage anybody to try whatever they have in the junk box as replacements. It's amazing what you can get away with.
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Old 14th Jun 2013, 10:33 pm   #4
PaulR
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Default Re: BC557B Cheap Replacement for AF117

Interesting as I have just paid £3.30 each for 3 x AF127s for my Motorola 828, but I suppose that it is at least cheaper than replacing rare valves.
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Old 15th Jun 2013, 1:32 am   #5
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Default Re: BC557B Cheap Replacement for AF117

PaulR, keep those AF127's in reserve for situations where silicon transistors won't work.
Alternatively use silicon transistors for the IF stages and the AF127 for the mixer stage and you might have an easier realignment task.
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Old 15th Jun 2013, 9:56 am   #6
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Default Re: BC557B Cheap Replacement for AF117

To be honest I am not really bothered about the price of the transistors.

I am only getting involved with them out of necessity as the radio in my classic car has stopped working and I want to get it repaired before we go on holiday in it. I usually do valve stuff and would be delighted if I could get 3 front end valves for a total of £10 plus pence for p+p. If I didn't fit them now they would probably still be there when I go to the great radiogram showroom in the sky!

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Old 15th Jun 2013, 10:03 am   #7
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Default Re: BC557B Cheap Replacement for AF117

Yep BC557s work well. I appear to have a fair quantity of them (can't remember where or when I got them!) and have replaced plenty of AF117s with them. Haven't noticed any great problems with alignment after changing them. Admittedly I haven't ever changed the biasing arrangements (I haven't done any AF117 replacements for a while and that was before I knew about the biasing differences!).
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Old 15th Jun 2013, 10:40 am   #8
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Default Re: BC557B Cheap Replacement for AF117

A couple of years ago I was given an old Dansette transistor radio dating from the early 60's. It was a mess, so rather than attempt a proper renovation I decided to convert it into a 'Topband Special'. For the non radio amateurs among us, Topband is an Ham Band covering 1810 to 2000 KHz, ie just above the MW band.
As I intended replacing the grotty audio stages with an LM380 and fitting an NE602 product detector (for SSB reception), I thought it wise to whip out the original PNP germanium transistors and replace them with more modern NPN BC547's, reverse the electrolytics etc and go for a negative earth design. All went well, but wow, the IF gain was much greater than before and I had to add a bit of un-bypassed emitter resistance to both IF stages to calm it down!
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Old 16th Jun 2013, 8:20 pm   #9
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Default Re: BC557B Cheap Replacement for AF117

Why replace the AF117s? 90 Volts and a suitable electrolytic capacitor will zap those tin whiskers into oblivion. I did my Bush TR130 some years ago and it's still working well today.
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Old 17th Jun 2013, 8:19 pm   #10
johncsparks
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Default Re: BC557B Cheap Replacement for AF117

What's the longest period anyone has found for a significant number of devices to stay OK after zapping?

If I could be confident that a set full (say 8) AF11x's would be OK for maybe 10 years or more then I would stick to zapping. Though if some were still to fail that could still be a reliability issue.

Given the effort dismantling, desoldering, zapping and putting it all back together; a price of 12p per device for hopefully many tens of years of service seems an attractive option.

The other possible option is to do a mini-zap in-situ, e.g. using an analogue meter on low resistance setting. This takes only a few seconds of effort but only works on some devices and for a limited period.

John
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Old 17th Jun 2013, 8:57 pm   #11
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Default Re: BC557B Cheap Replacement for AF117

You never know how long a zapped AF117 will last, because the zapping only burns out existing shorts, not developing tin whiskers which haven't yet bridged the gap.

I'm with you on this.
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Old 17th Jun 2013, 9:35 pm   #12
shortwaverx
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Default Re: BC557B Cheap Replacement for AF117

Quote:
Originally Posted by johncsparks View Post
Whats the longest period anyone has found for a significant number of devices to stay OK after zapping?

If I could be confident that a set full (say 8) AF11x's would be OK for maybe 10 years or more then I would stick to zapping. Though if some were still to fail that could still be a reliability issue.

Given the effort dismantling, desoldering, zapping and putting it all back together; a price of 12p per device for hopefully many tens of years of service seems an attractive option.

The other possible option is to do a mini-zap in-situ, e.g. using an analogue meter on low resistance setting. This takes only a few seconds of effort but only works on some devices and for a limited period.

John
Hi John,

I see your point and agree to a certain extent. I just like to see the original transistors in situ. My argument breaks down though, as I admit that I usually replace electrolytics and leaky decoupling capacitors. For some reason, I view vintage semiconductors in a different light.

Cheers,

Ian
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