Replacing an AF178 with a BC557?

[OldTechFan96]

In early 2017 I worked on my first vintage transistor radio, a Roberts R700. I did not really know anything about faultfinding or have any test gear bar a DMM. The R700 did not work so I decided to replace all of the RF/IF transistors since I'd read that the AF11*s can develop tin whiskers. The BC557 was the replacement of choice.

This included replacing the AF178 RF amplifier with a BC557. To my delight the radio worked well on FM. It was picking up everything I expected it to.

I don't think the BC557 is specified for VHF applications. I must have read somewhere that it can replace a AF178.

The two transistors look similar when I looked at the information provided by alltransistors.com.

I'm now aware that the AF178 does not suffer from tin whiskers and is generally reliable.

Was this replacement a good choice?

[Restoration73]

It is not possible to provide an answer as there are so many parameters that can affect
performance. For general RF/IF any 557 would do although 557C has the highest gain
in the grouping. For RF there are also other factors including transition frequency and
collector to base capacitance. If you have aligned the set for best gain at both ends of
the FM band then you have probably reached optimum performance.

[paulsherwin]

They are very different transistors, but you can often get away with these sorts of subs. BC 'audio' transistors do have decent gain at band II frequencies.

[Nuvistor]

I think if it works correctly and is reliable then in that circuit it’s a good replacement, unfortunately that may not be true in other VHF front ends.

[Mike. Watterson]

I'd go for a BF series. The BC557 is marginal.

[paulsherwin]

I have used BC series transistors to replace BF194/5 Lockfits in the past and they do seem to work acceptably well. However, I do normally use proper VHF types because the specs are closer and the cost difference is marginal. There's never any harm in experimenting with what you have to hand though. Silicon transistors are surprisingly interchangeable in typical circuits so long as the basic ratings are adequate.

[G6Tanuki]

I've done it the other way round - using BF-series to replace the lockfits in the tone-control and audio-power stages on a Roberts R707 "Because there were ten BFs in the pack and I didn't want to break into a new pack of BC-series".

So far no issues with parasitic oscillations!

Biggest issue I have these days is swapping-in a modern transistor and finding I need to rebias: did this with another R707's tone-control stage a while back and found the new transistors were so 'gainy' that the collectors of both transistors ended up sitting at about 2.5V and caused serious asymmetric clipping/distortion when turning the volume up past half-way.

[Mike. Watterson]

Originally Posted by G6Tanuki

I've done it the other way round - using BF-series to replace the lockfits in the tone-control and audio-power stages on a Roberts R707 "Because there were ten BFs in the pack and I didn't want to break into a new pack of BC-series".

So far no issues with parasitic oscillations!

I'd not expect it as the BC series are often 60MHz. The BF are usually lower power and originally more expensive. No real difference now in price.

[paulsherwin]

The BC557 transition frequency is over 300MHz. All the common general purpose types are pretty similar in that respect.

[Mike. Watterson]

transition frequency is over 300MHz: i.e. RF Gain is rubbish before that. However I agree it can work to replace VHF parts.

Anyway, the point is that an ordinary bipolar RF transistor is no more likely to oscillate in an audio amp than any Audio BC series because they too have LOTS of gain at least 60 MHz.

[Argus25]

Originally Posted by OldTechFan96

In early 2017 I worked on my first vintage transistor radio, a Roberts R700. I did not really know anything about faultfinding or have any test gear bar a DMM. The R700 did not work so I decided to replace all of the RF/IF transistors since I'd read that the AF11*s can develop tin whiskers. The BC557 was the replacement of choice.

This included replacing the AF178 RF amplifier with a BC557. To my delight the radio worked well on FM. It was picking up everything I expected it to.

I don't think the BC557 is specified for VHF applications. I must have read somewhere that it can replace a AF178.

The two transistors look similar when I looked at the information provided by alltransistors.com.

I'm now aware that the AF178 does not suffer from tin whiskers and is generally reliable.

Was this replacement a good choice?

I first drew attention to the AF178 being suited to the replacement of AF11x transistors back in 2016 on this thread post #34:

https://www.vintage-radio.net/forum/...=131935&page=2

Up until that point nobody I had heard of in the UK had thought to use these in vintage radios, likely because they are a transistor intended for television applications.

The choice came about after a series of investigations because I did not like the smaller physical size and short lead wires of the AF12x series.

I identified the AF178, 2N2084 and the Tesla resin filled OC169 as suitable replacements. After this cat was out of the bag the popularity of these transistors increased, 2N2084's are hard to get now and the Tesla parts sold out. The 2N2084 had very long lead wires and a TO-5 case.

You will never have any problems dropping the AF178 directly into a circuit that previously had a whisker affected AF11x or Mullard OC169/170/171 etc.

If you change to a silicon transistor, a lot of the time it will work, but due to the bias conditions and the higher Vbe of the silicon, the collector currents can be a little lower and it is better to adjust the base bias resistor.This is especially the case for a mixer stage.

If you want the best result for a circuit, with circuit constants that were chosen specifically for the AF11x series, the AF178 is perfect (not affected by whiskers), or the AF12x (not affected by whiskers) if you don't mind the smaller sized part.

There is another part that is quite good in the same case as the AF178, they were used in car radios for a limited time in the early 1970's, the AF185, but they are hard to get.

[Argus25]

Actually it was way back in 2008 on this thread that the AF178 came up, how the years pass.

https://www.vintage-radio.net/forum/...ad.php?t=34779