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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 30th Jun 2019, 6:14 pm   #1
TonyDuell
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Default AC188 tin whisker problem

Yes, the transistor is an AC188 according to the service manual. But Towers says nd AC128 is an equivalent.

The unit in question is a Philips N2211 cassette recorder. The initial problem was it would only play quietly and with very nasty distortion if you tried turning the volume up.

The audio amplifier is DC coupled with a silicon NPN driver transistor (originally a BC148, but the one in my machine is not Lockfit) drivng a complementary pair (AC187 and AC188) output transistors.

I checked the DC voltages, they were all over the place with the midpoint of the output pair being at about 1V (should be half supply voltage or around 3.75V). Removed the PCB, did some tests and found the voltages were normal. In the end I found the heatsink bracket of the output pair was sitting at about 3.6V. It's earthed ti the chassis when the PCB is fitted.

More tests showed there as a low value leak between the heatsink bracket and the base of the AC188. Yes, a tin whisker short. I zapped it with the bench supply, the machine now works fine -- for the moment.

But my experience is that while zapping the short will help with faultfinding, it's not a reliable cure, the problem will come back.

I'd like to replace the pair of output transistors, so my question is : Does anyone have any suggestions as to what to use?
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Old 30th Jun 2019, 6:43 pm   #2
paulsherwin
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Default Re: AC188 tin whisker problem

This is becoming more of a problem. Presumably it's taken longer to emerge because the cases aren't normally connected to anything.

You could convert it to a Si output pair using a BC327/337 or BD139/140, but it may not be straightforward. I guess it depends on your skill level.
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Old 30th Jun 2019, 7:10 pm   #3
Electronpusher0
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Default Re: AC188 tin whisker problem

Hi Tony, I have a NOS pair of AC128 / AC127 you can have FOC (some recompense for the Maplin wind unit).
These are NOT the types that are fitted inside aluminium blocks.

Since tin whiskers only grow in the presence of an electric field NOS ones should give years of service.

Peter
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Old 30th Jun 2019, 8:07 pm   #4
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Default Re: AC188 tin whisker problem

If you are considering going silicon, consider a BFY51 and either a BFX29 or a BFX88 as its complement.

I've used these in the past - with the press-on aluminium anodised "star" heatsinks - to deliver around 3 Watts to an 8-Ohm speaker when using 24V as the supply.

Of course if you do this you'll need to increase the bias - and also if there's a temperature-sensing diode in the bias network that's fitted close to the output-pair this will probably be a Ge device so you'd be wise to replace it with a Si one to get better thermal 'tracking'.

Alternatively - ditch the old output-stage and slot in something like a LM380 or LM384 IC. Go for the full 14-pin version rather than the cut-down low-power version with only 8 legs. With only four additional components [input/output-capacitors and a R=C Zobel-network] you'll get a couple of Watts easily. See http://sound.whsites.net/project160.htm
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Old 30th Jun 2019, 9:46 pm   #5
60 oldjohn
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Default Re: AC188 tin whisker problem

Quote:
Originally Posted by Electronpusher0 View Post


Since tin whiskers only grow in the presence of an electric field NOS ones should give years of service.

Peter

Have you metered Can to Leads? all my NOS AC127s bar one were useless.


John.
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Old 30th Jun 2019, 10:14 pm   #6
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Default Re: AC188 tin whisker problem

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Originally Posted by 60 oldjohn View Post


Have you metered Can to Leads? all my NOS AC127s bar one were useless.


John.

I have just checked both the AC127 and AC128 I put aside for Tony and both are completely open circuit to the can (leads connected together, highest ohms range on a digital meter)

Peter
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Old 30th Jun 2019, 10:38 pm   #7
Herald1360
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Default Re: AC188 tin whisker problem

Quote:
Originally Posted by Electronpusher0 View Post

Since tin whiskers only grow in the presence of an electric field NOS ones should give years of service.

Peter

I don't think that's the case.... though it may affect the rate, perhaps.
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Old 1st Jul 2019, 12:57 am   #8
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Default Re: AC188 tin whisker problem

Temperature might also affect the rate, not sure either.
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Old 1st Jul 2019, 5:00 am   #9
TonyDuell
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Default Re: AC188 tin whisker problem

My experience is that tin whiskers can form even when the device is not powered. I've had transistors with tin whisker faults, zapped them, and they seem OK. The device works. But if I turn it off, then try it again perhaps a week later, the whiskers are back.

My aim when doing a repair/restoration is to keep the circuit design as close to the original as possible. So no way would I put an IC amplifier in there. Going to silicon is going to be non-trivial, the bias would need changing, there is both a diode (which appears to be silicon, it has a 'BA...' number) and a thermistor in the circuit.

A friend (not on the forum) has suggeted something that I thought of as well. As these transistors seem to short base-can (and still work as transistors), simply isolate the can from the chassis. Possibly those 'greaseless washers' used with more modern power transistors could be adapted for this.
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Old 1st Jul 2019, 5:13 am   #10
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Default Re: AC188 tin whisker problem

Quote:
Originally Posted by Herald1360 View Post
Quote:
Originally Posted by Electronpusher0 View Post

Since tin whiskers only grow in the presence of an electric field NOS ones should give years of service.

Peter

I don't think that's the case.... though it may affect the rate, perhaps.
Having just checked again you are correct, I withdraw my statement ( the offer of the transistors still stands)

Peter
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Old 1st Jul 2019, 6:14 am   #11
Argus25
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Default Re: AC188 tin whisker problem

Mercifully, there are 2 designs of the AC187/188 style transistors. The originals (tin whisker prone) and the Telefunken style recognized from their smaller outer diameter. They are filled with hard resin and immune to whiskers, they come free like this example:

https://www.ebay.com/itm/AC187-NPN-A...oAAOSwab1b1tRL

or pressed into rectangular heat sink adapters like these versions:

https://www.ebay.com/itm/GC511-GC521...IAAMXQwMdREU4a
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Old 2nd Jul 2019, 1:32 pm   #12
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Default Re: AC188 tin whisker problem

Quote:
Originally Posted by TonyDuell View Post
simply isolate the can from the chassis.
That's exactly what I always do - works for me every time
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Old 4th Jul 2019, 7:28 pm   #13
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Default Re: AC188 tin whisker problem

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Originally Posted by Techman View Post
Quote:
Originally Posted by TonyDuell View Post
simply isolate the can from the chassis.
That's exactly what I always do - works for me every time
If there's a proven track record of them still functioning and not developing further shorts that's the route I'd take, but I have also had success carefully drilling 2 1.5mm holes in opposite sides of the can (I manage freehand but would really recommend a drill press and vice for safety) and then "washing" the can out with aerosol brake cleaner, or even Servisol, before sealing up the hole again with either a blob of solder or piece of heatshrink.

This has worked in a similar amplifier using AC176(whiskered) and AC128 (fine) as the outputs. No apparent issue with overheating despite the thermal grease being sluiced out. And as mentioned, I've had numerous NOS Germaniums be whiskered and even more dead than the one they were to replace. (I only buy them when I'd like a spare in case I mess up in cleaning them!)
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