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Old 15th Jan 2020, 2:49 pm   #1
dave_n_t
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Default Tester for AF117s (etc.)

I was recently restoring one of my favourite transistor sets, the Marconiphone T84B (which shares a chassis with the HMV 1421 and the Ferguson 626BT). This set looks like a valved set, and originally ran from the somewhat rare PP10 battery. The sound reproduction is excellent, and again reminds one of a valved set.

The radio has 2 OC171s and 3 OC170s on-board, so, not surprisingly, tin whiskers reared their ugly heads. I managed to recover the mixer/oscillator OC171 (strap together EBC, and discharge a 4700uF 25V electrolytic, charged to 20V, between the 3 strapped wires and the shield). Indeed, going thru' my stock of AF11x and OC17x transistors showed that this method is pretty successful(*)

But I hankered after a test to show the transistor was working well (so that, after replacing in a set - like the one I was working on - I wouldn't be disappointed). I ended up making a VHF oscillator (based on the mixer/osc circuitry which seemed to be almost universal in the early transistor VHF sets). It uses a small VU meter (and germanium diode rectifier) to indicate that the circuit is actually oscillating. I tweaked the tuned coil (by spreading its winding) so that the oscillator runs at around 100MHz. Activity can be verified by bringing a portable FM receiver close up (with the telescopic aerial collapsed). A working oscillator will blot out one area of the FM band.

Attached is a photo of the built-up oscillator ('ugly' style on a scrap of PCB), and the circuit diagram.


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Notes:
1. The tuned winding is 4 turns of thickish - maybe 20swg - enamelled copper wire, wound on a pencil (which is then removed).
2. The output coupling winding is 3 turns of solid core hookup wire, similar diameter, placed alongside the tuned winding.
3. The rf choke in the collector circuit was a 22uH commercial device of a similar size to a 1/4 watt resistor
4. The screen pin of the transistor goes to earth (i.e. battery +ve)
5. I used a 4-pin transistor socket to make it easy to swap in other transistors
6. The VU meter reads about 140uA full-scale; a good transistor gives a reading of about 30% of this (i.e. about 40uA)
7. In terms of ability to oscillate at around 100MHz, there was no discernible difference between any of the markings: AF114-5-6-7, and OC169-70-71.
Amongst the 'alloy diffused' types, AF102s were the 'strongest' (as expected, perhaps).

(*) only about 5-10% of my extensive stock of AF11x and OC17x transistors had whiskers from shield to EBC; typical DC resistance of the 'short' was in the range 25-75 ohms.
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Old 15th Jan 2020, 3:10 pm   #2
paulsherwin
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Default Re: Tester for AF117s (etc.)

A useful little project.

It's now generally believed that the AF114-7 and OC169-71 were all the same transistor, but given different codes for some odd marketing reason. Mullard even made them appear different in their data books by quoting different characteristics. It used to be thought that they were selected for particular characteristics before marking, but that now seems unlikely.

The trouble with zapping the whiskers is that they will grow back eventually, though perhaps not for many years. It's also the case that zapping will do nothing to whiskers which are well advanced but not quite causing a short.
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Old 15th Jan 2020, 3:17 pm   #3
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Default Re: Tester for AF117s (etc.)

Good project, for radios where I change transistors I put in turned pin IC socket pins, saves destroying the usually fragile PCB. It also enables experiments with PNP Si devices, which, surprisingly enough, work very well indeed.
 
Old 15th Jan 2020, 8:46 pm   #4
dave_n_t
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Default Re: Tester for AF117s (etc.)

Quote:
Originally Posted by paulsherwin View Post
The trouble with zapping the whiskers is that they will grow back eventually,
Agreed. But possibly not in my lifetime .

Quote:
Originally Posted by paulsherwin View Post
It's also the case that zapping will do nothing to whiskers which are well advanced but not quite causing a short.

Agreed. This may be the reason why some devices oscillate rather weakly - unwanted capacitances between sundry electrodes and case (earth).
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Old 15th Jan 2020, 8:50 pm   #5
dave_n_t
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Default Re: Tester for AF117s (etc.)

Quote:
Originally Posted by merlinmaxwell View Post
Good project, for radios where I change transistors I put in turned pin IC socket pins, saves destroying the usually fragile PCB.
So do I (although the specific set here has a useful B9A socket atop the screened VHF module, which can be used - albeit with no screening - at a pinch).

Quote:
Originally Posted by merlinmaxwell View Post
It also enables experiments with PNP Si devices, which, surprisingly enough, work very well indeed.
Indeed they do (in fact a couple of 'random' Si devices oscillated so strongly that the VU meter was overloaded! )
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