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Old 19th Sep 2021, 5:40 pm   #1
lloydwells
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Default Transistor identification help please .

I have a Discatron Combo made 1967 this is the one with a LW MW radio and a record player I have two dead transistors other than having the appearance of being made by Newmarket I can't find any writing on them one has N5 on it the other M3 please see attached pic these are the output stage and usually have heat sinks attached.
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Old 19th Sep 2021, 6:32 pm   #2
G.Castle
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Default Re: Transistor identification help please .

Reasonable equivalent to AC 187/188 1A 1W
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Old 19th Sep 2021, 11:48 pm   #3
lloydwells
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Default Re: Transistor identification help please .

Hi G.castle that still doesn't tell me what the originals are though.
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Old 19th Sep 2021, 11:50 pm   #4
Chris55000
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Default Re: Transistor identification help please .

Hi!

Unfortunately the "Discatron" S.M. from "up top" has no circuit info in it!

Data on Newmarket transistors is a bit hard to come by, but if your Discatron Amplifier panel is transformerless o/p., as I think they are, than the o/p pair is most likely to be NKT223 (PNP) and NKT772 or NKT773 (NPN), if these devices have failed, you can use AC128/AC176 or the more recent AC187/AC188 – note, if you can get ACxxx transistors suffixed "/01", these are fitted in a small square block with a mounting hole, allowing convenient mounting to a heatsink or chassis!

It was common practice for germanium TO–1 o/p devices to be coded with a red collector dot for PNP and a white or blue collector dot for NPN, but this wasn't totally consistent in some cases!

It's easy to find out which is which –if the amplifier is of the usual complementary–symmetry type where the o/p emitters are connected together by low value resistors (2.2 to 4.7 Ω typical), the transistor with its collector (spot) lead connected to battery +ve will be NPN, whilst the PNP collector will go to battery negative!

The mid–point voltage for a 9V battery supply is usually about 4.75V to 5V w.r.t. the positive battery terminal, and the quiescent battery current about 7-12mA.

Under no circumstances should any speaker of less than 15 ohm impedance be used with germanium o/p transistors unless the circuit is designed for low–impedance units!

Chris Williams
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Last edited by Chris55000; 20th Sep 2021 at 12:05 am.
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Old 20th Sep 2021, 5:05 am   #5
G.Castle
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Default Re: Transistor identification help please .

Quote:
Originally Posted by lloydwells View Post
Hi G.castle that still doesn't tell me what the originals are though.
Sorry, I thought you may have wanted to replace them if they are faulty with something that's not completely unobtainable.

The transistors you have if NKT are house numbers so anyones guess. They would have been selected on the required spec of the manufacturing customers request, so would not be expected to carry commercial type numbers.

The information I have shows NKT271 and NKT773 pair for the AF output.
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Old 20th Sep 2021, 12:27 pm   #6
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Default Re: Transistor identification help please .

Could be one of the Newmarket modules: http://www.wylie.org.uk/technology/s...rket/NKTPC.htm. Discatron used one of the more common Mullard modules in at least one model.
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Old 23rd Sep 2021, 5:43 pm   #7
lloydwells
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Default Re: Transistor identification help please .

This isn't a module the tuner and the amp are one pcb
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Old 23rd Sep 2021, 5:51 pm   #8
paulsherwin
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Default Re: Transistor identification help please .

As others have said, an AC187/8 pair should be OK. These early Ge output transistors are all much of a muchness. You should also be able to fit a BC327/337 pair with some bias changes.
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Old 24th Sep 2021, 6:44 pm   #9
G.Castle
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Default Re: Transistor identification help please .

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Originally Posted by paulsherwin View Post
As others have said, an AC187/8 pair should be OK. These early Ge output transistors are all much of a muchness. You should also be able to fit a BC327/337 pair with some bias changes.
..... And would cost but a few pence, (they would fit inside the TO1 case too if you're into re-stuffing).
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