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Old 31st May 2019, 11:21 pm   #1
'LIVEWIRE?'
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Question Information about a plastic encapsulated (2S)c107

A transistor marked c107 is used as the relay switching device in a TEAC AV Receiver model #CR-H257i. It operates a 12v relay whose colil resistance is 250 ohms, and which draws only a few mA when energized. It appears to be a conventional NPN transsitor, but both Towers and a couple of on-line data sheets I've looked at state that the 2SC107 is a metal cased transistor capable of handling up to 1500mA (Ic). The transistor in this receiver appeared to be O/C twixt base & collector. I've fitted a 2SC1815, but haven't yet applied power. Can anyone shed any light on the fact that two apparently totally different transistors have the same type no. HiFi engine doesn't, as I hoped, have a service manual for the CR-H257i
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Old 1st Jun 2019, 12:01 am   #2
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Default Re: Information about a plastic encapsulated (2S)c107

The only other explanation is that it might be a C107 thyristor in the transistor package?
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Old 1st Jun 2019, 12:18 am   #3
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Default Re: Information about a plastic encapsulated (2S)c107

It's not unknown for transistors which started off in a metal case to be later made in TO92 or something similar. The 2N2222 is like that.

Power it up and see what happens. The worst that can happen is it'll go bang, and there's no shortage of 2SC1815s around.
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Old 1st Jun 2019, 8:42 am   #4
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Default Re: Information about a plastic encapsulated (2S)c107

I'll follow your suggestion, Paul. It's not a thyristor, restoration 73, because on the PCB is clearly marked B-E at the two outer connections for the device, which surely must mean Base & Emitter. I did wonder, after fitting the 2SC1815, whether the 107 was a Darlington Pair, but why would such a high gain device be needed when the relay it operates draws only a few mA from a nominal 12v supply. The coil is rated for 12v so the voltage across it obviously shouldn't be any higher than that. The circuit is a standard relay switching one, presumably triggered from the standby supply. Given the above, and the fact that Towers states that the Ic(max) of a 2SC107 is 1500mA, I wondered why a transistor capable of supplying maybe >100x the current needed by the relay would have been chosen in the first place.

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Old 1st Jun 2019, 5:21 pm   #5
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Default Re: Information about a plastic encapsulated (2S)c107

Does the relay have a reverse biased diode wired across the coil?

The 'back EMF' created when the transistor is turned off can be much greater than the supply voltage.
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Old 1st Jun 2019, 6:47 pm   #6
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Default Re: Information about a plastic encapsulated (2S)c107

There is a silicon diode wired, as far as I can tell from the circuit layout, in parallel with the coil in the usual manner, but I admit I hadn't taken the back e.m.f. into account. Presumably the designer(s) did, though I don't know what caused the 2SC107 to fail partially o/c. Hopefully the 2SC1815 will stay the course, particularly as the CR-H257i is a pig to dismantle, and the supplementary (standby) PSU circuit is on a board beneath another one.
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Old 1st Jun 2019, 6:57 pm   #7
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Default Re: Information about a plastic encapsulated (2S)c107

It could be a 'digital transistor' as we called them

Is it this one?
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Old 1st Jun 2019, 6:58 pm   #8
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Default Re: Information about a plastic encapsulated (2S)c107

Which incidentally is why it could read OC.
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Old 1st Jun 2019, 10:37 pm   #9
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Default Re: Information about a plastic encapsulated (2S)c107

You could be right, Mooly, in which case the 2SC1815 I fitted won't work without added resistors. The circuit extract you posted appears very similar to that in the TEAC CR-H257i. The fact that the transistor in the circuit you posted is a KRC107M is too much of a coincidence. The TEAC was brought to me not working at all, supposedly because the internal fuse had failed (it hadn't!). I've known about those 'digital' transistors for years, it just never entered my head that the 107 might be one of them!. The relay wasn't being switched on, but it seems I may have jumped to the wrong conclusion, based on a meter reading which I assumed meant the base/collector junction was o/c. Only thing is that I was getting a normal base/emitter reading using the diode test function on my DMM.
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Old 2nd Jun 2019, 8:44 am   #10
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Default Re: Information about a plastic encapsulated (2S)c107

BTW, that circuit excerpt wouldn't be from a TEAC CR-H257i or a related model, would it?
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Old 2nd Jun 2019, 8:53 am   #11
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Default Re: Information about a plastic encapsulated (2S)c107

Yes it was, the lesser H255? I think.

Have you looked at what the base is doing, I would assume either a 0 or 5v level on there.

Also you could try manually shorting C and E to fire the relay and see what happens.
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Old 2nd Jun 2019, 9:55 am   #12
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Default Re: Information about a plastic encapsulated (2S)c107

Just looking now at the manual for the 225/255.

Have you checked that the auxilliary standby supply to the uP is OK? Standby transformers going OC could be a possibility if the thing is 100% dead.
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Old 2nd Jun 2019, 10:57 am   #13
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Default Re: Information about a plastic encapsulated (2S)c107

Thanks for confirming that the diagram is from one of that TEAC Series, Mooly. The standby transformer resistance measures OK, both on the primary and secondary, as do the rectifiers associated with it, but I haven't tried shorting the C-E- connections, though I know that the relay works. It'll be Thursday or Friday before I get time to go back the the TEAC, but it looks as if I'll have to remove the 2SC1815 and replace it with a 'digital' transistor - I have a few somewhere, though not of the exact type used-I can't refit the original as it's Base leg broke off when I was removing it!! B.T.W I've never known how to successfully ckeck transistors of that type, since a conventional meter (Digital or Analogue) won't check them due obviouslt to the internal series & shunt resistors. I've not tried one on one of those Chinese testers or in the transistor test socket on my DMM, but I suspect either would give false readings.
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Old 2nd Jun 2019, 2:37 pm   #14
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Default Re: Information about a plastic encapsulated (2S)c107

I think the only realistic static test is to confirm that C-E isn't short or leaky when connected to a suitable meter on a high-ish ohms range while observing correct test lead polarity.

Check the actual 5 volts supply is correct.
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Old 12th Jun 2019, 12:00 pm   #15
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Default Re: Information about a plastic encapsulated (2S)c107

Since my previous post, I've found that I need a KRC107M or equivalent, but cannot find a supplier, having tried CPC, Cricklewood Electronics, etc., other than two eBay sellers (One is DigiKey) who are quoting maybe 20-30 day delivery times.
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Old 12th Jun 2019, 1:51 pm   #16
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Default Re: Information about a plastic encapsulated (2S)c107

It appears to be just a PNP switching transistor with integral bias resistors. Could you fit something like a BC557 and fit the resistors externally?

https://www.alldatasheet.com/datashe...C/KRC107M.html

You can't use a 2SC1815 as that's NPN.

[Correction - it's NPN, not PNP. See following posts.]
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Old 12th Jun 2019, 2:50 pm   #17
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Default Re: Information about a plastic encapsulated (2S)c107

That description looks wrong Paul. Its definitely drawn as NPN (on the data sheet in your link) and also on the one I looked at (post 7).

The actual circuit of the unit shows NPN is the correct fitment.

It's dead easy to make something suitable. Assuming the base is logic driven from a uP then pick something that allows sufficient current to saturate the transistor. A 1k5 or 2k2 should be fine. The resistor to the emitter will be just to stop the base floating (if the uP has such a state) or simply there 'because it has to be' if you choose such a transistor type as these. 22k should be fine for that one.
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Old 12th Jun 2019, 3:00 pm   #18
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Default Re: Information about a plastic encapsulated (2S)c107

You are absolutely correct. The datasheet in my link clearly shows it to be NPN, and Alldatasheet have misdescribed it. Thanks for the correction.
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Old 12th Jun 2019, 10:40 pm   #19
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Default Re: Information about a plastic encapsulated (2S)c107

I did actually think of fitting external resistors, leaving the 2SC1815 in circuit. Hadn't spotted the error in Alldatasheet's description, though I knew that the transistor was a NPN type. Incidentally I found a manual for the CR-H257i on HFi Engine, but they list it under 'Combination Receivers', not as an 'AV Receiver'

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Old 13th Jun 2019, 7:17 am   #20
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Default Re: Information about a plastic encapsulated (2S)c107

I think that transistor is a FET
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