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Need transistor replacements for R2R amp boards. 2SC631A, 633A, 634A.
Need transistor replacements for R2R amp boards. 2SC631A, 633A, 634A.
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Old 28th October 2016, 08:27 PM   #1
ChopperCharles is offline ChopperCharles  United States
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Default Need transistor replacements for R2R amp boards. 2SC631A, 633A, 634A.

I have a Sony TC-377 with some noisy transistors. Left channel sounds like ocean waves, and the right channel just has a lot of hiss (way more on source than when playing tape too). Looks like it's a complement of 2SC631A (x10), 2SC633A (x2), and 2SC634A (x6) between the recording and playback amplifier boards. Plus two 2SC634A's on the bias oscillator board.

Oh, and a 2SD291, a 2SC634A, and a 2SC633A on the power supply board.

So, the million dollar question is, what are the replacements for the transistors? Also, is it worth messing with the transistors on the bias oscillator board and power supply? They're all the same noisy series... but does it matter on those boards?

Thanks a bunch!

Charles.
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Old 30th October 2016, 10:44 AM   #2
indianajo is offline indianajo  United States
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I've got a TC366 deck and never found it to be particularly noisy.
2SC631, 2sc633, 2sc634, all cross to RCA SK3250. This is an npn 50 v Vceo junction transistor. These are to92m case with e,c,b pinout, so you better pay the premium for the NTE equivalent. All the npn transistors I have experience with are e,b,c
Don't expect much from NTE parts. I find the datasheets have half the specifications of industry numbers they are equivalent to.
I wouldn't expect noise to come from transistors on oscillator or power supply boards. Sometimes paralleling a zener reference diode can help PS noise. Or more modern voltage reference IC's.
At this age resistors are as likely to be noisy as transistors, so check the noise source carefully before heating up the soldering iron. Modern metal film resistors are quieter than carbon comp or carbon film, especially over 100 kohms.
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Last edited by indianajo; 30th October 2016 at 11:03 AM.
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Old 31st October 2016, 07:49 PM   #3
ChopperCharles is offline ChopperCharles  United States
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RCA SK3250 is an obsolete part, I can't cross to anything with that.

I've gotten the 2SC633 and 2SC634s to cross to a KSC-1815 with a reasonably close match. Both of these have an Hfe of 65, and the KSC1815 is 70, so that's a good match.

The 2SC631 however has an Hfe of 130, and I can't find anything modern that comes close. The KSC1845 looks great, but the only version available has an Hfe of 300.

Thoughts?

Charles.
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Old 1st November 2016, 12:24 AM   #4
indianajo is offline indianajo  United States
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There are some industry number B,C,E TO 92 transistors around, but I don't keep datasheets on any of them. You could take one of those and flip it around backwards with the flat on the bottom instead of the top, to fit in a 2SC633 board.
You know that NTE stole the RCA catalog line and replaced all the SK numbers with NTE don't you? NTE never gave away any catalogs that I knew of, so everybody ordered the RCA numbers but received NTE parts until RCA was dead. You might find some NTE3250 around somewhere.
As far as the excess gain, that is the story of parts these days. Gain 300 or no part at all are your choices. I'm messing around this week with way too much gain through an APEX AX6 due to gain 400 2n5401 (On semi) and maybe gain 300 GE D44R2. There is no upper limit on gain on the datasheets but it is causing problems in my circuit. Volume knob at 20% is blasting me out of the room. You can fix gain by modifying the circuit to use the 4 resistor biasing, or by adding a negative feedback resistor, but I haven't done either successfully yet. I just connected my emiter to some random point, not the feedback capacitor, my ohmmeter tells me.
Happy shopping
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Old 2nd November 2016, 06:44 PM   #5
df_genius is offline df_genius  United Kingdom
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Use O grade for 2SC1815 if you can find them, HFE is 80-140 and you can then select the lowest gain ones with a multimeter. There is also a 2SC1815L (low noise version) which could be beneficial
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Old 14th March 2019, 07:02 AM   #6
latreche34 is offline latreche34  United States
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I just jumped in the same boat recapping the Sony TC-580 my transistor list is:
2SA678
2SC632A
2SC634A
2SC867
2SD291
Any changes since 2016?
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