Help with part replacement?

famous beagle

Well-known member
So I'm restoring an Akai X-150D reel to reel machine, and I'm looking for some replacements for some obsolete transistors:
Hitachi 2SC281.

I have found some originals on eBay, but the prices are very inflated ($10 or more for 1, etc.) compared to modern ones you'd get from Mouser, etc.

The problem isn't that I'm not able to find a replacement listed; it's that I'm finding too many listed. In three different sites, I've seen three different suggestions and have yet to see the same one repeated. (See attached.)

I'm wondering if anyone can help me sort out which one to trust (and maybe a little of the why)?

Thanks!
 

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That transistor is nothing special. In fact it is probably just a generic, Silicon planar traisitor, one of billions made and put in cans or plastic cases and stamped with the desired name an number.

You could use BC109,BC108, BC184L or one of score of other types numbers.
Why? The important parameters are,
VCE0 the operating voltage. That one is 30V I bet the circuit is 24V? So safe.

Ic max the current capability. 100mA, most small signal circuits work at 1 to 5mA WELL safe!

Hfe/Beta. Sort of current 'gain'. can never be exactly specified for transistors so they give you a range. The circuits therefore are designed so it does not matter.

The transistors in a tape recorder circuits are mostly non critical except maybe such areas as the bias oscillator? (they also VERY rarely fail! Wassup?)

Dave.
 
That transistor is nothing special. In fact it is probably just a generic, Silicon planar traisitor, one of billions made and put in cans or plastic cases and stamped with the desired name an number.

You could use BC109,BC108, BC184L or one of score of other types numbers.
Why? The important parameters are,
VCE0 the operating voltage. That one is 30V I bet the circuit is 24V? So safe.

Ic max the current capability. 100mA, most small signal circuits work at 1 to 5mA WELL safe!

Hfe/Beta. Sort of current 'gain'. can never be exactly specified for transistors so they give you a range. The circuits therefore are designed so it does not matter.

The transistors in a tape recorder circuits are mostly non critical except maybe such areas as the bias oscillator? (they also VERY rarely fail! Wassup?)

Dave.

Hey Dave,

Thanks for the info. I'm going to be recapping the audio cards on this unit (from 1967), and I thought while I did that I may as well do the transistors, too. Should I not?

What about the resistors? Should I bother with them? Should I at least measure them to see if they're within spec?

Thanks!
 
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Hey Dave,

Thanks for the info. I'm going to be recapping the audio cards on this unit (from 1967), and I thought while I did that I may as well do the transistors, too. Should I not?

Well I wouldn't. Transistors do not wear out, unlike electrolytic caps they have no chemicals in them.

I assume you have a service manual for the cards? That will give you the various DC voltages that should be on collector, emitter and base and if they are out by say more than 20% you MIGHT have a duff transistor but it is just as likely a leaky cap or a resistor* gone out of tolerance.

The manual will also give you the specifications: Levels, frequency response and distortion. The first three are easy to check BEFORE and after the re cap.

*I read of replacing capacitors all the time but never those crappy carbon composition resistors! They were noisy and tended to change value, especially the higher values > 100k say.

Dave.
 
Well I wouldn't. Transistors do not wear out, unlike electrolytic caps they have no chemicals in them.

I assume you have a service manual for the cards? That will give you the various DC voltages that should be on collector, emitter and base and if they are out by say more than 20% you MIGHT have a duff transistor but it is just as likely a leaky cap or a resistor* gone out of tolerance.

The manual will also give you the specifications: Levels, frequency response and distortion. The first three are easy to check BEFORE and after the re cap.

*I read of replacing capacitors all the time but never those crappy carbon composition resistors! They were noisy and tended to change value, especially the higher values > 100k say.

Dave.

Great thanks.

If I check and see that the resistors are within spec, do you think there would be any audio benefit to replacing them? If the unit doesn't seem too noisy to me now, should I bother you think?
 
Personally I wouldn't bother with resistors unless there's some problem or you have reason to believe they've been damaged or something.

The problem with checking to see if they're in spec is that you have to lift one end of each and then, if you do that, you may as well replace it anyway...You're half way there.
All that desoldering/checking/soldering is (in my clumsy experience) more likely to create a problem than solve one.

If there's a mic preamp path and you want to upgrade components there for noise then fair enough but for the rest I think I'd just be checking for expected voltages at a handful of places and just carrying on unless something's out.

Same for the transistors. If there's a fault or issue then, sure, find it and fix it.
If there isn't I'd just be voltage checking B-C-E and if they're in spec just leave well alone.
 
Weeeel. the great thing about digital meters is that you can check many resistors in situ because the meter does not output enough current to do any harm. Yes, you can reverse bias a diode or transistor junction but that is pretty obvious if 10k say reads 400 Ohms! If you know the value and it reads higher than tolerance indicates you got a bad'un.

But no, I would not suggest you check each resistor! I just mentioned the fact that old, carbon comp' Rs are not stable and noisier than modern MFs. You can check the noise against original spec and if worse look at collector loads.

Dave.
 
Weeeel. the great thing about digital meters is that you can check many resistors in situ because the meter does not output enough current to do any harm. Yes, you can reverse bias a diode or transistor junction but that is pretty obvious if 10k say reads 400 Ohms! If you know the value and it reads higher than tolerance indicates you got a bad'un.

But no, I would not suggest you check each resistor! I just mentioned the fact that old, carbon comp' Rs are not stable and noisier than modern MFs. You can check the noise against original spec and if worse look at collector loads.

Dave.

I was thinking more about parallel resistance from other components when in situ.
I've tested plenty of resistors and found them to be 'wrong' only to remove them and find they're right.
 
I was thinking more about parallel resistance from other components when in situ.
I've tested plenty of resistors and found them to be 'wrong' only to remove them and find they're right.

Yes, not having a pop but IF armed with a circuit it is fairly easy as a rule to work out a fault. I don't suggest it as a normal servicing process but such a quick 'cold check' can show up silly mistakes such as 'blobbed' tracks in home builds. Discrete transistor circuits are usually pretty simple.

I say again, I am NOT suggesting the OP does this.

Dave.
 
I have an old Akai 1800SD (Reel to Reel + 8 trk cartridge) I bought back in the late 60's, a 4000DS bought at a yard sale, and an M9 I got for free. The 1800SD works and functions fine except the 8 trk part is abit messed up. The 4000SD runs well also. The M9 I had to replace the caps that start and run the drive motor, but not much else. Akai built some good machines back then. If the one you have works maybe just replace the electrolytic caps, but not much else.
 
Nice!

Yes I already replaced the motor run cap because the playback speed started to slow down. That seemed to do the trick on that. I also lubed the motor, flywheel, and idlers. I just ordered the parts for a recap on the two audio cards and two other larger caps that are easy to get to (not sure what they do, but I figured while I'm in there). I think it should be good to go after that.
 
It would be good to notice if the transistors have silver or black leads on them. The Black leads did not start out that way but they are well known in some Akai decks for the black working up the lead to the substrate and then making the transistor defective in some way. Transistors can be replaced easily if you look at the transistor parameters and get something that will do the same job. There are circuits where a transistor that has an max Ic of 200mA can be replaced by one with a max Ic of 50 mA such as the KSC1845. The reason this can be done is if the circuit values are calculated the circuit only has a current of 8mA and so the 50mA transistor will not have a problem. I have stocked about 9 transistors that can be used in many places as long as their values are not exceeded even though I have about 60 cabinets with all kinds of transistors in them that might even be the one I need but knowing what will work by parameters will get you a lot farther along than looking for the exact part. A Fairchild KSC1845 is a much fresher transistor than some old 40-50 year old item that will cost you a lot and is lower noise I bet as well. Yes I am a BSEE.
 
It would be good to notice if the transistors have silver or black leads on them. The Black leads did not start out that way but they are well known in some Akai decks for the black working up the lead to the substrate and then making the transistor defective in some way. Transistors can be replaced easily if you look at the transistor parameters and get something that will do the same job. There are circuits where a transistor that has an max Ic of 200mA can be replaced by one with a max Ic of 50 mA such as the KSC1845. The reason this can be done is if the circuit values are calculated the circuit only has a current of 8mA and so the 50mA transistor will not have a problem. I have stocked about 9 transistors that can be used in many places as long as their values are not exceeded even though I have about 60 cabinets with all kinds of transistors in them that might even be the one I need but knowing what will work by parameters will get you a lot farther along than looking for the exact part. A Fairchild KSC1845 is a much fresher transistor than some old 40-50 year old item that will cost you a lot and is lower noise I bet as well. Yes I am a BSEE.
Agree with that but did not know of the 'creeping black rot! The BC184/BC214L will replace pretty much most small signal NPN or PNP transistors. Always check the BEC voltages before and after or with reference to the manual. Most transistor circuits are self stabilizing over a wide range of hfe's. (current gain). Farnell have them.

Dave.
 
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