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Does the 2sc2878 have a replacement?

7367ruffles

7367ruffles

New Member
I want to replace the 2sc2878 transistors in my Yamaha AX-390 amplifier’s tone controls, as there is an audible hiss through the speakers and headphone jack, which increases/decreases in intensity as I boost/cut the treble using the knob.

So far I’ve uses the Toshiba 2sc2878 datasheet and entered the maximum ratings and electrical characteristics (where applicable) into Digikey’s “Transistors- Bipolar (BJT)” page and came up with the ON Semi BC337, which matches or exceeds the specifications except Vbe. Where the Vbe of the 2878 is 25volts, the Vbe of the BC337 is only 5 volts.

Would it be okay to use the BC337, or do I have to use another substitute?
 
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This is a tough one.

your original device 2sc2878 is a TO-92 package (small plastic half-cylinder) and yet it has a vebo rating of 25 V. That’s really high for a typical TO-92 device. Finding a modern replacement TO-92 or TO-126 packaged device might be tough.

I’ve looked through Mouser.com and I don’t see a good match that meets 25V ebo.

the bc337 has a problem in that the pin out is CBE whereas your original device is ECB. If you wanted to use the bc337, you’d have to twist the legs or leads as you installed it and therefore have to put insulators in those legs to prevent shorting. Totally doable, just tricky. And, as you said, the bc337 ebo doesn’t match the 25 V spec of the original device.

so, the next best step would be to determine how much Vebo you actually need in that circuit at this device. If it’s 5 V or less, there are plenty of transistors that you could sub in there. If it is really high, more than 8 V or so, you choices will be really limited.

Also, you can check at bdent.com work o see if they have any of the original 2sc2878 left in stock.
 
You need a high V(EBO) if you want to use it as a muting transistor. If you use a BC337 it's just going to zener to maybe 7 V and will destruct if the base current becomes too high. I guess Yamaha used this device because the (un)mute voltage is -15 V and this transistor doesn't zener. If you suspect it's noisy just remove it because it is only used for muting which will not work with them (Q303/4) removed.

Tjerk, 9ZZ
 
Looking at the service manual the two 2sc2878s are indirectly (other non-transistor passive components in between) in the path of the treble potentiometer. If i take them out, the treble control would be open-circuit.

Bdent is unfortunately out of stock.
 
dlucy said:
If a genuine 2SC2878 couldn't be found, could something like the 2SD2704 work (after being mounted to an adapter)?
Click to expand...
Pc is only .2w though, half of the 2878 at .4w, and a slightly lower RON at .7 Ohm vs 1 for the 2878. But it would be worth a try if it survives the Pc. Maybe a small heat sink attached w/ thermal epoxy will help it.
 
eizner23 said:
2sc2878 here
Click to expand...

New old stock? Usually (when I’m only using them for fun) I love NOS but I’d prefer to have a modern replacement that is active on Digikey if possible.

dlucy said:
If a genuine 2SC2878 couldn't be found, could something like the 2SD2704 work (after being mounted to an adapter)?
Click to expand...

Now that’s pretty interesting! I wonder how I could make a TO-236 SMD to TO-92 through-hole adaptor that could fit inside the amp without mods, though.
 
7367ruffles said:
New old stock? Usually (when I’m only using them for fun) I love NOS but I’d prefer to have a modern replacement that is active on Digikey if possible.
Click to expand...

We would all like there to be modern replacements for everything. But here in reality, that is just not the case. NOS (when available) is drop-in with no mods or adapters needed. The problem with SMD is dissipation.. They just can't take the heat. :D
 
7367ruffles said:
so an adaptor is the only way?
Click to expand...
For substituting SMD for through-hole, it's the most practical way. You could solder leads directly to the device, but it would be a hassle, and you risk desoldering them when you solder the leads to the board. I suppose using a heat sink on the leads would help, but that would be impractical in some instances because of clearance/room to work. The adapters are uber cheap, and barely larger than a TO-92L too. :D
 
FootFungus said:
The 2SC3326 is the surface mount version of the 2SC2878 available at Mouser. You'll need a SOT-23 adapter for it like Avionic linked to.
Click to expand...
Oh my, that is an exact match spec-wise.

Also, how does the DC current gain affect the operation of the tone controls?
 
And for the often-maligned 2SA798 and 2SA979 devices, this is an similar option:



With one of these:

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