Yet another butchered 250

[Jose Jimenez]

Another pair would be the 2N4900/2N4912, but these devices are dinosaurs. The NPN devices are sometimes linear enough to be used in an audio circuit, but besides the gain mismatch, the PNPs leave a lot to be desired. I read this post and was curious enough to test some of these as to reinforce my opinion on them. Previously, I made comment that Marantz did a better job with their MJ and SJ devices in general when these amplifiers were new than Mcintosh with their transistors. The curve tracer is a valuable tool.

For the TO-66 drivers in Marantz amplifiers, I use MJ3238 and MJ3248 devices that are Motorola transistors designed for audio applications, especially drivers. They came out in the very late 70s and these are about the best you can use if you want the amplifier to perform, not just function. I save all the older TO-66 and TO-3 transistors and test them to see how well they held up since the 70s.

If the amplifiers attributes matter and you're not just seeking a good-looking sine wave on the scope, I would definitely move to replacing and adapting the newer MJE15032 (NPN), MJE15033 (PNP) devices or similar.

 

[Stoddrob]

Have you ever tried 2N6211 and 2N3583 in 240/250/1200 power amps that have TO-66 drivers?

Tom

Tom, I purchased Motorola 2N6211 and 2N3583 TO-66 Transistors. I assume you have some experience with them? I first installed them in the left channel. I then checked BIAS. Before, when I used 2SA1837 and 2SC4793, I got no BIAS reading at all. Of course, I then discovered Q511 was in backwards and it didn’t appear to be an easy method to mount it in properly. So I decided to go the TO-66 route with 2N6211 and 2N3583 as you “suggested.”

Now, I get 36.6mV at R531 and R532 with pot completely CCW. I can only adjust up. DC Offset is mostly under +/- .5mV but it jumps around a little between -1.2mV and +1.2mV. That’s actually an improvement since I wasn’t even getting a BIAS reading before. However, I also noticed the heatsink getting very warm.

Do you have time or care to make any comment and offer any suggestion?

Rob

 

[Stoddrob]

I didn’t trust these transistors. So I installed the original Marantz Motorola SJ2586 and SJ2585 TO-66 Transistors. BIAS was at 12.2mV and gradually went up after 30 minutes to 13.3mV.

 

[quadmatrix82]

I’ve had good success with the subs Tom recommended however they do need to be matched as per the service manual notes

 

[Stoddrob]

I’ve had good success with the subs Tom recommended however they do need to be matched as per the service manual notes

I think finding genuine and authentic new-old-stock sub components is priority one. Being able to buy enough to match as close as possible is certainly also as important. I will try some of the others I have and I will likely have good success. I just don’t believe the “NOS Motorola” ones that I got from vendor (not eBay) that were tested and matched were authentic and genuine. I think they were recently manufactured. Thanks for your feedback and comment.