I am visiting my brother, helping him with his Nak PA-7 amp. There is a no-bias condition on the Right amp, with the bias pot having no effect. Left amp is working fine. I am about to pull the bias transistor (2SC3333) on that side to check it. It is obsolete. I was able to find MPSA42 as a potential substitute. The main difference I see is that the max Vce (sat) for the 2SC3333 is 1.5V, vs. 0.5V for the MPSA42. Datasheets below.
Is a MPSA42 an acceptable sub for 2SC333 in the bias circuit of the PA-7(attached, showing the better labeled Right amp)?
Is a MPSA42 an acceptable sub for 2SC333 in the bias circuit of the PA-7(attached, showing the better labeled Right amp)?
Keep in mind that an MPSA42 will have pinout EBC, while a 2SC3333 is likely ECB. Some re-arrangement of leads will be necessary if you sub an MPSA42.
Thanks, yes, 2SC3333 is ECB. I have marked the bag of 10 of the MPSA42 that we bought yesterday with EBC, as a reminder, if we end up using them.
Turns out it was the bias pot that was bad. Replaced that and all good, so no need to test the transistor substitute.
Was it one of those rotten flimsy open-frame carbon pots? They can go bad - one of those in a non- fail-safe Vbe multiplier circuit opened up and smoked the output transistors on a small Sony receiver. I never looked at Sony the same way after that experience. Quality? Hah!
Yep, one of them. Tiny little thing. It is a 150 Ω pot, but was reading about 335 Ω across the ends. The same pots were in my PA-5, which I replaced with Bourns sealed pots a few years ago, and one of those originals had drifted way high as well.
When I touched the outer circumference of this one, I got small arcs of plastic in my hand, and then the whole ring of plastic came off. Looking at the remaining pots in the unit, they all have cracks in that small plastic ring. Obviously, for this particular plastic, 40 years is too long.
I have to say that I too have been surprised by Nakamichi's use of these pots, as they are rather flimsy, regardless of the plastic issue. I doubt that the issue of how long the plastic would last before such embrittlement was a consideration back then, and probably much less was known about that issue then. But still, for as much care as was obviously used in selection of most parts in this unit, better choices could have been made for these pots.
When I touched the outer circumference of this one, I got small arcs of plastic in my hand, and then the whole ring of plastic came off. Looking at the remaining pots in the unit, they all have cracks in that small plastic ring. Obviously, for this particular plastic, 40 years is too long.
I have to say that I too have been surprised by Nakamichi's use of these pots, as they are rather flimsy, regardless of the plastic issue. I doubt that the issue of how long the plastic would last before such embrittlement was a consideration back then, and probably much less was known about that issue then. But still, for as much care as was obviously used in selection of most parts in this unit, better choices could have been made for these pots.
What got me about the Sony receiver was not just the use of a flimsy pot in the Vbe multiplier, but the non-fail-safe design, which pretty much guaranteed that the output devices would smoke if the bias pot opened up. And yes, I replaced the flimsy pot with a decent cermet sealed pot.
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