New transistor smell :)

[Mike Sweeney]

So after recapping, potting and cleaning the main amp, I could not get the left channel dialed to 0 mv DC Offset .. it kept swinging about +- 1mv which is not much but it was bugging me. So I ordered up a whole set of transistors ( 4 PNP, 4 NPN) for the 2285B. Swapped them with new mica and goop and check it out.. both channels now zero out perfectly The PNPs are a bit hotter than the left side. Both PNP and NPN are hotter on HFE than the OEMs even though the mV is within 1% or so. The only adjustment was the idle current which went from 30mV to 55Mv with new transistors. I did note the unit on a whole now draws .5 watts more than before.

Old PNP HFE=63 average /571mv New PNP HFE= 80/547mv - MJ21193G
Old NPN HFE=51 average /569mv New NPN HFE=73/559mv - MJ21196G ( the 94s were not available when I ordered, they are now)



So note to self.. when DC Bias doesn't want to settle down, it could be a slightly flaky transistor. Someone had suggested some bad diodes but they all passed when compared R to L channel measurements.

 

[bryans12v]

A 1mV swing in DC would not warrant the purchase of all new finals ($40+), in my opinion. The place to start wouldve been the diff pair which costs 12 cents when you buy by the 1000 but even then at 1mv makes it a moot point.

Glad it worked out though!

 

[Dingman]

Ya, glad that worked out.
I also had a unit (not marantz) that I couldn't get a stable DC Offset - generally about 1mv fluctuation, sometimes 2mv. It bugged me, I did match up another set of 992's but that didn't help. I put originals back in, didn't help.
Put a 3rd set of matched 992's in, but no joy. I ended up just leaving it as is - sounds fine.

Now I know that maybe swapping outputs would correct that, but - not unless I have too.

 

[Mike Sweeney]

A 1mV swing in DC would not warrant the purchase of all new finals ($40+)

Oh, I agree but it was bugging me and this unit while being restored, has been abused so it will never be a "pristine" restoration. It will be pretty and sound good but it's been/is a test bed for some repair hacks. If you look at some of my other threads, I've been experimenting with how to mount the 4 replacement caps for the 2 in 1`OEMs, replace trimmer pots that are HUGE compared to todays and so on. The transistors were bought for two reasons.. one I can right now and I was honestly curious if I would hear any audio differences. I was pulling them anyways to put new thermal compound on so why not? There is a ton of myth and lore that the Marantz parts add a warmth/space/noise etc.. to my ears, nada. These transistors didn't change the sound at all. If I had a kick-ass speaker set.. maybe.. but I have an average set up that works well and I don't hear a difference. The fact that it fixed a flaw is a bonus and verified another post I had read where the writer had said pretty much the same thing. That in a few units with nagging bias issues, replacing the finals fixed the issue even though there was nothing "wrong" with the transistors. Maybe if you have the gear to generate a full set of curves and measurements you would find something off? Don't know.. not sure I care that much about it And in truth, given what this 2285B cost as a "fixer upper", 40 bucks is not very much at all to experiment. It's only 5 trips to starbucks Or four G&Ts on friday night.. so it's not that much.