Ok my curiosity got the better of me and I went back to the SX-950. Installed ALL NEW board transistors incl. perfectly matched complimentary transistors. I also changed out the ceramic caps for high voltage micas. Double checked stv4h's, diodes, trim pots and resistors and all are to spec. I was able to set Bias to 20mv on both channels (VR3 & VR4) stable. BUT NOW DC offset on both Left and Right channel (VR1 and VR2) is bouncing wildly all over the place and will not settle in.
What sort of swing are you seeing on the offset? Are we talking millivolts, or volts or even tens of volts?
Strange if both channels are identical... all new transistors... I wonder if there is some off stability issue. I'm not really a fan of blanket changing parts and many designs actually rely on the characteristic of the transistors as part of the stability equation.
Strange if both channels are identical... all new transistors... I wonder if there is some off stability issue. I'm not really a fan of blanket changing parts and many designs actually rely on the characteristic of the transistors as part of the stability equation.
Starting from scratch: This unit had a full recap with Nichicon's, new diodes, trimmers and new modern transistors (except outputs which were original) three years ago. It ran fantastic every day until this summer when I turned it on and I had nothing. Opened it up and had a single burnt transistor (R38) on the right channel (facing the front of the unit or left if facing the rear), the STV4H and three of the four outputs were shorted as well the bias trimmer V4. Before the problem Bias was set and running at 20mv and DC offset was set and stable at 0mv and the unit ran beautifully on a daily basis for 3 years. Replaced the the four outputs, both bias trimmers, the R38 & R40 resistors on the problem side. After repairs dim bulb test said no shorts or opens but I could not get the bias to below 27mv even with trim pot turned all the way clockwise. DC was ok but did bounce a bit on both channels. I checked ALL the components on the problem side and found nothing off. After suggestions from members I replaced transistors on bad channel with new, including perfect matched complementary in case I had a leaky one. The transistors are exactly the same part numbers as on the good channel. I also replaced small ceramic caps with high voltage micas. The good channel was left untouched. Now I have the ability to dial in bias perfectly but the DC is bouncing. I am only testing with DBT in place right now to protect outputs. When turned on the acts like nothing is wrong and get a good click from the protection circuit after a couple of seconds. I am using 5.1k dummy loads in the power amp in jacks as per service manual. Jumpers are pulled.
As a last resort I changed out the Bourns pots (even though they are new) and my problems are solved. I don't know if I got a bad batch of pots or not but the ones I was using were swinging from 150 to -150mv and would not settle. New pots in and I'm at 20mv bias and .2mv DC stable.
Yesterday a friend offered me a set of original Toshiba outputs in perfect condition for the 950. Now that this one seems to be OK I was wondering if it would be worth the effort to pop the originals back in?
That was me 🙂
So you are saying all is good at the moment 👍
It would be very unusual to have faulty new parts of a reputable brand unless they were fake of course... as with anything, only buy from authorised distributers.
It depends what you have actually got fitted and whether those are genuine (even if a different type number).
Good, so you used the recommende Bourns pots I posted and they solved your issues?
We blew those old Toshiba outputs many times years ago. But we used to blast the heck out of the amp too. Imo stick with the new onsemi mj2119x parts.
Make sure to turn bias down and readjust when applying full AC
We blew those old Toshiba outputs many times years ago. But we used to blast the heck out of the amp too. Imo stick with the new onsemi mj2119x parts.
Make sure to turn bias down and readjust when applying full AC
All my parts are from either Digikey or Mouser, without exception. Used the same pots (3362P 101 and 103) I had ordered (ordered 3 pairs) back in August but the first pair would not settle in at all. On the second pair the bias would not settle. It wasn't until I installed the third pair did they work as they should. So its ok to have the original Toshiba's on one side and the on-semi on the other.
Well neither side knows what the other is doing 🙂 so its fine in that sense.
I would say that there will be no audible difference... its up to you... have it all correct for the sake of correctness or be happy its all working and it doesn't really make nay difference that they are different.
I presume we’re talking about comparable parts. I wouldn’t pair MJL21193 with C5200, but I might pair NJW0302 with C5200(N). Or Toshiba 5200 with the Fairchild A1943.
Some of the old Sanyo “complementary” pairs were triple diffused NPN and epitaxial planar PNP. If you get sustained beta to 8 or 10 amps and fT 20-30 MHz it doesn’t really matter how you get it. You can still match hFE if you are so inclined to go to the trouble.
Some of the old Sanyo “complementary” pairs were triple diffused NPN and epitaxial planar PNP. If you get sustained beta to 8 or 10 amps and fT 20-30 MHz it doesn’t really matter how you get it. You can still match hFE if you are so inclined to go to the trouble.
If it is the same on both channels, it may be power supply. Can you hear the noise with a speaker connected?