Ok, looking deeper... so what I've seen is the rubber diode... with a BJT. Like in the NAD3150 I guess. Not a diode.
Use of diodes is the least common approach. Usually we use a transistor to do this.
There are always ways and means 🙂
This is how I mounted a transistor on a home built amp many years ago.
Post #18:
Can bias transistor mount on top(body) of output transistor??
Post #18:
Can bias transistor mount on top(body) of output transistor??
I don't have transistors with a screw whole without metal... I got 2SD669A which have metal on the backside, plastic on front, and TO-220 style ones, but they are metal around the screw whole. TO-92 would seem difficult to fix without whole.
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Aha! That's a way. So need to think about that spring.
There are unused screw holes in the heatsink, but they are a bit away from where the transistors mount. There I could use a TO-220 with a mica washer. But a bit away, around the corner.
Only you can decide what will fit OK. You could use a metal backed T0126 package like a BD139 or MJE340 type and mount it upside down and secured with a nylon nut and bolt.
Or a metal one with an insulating kit.
I have to leave that to you as it depends totally on having the unit in front of me to work on to know what would/could fit.
Or a metal one with an insulating kit.
I have to leave that to you as it depends totally on having the unit in front of me to work on to know what would/could fit.
We can always check the bias is controlled correctly once its up and running. As long as the transistor gets warm along with the output transistors then that should be fine.
Typically these transistors are mounted between the outputs on the main heatsink.
I'm also going to suggest that you make the transistor adjustable by using a preset instead of those fixed resistors. Dead easy to do and the preset can be soldered directly to the transistor leads.
We would then set the amp up as follows:
1/ The new transistor would be set for minimum bias.
2/ The original preset would be turned to approx half way.
3/ The new preset would be altered to get the correct bias.
A T092 style transistor is the best for this tbh, one with base in the middle.
Up to you though.
We would then set the amp up as follows:
1/ The new transistor would be set for minimum bias.
2/ The original preset would be turned to approx half way.
3/ The new preset would be altered to get the correct bias.
A T092 style transistor is the best for this tbh, one with base in the middle.
Up to you though.
Ok, so some decisions taken 🙂
I ordered some transistors in a TO-220F all plastic package, and I will mount them as SONY devises to. I also put all the relevant parts to build a DBT in my shopping cart, will only need to order (and pay for them, ugh).
So some days wait. I resist the urge for instant gratification 🙂
Thanks again, Mooly, for walking me here, or, to stay in the above metaphor, for pulling me out of the water when no shore was in reach. Huge gratitude here.
I'll report back with news and a DBT.
I ordered some transistors in a TO-220F all plastic package, and I will mount them as SONY devises to. I also put all the relevant parts to build a DBT in my shopping cart, will only need to order (and pay for them, ugh).
So some days wait. I resist the urge for instant gratification 🙂
Thanks again, Mooly, for walking me here, or, to stay in the above metaphor, for pulling me out of the water when no shore was in reach. Huge gratitude here.
I'll report back with news and a DBT.
Ah, I didn't see your last two posts in time. Ok let me ponder. Base in the middle is important for my dislexical way of mixing up transistor pinouts 🙂
That would mean that in principle I also could solder in a fixed resistor instead the old preset? And what size of pot would I need? 5k? In case I need to order more parts.
Maybe also ok to build a little PCB which attaches to the transistor leads (as to SONY's service amendment). there is space behind the heatsink. Fix the preset to the PCB, so you could change it from above without unmounting the heatsink with the power amp PCB.
An all plastic package will be fine... if you have ordered them then lets go with that.
The resistors (the fixed ones as per Sony mod) can be mounted direct on the transistor... again up to you but I would do that without a PCB tbh. Just be quick and neat with the soldering.
Reasoning for base in the centre with a T092 was that a small preset can be wired direct across the pins with the wiper in the centre (base).
My only concern is that Sony have given those resistor values as being applicable to that transistor they specify. Other devices, well they will be close but its not 100% guaranteed they will come out the same.
The resistors (the fixed ones as per Sony mod) can be mounted direct on the transistor... again up to you but I would do that without a PCB tbh. Just be quick and neat with the soldering.
Reasoning for base in the centre with a T092 was that a small preset can be wired direct across the pins with the wiper in the centre (base).
My only concern is that Sony have given those resistor values as being applicable to that transistor they specify. Other devices, well they will be close but its not 100% guaranteed they will come out the same.
Bulb tester... you can use a table lamp and just solder the mains plug wires across the fuse holder in the amp (remove fuse of course).