You should not be hooking up speaker loads when you have a DC offset on the output! That will draw all kinds of current and could blow the output transistors if they're are not already blown. In this receiver there are 2 bias adjustment pots per channel and they affect the DC offset as well. You could easily have 5V on the output if they are off. Follow the procedure in the service manual and adjust them. You have to go back and forth between them a few times, checking the bias current and the output voltage. Then tweak one of them so the output voltage is 0V.
If you wanted to protect the output transistors from going Fzzzzzzzzzzzzzzip....poof!! Install some 10watt 100Ω resistors in place of the emitter resistors. They will act as current limiters and save your ass when things go wrong, replace the bias trimmers as well if you already haven't.
I think Sansui want a load on the output to measure the DC offset, hence that 10Ω 1 Watt nonsense...
I've made some progress. The new outputs are in and the 1000uf caps for the output mod are installed. I ended up using 3 caps per side, for 3000uf each channel.
I did find what seems a workable method to bias this thing and get DC offset where it should be.
- Remove both fuses from one channel.
- using two meters, connect to measure mA across each of the two fuse holders
- Use a third meter to measure DC offset at a point before the cap mod
- adjust both bias pots for 80mA while watching the 3rd meter for offset
- you'll find a sweet spot where 80mA is achieved on both sides of the one channel while the DC offset lands where it should be.
- Repeat above steps for the second channel
I was able to do both channels this way and have a rock solid 80 mA on both channels and a DC offset of around 1mV each.
It looks like I need a new volume pot, as at full off there is enough volume to hear program material quite well. Measuring the low wiper on the pot shows 180 ohms and 320 ohms (L/R) to ground, where it should be at or closer to 0/ground.
However - it sounds great! A very sweet sound from the little testing I've done so far. I'm very pleased.
Pre-Power board with new diodes, resistors, and multi-turn pots for bias. And below that, the Driver amp board with new caps, pre-driver transistors, etc.
3000uf each channel for the output mod. Still need to find some 25mm fuses...
Ripple-Filter board with new caps.
Phono preamp with new caps. Input cap replaced with a film.
I don't have a picture of it, but I also pulled and recapped the Control amp board and did the caps and transistors there also.
So what y'all are saying is, None of us can fix this thing, but do exactly as we say.
Got it.
You go from saying 100 ohm 10 watt to agreeing with 10 ohm one watt.
I'm confused.
Having a hard time finding a kit. May have to piece it together myself.
Interesting ideas here. Don't think I'd call it a Sansui 3000a anymore, but it's something.
I have a 3000A as a future project on the shelf I may try to get it working this winter. Its also a 69 model does not look like it has had the output mod. The other project is a similar vintage SX-9000.
I was looking at Mouser and in stock options they have in 1000 uF non-polar highest voltage is 35V. Step down to 470 uF adds 50 and 63 V options. If you want higher voltage/capacitance options may require doing series/parallel combinations. Capacitors work opposite of resistors. Two caps in parallel double the capacitance. I think the capacitance controls the low frequency roll off point.
Their mod may be more like putting a high pass crossover filter ahead of the speakers to block the DC, like a first order tweeter crossover in a speaker. The capacitance increases as the crossover frequency decreases. High DC can burn out the woofer the tweeter will usually be okay.
If you can interpret what they are recommending the guy at select45rpm.com over in England likes this Sansui. He claims ignore published ways of biasing and describes another way to go about it.that could be tried. Work down this page to find the comments on the 3000A:
http://select45rpm.com/pages/hifi/vintage-hifi-reviews.html#9
I have used the recommendations of Conrad Hoffman on this site on setting the bias of my 2000A (1970) and it sounds great and has given me no problems. Of course it has had his amp mods done to it.