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Acrosound Stereo 120 rebuild

ccheung

ccheung

Member
I remember it sounds all right when I briefly powered it on many years ago. But I did not turn it on for long. It seems like coupling caps or diodes are not originals. I suspect the previous owner did some minimum restoration. But I may be wrong. I plan to change all the electrolytic capacitors first and coupling caps too. What more do you suggest?
 

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Just to the left of the SR in the last photo you can see a diode hanging in the air with two wires connected to it. That is probably the replacement for the SR, but that diode has one end hanging in the air. That is not safe construction practice! That diode either needs to be on a standoff or the entire diode with the connection to the two wires should be in heat shrink tubing.
 
I updated the power section and power on with 120v house voltage. The output is 520v without tubes. I expected it would drop when I put in tubes. However, it still stays on 518v with the tubes. Any suggestions?
 
This is high.

Are you sure the power tubes are loading the PS correctly?
What is the voltages at the 4.7R cathode resistors of each power tubes?
That will tell the current going through them.
 
strange, only one tube has 0.028v across the 4.7R cathode resistor, other 3 have no voltage. All tube has 528V in pins 3.
 
So indeed, the power tubes are cut off.
What is your bias voltage on the grid? I think it's way too negative.
 
That's good, what about at pin 5 (grid) of the power tube?

Have you tried to adjust the bias before? You should have around -45V here to get the tubes running correctly.
 
I adjusted pin 5 to -45V, now the pins 4 is 506V. the voltages across the not the same, one has 0.119v, other has 0.156v. Is that normal?

The meter seems dead, the needle does not move. But I measure two end, its 21ohm, not open.

I turn the top two pots, seems doesn't affect anything. What are their purpose of them?
 
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Check all the carbon composition resistors to be sure they are in tolerance. They tend to drift out of spec by large amounts as they age, especially plate load resistors.
 
The top pots are AC balancing pot. For now set them at mid course.

Ah it's getting better. The right voltage on all 4 4.7R resistors is 0.169V.

The 2 lower pots adjust the 2 left power tubes, and the 2 right pots for the right tubes.

I pot adjust the bias level and the other balance the tubes.

So with the balance pot, you can adjust the same voltage across both 4.7R, then with the bias pot you adjust it to be close to 0.169V.
You may have to go back and forth between balancing and biasing until you get it right. Then do the same with the other channel.

I believe with 4 tubes running a 36mA (0.169V at the 4.7R resistor) your B+ should fall below 500V. Then you're good.
 
I adjusted the bias pots. Now the voltage around the 4.7R cathode resistors are 0.169V. The amp sounds OK ( I haven't seriously listen it yet). I have two questions.
1. The bias meter moves in the middle position, in between 0 and A. Doesn't it supposes to reach A? and the schematic voltage chart said KT-77 pints 8 is 0.34V. Should I bias it to reach 0.34V?
2. Where do I measure to adjust the 2 top balance pots?
 
Great. So your B+ is now correct I suppose?

1) If you bias it at 0.34V, that will make the power tube conduct at 72mA. This is over 100% dissipation, not good. You'll most likely red plate. 36mA or so is what you should bias it to.
Now if you want the meter to be a A for 36mA, you'll have to adjust the resistor R51 off the meter. I would install a 50R trimmer and calibrate it that way.

2) If you have a distortion analyzer, that's simple, you adjust it to have the lowest distortion. If you don't, you'll need a scope, a signal generator and a 8R power load resistor. Connect the resistor to the 8 ohm speaker connection, and inject a very small 1K sin signal to the RCA jack, and set the scope to the 8R load resistor. You should see a beautiful sin trace. Then increase slowly the amplitude of the input signal and bring the amp to clipping. Then you adjust the pot for that channel so the clipping is even on top and bottom. That would be good enough.

Most of the time, with a decently balanced 12AU7, setting the pot at mid course is good. Unless you have a grossly mismatch 12AU7, you're good with both pots at mid range.
 
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