Sunn Beta Bias problem

[JohnRoberts]

Ah, o.k. Thanks. I don’t have a lot of experience with solid state.

This is how to get some..

It looks like I need to remove the active components to measure.

That can cause incidental damage, first measure in place. Only after you identify a component as possibly faulty remove it and measure it out of the circuit to confirm.

I thought Q5 might be bad since there seemed to be no voltage drop. I replaced that, but it didn’t change. I then removed Q7 and there was still 44v at the collector of Q5. Then I replaced Q13, Q14, and Q12 one at a time with no change. At that point I suspected C13 could be short, so I replaced that. Still problems. I then replaced Q7 and looked at signal on a scope. That is when I decided to start this discussion. I took another look at Q7 last night and it was bad, so I put another new device there. Now I get signal through the amp, but the bias is still off. I see -8v at the base of Q7, and -12v at the base of Q16

The base of Q7 should be roughly 3 diode drops above the output voltage, or nominally +1.5V DC with no signal. Likewise Q16 should be a symmetrical minus 3 diode drops. I suspect the output is -10V, and class A bias current is strong.

I am not a fan of the class A bias trim configuration. Those trim pots tend to fail open circuit which would be maximum bias current (not good). For debugging you can set the class A current to minimum. You can measure the class current by the voltage drop across the 0.33ohm emitter degeneration resistors (R18, R21, R48, R50). Generally you can measure only one, not all of them.

and the signal looks like a cowboy hat at those points. I’ll keep at this until I get it 100%. Then I can move on to the next one. The next one sounds good, but has several broken knobs, pot shafts. Where can I get pots and knobs for these?

If the output stage is working and output voltage is -10V when it should be 0V, inspect the negative feedback circuitry (R7 and R31) and input long tail pair (differential pair).

Another possible suspect is the DC servo IC1, again I am not a fan of using a polar electrolytic with indeterminate polarity, and it looks like I may have too much control range.

JR

 

[walter]

Where does the time go? I’m still at this… I focused on checking B-E junction voltages in the suspect areas and found Q13 bad, so replaced it again. I couldn’t find a 2N4250, so I used a 2SB737.
Now the amp is working good. The Bias procedure describes setting the voltage across R18 to 1mV by adjusting the trim pot R42. Sounds easy enough, How does one measure 1mV? Am I just making sure it is not more than 1mV because my DMM does not read less than 1mV. I remember learning about using different equipment for different ranges back in college, so I dug out the old Simpson meter. No, it’s lowest reading is 1mV and I don’t have test leads. The Bananna jacks are male on the Simpson, so now I need these unique test leads to use it! I can’t see any change when I turn the trim pot, I’m guessing it’s less than 1mV. I will look at the voltage at the Base of Q7 and compare that with the other amp.
I did do some work on the other amp. I located some 100k reverse audio pots for the treble, some 100k analog pots for the master, and some original looking knobs. I removed the broken pots, and moved a couple good pots. The original pots are D-shaft. The replacement pots are greater than D-pots, so I had to grind them to fit.
I added new thermal compound under the output devices, and cleaned the chassis and heatsink for the output devices. Checked bias, right at 1mV, and checked for crossover distortion. The schematic lists voltages at the test points, so I ran 1kHz into the input and set the volume so there is 2.2vP-P at TP-1(all other knobs at 12 o’clock). The signal readings at the rest of the testpoints match what’s on the schematic. Then switched from dummy load to speaker. It’s loud. Channel B is not loud enough, so I’ll get back in there. The power cord ground pin is busted off, so that needs replacing, and the power switch is not illuminating, so I’ll want to fix that.