SX-626 problem, at half volume DBT kicks in

[bradfro]

I went back and checked center voltage with no signal and zero volume. It was off a tiny bit but I got both Channels adjusted. The bios for both channels were around 14 MV, so I adjusted them back to 16mv

 

[merlynski]

Bias adjusted for both channels at 16mv and center voltages are right on. With the scope I get 10 volts peak to peak which I think is 12 watts per Channel . Any higher volume and the dim bulb starts to glow and all the voltages start dropping Voltage on the output collector at this point is 44volts and half voltage is 23.4. Bias is 77mv. Heatsink is warm. When I move to mains, the volts go up to 54.8 and 27.5

You got ahead of me, you are lucky you did not fry something.

You may have to start over with the settings, setting them on the dbt (if you did that) is a recipe for disaster, the bias will be set way too high and will cause problems when you go to full mains, you can blow your output transistors and more that way. Center voltage and bias must be set on full mains. It is ok to test functionality but not final settings. turn bias back to minimum before going to full mains, then set it correctly.

Of course the dbt starts to glow, its job is to limit the power getting into the amp, when you try to play music (or use dummy loads) it draws more current, the dbt limits it more, voltage sags and the amp does not work.

No more dbt.

Is center voltage and bias now set correctly while on full mains?

Need to set the input signal correctly before load testing and center voltage optimization.

 

[bradfro]

I did not do any adjustments while connected to dbt. I did the 20 minute burnin at full volume while connected to dbt because I was concerned, at first, that something would fail, or the fuse would blow. I monitored the heat sinks and around 20 minutes I was confident that nothing was overheating. I checked the voltages and they were lower than normal. I removed the dbt and I was surprised that when turned up full there was no blown fuse. Also that the voltages were fine.

 

[bradfro]

The center voltage a bios were set while on full mains. Today all the voltages are right on.

 

[merlynski]

Excellent,I am glad it is working now.
To set the center voltage under load on full mains is the only way to maximize performance.
The SM says that the Aux input sensitivity is 200mv RMS, which calculates to 566mv peak to peak. Use a sine wave of 400Hz (approximately) connected to the Aux input and adjust the level to be 566mV P-P at the input on your scope. Using dummy loads on both outputs look at one output and turn the volume up until you just get clipping. It will (probably) not be symmetrical. Adjust the center voltage for that channel to get symmetrical clipping. If it is clipping on one side only, and adjusting the center voltage eliminates the clipping, increase the volume control until you get clipping again, then readjust the center voltage again. One you have symmetrical clipping on that channel reduce the volume to just below clipping and measure the P-P voltage. Repeat this for the other channel. Once you have both channels set symmetrical recheck the bias. Don't leave it long at that level as it will overheat the amp. Once you have the bias set you can recheck the clipping level for being symmetrical. Once the clipping center voltage and bias are set you should not worry about readjusting the center voltage with the voltmeter, as that will undo the centered clipping adjustment done with the scope.
And you are done. 40vp-p into 8Ω is 24.4W

 

[bradfro]

Completed, thanks so much Merlnymski for your help. Your last step went well, I was able to get clipping adjusted for both peaks, just as you instructed for both channels. The bias was high for the right channel, so I readjusted, left was a tad high.

listening to this right now. Sounds beautiful

 

[merlynski]

Completed, thanks so much Merlnynski for your help. Your last step went well, I was able to get clipping adjusted for both peaks, just as you instructed for both channels. The bias was high for the right channel, so I readjusted, left was a tad high.

listening to this right now. Sounds beautiful

You are most welcome!
Congratulations on a repair well done. Looks like it is working well at its max rated output. From what you have learned through this I think you see and understand why it was not working correctly after the parts replacement, even though they were all replaced correctly, and with the right replacement parts.
If your friend you were doing this for continues to blow fuses you will KNOW you have it set up correctly.