If you can handle SO8 SMT ic's the op amp is opa1462. There is a an ESD chip and series resister at the input. It's not hard to work on this board.
I not sure what they used. I'd have to open it up and look.
QA scrubbed all the part numbers off any big enough. It's near the front BNC's just check to see if it's shorted and if the series resistor is open. You'll see once you're inside.
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Dynaco tube amp screen shots.
Ok, here are the screen shots of the Dynaco ST-70 tube amp I have been working on. Left and right channel. I have the amp hooked up to 8 ohm dummy loads. Across the dummy load I connected the QA190 probe at X100 and just used the voltage input and measured 1 channel at a time. I am not sure if I need to change setting within the QA400 for this or not and if this connection is correct. I am essentially just trying to measure distortion in old tube gear I repair and learn some new tricks along the way. Any direction greatly appreciated. This is all fun and games for me. Robert
Ok, here are the screen shots of the Dynaco ST-70 tube amp I have been working on. Left and right channel. I have the amp hooked up to 8 ohm dummy loads. Across the dummy load I connected the QA190 probe at X100 and just used the voltage input and measured 1 channel at a time. I am not sure if I need to change setting within the QA400 for this or not and if this connection is correct. I am essentially just trying to measure distortion in old tube gear I repair and learn some new tricks along the way. Any direction greatly appreciated. This is all fun and games for me. Robert
Attachments
Hello!
Thanks for the reply 1Audio! The powder cord is a 2 prong. The black speaker outputs are connected to chassis ground. Am I clipping an alligator clip from the chassis to somewhere else when I do the distortion measurement? If you could explain the 60 hertz modulation you see in the left channel that would be helpful also. Thanks for giving me the opportunity to learn something here. There is not a lot of support or documentation for this product. Robert
Thanks for the reply 1Audio! The powder cord is a 2 prong. The black speaker outputs are connected to chassis ground. Am I clipping an alligator clip from the chassis to somewhere else when I do the distortion measurement? If you could explain the 60 hertz modulation you see in the left channel that would be helpful also. Thanks for giving me the opportunity to learn something here. There is not a lot of support or documentation for this product. Robert
Just a clip lead from the PC chassis or the QA400 chassis to the amp chassis or ground. Check with a meter first to make sure there isn't a more serious leakage or hot chassis problem.
If you look att the fundamental and harmonics you will see sidebands on each side. I'm guessing they are 60 Hz from the fundamental. Closer inspection will confirm. If they are still there with the extra ground link I would suspect power supply hum. It usually gets worse under load.
If you look att the fundamental and harmonics you will see sidebands on each side. I'm guessing they are 60 Hz from the fundamental. Closer inspection will confirm. If they are still there with the extra ground link I would suspect power supply hum. It usually gets worse under load.
Hello!
I will change the average from 0 to 10. I see the knob for "average". What will that do? Yes, .02 distortion for a tube amp does not seem correct, which is one of the main reasons for posting. I do notice a wire in the left channel going past the rectifier tube and 2 electrolytic caps. I bet if I reroute the wire, some of that 60 hertz hum will clear up in that channel. I still need to figure out how to take a power measurement with the QA400 and QA190. The instructions are seriously lacking, but the price is great. Thanks for keeping this fun! Robert
I will change the average from 0 to 10. I see the knob for "average". What will that do? Yes, .02 distortion for a tube amp does not seem correct, which is one of the main reasons for posting. I do notice a wire in the left channel going past the rectifier tube and 2 electrolytic caps. I bet if I reroute the wire, some of that 60 hertz hum will clear up in that channel. I still need to figure out how to take a power measurement with the QA400 and QA190. The instructions are seriously lacking, but the price is great. Thanks for keeping this fun! Robert
At the rectifier and caps you don't have 60 Hz - it's 120 Hz and up at that point.
Jan
I just got my QA400 today. When calibrating, my AC voltmeter reads 0.8 VAC each channel. This is not acceptable to the calibration routine.
Should I have accepted the defaults? I think they were 1.29
TFM says to check with a scope, it looks ok, so I accepted the defaults.
The loopback test is excellent. Very little difference between channels.
Should I have accepted the defaults? I think they were 1.29
TFM says to check with a scope, it looks ok, so I accepted the defaults.
The loopback test is excellent. Very little difference between channels.
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