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Digital Designs D4.90 in protection
Digital Designs D4.90 in protection
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Old 27th October 2020, 06:56 AM   #31
mild2wild is offline mild2wild  Trinidad and Tobago
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So, I changed those 2 driver transistors. The amp powers up with no noticeable current draw as the limiters are not illuminated. The drive signal however is showing both traces locked together. Is this right?

Amp is playing all channels. Reconnecting Q517 causes the protection light to illuminate but amp plays unaffected. Rail voltage is now +31/-26.
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Old 27th October 2020, 07:00 AM   #32
Perry Babin is offline Perry Babin  United States
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Are you placing the probes precisely where you did when they were overlapping slightly?
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Old 27th October 2020, 07:14 AM   #33
mild2wild is offline mild2wild  Trinidad and Tobago
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Something weird is going on with the scope. The red trace is moving across the screen, but the yellow trace just shows a square wave without any movement. Swapping them doesn't make a difference...I will reboot and test again tomorrow.

How do I track down the protection and rail voltage issues?

Just asking, can this amp use IRF3205 and 47ohm resistors? I know the original IRF1404 has higher voltage and current ratings, but I put those ones from this amp into a different amp and I have no more IRF1404. Will IRF3205 not be strong enough for this amp?
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Old 27th October 2020, 07:19 AM   #34
Perry Babin is offline Perry Babin  United States
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The user will determine if the 3205s will survive. If there's no absolute rush, order the right parts.

The red trace sounds like it's not triggering the scope. Try adjusting the trigger level.

Are you using the secondary ground for the black probe when measuring the rail voltage?
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Old 27th October 2020, 07:30 AM   #35
mild2wild is offline mild2wild  Trinidad and Tobago
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I was using GND input terminal when rail measurements were taken. I will try the secondary ground. I will also check the scope tomorrow. What about the protection LED?
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File Type: jpg IMG_20201026_205047.jpg (662.1 KB, 15 views)
File Type: jpg IMG_20201026_205516.jpg (749.1 KB, 15 views)
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Old 27th October 2020, 07:37 AM   #36
Perry Babin is offline Perry Babin  United States
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I have absolutely nothing on this amp so I may not be able to help with the protection LED issue. Let's get through all of the other issues first.
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Old 27th October 2020, 03:27 PM   #37
mild2wild is offline mild2wild  Trinidad and Tobago
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Picture is drive signal at gates of FETs. No significant current draw. When measured using secondary ground, rail voltage is +/-29v. Q517 is marked ASG which should be a KTA1504, of which I have none. It measures the same in either direction from base to collector so I replaced it with a MMBTA56 which stopped the protection LED. Amp powers up and operates properly when tested. Just to fully populate the power supply and remove the driver board jumpers and test again.
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File Type: jpg IMG_20201027_095000.jpg (980.3 KB, 12 views)
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