« Last post by sdiazmun on April 20, 2026, 08:07:07 PM »
Hi!!
Im just received a second hand MP573 (I have other three), and after compare them with "Smaart", the last one has a -6dB drop across the entire frequency spectrum, and that difference remains even when modifying the gain or the output potentiometer.
Where should I start looking for the point of failure?
« Last post by JPK on April 20, 2026, 11:19:16 AM »
Now I lost ac voltage at TP1 and I still get 1.64vac at TP3 and 0.055vac at TP4 You can debug without the front panel PCB connected. This will help to narrow down the search area.
Just to confirm Normal. 2 pins are connected together. 10kohms max between the 2 other pins.
« Last post by Ben on April 18, 2026, 11:01:38 PM »
I'm stumped, Now I lost ac voltage at TP1 and I still get 1.64vac at TP3 and 0.055vac at TP4 I still have 30vdc at TP1 and 2 as well. Just to confirm, what value should I be getting on P1? the impedance stays at 0 when turning the pot on the right 2 pins when looking at the pot from the rear, It varies between 0 and 10k on the left 2, I get 10k on th outer 2 pins
« Last post by vwyf11 on April 18, 2026, 04:57:30 AM »
Well I think if there’s nothing left to check, I give up. I’ve replaced Q7 three times and still am coming up with the same results. I’ve looked over the schematic and retraced everywhere for continuity. Every part I’ve pulled out has tested fine. All of the Q7 transistors have tested identical out of circuit. I haven’t had trouble like this with other diy kits so I’m feeling a little defeated. There must be something small I’m overlooking somewhere.
« Last post by JPK on April 17, 2026, 09:43:04 AM »
So the issue is in the final gain stage around U1.2. Check the value, direction and solder joints of U1, R14 to R16, R18, C9, C10, C11. Do you get a signal on TP4 when the front PCB is completely removed, with power on? Also try with U2 removed. All voltages should be measured against 0V.
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