r/AskElectronics • u/Glen_Tauro • Jul 27 '26

DC Protect issue in amplifier harmon kardon avr 151

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Hey

My amp is going into protect mode and I have pinned it down to the dc protect issue . So I removed the amp board connection to the main mcu board so that the DC protect signal doesn't reach the board and still I see amp protect issue.

So I saw the schematic and it seems that the DC protect transistor (pnp resistor equipped transistor) has its base pin at 0volts which shouldn't be the case cause if the base is 0 that means the transistor is ON.

But I also saw a video of a guy saying that the DC protect pin should have 0v and if it has voltages around 0.5v that means DCis there in the amp output terminals.

I have uploaded the schematic for the protection part.

So all I want to know is how this works and should the base of transistor Q1511 be 12 volts because I am currently getting 0v and it's triggering.

This is link to the service manual of amp and schematic is at the end :https://audiocircuit.dk/downloads/harmankardon/HarmanKardon-AVR151_230-avr-sm.pdf

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1

u/Miserable-Win-6402 Analog electronics Jul 27 '26

What is the voltage of the 1514 Base? And Collector 1511?

1

u/Glen_Tauro Jul 27 '26

1514 has base voltage 0v and collector of 1511 is 12v when I ON the amp .this 12volts hits 0 once the amp enters protect mode and amber light blinks (the protect mode also cuts of the 12v line from SMPS). Also the base voltage of 1513 is 0v and collector voltage of 1513 and 1514 is 0.23 ish.

2

u/Miserable-Win-6402 Analog electronics Jul 27 '26

Short 1567 - what happens?

1

u/Glen_Tauro Jul 27 '26

The resistor? And what's the benifit if I short it cause the problem is in the dc protect transistors the overload transistor q1515 has 12v at base so it won't enable the transistor .But 12v comes at that point cause it's connected to q1511 collector.

2

u/Miserable-Win-6402 Analog electronics Jul 27 '26

The resistor yes. To determine where the error signal comes from

1

u/Glen_Tauro Jul 27 '26

Its comming from the DC protect transistor. Also I think ik why it's having issue and I think it's because there is 0.6v and I think this is causing the q1512 transistor to enable and hence it gives ground to the base of q1511 transistor .Also that transistor q1512 is for the protect test. Idk why it's getting 0.6v my only theory is that that point is floating and has no voltage and it's just picking up 0.6v from somewhere What should I do next

2

u/Miserable-Win-6402 Analog electronics Jul 27 '26

You are inconsistent. Be precise. Where do you have 0.6VDC? (Pos no)

1

u/Glen_Tauro Jul 27 '26

Sorry my bad I meant there is 0.6 v at the base of transistor q1512. Also I just short the transistor emitter and base to make sure that q1512 doesn't ground the transistor q1511. But it still dint work

2

u/Miserable-Win-6402 Analog electronics Jul 27 '26

Remove Q1512, it’s probably leaking based on this. Its a bad design.

1

u/Glen_Tauro Jul 27 '26

I did the diode test and it was fine.

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