Hi all
Perhaps someone much smarter than me can solve this on the Harmon Kardon 730 Vintage Receiver.
HK730 Service Manual
I have done a complete restoration and calibration - Nichicon HE KA, Panasonic FC, Wima MKS2 Caps, Bourns 3386 Potis, Vishay Resistors etc.
There is one problem remaining.
On the right channel, there is excessive DC for less than a second on startup, which causes the (added) protection circuit to activate.
At the Output Amp Speaker connector, the DC spikes up to about 7 volts (and buzzes the speaker), before the protector clamps.
After a couple of seconds, the DC stabilizes to <120mV, and so the protection circuit releases, and the machine operates normally.
Here’s the kicker.
I disconnected the U-connectors at the rear (signal path between the PreAmp and Power Amp).
I then fed an audio signal from Another Preamp directly into this HK730's Power Amp, and monitored DC at the speaker connectors.
No DC spike (max DC about 150mV), and no protection shutdown.
So, the Power Amp stage is just fine.
The problem is in the Right PreAmp, whose output (Green Wire at LP510 on the schematic) spikes up to 1.5VDC.
Here I’m lost.
I have traced the entire signal path on my scope, and it all looks quite normal (obviously), because the DC spike happens only right at startup, and doesn’t repeat, so it is incredibly difficult to trace under normal conditions.
Any hints at where to look?
Much appreciated
Menahem Yachad
CondorAudio
Perhaps someone much smarter than me can solve this on the Harmon Kardon 730 Vintage Receiver.
HK730 Service Manual
I have done a complete restoration and calibration - Nichicon HE KA, Panasonic FC, Wima MKS2 Caps, Bourns 3386 Potis, Vishay Resistors etc.
There is one problem remaining.
On the right channel, there is excessive DC for less than a second on startup, which causes the (added) protection circuit to activate.
At the Output Amp Speaker connector, the DC spikes up to about 7 volts (and buzzes the speaker), before the protector clamps.
After a couple of seconds, the DC stabilizes to <120mV, and so the protection circuit releases, and the machine operates normally.
Here’s the kicker.
I disconnected the U-connectors at the rear (signal path between the PreAmp and Power Amp).
I then fed an audio signal from Another Preamp directly into this HK730's Power Amp, and monitored DC at the speaker connectors.
No DC spike (max DC about 150mV), and no protection shutdown.
So, the Power Amp stage is just fine.
The problem is in the Right PreAmp, whose output (Green Wire at LP510 on the schematic) spikes up to 1.5VDC.
Here I’m lost.
I have traced the entire signal path on my scope, and it all looks quite normal (obviously), because the DC spike happens only right at startup, and doesn’t repeat, so it is incredibly difficult to trace under normal conditions.
Any hints at where to look?
Much appreciated
Menahem Yachad
CondorAudio
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It's very nice of you to change such good components.
will you consider this might be the problems of the old transistors on the pre-amp,
I suggest check and compare the voltages of the transistors on both channels when start up. if difference is found then swap it one by one to see the result.
also check the circuit board completely for bad, broken or loosen joints, it happens sometime.
will you consider this might be the problems of the old transistors on the pre-amp,
I suggest check and compare the voltages of the transistors on both channels when start up. if difference is found then swap it one by one to see the result.
also check the circuit board completely for bad, broken or loosen joints, it happens sometime.
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