Hi,
so I have this amp to fix - Palladium Prestige (aka Lotte Prestige, also sold under other names).
I can't find a service manual anywhere.
The amp has distortion on both channels after a while. It is OK when set to DIRECT (bypass bass and treble controls). The fault is in the pre-amp.
The pre-amp has 2 NJM4558DD. One for each channel and half of each handles the direct path and the other half the tone control part. There is high voltage on the outputs on the non-direct side. And after a while the capacitor on the output of the preamp can't filter it and the sound is distorted.
The input is the same for direct/tone control. I have drown the schematics of the circuit around the opamps.
The opamps themselves work fine, I have replaced with new ones. If I remove the connections to the bass and treble pots, the issue remains. So the only difference really is the circuit around the opamp. I can get is more or less stable and output under 50mV when i replace the 470kOhm resitor with 166kOhm. It works fine, but does not stay on one value like the direct side.
As far as I can see the ground is connected where it should be and the components around test ok. Which brings a question why is it not working as the circuit was designed like this and should be working🙂
so I have this amp to fix - Palladium Prestige (aka Lotte Prestige, also sold under other names).
I can't find a service manual anywhere.
The amp has distortion on both channels after a while. It is OK when set to DIRECT (bypass bass and treble controls). The fault is in the pre-amp.
The pre-amp has 2 NJM4558DD. One for each channel and half of each handles the direct path and the other half the tone control part. There is high voltage on the outputs on the non-direct side. And after a while the capacitor on the output of the preamp can't filter it and the sound is distorted.
The input is the same for direct/tone control. I have drown the schematics of the circuit around the opamps.
The opamps themselves work fine, I have replaced with new ones. If I remove the connections to the bass and treble pots, the issue remains. So the only difference really is the circuit around the opamp. I can get is more or less stable and output under 50mV when i replace the 470kOhm resitor with 166kOhm. It works fine, but does not stay on one value like the direct side.
As far as I can see the ground is connected where it should be and the components around test ok. Which brings a question why is it not working as the circuit was designed like this and should be working🙂
Attachments
I fixed it, but had to change the design of the circuit, which is just wierd on a 30 yrs amp... I had to move the 10 uf capacitor on the tone side outside of the negative feedback loop. Same way it is done on the direct side. Then the output is stable at 7mV, instead of drifting to -12 volts.