Hi mates !
I did read all of the old threads about the amp.
Amp came in with a blown output mosfet - FS70SMJ - but sadly this one cannot be found anywhere. I've tried to replace this with IRFP150N but this always makes the amp to go in protect (offset + overcurrent).
Pre stage power supply is fine. No worries there. Checked with DMM most of the transistors, diodes and so on. Nothing shorted.
Defeating the protection circuit makes the amp come on live with the IRFP150N and it plays just fine, but channel 3 level where the IRFP150N is located is way lower than all of the other channels.
If channel 4 is at 20vrms clean sinewave, channel 3 stays at 15vrms. So far i've only looked at the pre amp section and didn't see any channel disbalances so both channels 3 and 4 should receive the same pre-amp level.
Haven't looked in details into the differential and drive amp so far. The bias pots have been touched before, so i've set all of them at their lowest level (counterclockwise) just for initial testing.
Is my issue related to the fact that the power transistor is not the right one, the bias adjustment or I should dig deeper ? (i did managed to find the service manual in other thread)
I did read all of the old threads about the amp.
Amp came in with a blown output mosfet - FS70SMJ - but sadly this one cannot be found anywhere. I've tried to replace this with IRFP150N but this always makes the amp to go in protect (offset + overcurrent).
Pre stage power supply is fine. No worries there. Checked with DMM most of the transistors, diodes and so on. Nothing shorted.
Defeating the protection circuit makes the amp come on live with the IRFP150N and it plays just fine, but channel 3 level where the IRFP150N is located is way lower than all of the other channels.
If channel 4 is at 20vrms clean sinewave, channel 3 stays at 15vrms. So far i've only looked at the pre amp section and didn't see any channel disbalances so both channels 3 and 4 should receive the same pre-amp level.
Haven't looked in details into the differential and drive amp so far. The bias pots have been touched before, so i've set all of them at their lowest level (counterclockwise) just for initial testing.
Is my issue related to the fact that the power transistor is not the right one, the bias adjustment or I should dig deeper ? (i did managed to find the service manual in other thread)
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You could swap the transistor between this channel and one of the others to confirm that it's not the transistor. I don't think it is.
The level problem could be due to an out of tolerance capacitor in the feedback circuit.
It's also possible that one of the NFB relays has dirty contacts. Does switching the NFB switch change anything?
The level problem could be due to an out of tolerance capacitor in the feedback circuit.
It's also possible that one of the NFB relays has dirty contacts. Does switching the NFB switch change anything?
Okay, the only thing i found is this.
VR301 RES, ADJ, CERMET 470 (IDLING) is showing open between 1-3 and 2-3 legs.
So I can not bias properly channel 3.
Anything specific to look up when ordering 470 ohm single turn cermet type trimmer except mounting dimensions ?
NFB switch is good, switching it ON lowers the output level a bit equally for all 4 channels.
I'm still not 100% that's my only issue...if this is responsible for the offset protection, why is the over current one also triggering ?
VR301 RES, ADJ, CERMET 470 (IDLING) is showing open between 1-3 and 2-3 legs.
So I can not bias properly channel 3.
Anything specific to look up when ordering 470 ohm single turn cermet type trimmer except mounting dimensions ?
NFB switch is good, switching it ON lowers the output level a bit equally for all 4 channels.
I'm still not 100% that's my only issue...if this is responsible for the offset protection, why is the over current one also triggering ?
Did you confirm that the Cx15 and Cx16 caps were within tolerance?
I don't see anything special about the bias pots.
Are you sure that ch3 is causing the over-current to trip?
Did you try swapping outputs from 3 to another channel to see if the problem followed the IRFP150?
I don't see anything special about the bias pots.
Are you sure that ch3 is causing the over-current to trip?
Did you try swapping outputs from 3 to another channel to see if the problem followed the IRFP150?
C315 or C316 don't remember which exactly is bad, one of them was in tolerance but the other one...changed both of these with new ones same value and same brand.
I will try to swap the IRFP150 once the bias pot arrives.
Are you sure that ch3 is causing the over-current to trip? - I think it's channel 3 but I'm going to confirm that later.
I will try to swap the IRFP150 once the bias pot arrives.
Are you sure that ch3 is causing the over-current to trip? - I think it's channel 3 but I'm going to confirm that later.
