The 3.3kohm 5w resistors next to D108 & D121 should have the -rail voltage on one side. To illustrate look at the the two 3.3kohm resistors feeding D107,D120. The voltage on one side will be the + rail (71volts with a 14.4 battery term voltage) the other side will be 12volts less or approx 59 volts,using the transformer center tap.
Correction: the resistors are grounded and the rails are tied to the zener diodes, so what I said is correct but needs to be seen from diodes perspective!!
Okay, so the reason why you are not seeing the 12 volt drop across D108,D121 is either two open resistors (not likely) or the -rail is missing. Dollars to Doughnuts you have broken leg/s on the power supply filter inductor. Fairly common issue with this amp. If this is the case, you may be able to stuff leg/s back in board and resolder.Keep your fingers crossed.
So what you are seeing is not missing rails, but what appears to be shorted zener diodes or the capacitors across them!!
I've seen this problem before and the fact that it's happening on the negative rail side and both zener diodes may be leading us astray. Having said that first check to see if there is a short across the zeners. If so remove zeners and check again.C153 & C154 across D121 and C143 & C144 across D108.
There is also the possibility that a solder bridge on one of the drivers you replaced or somewhere else on the board, or a shorted sm transistor/s is causing this. Keep in mind a broken or cracked trace, or other possibilities. Nothing should be ruled out.
Whatever is causing this is staring us both square in the face, we just need to open our eyes!! Hang in there!
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Correction: the resistors are grounded and the rails are tied to the zener diodes, so what I said is correct but needs to be seen from diodes perspective!!
Okay, so the reason why you are not seeing the 12 volt drop across D108,D121 is either two open resistors (not likely) or the -rail is missing. Dollars to Doughnuts you have broken leg/s on the power supply filter inductor. Fairly common issue with this amp. If this is the case, you may be able to stuff leg/s back in board and resolder.Keep your fingers crossed.
So what you are seeing is not missing rails, but what appears to be shorted zener diodes or the capacitors across them!!
I've seen this problem before and the fact that it's happening on the negative rail side and both zener diodes may be leading us astray. Having said that first check to see if there is a short across the zeners. If so remove zeners and check again.C153 & C154 across D121 and C143 & C144 across D108.
There is also the possibility that a solder bridge on one of the drivers you replaced or somewhere else on the board, or a shorted sm transistor/s is causing this. Keep in mind a broken or cracked trace, or other possibilities. Nothing should be ruled out.
Whatever is causing this is staring us both square in the face, we just need to open our eyes!! Hang in there!
__________________
sorry bill, been a bit busy lately. back at this now. D120 and D107 show the voltage drop like you said, I'm seeing 61v on one side of the diode and 49v on the other. D121 and D108 I show 61v and 61v, no voltage drop across them. I've looked for solder bridges on the drivers and found none, removing the diodes now as well as the smt caps to test them out of the board. could you tell me what Q104, Q113, Q126 and Q135 are again, maybe I put them in the wrong pnp/npn order.
Check to see if D108 and D121 are grounding through R156-3.3K ohm and R216-3.3k ohm resistors. With an ohmmeter and one probe on the cathode and the other probe on the transformer center tap you should see 3.3k ohm.
You may want to check to see if anodes of each diode is connected to L02 (should be the green winding)
The fact that both zener diodes, which both are common to the negative drive's for the IRF640's on opposite sides of the sink, suggests that either they aren't getting the negative rail voltage (according to your prior posts this isn't the case) or an open trace or bad via which both sides share.
Sometimes I wish I can reach in and poke around myself!!! It's glaring at us, there is a solution to this problem!
You may want to check to see if anodes of each diode is connected to L02 (should be the green winding)
The fact that both zener diodes, which both are common to the negative drive's for the IRF640's on opposite sides of the sink, suggests that either they aren't getting the negative rail voltage (according to your prior posts this isn't the case) or an open trace or bad via which both sides share.
Sometimes I wish I can reach in and poke around myself!!! It's glaring at us, there is a solution to this problem!
correct, just double checking here. with just the diodes in place and caps c143, 144, 154, 155 removed i still dont have a 12 volt difference from either side of the diode. with the diodes removed as well as all the caps, measuring the solder pads where the diodes go i get -61v on both pads. could q114-117 and q136-139 be faulty? also, i checked for broken traces and all that and i have none. one side of the 3.3k resistors is grounded properly and the other is tied to the diode properly.
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