Received the amp in pretty beat up condition. Power supply FETs had been hot enough to melt to the heat sink but somehow had not failed. One front channel had shorted outputs, one rear channel had burnt open both of the 10ohm resistors that feed power from the power supply, all the bias and dc pots where damaged with most being inoperable, (appeared someone got carried away with a screw driver)
With the power supply FETs out of circuit there was good drive signal present at all gates. The secondary power supply that feeds the op-amps and the transistors for the outputs also appears to be in OK shape with the exception of the +/-15 its right around +14 and -19, I'm not sure if this is OK, I'll try and find if a regulator or something on the negative side is out of whack.
I replaced the 10 ohm resistors on the channel they had failed on, as soon as the auxiliary power comes up on voltage it instantly smokes them. I removed the two transistors next to the 10 ohm resistors and they no longer smoke when power is applied. The two transistors measure OK when checking them with my meter.
I put in new power supply FETs in to try and get the amp up and see if the remaining channels would work. The amp trips my supply and blows a 10 amp fuse. I removed all the outputs thinking maybe they are being driven on and causing a short. Same thing happens the main supply is drawing excessive current. Thought that perhaps a diode was shorted so I pulled them all to check and they test OK. With the diodes removed the main supply powers up and drives the transformer as it should.
I'm struggling trying to find why the amp is drawing excessive current with all the outputs removed from the circuit.
With the power supply FETs out of circuit there was good drive signal present at all gates. The secondary power supply that feeds the op-amps and the transistors for the outputs also appears to be in OK shape with the exception of the +/-15 its right around +14 and -19, I'm not sure if this is OK, I'll try and find if a regulator or something on the negative side is out of whack.
I replaced the 10 ohm resistors on the channel they had failed on, as soon as the auxiliary power comes up on voltage it instantly smokes them. I removed the two transistors next to the 10 ohm resistors and they no longer smoke when power is applied. The two transistors measure OK when checking them with my meter.
I put in new power supply FETs in to try and get the amp up and see if the remaining channels would work. The amp trips my supply and blows a 10 amp fuse. I removed all the outputs thinking maybe they are being driven on and causing a short. Same thing happens the main supply is drawing excessive current. Thought that perhaps a diode was shorted so I pulled them all to check and they test OK. With the diodes removed the main supply powers up and drives the transformer as it should.
I'm struggling trying to find why the amp is drawing excessive current with all the outputs removed from the circuit.
I removed all the FETs from the output section (530's and 9z34's) thinking that part of their drive circuity in the channel was turning them on when they shouldn't be. Power through a current limiting resistors provides around 8 volts at the +terminal and the supply just makes odd noises and never comes online. Should I drop to a lower resistance so the voltage is higher and allow more current into the amp?
Sometimes, the resistor prevents the amp from powering up completely but it allows enough supply output to cause the defective components to heat up.
In the future, use the heating of the outputs (being careful not to let any transistors overheat while testing) or the voltage across the emitter/source resistors (can be done with all components clamped tightly to the heatsink) as a guide as to which channel is drawing excessive current.
Are the output still out of the amp?
What is the value of your current limiting resistor?
In the future, use the heating of the outputs (being careful not to let any transistors overheat while testing) or the voltage across the emitter/source resistors (can be done with all components clamped tightly to the heatsink) as a guide as to which channel is drawing excessive current.
Are the output still out of the amp?
What is the value of your current limiting resistor?
Yes, the outputs are still out of the amp, they all test OK and nothing on the board is getting warm but I've not let it run for more than about 30 seconds. First was powered through an automotive lamp, then through an 8 ohm resistor results were the same both ways. I'll see if I can find a lower ohm resistor to power it up with. Prior to inserting the power supply FETs the amp would turn on and the only items on the board that would warm up were the bf422 and 423 transistors which are powered by the +/- ~50 volts off the smaller transformer.
I'll put outputs back in the amp and let it run and see if anything warms up.
I'll put outputs back in the amp and let it run and see if anything warms up.
I almost picked some of those up the other day just because... Not sure if they are still open or not to get them from... I'll order something online from future or digikey... all the radio shacks near me are empty... 95% off everything... which I wish I'd have known sooner or I would have went early enough to get some decent things. The one thing I'm not getting is how could the amp be drawing so much current with no outputs installed? The inductor caps and the outputs are about the only thing powered by the main rail supply right?
Ok, I found a couple of 1 ohm 5 watt resistors in my collection of parts, put them in series to get near 2 ohms and hooked the amp up. Most voltage I'm able to get in the B+ terminal is 8.8vts the resistors heat pretty rapidly the power supply transformer or inductor makes noise like it's trying to power a dead short. The power supply fets warm up and the diodes warm up slightly. I've still not out the outputs in because I don't think the power is making it that far anyways. Seems like possibly the diodes are bad but they checked on out of the circuit. Should I pull the inductor to check it for a possible short? It looks like the +\- rails both go to it. It just doesn't appear to have been overheated and shorted out. And it's not getting warm so it seems that it would be easy to rule it out.
Last question for the night, should I swap the gate resistors from 22 ohms to 47 ohms, at this point I wouldn't think it would cause too much of a difference as the rails should be virtually unloaded with no outputs installed.
Last question for the night, should I swap the gate resistors from 22 ohms to 47 ohms, at this point I wouldn't think it would cause too much of a difference as the rails should be virtually unloaded with no outputs installed.
The problem was with the inductor. I must have moved it just right when handling the board replacing the diodes. There is a short section of 2 wires that has the insulating layer removed. Just so happens one is on the negative rail and one on the positive rail. It's been partially unwrapped isolated and rewound. No problems when moving it around now. I'll get outputs in tonight and find out if any of the channels are operational, my guess is not with the condition it was in when I received it.
Thanks for the insight on double checking the inductor.
Thanks for the insight on double checking the inductor.
Perry, could I get your opinion on using KSC23830OTA in place of the BF422 and the KSA1013OBU for the BF423. I guess another option is going through another provider which I've not purchased parts from before, what are your recommendations for these. I have several that leak and a few that are flat shorted out.
Insert leg 1 normally, swap the two other legs and twist the body so that no legs touch. If you want, you can place a piece of small heatshrink tubing or a piece of stripped wire insulation over one of the crossed legs. This will allow you to leave the body of the transistor straight without having the legs short.
I don't remember but those would have the ones I would have used if I needed high voltage subs and the originals weren't available. For low voltage, I'd likely have used the A06 and A56. I may have used the KSC2690A and the KSA1220A as well. Sometimes the legs are too large to fit or the body is too large.
So I used parts from one of the other channels to attempt to create at least one fully operational channel on this amp and I've not been able to pin down whats causing 2 resistors to overheat and fail.
Once the resistors fail the channel still produces audio and the dc offset can still be adjusted. These 2 resistors in all but one of the channels were either completely burned open or looked OK just didn't measure any resistance.
Once the resistors fail the channel still produces audio and the dc offset can still be adjusted. These 2 resistors in all but one of the channels were either completely burned open or looked OK just didn't measure any resistance.
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Ok Perry, I found I have a few bad 3B transistors, which also correspond to the channels that are burning up the 100ohm resistors. On the 1N5 Zener diodes, how do I know what the correct value of them should be? Should I pull all of them out of the amp and measure them to see if they are consistent between channels?
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