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Still trying to resolve some issues with one channel of an SVT 4 Pro. I posted a week ago about what I found in it and currently am at an impass trying to find the source of weird voltage readings.
All the issues are contained in channel A only (B works fine): The hi side had blown all the outputs, ballast, 47 ohm & 1.5k resistors. I replaced those along with several of the ballast resistors on the Lo side (the 47ohm & 1.5k were good). R118 which feeds the V+ to the driver section was also blown/scortched. I pulled the Hi side heatsink with all the devices to replace the ballest resistors and have it out while I'm trying to normalize the voltage.
What I'm seeing is a very low V+ on the outputs (.4 volts instead of 3.6-4.5v), and a very weird -12.5v at TP4 which should be +/- 1 volt w/o load. I think that this is shutting everything down in the driver section and giving the low volts on gates of the outputs. The center rail (Output A) between the two sides also has a -1.7v where I'm assuming it should be zero. I realize that Output A feeds back to the pin 3 of the IC1 and is probably generating the -12v that I'm seeing on pin 1 (output of IC1).
At this point I've replaced all the diodes in the driver stage and the output stage (D114/115, D116/117, D110/D111, D112/D113, D106/107, D108/109). I've also replaced all the transistors in the driver stage (Q102/103, Q104/105, Q106/107 and Q108/109). Q102 which is where I first spotted the blown resistor has the right voltage on the E=77v and B=78v, but only putting out .3-.4 v on the collector. Same is true for the neg side although the neg voltages are a bit higher than the positive ones. I should say that Q103 & Q104 (the inside pair) seem to be about right: Q103B=4.5v, E=4v, C=77v. While Q104 B=-5.3, E=-8.3v, C=-78v.
Up to now I also resisted the reflow all solder joints process, but as a last resort tonight I did that also. At least on the "A" side components. I've been comparing down the line the voltage readings from the B channel and checking it against A, but I can't seem to figure out why they are different, just that they are. All the voltage supplies seem good and consistant from one side to the other. I also pulled the 5532 and socketed it and put in another, but I get the same reading of -12v on pin one, so I put the old part back in. I know that both channels share that device, but the B side reads zero on pin 7 (it's output).
Any thoughts or aha's? I feel like I'm flailing at this point. Also the readings up to IC1 seem about right.
Matt
All the issues are contained in channel A only (B works fine): The hi side had blown all the outputs, ballast, 47 ohm & 1.5k resistors. I replaced those along with several of the ballast resistors on the Lo side (the 47ohm & 1.5k were good). R118 which feeds the V+ to the driver section was also blown/scortched. I pulled the Hi side heatsink with all the devices to replace the ballest resistors and have it out while I'm trying to normalize the voltage.
What I'm seeing is a very low V+ on the outputs (.4 volts instead of 3.6-4.5v), and a very weird -12.5v at TP4 which should be +/- 1 volt w/o load. I think that this is shutting everything down in the driver section and giving the low volts on gates of the outputs. The center rail (Output A) between the two sides also has a -1.7v where I'm assuming it should be zero. I realize that Output A feeds back to the pin 3 of the IC1 and is probably generating the -12v that I'm seeing on pin 1 (output of IC1).
At this point I've replaced all the diodes in the driver stage and the output stage (D114/115, D116/117, D110/D111, D112/D113, D106/107, D108/109). I've also replaced all the transistors in the driver stage (Q102/103, Q104/105, Q106/107 and Q108/109). Q102 which is where I first spotted the blown resistor has the right voltage on the E=77v and B=78v, but only putting out .3-.4 v on the collector. Same is true for the neg side although the neg voltages are a bit higher than the positive ones. I should say that Q103 & Q104 (the inside pair) seem to be about right: Q103B=4.5v, E=4v, C=77v. While Q104 B=-5.3, E=-8.3v, C=-78v.
Up to now I also resisted the reflow all solder joints process, but as a last resort tonight I did that also. At least on the "A" side components. I've been comparing down the line the voltage readings from the B channel and checking it against A, but I can't seem to figure out why they are different, just that they are. All the voltage supplies seem good and consistant from one side to the other. I also pulled the 5532 and socketed it and put in another, but I get the same reading of -12v on pin one, so I put the old part back in. I know that both channels share that device, but the B side reads zero on pin 7 (it's output).
Any thoughts or aha's? I feel like I'm flailing at this point. Also the readings up to IC1 seem about right.
Matt
. If you just try to calculate base-emitter voltages, you get: Q103 0.5V, Q104 3V. For me it's very far from beeing "about right". The circuit is symmetrical. Shouldn't the voltages be symmetrical too (at least to some extents)?
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