Hello all,
This is my first post on this forum. I have been a long time member at Diyaudio and was referred by a member.
I have an SVT-4 Pro that is being really difficult to work on. I have seen a few of these, but this one is a head scratcher.
Quick background- This amp was given to me after two people tried to fix it but hit a wall. I think I have restored it to where the wall was hit.
I have replaced all outputs with UNMATCHED devices as per the owners instructions. He does not believe they are necessary. (Thats a whole other thing)
I replaced all emitter resistors and any gate resistors that were burnt or out of spec. I have replaced the bias transistors and tested all diodes around them. I did replace one 10v zener. I have also replaced the other MPSA and MJE transistors and the 5532 behind them. I also checked out all diodes and resistors in front of the 5532 for shorts/open.
Here's whats odd. When the amp is brought up via a variac the unit starts drawing excessive current early on. ~25vac at the primary and about +-15v dc on the output rails. The rails look symmetrical.
I find one transistor conducting on its own and the opposite bank of 5 conducting evenly. Hence the heavy current draw.
If I bleed the rails down and bring it up again the same transistor, or another single transistor will start conducting and it's opposite bank will start evenly.
Which single transistor starts first seems to be random.
My confusion is that the amp should come up until the bias starts with no problem as the gates should be held off by design, but the +-15v rails that the bias runs off is only at 3v or so and the bias is at ~2mV when a transistor latches, so bias isn't awake yet.
I should add that the output section will behave the same way with the low voltage rails connected or not.
Any ideas?
Thanks!!
This is my first post on this forum. I have been a long time member at Diyaudio and was referred by a member.
I have an SVT-4 Pro that is being really difficult to work on. I have seen a few of these, but this one is a head scratcher.
Quick background- This amp was given to me after two people tried to fix it but hit a wall. I think I have restored it to where the wall was hit.
I have replaced all outputs with UNMATCHED devices as per the owners instructions. He does not believe they are necessary. (Thats a whole other thing)
I replaced all emitter resistors and any gate resistors that were burnt or out of spec. I have replaced the bias transistors and tested all diodes around them. I did replace one 10v zener. I have also replaced the other MPSA and MJE transistors and the 5532 behind them. I also checked out all diodes and resistors in front of the 5532 for shorts/open.
Here's whats odd. When the amp is brought up via a variac the unit starts drawing excessive current early on. ~25vac at the primary and about +-15v dc on the output rails. The rails look symmetrical.
I find one transistor conducting on its own and the opposite bank of 5 conducting evenly. Hence the heavy current draw.
If I bleed the rails down and bring it up again the same transistor, or another single transistor will start conducting and it's opposite bank will start evenly.
Which single transistor starts first seems to be random.
My confusion is that the amp should come up until the bias starts with no problem as the gates should be held off by design, but the +-15v rails that the bias runs off is only at 3v or so and the bias is at ~2mV when a transistor latches, so bias isn't awake yet.
I should add that the output section will behave the same way with the low voltage rails connected or not.
Any ideas?
Thanks!!
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