Hello all,
New member here. I have replaced boards on several GFA-565 amps now and I had a peculiar issue on one of them. The board worked fine for about a month or so, and then the unit came back to me with the user complaining that it was running hot - really hot. I checked the bias and it was way off - inexplicably so. I adjusted the bias to the limit but couldn't bring it back into spec.
For background, I'm still using the "older" BFA-565 boards and not the updated EBFA-565 - this particular board was done about 2 years ago; adjusting the bias to the limit and telling the user to turn off the amp while not in use was an acceptable solution, but he wanted a complete fix that I didn't have the time for then, but I finally got the time now.
I went through both output blocks and everything tested out correctly. I then went back to the main board and re-tested all voltage points. Most everything tested within spec except that the bias voltage range (reference his linked spreadsheet on his site: https://www.dropbox.com/s/kr8w2nrcx...85 Input board performance checklist.xls?dl=0) is about .05v out of spec and about .1v from where it was when this particular board was installed. I highly suspect that this is the root cause of the bias jumping. However, I have no idea what the root cause of this "jump" was. I could simply change out the two resistors (R141 and R162) until I'm able to get the bias adjustment into an acceptable range. This is not an ideal solution as I'm worried that I may just be wallpapering over a potentially larger issue. I would much prefer to get at the root cause of this bias jump.
Any help would be greatly appreciated. My electrical engineering background is primarily in high-speed digital signals with minimal exposure to power electronic circuits (and I'm 25+ years removed from school at this point) so I'm a bit unsure of exactly how the biasing circuit is supposed to work.
New member here. I have replaced boards on several GFA-565 amps now and I had a peculiar issue on one of them. The board worked fine for about a month or so, and then the unit came back to me with the user complaining that it was running hot - really hot. I checked the bias and it was way off - inexplicably so. I adjusted the bias to the limit but couldn't bring it back into spec.
For background, I'm still using the "older" BFA-565 boards and not the updated EBFA-565 - this particular board was done about 2 years ago; adjusting the bias to the limit and telling the user to turn off the amp while not in use was an acceptable solution, but he wanted a complete fix that I didn't have the time for then, but I finally got the time now.
I went through both output blocks and everything tested out correctly. I then went back to the main board and re-tested all voltage points. Most everything tested within spec except that the bias voltage range (reference his linked spreadsheet on his site: https://www.dropbox.com/s/kr8w2nrcx...85 Input board performance checklist.xls?dl=0) is about .05v out of spec and about .1v from where it was when this particular board was installed. I highly suspect that this is the root cause of the bias jumping. However, I have no idea what the root cause of this "jump" was. I could simply change out the two resistors (R141 and R162) until I'm able to get the bias adjustment into an acceptable range. This is not an ideal solution as I'm worried that I may just be wallpapering over a potentially larger issue. I would much prefer to get at the root cause of this bias jump.
Any help would be greatly appreciated. My electrical engineering background is primarily in high-speed digital signals with minimal exposure to power electronic circuits (and I'm 25+ years removed from school at this point) so I'm a bit unsure of exactly how the biasing circuit is supposed to work.
@BSST : Yes, that is the correct manual. The issue is that the bias suddenly "jumped" and the voltage range has slipped quite a bit from when I had installed the boards. So I'm wondering what could have caused that. Trying to trace down possible causes. I verified both compensation transistors and they seem OK at least on a diode checker (maybe there's some other failure mode that I'm unaware of?).
Hey there, sorry I didn't get back to you, I got a lot cooking right now.
How is the condition of the bias comp cable? Any corrosion? What happens when you wiggle it? I've seen these corrode and cause DC offset, and I suppose it would mess up the bias too.
The transistor mounted on the heatsink is a Vbe multiplier, and part of the bias-spreader circuit.
Do you get equal and opposite DC voltages at the drive outputs?
I attached an annotated schematic for the EBFA-565, and it has all kinds of DC operating points and troubleshooting tips in it. (The EBFA circuit isn't any different, it's just the layout is better.)
How is the condition of the bias comp cable? Any corrosion? What happens when you wiggle it? I've seen these corrode and cause DC offset, and I suppose it would mess up the bias too.
The transistor mounted on the heatsink is a Vbe multiplier, and part of the bias-spreader circuit.
Do you get equal and opposite DC voltages at the drive outputs?
I attached an annotated schematic for the EBFA-565, and it has all kinds of DC operating points and troubleshooting tips in it. (The EBFA circuit isn't any different, it's just the layout is better.)
Hey, Chris. I had heard through the grapevine that you had stepped away so I went here. Thanks for the reply, really appreciated.
So, to answer your question, yes, they are equal - opposite; however the range is off. Now, there's one difference that I see from your attached schematic and the test points that I added here. Your test points are at the opposite ends of R136 and R135. I tapped directly off of the positive (cathode) side of D101 and negative (cathode) side of D102 which is where you had marked test points 4 and 5 respectively on the BFA-565 schematic. I this won't make any difference as those are "open" right now, but just mentioning for completeness.
Basically I'm getting +/- 1.72 - 1.95v adjustment range. When originally commissioned, the bias range was 1.59 - 1.81v.
The cables themselves look in good condition and the connectors were new. I still have one more spare set of cables that I can swap out if necessary, but I don't think that this is the problem. I just double-checked and it looks like I did replace these cables already so these aren't the (possibly corroded) originals. I didn't try a wiggle test, can try that later, but these have been disassembled and reassembled a few times now and the behavior is consistent so I would guess that isn't the problem.
Like I had said in the e-mail and earlier message; since the voltage is "close", I could just change out R141 and R142 (I had called it out as R162 but that's my attempt at reading a "black dot" from the original manual) to give me something that gets me a better range. But I'd much rather know if there's something else possibly suspect and could fail. I don't really have a good transistor trace checker to verify those heat sink mounted transistors. I can re-run the diode checks on them and see if the voltage seems "out of whack" but I didn't notice that the first time through when I tested all the parts.
VR,
Andrew
So, to answer your question, yes, they are equal - opposite; however the range is off. Now, there's one difference that I see from your attached schematic and the test points that I added here. Your test points are at the opposite ends of R136 and R135. I tapped directly off of the positive (cathode) side of D101 and negative (cathode) side of D102 which is where you had marked test points 4 and 5 respectively on the BFA-565 schematic. I this won't make any difference as those are "open" right now, but just mentioning for completeness.
Basically I'm getting +/- 1.72 - 1.95v adjustment range. When originally commissioned, the bias range was 1.59 - 1.81v.
The cables themselves look in good condition and the connectors were new. I still have one more spare set of cables that I can swap out if necessary, but I don't think that this is the problem. I just double-checked and it looks like I did replace these cables already so these aren't the (possibly corroded) originals. I didn't try a wiggle test, can try that later, but these have been disassembled and reassembled a few times now and the behavior is consistent so I would guess that isn't the problem.
Like I had said in the e-mail and earlier message; since the voltage is "close", I could just change out R141 and R142 (I had called it out as R162 but that's my attempt at reading a "black dot" from the original manual) to give me something that gets me a better range. But I'd much rather know if there's something else possibly suspect and could fail. I don't really have a good transistor trace checker to verify those heat sink mounted transistors. I can re-run the diode checks on them and see if the voltage seems "out of whack" but I didn't notice that the first time through when I tested all the parts.
VR,
Andrew