Hello Folks,
I'm having a very weird issue with an ADCOM GFA-5800, I have another thread in the Solid State Forum but I'm not getting much of attention, hopefully here Mr Pass or somebody else familiar with his designs will have a clue.
Basically for a long time I thought that the Left Channel was damaged because its at this point going full + 83VDC a the speaker output, so I de-soldered every single component and tested it and re-soldered them back, I even replaced some MOSFETs that I know usually get damaged, based on other GFA-5500s and 5800 that I have fixed, I also changed some resistors that were very close to the 1% tolerance and of course the little electrolytic caps.
It wasn't until I finished all of that yesterday that I thought, OK silly why don't you connect the right channel assembly on the left side of the power supply and vice-versa and see what happens...
Well, I'm glad I did that, it turns out that that other channel had the same behavior and the previously thought damaged channel is perfectly fine in the other side of the power supply with only 12 mV DC offset.
I removed the power supply and again de-soldered every component from that left side and checked them, well only the ones on the regulated +/- 91VDC area, resistors, zener diodes, Transistors, voltage regulators, capacitors... all apparently good.... I have compared voltages with the other side and they all match.
Other observations I have seen are:
1. If I connect both of the Unregulated +/- 81VDC all good, no DC voltage being leaked to speaker outputs nor to the pre-ampfification area.
2. If I connect either of the regulated +/- 91VDC separately everything seems to be fine.
3. If I connect both of the regulated +/- 91VDC that’s when it goes bananas, with or without the +/- 81VDC rails connected...
Any help would be really appreciated.
Thanks,
Alex
I'm having a very weird issue with an ADCOM GFA-5800, I have another thread in the Solid State Forum but I'm not getting much of attention, hopefully here Mr Pass or somebody else familiar with his designs will have a clue.
Basically for a long time I thought that the Left Channel was damaged because its at this point going full + 83VDC a the speaker output, so I de-soldered every single component and tested it and re-soldered them back, I even replaced some MOSFETs that I know usually get damaged, based on other GFA-5500s and 5800 that I have fixed, I also changed some resistors that were very close to the 1% tolerance and of course the little electrolytic caps.
It wasn't until I finished all of that yesterday that I thought, OK silly why don't you connect the right channel assembly on the left side of the power supply and vice-versa and see what happens...
Well, I'm glad I did that, it turns out that that other channel had the same behavior and the previously thought damaged channel is perfectly fine in the other side of the power supply with only 12 mV DC offset.
I removed the power supply and again de-soldered every component from that left side and checked them, well only the ones on the regulated +/- 91VDC area, resistors, zener diodes, Transistors, voltage regulators, capacitors... all apparently good.... I have compared voltages with the other side and they all match.
Other observations I have seen are:
1. If I connect both of the Unregulated +/- 81VDC all good, no DC voltage being leaked to speaker outputs nor to the pre-ampfification area.
2. If I connect either of the regulated +/- 91VDC separately everything seems to be fine.
3. If I connect both of the regulated +/- 91VDC that’s when it goes bananas, with or without the +/- 81VDC rails connected...
Any help would be really appreciated.
Thanks,
Alex
for start , check continuity between central GND and local GND of +/-91V supplies
check also all connections with main supply - these connecting 91V rails (pre-reg) with main rails
after that , replace all zeners, they're cheap anyway
while you're there, you can replace bridges too, they're also cheap
check also all connections with main supply - these connecting 91V rails (pre-reg) with main rails
after that , replace all zeners, they're cheap anyway
while you're there, you can replace bridges too, they're also cheap
Thanks for your suggestions Zen Mod, well I checked all connections to ground on the power supply and they were all good....
There was only 1 thing left to disassemble and analyze, yeah, the balanced input board, I was really hesitant to do that because its a pain to remove it but I said, I must because that thing has a couple of loops to ground so I did it, and YES, that was freaking it!!!!!
An open ground, see below.... JEeeeeezzzzzzz!!!!
There was only 1 thing left to disassemble and analyze, yeah, the balanced input board, I was really hesitant to do that because its a pain to remove it but I said, I must because that thing has a couple of loops to ground so I did it, and YES, that was freaking it!!!!!
An open ground, see below.... JEeeeeezzzzzzz!!!!
I recently repaired a GFA-565 with the infamous leaky capacitors. After cleaning the board of electrolyte it "seemed" to work OK but only with a load connected. Without a load the DC offset was a couple of volts, with a load the DC offset would start higher than normal but eventually get down to an acceptable level. Finally found R121 was open, a 10 Ohm resistor that connects the ground circuits together. Replaced it and it's back to normal.
Craig
Craig