Hi. Got the parts, got some time and put the amp back together. Had real trouble with tired old traces on the bottom of the pre-driver board but finally got everything nice and tucked in. Checked all connections again. I had removed all power transistors on the right (bad side) and tested them and they were all fine. They were reinstalled with new sil-pads each.
I replaced Q106/156 and Q101/102 & Q151/152 (after using my new transistor tested to get matching units. I had previously replaced Q157 and Q107.
I put in on the variac and started bringing her up real slow and no noise/no hum and then when I got to about 60v something sparked down low on the right side power rails. I do not know what but I suspect that it was Q201 or Q202. It was late so I did not have the energy to take it all apart again. Damn! Could it be tightening them down too tight/too loose?
So, if it was Q201/202 I wonder if I messed them up when I removed them for testing? There is one bolt/nut that goes through the both holes and bolts it down to the heatsink. I made sure it had good sil-pad insulation under each. I am not looking forward to starting again.
I so want to get it running.
I will have to let it sit for a couple of days due to family obligations. Perhaps I will think of something.
I replaced Q106/156 and Q101/102 & Q151/152 (after using my new transistor tested to get matching units. I had previously replaced Q157 and Q107.
I put in on the variac and started bringing her up real slow and no noise/no hum and then when I got to about 60v something sparked down low on the right side power rails. I do not know what but I suspect that it was Q201 or Q202. It was late so I did not have the energy to take it all apart again. Damn! Could it be tightening them down too tight/too loose?
So, if it was Q201/202 I wonder if I messed them up when I removed them for testing? There is one bolt/nut that goes through the both holes and bolts it down to the heatsink. I made sure it had good sil-pad insulation under each. I am not looking forward to starting again.
I so want to get it running.
I will have to let it sit for a couple of days due to family obligations. Perhaps I will think of something.
Hello All,
I am happy to report a full success on this amp. I got replacement parts, was able to match the differential transistors, replaced all small electrolytics and the pre-driver transistors.
All done, adjusted and put back together. What a beast this is but with a great clean accurate sound. I am really impressed with it.
Thanks for all the help here. If you care to, here is a Youtube of it and my B & O Beogram turntable. https://www.youtube.com/watch?v=oxZojqsepWI&feature=youtu.be
Cheers!
I am happy to report a full success on this amp. I got replacement parts, was able to match the differential transistors, replaced all small electrolytics and the pre-driver transistors.
All done, adjusted and put back together. What a beast this is but with a great clean accurate sound. I am really impressed with it.
Thanks for all the help here. If you care to, here is a Youtube of it and my B & O Beogram turntable. https://www.youtube.com/watch?v=oxZojqsepWI&feature=youtu.be
Cheers!
Very Interesting and Encouraging Thread, I just got one of this but the left channel is not putting any output at all, I wish I had got it for $50 bucks though, I think I over paid for it but it is what it is, so now I'll have to try to fix it.
Where would you start testing?
I have checked everything inside and there's no obvious or visible evidence of anything burned, capacitor leaks, etc.
I hope you can help me to figure this out, I have been an audio aficionado my whole life and I'd really love to learn how to fix this.
Thanks in Advance!
Where would you start testing?
I have checked everything inside and there's no obvious or visible evidence of anything burned, capacitor leaks, etc.
I hope you can help me to figure this out, I have been an audio aficionado my whole life and I'd really love to learn how to fix this.
Thanks in Advance!
Adcom GFA 555 II hum issue
I've been reading this thread with interest as I just bought a GFA 555 II with a right channel problem. On my DBT I was unable to adjust bias on the right channel. At this point I didn't place a load on the amp but decided to find the bad components on the driver board.
I found Q158 2SC2912 was shorted B-E and replaced with KSC3503, and found R157 was dark in color was replaced with a 3.9KΩ 1/4W.
Powering up on DBT I was able to adjust bias on both sides now. I tested for sound on the DBT and got undistorted sound from a CD source on both sides, but have hum at zero volume.
My question is, where should I look from here?
I've been reading this thread with interest as I just bought a GFA 555 II with a right channel problem. On my DBT I was unable to adjust bias on the right channel. At this point I didn't place a load on the amp but decided to find the bad components on the driver board.
I found Q158 2SC2912 was shorted B-E and replaced with KSC3503, and found R157 was dark in color was replaced with a 3.9KΩ 1/4W.
Powering up on DBT I was able to adjust bias on both sides now. I tested for sound on the DBT and got undistorted sound from a CD source on both sides, but have hum at zero volume.
My question is, where should I look from here?
I think I figured it out, the hum might've been caused by the rear bridging/stereo switch being in Bridge mode. I also attached some screws to the top cover that holds the driver board assembly and retested with another preamp, GTP-450, and have no hum. All is good, sound is clean after adjusting bias to 10mV.
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