Hello Guys,
I thought the 700b was ready for years of renewed life, but I am now dealing with a new issue. This amp was built as a present for my nephew who graduated from college with his Master’s Degree recently. The family presented the amp yesterday afternoon, I had it playing through the speakers when he and my brother and his wife arrived. My nephew was quite impressed, and then we “handed the keys” to him, and congratulated him on all the work he has put in. Of course he was shocked. He took it home and set it up, called me and said it sounded amazing, and he was so surprised, and grateful. About 9pm he called me and said that he shut it off about 8:30, then went to turn it on again about 8:50 and heard a pop, like a fuse had blown. He checked the fuses, and the left channel ground was blown (AGX-5). I told him to bring it over this morning and I would look at it. Early this morning, I had a chance to replace the fuse, and bring it up, with a DBT and Variac, and it came up as expected, relays kicked in as they should, and everything seemed okay. I checked the bias, and I have .360 in the right channel, and .008 in the left channel. DC offset in both channels is .001. I still have the +/- 15volts on both boards at those test points. I noticed that while I had the amp up, I had 86.8v DC at the caps separately, and 173.6 across both. I also have 173.6v DC across points 11, and 12 on the G1 board for both channels, no fuses blown on the backplane boards. Checking the collector case to ground, under power, I have 86.8v DC on all of the Right Channel outputs, and all 6 of the third row of 95’s on the Left channel. The row next to the transformer (96’s left channel) all show .003v DC when I tested them.
I pulled all of the 96’s in the row next to the transformer. They all test perfectly across Emitter/Base/Collector testing .514/.521 avg. and O/L as they should where appropriate. I did find one of these that had cut through the sil-pad ever so slightly, and there was evidence of arcing to the heat sink. This I figure is what blew the fuse. I checked all of the heat sinks carefully and chamfered accordingly before I initially installed the outputs. Some of the transistor case holes were quite sharp, and I took that edge off as well when I initially installed, to prevent cutting the sil-pads. I did observe the 5-inch pound final torque. So on that one I cleaned up the case arcing, and it tested fine. I reinstalled with a new sil-pad. The amp powers up fine with the DBT and Variac, but I still have the bias at .008 in the left channel.
I won’t have a chance to look at this until later this afternoon, or this evening. I was going to work my way through the Left backplane board and test as I go. You guys have much more experience in this area than I do. It looks like that output arced through the sil-pad, after a few days of working perfectly.
I am a master Tool and Die maker, so I was very careful checking burrs, and chamfering where appropriate, and base/emitter pin/screw alignment on the initial assembly , checking both transistor cases, and heat sinks. I did find several transistors that had the full burr on the punched holes, which surprised me, but as I said, I cleaned them up, before installing, and I checked each one for a short per the start up procedure. I was very careful not to tighten the SHCS to tight (5 in/lb), and I took my time as I assembled. The amp has worked perfectly for a couple days now, without even a hiccup.
Since the transistors all check okay on that offending row (LH channel next to the transformer) and I still have 86.8v dc on the cases of all 18 in the 3 other rows, but only .003v dc in row 4 by the transformer, any help on where to start troubleshooting? Nephew feels it is something he did, I am sure it was not. All he did was turn it on, and when he heard the “pop” of the fuse, he immediately shut it off.
Thank You
I thought the 700b was ready for years of renewed life, but I am now dealing with a new issue. This amp was built as a present for my nephew who graduated from college with his Master’s Degree recently. The family presented the amp yesterday afternoon, I had it playing through the speakers when he and my brother and his wife arrived. My nephew was quite impressed, and then we “handed the keys” to him, and congratulated him on all the work he has put in. Of course he was shocked. He took it home and set it up, called me and said it sounded amazing, and he was so surprised, and grateful. About 9pm he called me and said that he shut it off about 8:30, then went to turn it on again about 8:50 and heard a pop, like a fuse had blown. He checked the fuses, and the left channel ground was blown (AGX-5). I told him to bring it over this morning and I would look at it. Early this morning, I had a chance to replace the fuse, and bring it up, with a DBT and Variac, and it came up as expected, relays kicked in as they should, and everything seemed okay. I checked the bias, and I have .360 in the right channel, and .008 in the left channel. DC offset in both channels is .001. I still have the +/- 15volts on both boards at those test points. I noticed that while I had the amp up, I had 86.8v DC at the caps separately, and 173.6 across both. I also have 173.6v DC across points 11, and 12 on the G1 board for both channels, no fuses blown on the backplane boards. Checking the collector case to ground, under power, I have 86.8v DC on all of the Right Channel outputs, and all 6 of the third row of 95’s on the Left channel. The row next to the transformer (96’s left channel) all show .003v DC when I tested them.
I pulled all of the 96’s in the row next to the transformer. They all test perfectly across Emitter/Base/Collector testing .514/.521 avg. and O/L as they should where appropriate. I did find one of these that had cut through the sil-pad ever so slightly, and there was evidence of arcing to the heat sink. This I figure is what blew the fuse. I checked all of the heat sinks carefully and chamfered accordingly before I initially installed the outputs. Some of the transistor case holes were quite sharp, and I took that edge off as well when I initially installed, to prevent cutting the sil-pads. I did observe the 5-inch pound final torque. So on that one I cleaned up the case arcing, and it tested fine. I reinstalled with a new sil-pad. The amp powers up fine with the DBT and Variac, but I still have the bias at .008 in the left channel.
I won’t have a chance to look at this until later this afternoon, or this evening. I was going to work my way through the Left backplane board and test as I go. You guys have much more experience in this area than I do. It looks like that output arced through the sil-pad, after a few days of working perfectly.
I am a master Tool and Die maker, so I was very careful checking burrs, and chamfering where appropriate, and base/emitter pin/screw alignment on the initial assembly , checking both transistor cases, and heat sinks. I did find several transistors that had the full burr on the punched holes, which surprised me, but as I said, I cleaned them up, before installing, and I checked each one for a short per the start up procedure. I was very careful not to tighten the SHCS to tight (5 in/lb), and I took my time as I assembled. The amp has worked perfectly for a couple days now, without even a hiccup.
Since the transistors all check okay on that offending row (LH channel next to the transformer) and I still have 86.8v dc on the cases of all 18 in the 3 other rows, but only .003v dc in row 4 by the transformer, any help on where to start troubleshooting? Nephew feels it is something he did, I am sure it was not. All he did was turn it on, and when he heard the “pop” of the fuse, he immediately shut it off.
Thank You