Intro and Question about Quad Cap
Posted: Wed Dec 30, 2015 7:23 pm
Hi all,
I just joined the forum after lurking for a few days and reading/digesting several posts. I came by the (mis)fortune of the world's best Xmas gift from my father...his old Dynaco ST-70 combo with a PAS-2 pre-amp. Before powering up either unit, I took a couple of days to research them and to see what potential upgrades are available or recommended given that they are completely untouched since their original build. In my foolishness, I decided not to do the main board CAP upgrade on either until I had a chance to plug the units in and to take some measurements with my trusty multimeter so I could order all replacement parts in one go. Here was the result:
1. PAS-2: Powered up and left on for 30 minutes with no noticeable issue other than the power switch and unit received power regardless of switch position. I traced it back to a bad 0.02 mfd cap and soldered in a new one that I had and voila...problem solved! Though I noticed some seepage from the 2000 mfd cylinder caps so added those to the list. Now for the important part...
2. ST-70. Powered up and watched the tubes slowly come to life while monitoring the tube bias. As expected, it fluctuated slightly through warm-up until the 10 min mark when the GZ34 shot a blue arc and blew the slow-blow 3A fuse. Per factory rec, I yanked the GZ34, replaced the fuse, and fired it up without the GZ34...all worked as expected until I replaced the GZ34 and the fuse blew again...bad GZ34. Fast forward three days...new JJ GZ34 showed up and in the meantime, I switched the red power leads (verified position three times before soldering) and added the diodes to the tube socket per internet recs. I plugged in the new GZ34 and powered up...all good for the first 10 min. I adjusted/monitored the bias the whole way until I heard a pop and gray smoke POURED out of the right front jack and my house filled with noxious fumes/smoke. My first reaction, was "oh sh*$, I just fried the selenium rectifier and doomed myself to a slow painful death." Post-mortem teardown revealed a brown goo coating over the internals as shown in the pics below:
https://drive.google.com/folderview?id=0B_zD_oadVZipcEdBdDMyaTh3TUU&usp=sharing
I suspect that I fried the quad cap, but the question is why? I've searched forum, but didn't see any ideas. That said, I turn to the tube gods of the forum to help point me in the right direction. Aside from comments about my foolishness in frying a perfectly good amp and potentially killing myself (and my family), I 'd be interested to hear speculation about potential causes and next steps. I'm a noob in terms of tube amps, but have the usual amateur tools and experience with crude circuit design (555 timers, leds, etc).
I'm putting together a full upgrade list at this point, and have seen several options, but would love to hear from those with first-hand experience. Thanks all!
All the best,
Jeff
I just joined the forum after lurking for a few days and reading/digesting several posts. I came by the (mis)fortune of the world's best Xmas gift from my father...his old Dynaco ST-70 combo with a PAS-2 pre-amp. Before powering up either unit, I took a couple of days to research them and to see what potential upgrades are available or recommended given that they are completely untouched since their original build. In my foolishness, I decided not to do the main board CAP upgrade on either until I had a chance to plug the units in and to take some measurements with my trusty multimeter so I could order all replacement parts in one go. Here was the result:
1. PAS-2: Powered up and left on for 30 minutes with no noticeable issue other than the power switch and unit received power regardless of switch position. I traced it back to a bad 0.02 mfd cap and soldered in a new one that I had and voila...problem solved! Though I noticed some seepage from the 2000 mfd cylinder caps so added those to the list. Now for the important part...
2. ST-70. Powered up and watched the tubes slowly come to life while monitoring the tube bias. As expected, it fluctuated slightly through warm-up until the 10 min mark when the GZ34 shot a blue arc and blew the slow-blow 3A fuse. Per factory rec, I yanked the GZ34, replaced the fuse, and fired it up without the GZ34...all worked as expected until I replaced the GZ34 and the fuse blew again...bad GZ34. Fast forward three days...new JJ GZ34 showed up and in the meantime, I switched the red power leads (verified position three times before soldering) and added the diodes to the tube socket per internet recs. I plugged in the new GZ34 and powered up...all good for the first 10 min. I adjusted/monitored the bias the whole way until I heard a pop and gray smoke POURED out of the right front jack and my house filled with noxious fumes/smoke. My first reaction, was "oh sh*$, I just fried the selenium rectifier and doomed myself to a slow painful death." Post-mortem teardown revealed a brown goo coating over the internals as shown in the pics below:
https://drive.google.com/folderview?id=0B_zD_oadVZipcEdBdDMyaTh3TUU&usp=sharing
I suspect that I fried the quad cap, but the question is why? I've searched forum, but didn't see any ideas. That said, I turn to the tube gods of the forum to help point me in the right direction. Aside from comments about my foolishness in frying a perfectly good amp and potentially killing myself (and my family), I 'd be interested to hear speculation about potential causes and next steps. I'm a noob in terms of tube amps, but have the usual amateur tools and experience with crude circuit design (555 timers, leds, etc).
I'm putting together a full upgrade list at this point, and have seen several options, but would love to hear from those with first-hand experience. Thanks all!
All the best,
Jeff