babadono Posted February 14, 2022 Share Posted February 14, 2022 Something was a short or close to it and now it is burned open? Quote Link to comment Share on other sites More sharing options...
JohnW Posted February 14, 2022 Author Share Posted February 14, 2022 Dumb luck, man. Lol. Story of my life: Quote Link to comment Share on other sites More sharing options...
babadono Posted February 14, 2022 Share Posted February 14, 2022 8 minutes ago, captainbeefheart said: Should have, the maximum current I saw was around 350mA. 3.3 or .4 volts so around 400 mA and 1.3 watts across 8.2 ohms. If the current stays as low as we saw near the end of the video it is time to start checking B1,B2,B3 and B4 Sorry I was typing while you posted your video. Definitely a loose grounding screw could cause problems. Are you sure all functions are good now? Quote Link to comment Share on other sites More sharing options...
captainbeefheart Posted February 14, 2022 Share Posted February 14, 2022 Just now, babadono said: 3.3 or .4 volts so around 400 mA and 1.3 watts across 8.2 ohms. If the current stays as low as we saw near the end of the video it is time to start checking B1,B2,B3 and B4 I edited what I wrote after watching it again. Ya it went up to 4v which is around a 500mA. The fuse looked like it lived but it was probably close to popping for a few seconds there, must be a slow blow. If it was a dead short the voltage across R9 would just keep increasing. I doubt it's a transistor as they never come back to life. My guess is a dry capacitor where the oxide layer is not forming well all the time. Probably C4 as that one sees the most ripple current. Quote Link to comment Share on other sites More sharing options...
babadono Posted February 14, 2022 Share Posted February 14, 2022 Good point. @captainbeefheart @JohnW do you know how old the unit is? If it is +20 years I would go ahead and replace the electrolytic capacitors especially in the power supply section i.e. the rectifier boards. And get rid of that glue stuff as best you can. If you really insist on mechanically attaching use a little dab of RTV (silicone). Just my $0.02 Quote Link to comment Share on other sites More sharing options...
captainbeefheart Posted February 14, 2022 Share Posted February 14, 2022 16 minutes ago, JohnW said: Dumb luck, man. Lol. Story of my life: Awesome job!! Okay so we know during normal operation you want to read around 700mV across R9, good to know. You could have fixed a bad connection. I would go ahead and order the big gray capacitors on the rectifier boards and keep them on hand anyway, probably all of $20 including shipping. R9 is saving other damage from happening, for instance if a capacitor is leaking not enough current to pop the fuse it can still build heat internally and that's where you will see capacitors bulge out or explode. Instead the resistor is designed with such a low wattage rating that it will burn up with a fault telling you something is wrong. Happy endings are always the best. Quote Link to comment Share on other sites More sharing options...
JohnW Posted February 14, 2022 Author Share Posted February 14, 2022 33 minutes ago, captainbeefheart said: Awesome job!! Okay so we know during normal operation you want to read around 700mV across R9, good to know. You could have fixed a bad connection. I would go ahead and order the big gray capacitors on the rectifier boards and keep them on hand anyway, probably all of $20 including shipping. R9 is saving other damage from happening, for instance if a capacitor is leaking not enough current to pop the fuse it can still build heat internally and that's where you will see capacitors bulge out or explode. Instead the resistor is designed with such a low wattage rating that it will burn up with a fault telling you something is wrong. Happy endings are always the best. Right after that video I posted, voltage stabilized at .6 to .7--right where it should be. So I wrapped a piece of electrical tape around the resistor (original was insulated) and soldered it in. Still a bit of ozone smell coming out of the unit, but that may just be all the dust that's been knocked loose on it's journey from Arizona to here (But I'll keep it in the garage, with a fire extinguisher nearby, until I feel certain about it. There'a s $99 complete recap kit available on Ebay..but if you think all I need to do is replace the caps on the rectifier boards, that sounds like a lot less to screw up. This thing sounds tremendous! Previously I was using a sub with my RB5s here in the garage, but this really doesn't need it! Thanks again! 2 Quote Link to comment Share on other sites More sharing options...
JohnW Posted February 14, 2022 Author Share Posted February 14, 2022 40 minutes ago, babadono said: Good point. @captainbeefheart @JohnW do you know how old the unit is? If it is +20 years I would go ahead and replace the electrolytic capacitors especially in the power supply section i.e. the rectifier boards. And get rid of that glue stuff as best you can. If you really insist on mechanically attaching use a little dab of RTV (silicone). Just my $0.02 I believe these were only made 1976-77. Quote Link to comment Share on other sites More sharing options...
babadono Posted February 15, 2022 Share Posted February 15, 2022 76? 77? yea I'd say those caps are due for replacing. But man it is up to you. Quote Link to comment Share on other sites More sharing options...
captainbeefheart Posted February 15, 2022 Share Posted February 15, 2022 11 minutes ago, babadono said: yea I'd say those caps are due for replacing. I agree. Start with C1 though C9. That's the power supply caps on all three rectifier boards (A,B,C). Quote Link to comment Share on other sites More sharing options...
JohnW Posted February 15, 2022 Author Share Posted February 15, 2022 12 minutes ago, captainbeefheart said: I agree. Start with C1 though C9. That's the power supply caps on all three rectifier boards (A,B,C). Will do, eventually. In the meantime, I have some raw birch La Scalas to stain and recap, maybe even build some AAs. I'll do a thread about the recap when I get it. 2 Quote Link to comment Share on other sites More sharing options...
JohnW Posted February 15, 2022 Author Share Posted February 15, 2022 I don't think I mentioned some pertinent, important details: I bought this from some guy in Arizona for $200 who knew nothing about stereo gear. He sent it to me in an old Epson printer box with "EASTER" magic markered on the side, and the receiver was protected by laundry. That's right, no bubble wrap, no wadded paper, not even a blanket. Just old, XXXL tshirts, shorts, and yes...underwear. Three pair. I am not joking. I didn't check how well laundered they were, but there was a faint scent of detergent when I opened the box, and I'm going to tell myself that it came from the clothes being freshly laundered and that is the last thought I will give it, for obvious reasons. The box was taped shut with silver air conditioning insulation tape. The fact that the unit actually works is a miracle. 1 Quote Link to comment Share on other sites More sharing options...
JohnW Posted February 15, 2022 Author Share Posted February 15, 2022 Ah. Not so fast. Went back out to the garage to listen to it and R9 fried again. So I’m guessing that initial voltage spike at power up is an issue. looks like I’m recapping sooner than later. I know I need electrolytics…any particular brand? Quote Link to comment Share on other sites More sharing options...
billybob Posted February 15, 2022 Share Posted February 15, 2022 Were you able to find and check the diodes... Quote Link to comment Share on other sites More sharing options...
JohnW Posted February 15, 2022 Author Share Posted February 15, 2022 16 minutes ago, billybob said: Were you able to find and check the diodes... Diodes all tested functional, at l ast on the rectifier A board. 1 Quote Link to comment Share on other sites More sharing options...
JohnW Posted February 15, 2022 Author Share Posted February 15, 2022 Ok. I’m a bit done for the day, but I believe this is where we left off. 1 Quote Link to comment Share on other sites More sharing options...
captainbeefheart Posted February 15, 2022 Share Posted February 15, 2022 You can pretty much rule out diodes and transistors because when they fail they are gone and it would behave like a hard short showing the voltage across R9 continually increasing and not just stop at 5.5v. That is 670mA and almost 7 watts dissipation for R9. I got my money on a capacitor that is showing high leakage current from the oxide layer not forming properly when powering up. If you are bored waiting for parts start by pulling C4 off the board and short across the leads for 10 seconds. After you do that measure it's resistance and see what happens, my guess it will be a very low resistance at first and then start to form but stop somewhere in the 20k-30k area of resistance. If that's the case you can then try and slowly reform it by current limiting it with a DC bench supply but I would say it's not worth it. BTW that's how you can check all the power supply capacitors when removed from their boards, short the leads for 10 seconds and then measure their resistance. A normal capacitor will read low for a second or so and then quickly jump to infinite resistance once the oxide layer forms on the anode. If you find a capacitor reads below say 1Meg resistance it will pull too much leakage current to use in circuit. Quote Link to comment Share on other sites More sharing options...
JohnW Posted February 15, 2022 Author Share Posted February 15, 2022 18 minutes ago, captainbeefheart said: You can pretty much rule out diodes and transistors because when they fail they are gone and it would behave like a hard short showing the voltage across R9 continually increasing and not just stop at 5.5v. That is 670mA and almost 7 watts dissipation for R9. I got my money on a capacitor that is showing high leakage current from the oxide layer not forming properly when powering up. If you are bored waiting for parts start by pulling C4 off the board and short across the leads for 10 seconds. After you do that measure it's resistance and see what happens, my guess it will be a very low resistance at first and then start to form but stop somewhere in the 20k-30k area of resistance. If that's the case you can then try and slowly reform it by current limiting it with a DC bench supply but I would say it's not worth it. BTW that's how you can check all the power supply capacitors when removed from their boards, short the leads for 10 seconds and then measure their resistance. A normal capacitor will read low for a second or so and then quickly jump to infinite resistance once the oxide layer forms on the anode. If you find a capacitor reads below say 1Meg resistance it will pull too much leakage current to use in circuit. Was just shopping for new capacitors from Parts Conexxion. It's not like they don't need to be replaced, right? I'll pull the C4 and test it, but it'll probably be later this week. Here's my Parts Connexion shopping cart: will these work? https://www.partsconnexion.com/cgi-bin/sc/order.cgi?storeid=*20b7b4534d0a875b77f1157d6bb78322a865&function=show Quote Link to comment Share on other sites More sharing options...
captainbeefheart Posted February 15, 2022 Share Posted February 15, 2022 1 hour ago, JohnW said: Here's my Parts Connexion shopping cart: will these work? The shopping cart must be saved in your browser program's temporary cache file because I don't see anything when clicking the link. I don't have any experience with parts connexion as I typically purchase from Digikey. Although any general purpose electrolytic will do I like to get the long life caps rated for 105°C temperature, Nichicon, Panasonic, CDE etc.... 2 Quote Link to comment Share on other sites More sharing options...
JohnW Posted February 15, 2022 Author Share Posted February 15, 2022 When testing resistance of the cap, what should I set my Multimeter to? Quote Link to comment Share on other sites More sharing options...
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