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This 5150 head has 4 relays on the preamp board. The Post Gain controls will not fully shut the sound down. With the controls all the way down it still seems to be on 3 and passing a loud signal. I also posted for a schematic with circuit board parts lay out..... Anyone recognize this problem? ... It's on the bench waiting....
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I've never serviced one of these amps (though I've read that it's a PITA). And so I don't have any familiarity with common problems with them. But the first thing I would check is the relay power supply. Testing voltages all the way to the relays. If three relays are blown I would be surprised. It seems more likely a fault in the switching power supply."Take two placebos, works twice as well." Enzo
"Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas
"If you're not interested in opinions and the experience of others, why even start a thread?
You can't just expect consent." Helmholtz -
Chuck, have a coffee. NO SMPS in a 5150.Education is what you're left with after you have forgotten what you have learned.Comment
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Wha wha wha whaaaa... Badump bump."Take two placebos, works twice as well." Enzo
"Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas
"If you're not interested in opinions and the experience of others, why even start a thread?
You can't just expect consent." HelmholtzComment
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How can you have gotten along this far and never encountered a 5150? I don't find them particularly difficult to work on, though the 5150Combo is laid out like the Classic 30 - three sided board thingie.
I don't think he was having relay ttrouble, I think he mentioned four relays to differentiate from the other model with three relays. (Actuaslly there is a fourth there too, it is just tucked out of the way.)Education is what you're left with after you have forgotten what you have learned.Comment
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Thx for your help... I don't have the ft sw, so I'm not sure if I can get it into the "Ultra" mode. The channel switching button on the ft switches between "Rythm" & "Lead". I've been using the "Normal Gain" input. The "Pre Gain" controls work for both channels fine. The "Post Gain" controls don't shut down the sound completely on both channels. Other wise it works fine. So I think something with the relays are out of wack. The voltages from the power supplies are correct. No burnt/open resistors. No blown fuses. It looks very clean. Tracing the schematic is difficult for me with all the relays. I'm also not sure of the solder connections on the relay where they go?... I did take the board out and gave it a good looking over and then reinstalled and have the exact same problem (of course)..... So what the hey???Comment
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Could bad electrolytic decoupling caps in the power supply leak the signal into other stages/circuits? It's never been recapped. I put my scope on it and didn't see any signal coming out of the Post Gain pots. The grounds all measure good...Comment
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Oh that is possible, tack another cap in parallel and se if it helps. You could also have a crosstalk situation.Education is what you're left with after you have forgotten what you have learned.Comment
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GOT IT!!! Tacked on new electrolytics to the first 2 stages. Bingo! It was feeding signal thru the power supply..... Enzo, put another feather in your hat.... Geesh, what a pain to find... Thx...Comment
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Yupper. That's a high gainer alright. I remember building a Prosonic clonish thingy into a Vibro Champ once and having trouble with power supply signal coupling. When you consider that the gain of an amp like this is typycally over 100,000.!!! It really doesn't take much coupling to gum things up.
I think it would be best to REPLACE the original caps. Rather than just parallel them. Caps typically short rather than fault. So a cap that's defective in any way is a liability."Take two placebos, works twice as well." Enzo
"Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas
"If you're not interested in opinions and the experience of others, why even start a thread?
You can't just expect consent." HelmholtzComment
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The tack on caps were for testing purposes. A young Emo fellow owns it and I shot him a decent quote on a complete recapping. There are several snap mount caps I don't stock. The original caps were the cheap IC caps. I should have known just seeing them in there. I'm just happy to have the problem solved... I don't get very many Hi Gain amps in for repair. It's impressive that these amps don't self destruct with all the gain, buttons, knobs, relays, tubes, mulitple grds. They sure did dampen the preamp tubes...Comment
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I will respectfully disagree with you here Chuck. I replace more e-caps that have opened up (lost value) and caused the amp to hum than those that have shorted and killed the power supply.Originally posted by Chuck H View PostComment
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I'll agree with your disagreement. And add that I was thinking of my personal experiences with film caps. Which isn't applicable here. My bad."Take two placebos, works twice as well." Enzo
"Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas
"If you're not interested in opinions and the experience of others, why even start a thread?
You can't just expect consent." HelmholtzComment
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