thebes Posted January 26, 2017 Share Posted January 26, 2017 Dusted off my old Conrad Johnson PV3 tubed preamp to sell to help pay for Craig's NBS I bought. Hooked it up and I'm getting almost a metronome. The sound cuts in an out on a regular beat. Swapped out all the tubes. Same same. Happens on line and phono stages. Any guesses as to where I should start looking. Gross physical examination shows nothing. Just trying to figure out where to put the probes first. Here's the schematic: Quote Link to comment Share on other sites More sharing options...
dBspl Posted January 26, 2017 Share Posted January 26, 2017 I'm no expert, but I would start with the power supply to confirm all supply voltages are good. But it sounds like a leaky coupling capacitor in the output stage (charging & discharging). I've owned a PV3 since the mid-80's. It's a great little pre-amp. I still use mine regularly. Kerry 1 Quote Link to comment Share on other sites More sharing options...
mike stehr Posted January 26, 2017 Share Posted January 26, 2017 What Kerry said, as it seems good practice to start at the PS. Could be a bunk PS, or de-coupling cap. You can check for DC at the outputs I would think. Anything over 1 volt means something is wrong. Quote Link to comment Share on other sites More sharing options...
dBspl Posted January 26, 2017 Share Posted January 26, 2017 If the problem is common to both channels, then it's likely the power supply since that's the only part of the circuit both channels appear to share.. 1 Quote Link to comment Share on other sites More sharing options...
dirtmudd Posted January 27, 2017 Share Posted January 27, 2017 power supply issues are common problems with old cj gear.. Quote Link to comment Share on other sites More sharing options...
thebes Posted January 27, 2017 Author Share Posted January 27, 2017 Cool. Hopefully I'll get a chance to get at this tomorrow. I recapped it a few years ago when I was just learning to solder so I might have fried a cap. Always love the breath of knowledge in this place. Quote Link to comment Share on other sites More sharing options...
dirtmudd Posted January 27, 2017 Share Posted January 27, 2017 2 minutes ago, thebes said: Cool. Hopefully I'll get a chance to get at this tomorrow. I recapped it a few years ago when I was just learning to solder so I might have fried a cap. Always love the breath of knowledge in this place. was you pv3 factory built or kit form ? Quote Link to comment Share on other sites More sharing options...
thebes Posted January 30, 2017 Author Share Posted January 30, 2017 Sorry about the delay in responding. It was factory built but I recapped it several years ago. Quote Link to comment Share on other sites More sharing options...
dBspl Posted January 30, 2017 Share Posted January 30, 2017 It probably wouldn't hurt to go through it and check for a cold solder joints, too. Kerry 1 Quote Link to comment Share on other sites More sharing options...
babadono Posted January 30, 2017 Share Posted January 30, 2017 What is B+1 and B+2, I mean the voltage at these points. I love schematics that make you guess Quote Link to comment Share on other sites More sharing options...
mikebse2a3 Posted February 2, 2017 Share Posted February 2, 2017 On 1/26/2017 at 1:48 PM, thebes said: Hooked it up and I'm getting almost a metronome. The sound cuts in an out on a regular beat. Swapped out all the tubes. Same same. Happens on line and phono stages. To clarify a little do you have any power supply hum or is the sound normal but varying in and out? As others have mentioned if the problem is in both channels then troubleshoot the power supply. Measure the B+1 voltage and note if it varies with the cycling of the sound. If so D1,D2,D3,D4 should be checked to confirm one isn't open/leaky and of course capacitors C11, C12, C13, C15 should be checked as well as components in the regulator circuit (R20, R30, Q1, Q2, D15, D16 and D6-13 Zener Diodes). Edit: The Zener Diodes D6-13 look to be 1N5257 (8 of them in series) which if so they are a 33v zener so you should measure very close to that across any of the zeners individually and all together they will set a potential at the base of Q2 to ground of approximately 264 volts. Good Luck and don't smoke anything buddy miketn Quote Link to comment Share on other sites More sharing options...
dirtmudd Posted February 3, 2017 Share Posted February 3, 2017 On 1/26/2017 at 1:48 PM, thebes said: Dusted off my old Conrad Johnson PV3 tubed preamp to sell to help pay for Craig's NBS I bought. Hooked it up and I'm getting almost a metronome. The sound cuts in an out on a regular beat. Swapped out all the tubes. Same same. Happens on line and phono stages. Any guesses as to where I should start looking. Gross physical examination shows nothing. Just trying to figure out where to put the probes first. Here's the schematic: how did you make out ? Quote Link to comment Share on other sites More sharing options...
thebes Posted February 4, 2017 Author Share Posted February 4, 2017 My sad tale of my troubleshooting requires more thought than a Friday night post. The short Friday night version is that some, only some, of the magic smoke escaped. More to follow but first a nod to Mike whose knowledge still leaves me in awe. Quote Link to comment Share on other sites More sharing options...
mikebse2a3 Posted February 4, 2017 Share Posted February 4, 2017 Hey Thebes does your unit's parts values match the schematic you posted..? Here is another schematic that appears to be a version of the PV-3 also with some slight variation in component values and parts reference designator differences in case it matches closer to your unit version. It might also help you to visualize the circuit better even if it's not the exact version of the PV-3 you have. Quote Link to comment Share on other sites More sharing options...
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