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    Need some help with a Conrad-Johnson tube preamp

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    deepee99

    Posts : 1337
    Join date : 2012-05-23
    Location : Wallace, Idaho

    Need some help with a Conrad-Johnson tube preamp

    Post by deepee99 on Mon Jan 25, 2016 3:03 pm

    A friend who mistakenly thinks I'm smart, brought over a 4-tube Conrad Johnson PV3 pre-amp that is DOA. Filaments light up but all it puts out is a fairly loud mid-frequency roar, tried it with both the Boppers and the s/s Bedinis; Not 60 Hz, so no easy fix like an open ground.
    My first guess was a bad tube but the channels are separately wired and the racket's the same on both channels and it does not pass an audio signal, so I'm guessing it's not a single lame tube and may be in the pwr supply or some other commonality. The caps all appear to be OK by visual inspection, though I've no real way to test them. The xformer looks OK, no bubbles. It uses 12XX7s in all four holes.
    Might one of you be so kind as to steer me towards a diagnostic fix? Or should I just stuff it with new tubes?
    Thanks
    Dave

    tubes4hifi
    Admin

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    Join date : 2008-11-30

    Re: Need some help with a Conrad-Johnson tube preamp

    Post by tubes4hifi on Mon Jan 25, 2016 5:03 pm

    there is a schematic here:  http://www.freeinfosociety.com/electronics/schemview.php?id=2337
    V1 & V3 are the phono amp, V2 & V4 are the line amp.
    most likely the power supply is bad.   There is a 3 section can cap in the power supply, check the voltages on sections A, B, & C
    B & C should be around 265vdc.   A is likely in the neighborhood of 300vdc

    deepee99

    Posts : 1337
    Join date : 2012-05-23
    Location : Wallace, Idaho

    Re: Need some help with a Conrad-Johnson tube preamp

    Post by deepee99 on Mon Jan 25, 2016 5:24 pm

    tubes4hifi wrote:there is a schematic here:  http://www.freeinfosociety.com/electronics/schemview.php?id=2337
    V1 & V3 are the phono amp, V2 & V4 are the line amp.
    most likely the power supply is bad.   There is a 3 section can cap in the power supply, check the voltages on sections A, B, & C
    B & C should be around 265vdc.   A is likely in the neighborhood of 300vdc
    Many thanks, Roy. I had a Heathkit AR-15 go haywire just like this, and it was the center power supply cap too. Heck, it's raining and I wear iron shoes and a lotta jewellery on a steel floor, so I'll go checkin'. Smile Nothing better to do today than electrocute myself.
    Presume a new can-cap can be had from about anybody. I'll look around. Hope that guy didn't fry the tubes as well.
    Cheerio,
    Dave

    deepee99

    Posts : 1337
    Join date : 2012-05-23
    Location : Wallace, Idaho

    Re: Need some help with a Conrad-Johnson tube preamp

    Post by deepee99 on Tue Feb 02, 2016 1:29 pm

    Yep Roy, it was the electrolytic can-multi-cap. Dried out and worthless. Couldn't find a drop-in replacement that would fit inside the cabinet top, so we've replaced it Holger-style with new stand-alone caps -- not a lot of room in that tiny chassis, but enough. Everything else appears within value so it's time to get the fire-extinguisher out and light 'er up.
    Lesson here: don't let your grand-dad's gear sit up in the attic un-usued for a decade or two and expect it to behave well on first light-up. That goes for solid-state as well as, in this case, a tube instrument.
    Again, thanks for the advice and the link to the schematic.

    GP49

    Posts : 735
    Join date : 2009-04-30
    Location : East of the sun and west of the moon

    Re: Need some help with a Conrad-Johnson tube preamp

    Post by GP49 on Wed Feb 03, 2016 12:45 pm

    deepee99 wrote:I had a Heathkit AR-15 go haywire just like this, and it was the center power supply cap too...

    If anyone still has an AR-15 kicking around, the output coupling capacitors would often go bad, resulting in "no signal" on one, the other, or both loudspeakers.  They were the infamous "Nashville" or Tennessee" electrolytics that plagued a lot of American equipment from that era.  Good thing is that they are easy to replace now, with much smaller, more reliable capacitors.

    Both went bad in the AR-15 my late father-in-law had.  He was told by a skinny punk kid at Radio Shack that it couldn't be fixed and was the equivalent of a boom box.  Luckily he took the skinny punk kid for what he was, a cellphone huckster and not a good one at that. He put away the AR-15 and I fixed it with surplus-store parts on my next visit.  It's now mine...but it's 3000 miles away!


    Last edited by GP49 on Wed Feb 03, 2016 8:26 pm; edited 2 times in total

    deepee99

    Posts : 1337
    Join date : 2012-05-23
    Location : Wallace, Idaho

    Re: Need some help with a Conrad-Johnson tube preamp

    Post by deepee99 on Wed Feb 03, 2016 1:46 pm

    The AR-15 was a marvel for its time. $400 in kit form. 69 transistors and two of the new magic ICs, each hand-soldered (well, their sockets were) to the board, plus the Oaks tuner design. It took me a week of full-focus (back in college days when I was capable of focus) to build the thing, but absent its main power supply cap going haywire after 5 years or so it is still in service. Its only flaw was that if you cranked it up enough into clipping mode it shot DC out to the speakers and roasted them in about a second or two.

    audiobill

    Posts : 270
    Join date : 2014-03-13
    Location : Philadelphia

    Re: Need some help with a Conrad-Johnson tube preamp

    Post by audiobill on Wed Feb 03, 2016 5:55 pm

    Heat-Kit

    Tube Nube

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    Age : 53
    Location : Calgary, AB

    Re: Need some help with a Conrad-Johnson tube preamp

    Post by Tube Nube on Thu Feb 04, 2016 2:02 am

    5 years since college, Dave? Geez, I know flying is stressful but . . . .

    Tube Nube

    Posts : 643
    Join date : 2008-12-06
    Age : 53
    Location : Calgary, AB

    Re: Need some help with a Conrad-Johnson tube preamp

    Post by Tube Nube on Thu Feb 04, 2016 2:03 am

    Ahhhh, sorry, I think I mis read that.

    tubenutr

    Posts : 62
    Join date : 2015-01-25
    Age : 59
    Location : Northern Ontario

    CJ pre

    Post by tubenutr on Sun Feb 14, 2016 5:05 pm

    Take it to the river and chuck it in the water. lol!

    Tube Nube

    Posts : 643
    Join date : 2008-12-06
    Age : 53
    Location : Calgary, AB

    Re: Need some help with a Conrad-Johnson tube preamp

    Post by Tube Nube on Sun Feb 14, 2016 8:08 pm

    Isn't that a line from one of the great mo' town hits?

    tubenutr

    Posts : 62
    Join date : 2015-01-25
    Age : 59
    Location : Northern Ontario

    CJ pre

    Post by tubenutr on Sun Feb 14, 2016 8:24 pm

    Its from a Talking Heads song.

    deepee99

    Posts : 1337
    Join date : 2012-05-23
    Location : Wallace, Idaho

    Re: Need some help with a Conrad-Johnson tube preamp

    Post by deepee99 on Sun Feb 14, 2016 9:21 pm

    Eskimo wrote:Take it to the river and chuck it in the water. lol!
    Too late. Replaced the dried-out quad-cap with discrete new ones and it works like a chump, er, champ. Am wondering how it will handle a sand-amp.

    tubenutr

    Posts : 62
    Join date : 2015-01-25
    Age : 59
    Location : Northern Ontario

    CJ pre

    Post by tubenutr on Mon Feb 15, 2016 5:51 pm

    deepee49,

    Not sure how big Wallace is, but word travels fast.
    You may end up being the go to guy for all things tube. Not a bad thing, but when they are lined up at
    the door.....................
    Some years ago the word got out that I was handy repairing ATVs.
    Word spread and I was getting 6 - 10 phone calls, emails per day.
    Eventually I had to shut down because my personal time was too important.
    Except of course if you "Smoker you drink, the player you'll get" (Joe Walsh)

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