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Conrad Johnson PV-7 noise

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Hi all, I did a little research on this and couldn't find a satisfactory answer:

I have noise on my CJ PV-7 tube preamp that is well beyond what it should be, and much more than it used to be (it's volume is independent of the volume pot.) It is a hiss/crackle not a hum. Changing out every tube makes no difference. I e-mailed the CJ repair department a long time ago (long standing problem that I am getting around to fixing) and they suggested if switching out tubes makes no difference, it is probably a bad power transformer.

Is there any possibility that it might be something else? I'd like to know before I go off and replace the power transformer.
 
See what the B+ rail and filament voltages are (if you don't know your way around high voltage gear, don't listen to me! Serious safety issue!:att'n: ). If they're in the normal range, your power transformer is OK.

Could they have said "power transistor"? The ones in the regulator could certainly cause a problem like this.
 
Thanks for the ideas everyone. I contacted the CJ repair department and they suggested it might be a capacitor in the power supply. I tested every one of the supply caps and they looked ok (high dc resistance and proper capacitance). I guess it's on to other components (diodes maybe?)
 
This surely can't be so difficult. CJ preamps are very simple. I don't have a circuit for PV7 but looking at PV6 it should be easy to narrow the culprit down. Why do you think it's the PS? Is the noise equal in both channels? It should be possible to remove all tubes bar the output cathode follower. Is the noise still there?
Have you checked the bypassing cap on the zener array? Have you cleaned all tube sockets?

cheers
peter
 
The pv7 schematic is now posted on my webspace for your reference here. It's a little hard to read, but far better than not having it.

The cap that I think you mean by the bypassing capacitor on the zener array (c4 on the diagram) was one of the ones I checked, it was fine.

I took out all the tubes except for V5 which appears to be the output tube. (V1-3 are just the phono stage which I don't use anyway) The noise was present and a bit louder than with all tubes in, and somewhat more pronounced in the left channel. With V4 and V5 both in, the noise is a little lower, and basically balanced between channels.

I haven't cleaned the tube sockets, I suppose given that the noise was there even with only V5 in, if it is a socket it has to be that one, but I am too pessimistic to think that it is that simple. Any tips on cleaning sockets?
 
diyAudio Senior Member
Joined 2002
Hi,

Any tips on cleaning sockets?

Just pulling the tube out and reinserting it a few times should have sufficient cleaning effect already.

BTW, don't use the preamp without all the tubes in place for that will reduce the load on the PSU making the B+ rise to probably unsafe values for the surroundig components.

Cheers,;)
 
Kid Charles

Everything points out towards the PS. Luckily not that many components to check. The cap bypassing the zener array is C5, not C4. It wouldn't hurt to check all the transistors as well, although you may get a good indication that the regulator is working by measuring input and output voltages. If the pass transistor is short-circuited, output will be practically the same as input. If the regulator is fine you should see a drop in excess of 10v. If c5, c6,c7 and the transistors are fine you may be having a zener gone awol and generating excessive noise. Assuming that only one of the zeners is the source of the noise you may be able to locate it by shorting out each zener in turn. The output voltage will drop by the zener voltage (not quite sure what it is, but in the region of 30v) and the circuit will certainly work almost normally, thus allowing you to hear if any of the zeners contributes excessively to the noise. Keeping just V5 in the socket is not a good idea as it can't get proper bias without V4.

good luck
peter
 
Pv 7 Noise

Hey Kid Charles,
I saw a very old note you posted about a hissing noise on your Conrad Johnson PV-7 preamp. I have a PV-7 too, and have the same problem. I bought it recently and when I first hooked it up it had a very audible hiss that was independant of volume. I have since recaped all the line and phono caps, and added updated the power supply with a rebuild kit from Conrad Johnson. After all this it still hisses. So I know this must be a power supply resistor or even a tansistor. Let me know if you ever found out waht was wrong with yours. Thanks!

Justin
 
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Joined 2004
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Hi Justin,
I love it when people use the search function! Welcome to DiyAudio!

Did you replace the transistors in the regulated supply? Also clean the circuit board top and bottom in that area. This can also be caused by bad connections in the solder joints.

Add a little solder and use a solder sucker to remove all of it, follow with electronic solder flux and solder wick to clean it right up. Clean the area with a little lacquer thinner and a Q-tip or soft tooth brush. Go lightly so you don't damage the traces. At this point the pads should look like new. Install the new parts (probably TIP50 for the pass transistor) as they are not expensive. Clean the leads before you resolder and tin them. Add a little more flux and enough solder so the joints look like little volcanoes. Clean the same way you did before. Look for other bad connections. Anything that runs hot is suspect.

Don't forget that the heater capacitor(s) often open up. Simply change them for new 105° types, the same values. Higher voltage is okay.

-Chris
 
Well Ive got a PV-1 with the same problem. Should I start a new thread? I dont know how similar the 7 and 1 are. I do have the schematics though.

I've owned it for about ten years now, and pretty sure its all original(minus tubes of course). I havent contacted CJ yet, but can anyone point me in the right direction for possible fixes/upgrades?

Thanks....

Nate
 
Noise PV-7 / PV-2

I also had trouble with noise in my PV-7. I went ahead and changed all of the transistors in the HV supply and that helped. I am not sure it cured the problem as I became more sensitive to listening for the noise. As long as the power supply voltage is satisfactory, about 340V I would doubt that any of the zeners are bad. I also paralleled the final capacitor in the supply with a .01 polypropylene cap. My logic (or lack of it) is that even quality capacitors may not shunt higher frequencies due to (?). This logic may be true with the older electrolytics, but probably is not valid with the CJ units. While you have the unit torn apart, go ahead and replace the electrolytics. I had the HV electrolytic open and supply unfiltered HV through the supply. This was an unexpected problem as I usually find that the old electrolytics will leak voltage so the HV supply will drop and draw too much current through the main supply resistor. I wonder if Sprague developed a new failure mode for their capacitors so that they would open if enough current were drawn? I would be interested if you have any recommendations regarding the "best" tubes for the CJs.
 
Well if your asking me about the "best tubes", I've used Sovteks and been happy, without breakin the bank too. It came with some worn out Tele's, when I replaced them it was like a whole new preamp. I feel that tube selection, much like the rest of audio is fairly subjective, individual tastes, along with the other specific components are more important than just the "best tube". Which ever tube sounds the best to you, is the best tube...sorry if thats not really an answer....

So where is the best place to get parts for these units? CJ would be logical, but I'm afraid to ask "how much"...lol.
 
PV-2 / PV-7 noise

Thanks for the input regarding tube selection.

The power supplies for the PV-2 and PV-7 are different, although they are both types of closed loop regulators. The beauty of the CJ units is that they use common parts. I believe the power supply uses common Motorola devices, such as the MJE340 (PV-7). I have the PV-2 schematic, but not the parts numbers for your transistors. I think they could even be replaced with generic transistors as long as the VCE is okay, but don't quote me on that. Let me know how things work out.
 
Thanks for the info, though I probably wont tear into it for a while(at least a couple months). I will post back here when I do though.....I'm sure I'll need help somewhere.....lol. I do have the schematic for the 1, so at least its a start.

Anyone have a parts list for the PV1, or know where to look for one?
 
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