Early PV 10 hum in right phono channel

[emdude]

Hi everyone,

Hi everyone,

New member here that just acquired a PV-10. I have the very early model with balance control that can be switched out. This also has the early turbo line stage with a whopping 31 dB of gain that I have already converted to the later 12AU7 line stage with a more moderate 18 dB gain. No brainer here really, although it seems CJ made more changes to the line input on later B models but have I not been able to find what those are.

Now so far I'm enjoying the preamp but have a small have amount of hum in the right phono channel ONLY. With the lid removed both phono channels have equal hum, sort of stands to reason I suppose. Now even just a small amount of aluminum grounded shielding applied to the left side of the left phono input completely removes the hum in the left channel. I have not been able to find ANY way of shielding the right phono channel. No matter what I've tried I cannot get rid of the hum in the right phono channel. I've done all the logical stuff and all testing was done with shorted phono inputs.

I sort of think that this may have been the case on early early PV10 units but cant find any evidence of this on the net. It may have been sorted out in later models, or is low enough to not bother most people? I believe at this point it's induced via the power transformer, which incidentally sits at a very strange and not vertical angle in the chassis. This is NOT the result of any blunt force trauma for sure, it was installed this way. I'm going to go through the process of finding out in next few days if the hum is in fact induced by the on board transformer.

The right phono input does sit farther from the rear panel inputs so it may be that noise is induced in to the necessarily longer traces on the PCB? Will test that tonight.

But for now maybe if anyone has any information on this right channel only phono hum, causes, fixes or whatever I appreciate the info. I contacted CJ with the above and maybe they will respond, sadly they don't answer the phone.

Regards

Marc

[admin]

First, welcome to the site Marc. Nice to have you with us.

Considering you can remove the hum via EM shielding from one channel, it makes me think this is interference. Is the unit, or the wires coming in and out near other components that may be causing the issue?

As a 2nd measure, have you tried changing the tubes? Also, any chance that hum is 60 hz which would suggest a ground loop issue?

[roberto]

Hola,

Just a quick test. If you connect the left channel turntable RCA to the right input, do you have the hum? if not, then it is not the preamp, the RCA cable is contaminated with AC line adjacent cable touching the coax turntable cable or a bad cable. Poor cartridge cable contact too. Just check the colours of the cartridge cables, just thinking loud.

Happy listening!

[goonybird]

On my Late version PV10A there is an earth wire from the phono socket ground to the front at the volume pot.
All the pictures on internet don't show this. I'll see If I can take a pic of mine with top off.

[roberto]

Hola goonybird,

Did you swap the cables? Did the hum stay in the right channel? This test is to find if you have an external contamination or if the right channel in the preamp is the culprit.

The volume pot might be getting its ground from the RCA, check if that RCA ground is isolated from the main chassis. Some preamps might have a floating ground...and it is not connected to the chassis.

Happy listening!

[admin]

I agree with roberto. A grounding issue must be ruled out and we still haven't 100% narrowed the faulty component down yet. Switching the L/R inputs/outputs before and after preamp can easily tell you the component at fault. Grounding is done variably with turntables and preamps. Some have an accessory grounding wire, some don't, some preamps are grounded to the main others not,...

[emdude]

Thanks everyone for the responses. This helped in that had there been a problem inherent to maybe some early units as I had suspected, this would likely have come up but did not. So I assume now my case of low level hum in the right channel only affected only my PV-10.

Now the problem is actually fixed but I have no idea what exactly did it...I hate when that happens!!!. But I did do some thing to try and find out.

To reiterate, the hum appeared only in the right phono channel with the lid installed and the phono inputs shorted so as to eliminate the turntable entirely. With the lid removed there was hum in both channels but the left channel could be silenced easily with a small piece of aluminum placed near where the lid would be. I was never able to silence the right channel. The tubes were swapped and others tried to no avail.

I had assumed the hum might be induced directly from the field of the power transformer. In order to prove or disprove this I “or'ed” in a high voltage lab supply, got it all running into an amplifier from the record out, unaffected by the mute relay, turned the power on the PV-10 off and the hum persisted. So it was NOT being induced via the power transformer residual field.

At some point however the hum was simply gone!!! Completely gone, but I have no clear idea why but assume the following:

First off the person I got the unit from said it had been sitting in a closet for many years. I think the reason it was taken out of service is that every single tube in the unit was utterly shot. The tubes appeared original from what I've seen in pictures, or had been replaced by ones from CJ. I also checked the preamp out on my bench and converted the line stage form the 31 dB gain to 18 dB gain configuration as I knew I would not be able to deal with the higher gain. In the process of replacing the parts needed I had to get at the the bottom of the PCB, necessitating a substantial disassembly of the chassis. At this point I did discover some minor rust and corrosion around some the of screws that came out. Clearly the unit had been exposed to some degree of moisture at one time.

While I was checking the preamp out on my bench, I had to find replacement tubes from my stash to run the phono of equal performance and had to choose some 12AU7's for the line stage, I did see the hum, greater in the right channel on my spectrum analyzer. I also saw the the left channel hum completely disappear with the lid in place but thought maybe the right channel hum left would go away when moved from the possibly noisy test bench environment. Didn't happen.

With the hum now completely gone I can only assume there was some insufficient chassis ground connection due to some of the corrosion I found. In moving the preamp around during all of my inspection there may have been enough mechanical stress to have a possible poor connection make again. I have not taken the preamp back to the test bench but will do so at some point because I will be installing the “naked” Z-foil resistors CJ used in later models of their preamps. At that point I'll take another real close look at all ground connections. I did everything I could possibly check but everything seemed to ohm out fine. Maybe I missed some screws or bolts?

If I find anything conclusive I'll report it. In the mean time I'm really enjoying the preamp a lot!

Regards

Marc

[admin]

Glad to hear it is working well.

I think the two most likely culprits are a bad connection (either to a ground or maybe even a solder joint) and some kind of interference issue. Other than EM interference, I can't explain how you were able to influence the left channel hum with the "aluminum test."

[roberto]

Good job Marc!

Happy listening!