PV1 developed buzzing

[we350b]

Hello all,

Just picking the collective CJ braintrust as to what could be causing my much-loved PV1 to suddenly developed buzzing. It seems to be a bit volume-dependent, meaning buzz/hum gets louder as you increase the volume. I’ve tried changing inputs, source, interconnects to shielded, switching tubes, power outlets, but the buzz/hum remains.

The PS caps were replaced by a competent but non-CJ tech probably around 15 years ago. I see no bulging caps, burnt resistors but I’m no electronics maven. My initial impulse is the pot going bad. The sound, aside from the grating buzz, remains just as good as before.

If it’s indeed the pot, how easy is it to replace, and what brand/type should be used? (250k?)

Thanks for any help. I’ve used this PV1 with myriad of equipment, from Bedini 25/25 to Wright 2A3s to Threshold CAS to custom 300B mono’s and it never fails to impress.

[admin]

First, welcome to the site. Great to have you with us.

When I troubleshoot, I always go for the low hanging fruit first. Are the tubes failing, is the hum 60 hz to suggest a ground loop, any bulging caps or burn marks to suggest failure. Sounds like that is the way you approached it as well.

We have the PV1 schematic in our database so taking a look at that may give some answers: https://www.conradjohnsonowners.com/vie ... f=17&t=128

If you think it's the pot, you can remove it and test it with a multimeter or bypass it with a fixed resistor for testing purposes.

[roberto]

Hola and welcome,

I am with Admin that you have a ground loop. You never know when this problem is going to happen. You must track what it is causing to you this unwanted hum.
For not to chasing the rabbit tale, turn the amp off and disconnect all the inputs, just connect the output cables to the amplifier...no other connection to the preamp. Now turn on the the preamp. Still have the hum? Yes, it is coming from the preamp.

Now connect the DVD player only or the TV audio. Have the hum? Yes, disconnect the cable TV to the little box TV, the coaxing coming from the post outside of the house. Connect the TV audio but not the cable box. Yes you are not going to have TV for a moment, Do you still have the hum? Usually this is the 99% of the hum. If you still have the hum, check the RCA cables...is the present at both channels or only one?

Please tell us more about it...

Happy listening!

[AnotherJohnson]

Power supply caps 15 years old = good place to look.

They do not have to bulge or leak to fail to pass 60 Hz.

[we350b]

Thanks for the input. I’ve tried the above trial-and-error methods, except the pot dismantling, and the hum/buzz is still present. I’ll try the pot next, and if it’s still present, I’ll take the PV1 to a local tech. I could replace the caps myself but I recently got burned by my attempt to do the same with an old Sugden amp, and I ended up with a burnt resistor.

[admin]

I know I said this before, and it's low hanging fruit, but are you sure it's not a 60 hz buzz? You can get a ground loop isolator RCA plug for under $10 and it would rule out this issue.

For example: https://www.amazon.com/JacobsParts-AFIL ... E7CFXMQR04

[we350b]

I did some more low/level testing (which is my testing level lol). I had just the turntable connected using the phono inputs, no other inputs being used. I then hot-rodded the PV-1, using the rec-outs, effectively bypassing the volume control, going into to a passive line control into Quicksilve Mini’s, and there it is…glorious music and no hum. Some normal tube hash but clear music playing. I then used the regular preamp outputs, and hum again. I tried some other tubes, new AX and good 5751’s, and they made no difference to the test above. So, should I conclude it’s a bad pot? I should also add that signal coming in, and just turning the volume to the max, the very end of the pot gives a very oscillating kind of hum, like “ho-hum, ho-hum”. I’m making a scene here, but just trying to be accurate.

[roberto]

If you bypass the volume control, you are using only no active tubes sound. Please disconnect the turntable and have a listen. Have a listen for hum. If there is no hum, plug again the turntables RCAs to the phono input. Move the selector switch to the preamp to aux, and have a listen. It there is no hum, you have the RCAs of the turntable causing the problem. If you have a hum, the turntable RCAs are touching an AC cable. Route or address the turntable RCAs in a such way to be free of any adjacent cable. If you still have hum, are you using any other preamp for MC or similar to the turntable?

Please answer my questions so we can track together from where you are having this unwanted hum.

[we350b]

But wouldn’t “hot-rodding”, meaning using the rec-outs of the PV1 while the using the phono input (TT hooked up) mean that the phono pre part and power supply part are being used even if the line amp with the volume control bypassed? Used as such (rec-outs going into an external line volume control), there is enough gain listening to the turntable without any hum or buzz.

Following your advice, in all the steps, I get the hum/buzz. So, it’s there when I use all the functions the PV1, input or no input used.

[we350b]

BTW, ‘ppreciate the continued help, Roberto.

[roberto]

Yes, phono preamp is the only active working. The phono preamp is V1,V2 and V3. The line preamp, where the hum is, V4 and V5 are 5751. Then V6 is a 12AX7. Make sure you have these tubes in a correct position.

To me, it looks that you have a power supply bad capacitor. The part of the power supply for the three V4, V5, and V6. If you have an oscilloscope you could check the Bias Voltage to be free of hum. I will change all three caps. Conrad Johnson might have spare caps and also perhaps an upgrade for the power supply... service@conradjohnson.com. What bugs me is that the phono section is free of hum. Just check the bias voltages and also tube position. V1,2,3 and 6 are 12AX7 (7025).

Hum tells you that there is a bad cap at the power supply, Measure the capacitance and there could be a double or triple cap too. Those big three metal caps, one or two of them might be bad.

I wish this might help you to track your problem. Great idea what you had, using the phono pre section via recording out.

We are here to help...please tell us your findings...

Happy listening!

[roberto]

I forgot to mention that you could find a voltage regulator stage too. I can not know the values of the components. Just check the regulated voltage is stable...there could be a bad transistor. This cause hum too. See the diagram for other Conrad Johnson preamps, usually they have the same voltage regulation stage. The filament power supply might cause hum too. So you have to seek and look that it is free of hum. The transistor is leaking between emitter and collector...if it measures ok, it is always better change all three transistors. These transistors are not too expensive.

Another clue: is the hum is on both channels, definitely the problem is at the power supply.


Good luck! and soon you will be enjoying your PV1,

Happy listening!

[roberto]

Can you provide a pic of the power supply unit? Where the big metal caps are. I want to see the components there.

Thanks,
Roberto.

[we350b]

Don’t know if the pics will show up. I have not opened the PV-1 in a long time, and now just realized that most of the caps look to still be original.

[admin]

Nothing strikes me as a clear bulging cap but it can be subtle. Those caps are pushing 4 decades, I would go over them very carefully.

[roberto]

Hey Admin,

Looks that you are with me too. (LOL)..

Yup, those are the ones, but I also see two blue caps tied together. These caps are for the filament voltage. Change them too. They are not expensive and fresh caps will turn your PV-1 back to life. Hum is a sign of bad caps. Do your best and don't change only the bad one...change them all. The change will be the best investment into your PV-1.

Those metal cylinders must be replaced. They can be single, double or triple caps...in the metal are written the capacitance of each capacitor into the cylinder, and the working voltage. Keep in mind that they must be replaced with the same or a little more capacitance and the same or more working voltage. Never less!!!

That's what is causing your hum for sure!

Thanks for the pic and I don't see any regulation stage. It is OK. You have in your system, the first Conrad Johnson preamp!...a jewel!

Happy listening!

[roberto]

I forgot to mention that your tech man did a great job cleaning it. The unit is almost untouched. And looks neat!
Happy listening.

[roberto]

I am thinking in possibilities...the bridge diode is another component (leaking voltage) that could cause hum. It is the black rectangular shape device near the green big resistor. They are not expensive and fresh rectifiers are good too. I will renew all those components, even that they are measuring OK.

Just thinking loud! And it is a good money spent there. The rest of the preamp will be very happy having all fresh working voltages, fresh and clean.

Happy listening!

[AnotherJohnson]

I am thinking in possibilities...the bridge diode is another component (leaking voltage) that could cause hum. It is the black rectangular shape device near the green big resistor. They are not expensive and fresh rectifiers are good too. I will renew all those components, even that they are measuring OK.

Just thinking loud! And it is a good money spent there. The rest of the preamp will be very happy having all fresh working voltages, fresh and clean.

Happy listening!

^^^^^
What he said.

[roberto]

^^^^^
What he said.


You're too much AJ!