PV-12AL Dropout Issue

[epiengineer]

Hello, longtime owner, but CJO newbie here. I have a PV-12AL I bought new in 1998, with some mods (power supply and bias circuits w/V-Caps, AC-In connector) made by Bob and Gary Backert at RHB Sound Dezign a few years ago. At lower volumes, I'll occasionally have a channel drop out, usually the left, but it will come back up if I crank the volume and bring it back down, although it can drop again. The volume control does not cause any crackling or other noise, and I have tried cleaning/blowing it and the other controls as well as resoldered various solder joints. I've also swapped the RCA output connectors with another set that was never used, even though wiggling the interconnects never made a difference. I'm currently using Mullard tubes, and it happens with other tubes too. I do know that it is the preamp and not my Classe CA-150 amp or the sources, since I used a scope to verify that an input signal was there when there was no output at one of the channels.

Does anybody have experience with a similar problem? Any components in the signal path that might be suspect? I'm not afraid to replace parts myself. I had this issue before the mods were done, and having had the mods done means C-J won't touch the unit again anyway, other than putting it back to original configuration.

[admin]

Hello fellow PV-12 owner. Sorry to hear about the problems. Some things I would do first. Did you test the balance controls? Just asking as if there is a problem there it could potentially effect the output in one channel. If all else fails you can download the schematic for the unit (we have it in our database) and trace the left channel back with a multimeter and compare values to the right channel. When you notice a change in values, that may be the faulty component.

You mention that it is "usually the left", does that mean it is sometimes the right? If so, then it would narrow the components down to those which are shared. Again, you would have to reference the schematic to narrow it down.

Let us know what happens.

[epiengineer]

Hello, yes it does happen on the left channel too, but not as much. I have sprayed/blown out the balance control, and adjusting it during the issue has no effect. Actually, nothing seems to affect it except turning up the volume. I've downloaded the schematic and will see where it leads me. Thanks!

[epiengineer]

An update on the dropout issue. I wasn't able to get any advice from CJ, other than that I should get the Cap upgrade, which I pretty much already have, but did speak to Bob Backert at RHB, and he suggested it could be the volume control even without crackling or other noise. I resoldered the potentiometer connections but this did not help. He also said to try to torque the potentiometer case when the problem occurred, and at one point it seemed that I could impact, but not altogether eliminate, the issue by doing this, so I ordered a new one ($15) from partsconnexion.com and in the meantime ordered and replaced the relay ICs. Since I replaced them a few days ago, the problem has not reoccurred. Although these relays do short the outputs to ground when off, for the life of me I cannot figure out why volume would impact them, because according to the schematic they are driven by a voltage across a diode coupled to a rectified voltage directly from the main transformer.

Either way, I'm going to wait until the problem reoccurs before replacing the volume control.

[epiengineer]

A second update on this issue - the dropout reoccurred, so I replaced the volume control. The pins on the new one were too short, so I had to solder extending pins. Based on the "grooves" in the resistive element of the old volume control seen in the attached pictures, I was convinced that the issue was going to be resolved. Unfortunately, it was not. It did seem to improve, but I'm still getting dropout at lower volumes. I also replaced a couple original electrolytic capacitors, even though they aren't in the signal path. I'm not able to trace the circuit using the schematic on the site as it doesn't seem to match this unit.

[admin]

Sorry to hear that it did not fix your problem. Our PV12 schematic doesn't match your unit? Let us know if you come up with a solution. Best wishes.

[epiengineer]

Ok, so another (hopefully final) update on this issue. I do think that replacing the volume potentiometer did improve things, as it was no longer dropping out in either channel, but was just the left channel, and it seemed to be interconnect related, because I was still seeing a signal on my oscilloscope at the terminals for the RCA outputs. I had already swapped the RCA jacks in the C-J, but not on the power amp, which fortunately also has balanced XLR inputs, so I ordered a pair of RCA-to-XLR interconnects from JW Audio (jwaudio.net), which while also greatly improving the sound also appears to have eliminated the dropout issue. Keeping my fingers crossed.

Oh, and yes, my PV-12AL is different from the circuit schematic for a few component values, but seems to be largely correct.