Curcio PAS 3 vibration

[theurbanshaman]

I have had this PAS 3 with full Curcio Mods for 25 years. It started to develop a hum or vibration from the unit, not in the signal path. I should have sent it to Curcio but I sent it off to someone whom I thought would identify the issue, but they didn't. When it was returned I set it on a turntable dust cover for initial tryout and heard a strong vibration when I powered up that continues as long as it's on, even though the preamp still sounds sweet. Anyone know what's the likely cause? (FYI I'm not good at fixing gear or reading schematics.)

[Speedskater]

Tell us about the power supply transformer.

[anatech]

Hi theurbanshaman,
Kevin is right, it is most likely the power transformer. I suspect it is an EI type, and maybe the bolts have loosened off over time.

-Chris

[llwhtt]

No bolts on a PAS xfmr, bracket like a open frame choke. If the xfmr is vibrating due to current draw I can't see it lasting very long. The heater supply needs to have the selenium rectifier and filter capacitors replaced if not already done. The OP can pull the HV rectifier to see if the vibration stops. If that doesn't stop it pull the 12AX7s one at a time to see if that cures the vibration.

Craig

[anatech]

Hi Craig,
Then maybe its the varnish (or whatever) letting go and allowing normal magneto-restriction to vibrate the loose plates? If it was drawing that much current, it would begin to smell at some point and the O P would have mentioned that.

If it has a selenium rectifier, definitely get it replaced. They normally go high resistance in low voltage applications. At higher voltages I have seen them short, so normal diodes will result in a slightly higher B+ voltage. I completely agree with you on replacing the capacitors in the heater circuit, but include the B+ circuit too.

-Chris

[llwhtt]

The xfmr is so small it's hard to believe it's vibrating very much but that's the only thing that can vibrate. I didn't go look at the Curcio mod. but I'd bet the heater supply is already up to date. Not sure if the HV rectification is still tube or not. We'll have to wait for the OP.

Craig

[theurbanshaman]

More clues:

I believe the power transformer is the original Dynaco stamped "PA 211" on the top. There is no longer a rectifier tube - instead there is Curcio's power supply board CURCIO AUDIO STEREO-70 AMPLIFIER MODIFICATION

When I removed the unit from my system the hum and vibration is not obvious, but in the system even with everything else turned off there is a strong hum.

The line stage signal is very quiet, the phono has some hum when turned way up.