Preamp stuck in phono selection

[rkidLG]

I’ve been dealing with this issue on and off for months now. My Parasound P5 preamp seems to intermittently get stuck in the phono selection. This causes the other inputs to be completely unusable. The other frustrating part is that when it’s stuck on the phono input it sometimes doesn’t play anything. I’ll drop the stylus and adjust volume and change to different inputs but all I’ll hear is static and crackling. When I remove the stylus this sound goes away. Sometimes when I unplug and turn off the preamp it resets and the issue subsides while other times it does not. I brought this in to a tech and the problem didn’t replicate and they couldn’t figure out what the problem was. Not sure I trust that techs ability much. I was looking to see if anyone had any advice on what I could do or if I’m missing something. Any help would be much appreciated.

 

[slovell]

Spray the input switch with contact cleaner a few times while working it back and forth then blow it out with canned air.. Sounds like it might be full of dirt.

 

[dr*audio]

This looks like it has a microprocessor in it with digital switching of the inputs. The switching IC's don't fail in this way, they either work or go shorted and pull the power supplies down. The microprocessor sends data to the switching IC's and tells them what input to select. It sounds like the data is corrupted. Could be a power supply problem. Usually microprocessors don't send out corrupt data some of the time unless something causes it, like the power supply voltage is low or has noise on it.

 

[rkidLG]

Spray the input switch with contact cleaner a few times while working it back and forth then blow it out with canned air.. Sounds like it might be full of dirt.

Just cleaned out the input switch. Same issue still occurring.

 

[rkidLG]

This looks like it has a microprocessor in it with digital switching of the inputs. The switching IC's don't fail in this way, they either work or go shorted and pull the power supplies down. The microprocessor sends data to the switching IC's and tells them what input to select. It sounds like the data is corrupted. Could be a power supply problem. Usually microprocessors don't send out corrupt data some of the time unless something causes it, like the power supply voltage is low or has noise on it.

Sounds like I’ll need to replace a part then… why do you think the preamp sometimes works and other times has issues?

 

[gadget73]

maybe marginal power supply caps or a dying regulator, sometimes its just clean or dirty enough to act dumb. I've seen this on monitors and TV sets with dying caps. Sometimes it works, sometimes not, or it will work for a while and then bail out when the temperature changes enough to push it over the edge.

 

[dr*audio]

Sounds like I’ll need to replace a part then… why do you think the preamp sometimes works and other times has issues?

It's a good idea to test the microprocessor supply (usually 3.3V or 5V) with a scope to see if there is ripple or noise on it. A low voltage or noise can cause this type of failure.