Marantz 7C noise

[JMCMAN 0]

I have a Marantz 7C preamp that recently made a sound like a car engine revving up. Quickly shut it down. After looking inside the preamp I noticed some stray voltage when using the high line input selected. When I switch to low line level it zero's out. Not sure where to look for bad resistors or capacitors or switch in that high line input circuit. Any help would be greatly appreciated.

thank you

[tube fanatic 1186]

11 hours ago, JMCMAN said:

I have a Marantz 7C preamp that recently made a sound like a car engine revving up. Quickly shut it down. After looking inside the preamp I noticed some stray voltage when using the high line input selected. When I switch to low line level it zero's out. Not sure where to look for bad resistors or capacitors or switch in that high line input circuit. Any help would be greatly appreciated.

thank you

I am not sure what you mean by stray voltages.  Has it been professionally restored?

 

The first thing I would do is remove the tubes, treat the pins with Deoxit, and re-insert.  The sound you heard is oscillation.

 

Maynard

[JMCMAN 0]

Stray voltage= 0.35 ac volts measured on main preamp out to amplifier

the voltage will change if I lower the bass tone control to 0.1 ac volts, likewise if I use the high or low filter a slight variation in voltage.

the preamp is close to original the only changes were a new rectifier and volume control.

[tube fanatic 1186]

I would replace all the power supply electrolytics and then the coupling caps.  Do you have an oscilloscope which you can put on the preamp output?

 

Maynard

[JMCMAN 0]

unfortunately I do not have an oscilloscope.

 

[tube fanatic 1186]

Where are you located?

 

Maynard

[JMCMAN 0]

wisconsin

 

[tube fanatic 1186]

If you don’t know any qualified technicians, try to find local antique radio clubs.  They usually have members who are audiophiles with an extensive background in tube electronics.  
 

Maynard