Sansui 7070 ... Right Channel Static ... midrange makes it louder / softer

[perry167]

Hello! First timer here
Problem w my Sansui 7070.

Has worked well for a few years but now hearing a CONSTANT static / noise in the right channel, of moderate volume.

Nothing I've tried has ANY effect on it, EXCEPT for the midrange control ... when I adjust the midrange, the sound smoothly gets louder or softer as i turn it up or down, like a volumne control ... not like the pot needs cleaning.

Any ideas?

Here are a bunch more details and troubleshooting steps I've taken ...


I cleaned the pots when I first bought it a few years ago.
Other than lamps, have had no issues.

I recently had it serviced by a young electrical eng. student. He cleaned all the pots w deoxit, replaced lamps, adjusted bias & DC offset. One of the lamps is still intermitent, but other than that works great.

A few weeks after this, I started hearing constant static / noise in the right channel only.

I unpluged all sources
moved power cord around
reversed how it the power cord is pluged in

verified ground wire is connected
tried without ground wire

I've tried ALL the controls
Balance control has no effect on it at all
noise is always present in the right channel only, even if balance is set all the way to the left

I hear it through speakers and headphones.
I removed the speaker leads
The main volume control has no effect on it.

Nothing has any effect on this noise I'm hearing ... EXCEPT the midrange control.

When I turn the midrange all the down, the noise is almost gone. When i turn it up, it smoothly gets louder ... like the midrange control is acting as a volume control.

Help please?
Thanks in Advance!

 

[The Fuxtor]

maybe start checking transistors in the preamp section?

 

[Skywatcher]

Hello! First timer here
Problem w my Sansui 7070.

Has worked well for a few years but now hearing a CONSTANT static / noise in the right channel, of moderate volume.

Nothing I've tried has ANY effect on it, EXCEPT for the midrange control ... when I adjust the midrange, the sound smoothly gets louder or softer as i turn it up or down, like a volumne control ... not like the pot needs cleaning.

Any ideas?

Here are a bunch more details and troubleshooting steps I've taken ...


I cleaned the pots when I first bought it a few years ago.
Other than lamps, have had no issues.

I recently had it serviced by a young electrical eng. student. He cleaned all the pots w deoxit, replaced lamps, adjusted bias & DC offset. One of the lamps is still intermitent, but other than that works great.

A few weeks after this, I started hearing constant static / noise in the right channel only.

I unpluged all sources
moved power cord around
reversed how it the power cord is pluged in

verified ground wire is connected
tried without ground wire

I've tried ALL the controls
Balance control has no effect on it at all
noise is always present in the right channel only, even if balance is set all the way to the left

I hear it through speakers and headphones.
I removed the speaker leads
The main volume control has no effect on it.

Nothing has any effect on this noise I'm hearing ... EXCEPT the midrange control.

When I turn the midrange all the down, the noise is almost gone. When i turn it up, it smoothly gets louder ... like the midrange control is acting as a volume control.

Help please?
Thanks in Advance!

This has the tell-tale signs of a noisy transistor somewhere. More than likely on the tone control board. I would personally replace the known to "go noisy" transistors, and all the electrolytic capacitors. Look for any of the transistor leads to have become black. That black has been known to creep up inside the transistors and you'll hear it as "static" as it "leaks" electricity between them. I would recommend to have it taken care of by someone experienced in the older equipment, not by a "young student", who is probably very smart, but doesn't have the experience on aging electronics. Where are you located? Chances are someone from here could be nearby and could fix you up. The 7070 is well worth having it fixed right.

 

[The Fuxtor]

If you feel up to the challenge, on the tone board (preamp) , there are a couple transistors labelled 2sc1313.
Change these out with 2sc1845 replacement transistors. Most likely fix your problem. You can sign up on Hifiengine website and download the service manual which have pics of the boards and component lists etc