Kenwood KA-405 Left Channel Always on

[jcsul]

Around six months ago, my KA-405 started exhibiting some weird behavior. While turning the volume knob, the left channel would sometimes randomly jump up to almost full volume before dropping back down to the expected level. I assumed this was probably just a dirty pot issue. Since the top of the KA-405 case and the volume potentiometer have slots that face the top of the case, I assumed that something slightly conductive must have fallen through the slots on the top of the case and into the volume potentiometer. I put the amp in a closet and planned to give the controls a good cleaning in a few weeks. Well, it ended up being about six months, but I gave all the controls the Deoxit cleaner and lube treatment. I just plugged it in this morning to find out that the left channel is now always on. There's no noticeable static or noise when I'm not feeding a single into the amp, but when I do input a signal, I'm always getting a strong signal through the left channel. I'll bullet point some things below.

- Channel and speaker selectors have to be in the correct position. If I run a signal into the Aux input and set the amp to the phono channel, I don't get any. Same with any other combination of mismatched inputs and channels. So it doesn't seem like a channel or speaker selection switch issue.
- Any signal coming out of the left channel is somewhere around 50%-75% max volume. The volume control has no audible effect (or perceivable on the VU meters) until I crank the volume up around 75% of the way or more. When I crank the volume knob up most of the way, it does make the left channel louder, and that is reflected on the VU meters as well.
- In general, all the controls still work as expected for both channels, other than the weirdness with the volume control. Fiddling around with all the other controls doesn't cause the problem to cut in and out, which leads me to believe this isn't an issue with dirty controls. Additionally, I cleaned all the controls a couple of years ago and everything worked without issue for a solid year or so after that.

I'm considering pulling the volume potentiometer and hooking it up to my multimeter to see if the resistance does anything whacky while rotating the potentiometer's shaft. Other than that though, I'm out of ideas as to what could be the issue. Anyone have any ideas on what could be the issue here? I've repaired a handful of 70s and 80s amps before, but I've never run into an issue like this. Any help would be appreciated.

 

[rickl]

You may need to clean the pot again.

My sansui volume was similar and it took a few cleanings. And I have to clean the other switches like the mono and reverse buttons.

I'd clean them all

 

[llwhtt]

If the volume control is mounted on the circuit board check for a broken ground solder joint.

Craig

 

[rickl]

If the volume control is mounted on the circuit board check for a broken ground solder joint.

This is a good idea

 

[jcsul]

You may need to clean the pot again.

My sansui volume was similar and it took a few cleanings. And I have to clean the other switches like the mono and reverse buttons.

I'd clean them all

When I cleaned the volume pot over the weekend, I grabbed a Qtip and lightly dabbed it in some of the Deoxit leaking down through the bottom of the volume pot. It was completely free of any grime, which is normally how I check that pots have been thoroughly cleaned/flushed on other gear I've worked on. That, and the fact that I cleaned these out a year or two ago and everything worked fine, is what led me to believe that dirty controls weren't the issue. There's not really any harm in giving the volume pot another cleaning though, so I'll give it another couple rounds of Deoxit later tonight to see if anything changes.

 

[jcsul]

If the volume control is mounted on the circuit board check for a broken ground solder joint.

Good suggestion. Your comment in general got me thinking about the signal path within the amp in general. Took a look at the schematic, and all three of the inputs (Phono, Tuner, Aux) all go straight from the RCA jacks>Input Selector Switch>Volume Pot>Tape Dubbing & Monitoring Switches. The volume pot is on a small daughter board, with nothing else besides three wires going directly to the input selector switch, and a ribbon cable that connects down to the tape dubbing and monitoring switches. With the case removed, it won't be difficult to get the multimeter at the contacts for all of those wires and the volume pot. So, I think I'm going to do some continuity/resistance testing between the volume pot and all of those three switches in addition to looking for bad solder joints. Hopefully, I find something out of sorts.

 

[jcsul]

Good news, managed to identify the problem and get this KA-405 back in perfect working order. I ended have to wait until last night to give the controls another round of Deoxit and lube. I hooked the amp up this morning and the issue was still there and completely unchanged. So, I just set the balance control most of the way to the right channel, and kept the volume low and turned on Spotify while handling some work stuff. Around 1, I got a call and went to turn the volume all the way as I also stood up out of my desk chair and accidently yanked on the volume knob a little. As I was doing that, I noticed the meters jump around erratically. So after the call I checked things again by playing some music while putting some moderate force in various directions on the knob. The volume jumped all around, and but I could get things working properly by pushing the volume knob down with a moderate amount of pressure. That prompted me to finally partially disassemble the amp and pull out the volume pots daughter board. I had to bust out a magnifying lens, but I managed to find three solder joints on the volume pot that had gone cold. I redid all the soldering on the volume pot about half an hour ago, and the volume control has worked flawlessly since then.

Thank y'all for the input, I really appreciate it. If anyone else stumbles across this post while having some volume control issues with a KA-405, check that volume potentiometer daughter board; it might just be your problem.