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Kenwood KA-3500 hum problem through speakers (not transformer)

TomTom4049

TomTom4049

Member
Had a problem with one of the batwings so I replaced them with the recommended OnSemi replacements suggested by SilverFace. Except for distortion in the right channel it was working. I discovered when I pressed down slightly on a new capacitor C14 the distortion went away for a few minutes. I touched up many solder joints but still had the distortion. After wiggling C14 a bit it played for 10 minutes without distortion, really sounding great. All of a sudden a hum began through both speakers,

Interestingly when you flip the Power switch on, immediately the left channel produces hum (no delay whatsoever) and after the protection circuitry does its thing the right channel joins in with a hum. Change in volume control doesn't affect the hum. I ordered new Zeners for the power supply having read another person had luck replacing those and eliminating hum. Large and small power supply caps have been replaced.

Any ideas?
 
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I would say you are on the right trail troubleshooting this Kenwood. Maybe a shot of freeze spray on De3 De4 will prove something faulty. Are you measuring +/-39v on anode side of De3 and cathode side of De4?
 
eizner23 said:
I would say you are on the right trail troubleshooting this Kenwood. Maybe a shot of freeze spray on De3 De4 will prove something faulty. Are you measuring +/-39v on anode side of De3 and cathode side of De4?
Click to expand...

Jumpers are installed in place of De3 & De4. There are only D2, D5, & D6.
 
Just saw after being on 10-15 seconds faint smoke coming from somewhere on the left channel of the power amp, not sure where from.
 
Uh oh, better get yourself a dim bulb tester while you iron out the shorts.

Dim Bulb Tester Build and How-To

I put this together based on the pics that @Jailtime posted a while back, and figured I'd take some in process shots. There were a number of threads on these with dead links to imageshack or photobucket (the scourges of the internet) so this should be a good post on the issue that won't have...
audiokarma.org
 
I pulled zener De5 (on schematic) simply D5 on PCB. It is testing .002v in both directions (meter set for diode test). 1.3 ohms resistance test in both directions. On my Multifunction tester it reads as a resistor. Obviously this one is bad. Six are on the way from Newark probably getting here next week. Won't get a chance to install new one until next weekend, Drat.
 
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By the way as well all know these units are not the easiest to work on getting to the main PCB I came to the conclusion a service mod was in order.

Where I work we use connectors called WAGO and I salvaged some. They are normally opened using WAGO pliers then you insert a wire into a cavity and release the pliers and it grips the wire. I used three inserting them roughly half way in each wiring harness. Now I only need to unplug the male and female WAGOs and remove PCB screws. Pretty quick and I can stand the PCB on it's left side or even flip it over with the solder side facing up. A nice workaround.

DigiKey sells them. They come in a huge variety of sizes and number of pins. The ones shown below are similar but not identical to what I used. Using these it would even be possible to make extender wiring harnesses so you could "remote" the whole PCB as far away from the amp as you desired and work on it extensively outside of the unit while hot. Just a thought. :)

WAGOs.jpg.png
 
I've a KA-3500 that developed a hum. The hum starts in the left channel immediately when the power switch is turned on, and the hum begins in the right channel as soon as the protection circuit allows the right channel to come on. The hum is unaffected by volume control setting changes, or input selection changes.

In troubleshooting it I feel that because of the now and later behavior of the hum the problem is in the left channel. Along those lines I'd like to kill power from reaching the left channel to see if the right channel then operates normally. I'm not having an easy time figuring where I can do this. Any help from out there?

Thanks.
Thomas
 
Indeed, I merged these two threads. Let's not have multiple threads concerning the same issue.
 
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