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Kenwood KA-2500 60Hz hum

K

Kfc

New Member
I have been trying to fix this unit's background hum (60Hz). I replaced all the electrolytic capacitors (believing it was a bad filtering circuit issue) with Nichicon from Digikey, which did not improve the output (still the same level hum). The hum tone and volume are the same from 2 different speakers on all channels regardless of any volume or other control switch adjustment. However, I cannot detect the hum using headphones. Would you be willing to suggest a troubleshooting next step? thanks

IMG_2422.jpg IMG_2420.JPG IMG_2421.JPG hfe_kenwood_ka-2500_tk-250u_schematic_Page_2.jpg
 
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The original D1 and D2 were replaced with 1N5408 (the ones displayed in the first post image). Purchased from Digikey. Are these OK?

I spent time checking the continuity points of the electronics to body grounds. Everything has good continuity.

Then I used this jumperwire (below) to test different ground connections further and I listened to any changes in noise volume. This connection shown below had a noticeable decrease in noise.
What is your opinion on how I should proceed?



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upload_2023-1-21_16-19-16.jpeg


upload_2023-1-21_16-18-37.jpeg
 
Are you certain it’s 60hz and not 120hz?
Seems like a ground issue since it helped reduce hum with jumper on main filter cap. Poke around with non conductive stick for bad solder joint.
Not a fix but Try flipping the power plug plugged into the AC socket. Try another circuit in your household to rule out other appliances.
Resolder all ground points.
 
I went through the unit with a small wooden dowel giving a firm tap to all the components and joints on both sides of the boards and also the switch connections. I tugged at the wires also. No changes in the humming volume. I also switched the cord connection at the outlet and tried different outlets and there was no difference.

Yes, sorry, you are right. My sound app reads signals that are 120 and 240Hz. I need to figure out how the edit the post title.

Would a failed transistor or ceramic capacitor result in a background humming?
 
Kfc said:
I cannot detect the hum using headphones.
Click to expand...
The audio circuit is common for both speakers and headphones with the exception of the speaker selector.
Try rocking the speaker selector slightly, does it have any impact on the hum, try giving it a clean, also post a pic. The -ve speaker terminals are switched via the selector. With selector set to Spkr A. measure resistance between -ve spkr terminal and other GND points.
 
I measured the resistance of -ve with the selector on speaker a and there was good continuity with the chassis. I rocked the speaker selector and sprayed generously with contact cleaner but the background remained constant.

Here are some pictures of the back of the selector switch. Do these provide detail?
upload_2023-1-23_21-21-13.png
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I hooked up my speaker to the red x points on the circuit and to the ground and the result is below. I could hear the hum.
red points.jpg

Then I hooked up these same speakers to the A speakers terminals (switched to speaker A) and I got this response:
speaker ports.jpg

I also tried the headphone port (switched to phones) again and my ear could not detect this hissing using the same speakers.
 
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Kfc said:
I measured the resistance of -ve with the selector on speaker a and there was good continuity with the chassis
Click to expand...
Select the resistance function on your multimeter, short the probes together, this is the resistance of the probes and leeds, approx 0.5ohms(?).
Then measure from -ve spkr connector to chassis, what does the MM read? an increase of 0.5ohms, ie a 1ohm reading can be enough to cause hums.

Kfc said:
I also tried the headphone port (switched to phones) again and my ear could not detect this hissing using the same speakers.
Click to expand...
Need a clear problem description, are you talking about a 120Hz hum/buzz or hissing.

Use something non-conductive, like a chop stick and put pressure on each wire and solder connection in the speaker selector switch, note any change (louder/quieter) in the hum.

I need to go through results in post #9.
 
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mbz said:
Need a clear problem description, are you talking about a 120Hz hum/buzz or hissing.

.
Click to expand...

That is correct it is not 60Hz noise. I cannot figure out how to edit the title. It is 120 and 240 Hz noise measured thought a sound app.
I listened and measured the audio output at the Xs below with reference to the chassis ground. The noise is the same as the speaker A and Speaker B output. (120 and 240 Hz). I believe the noise is present before it reaches the selector switch? But my ear cannot detect this noise when I attach the same speakers to the headphone jack.
noise.png
 
Last edited:
mbz said:
Select the resistance function on your multimeter, short the probes together, this is the resistance of the probes and leeds, approx 0.5ohms(?).
Then measure from -ve spkr connector to chassis, what does the MM read? an increase of 0.5ohms, ie a 1ohm reading can be enough to cause hums.
Click to expand...
I measured the resistance between various ground points in the picture below and the negative speaker A terminal both L and R with the selector in the speaker A position. No differences found between the resistance of shorted probes (0.2ohms) and these points. Are there other ground points I should check?

upload_2023-1-24_15-51-32.png
 
mbz said:
Use something non-conductive, like a chop stick and put pressure on each wire and solder connection in the speaker selector switch, note any change (louder/quieter) in the hum.
Click to expand...

I tried this again and the same result. There was no change in noise output when pressing firmly against solder connections using a small wood rod (pencil size).
 
Yeah, the output caps are electrolytic, looks like you used Nichicon KG series.

I'm wondering if the headphone behaviour is caused by the higher impedance of the headphone (500-1kohms? note 330ohm series resistor) so much lower supply current than the speaker output, nominal 8ohms, ie, it's load related. Your GND/earth tests look fine. Maybe forget about the headphones as it may be a red herring.
 
Regarding the jumper wire in post #3.
One end connected to -ve terminal of 2200uf cap, where did you connect the other end? floating?
You say that amp would not work if other end was connected to chassis... Strange as the black
wire should go to chassis. Want did not work when jumper connected to chassis?
 
Since it is unaffected by the volume control, the hum must be coming from the output stage. Can you short pins 1 and 2 of the output stage to ground and verify that the hum is still there?
 
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