KA-8100 Humm (60hz)

[aatallah]

Hello,

A friend just gave me a KA-8100 with a strange hum problem. It hums at 60 hertz, regardless of volume on the control. The only setting that seems to affect the hum is the frequency filters on the front panel (they slightly change the pitch).

I put in new filter caps, and it didn't change anything.

I then noticed that if I put a source directly in the Amp In (at the back, bypassing the preamp) it sounds perfect. When I put the little U-shape connectors between Pre-amp and Amp again, it humms.

I checked the grounding, meaning using a continuity tester and beep between the ground between the filter caps and the various metal pieces of the enclosure and it's grounded. I also checked between the 3 boards (amp, preamp and control) and all grounds are common.

Anyone has any hints to debug further? It's quite annoying

Thank you!

 

[Semanon]

From the symptoms, its a power supply issue with the preamp. Few questions to avoid assuming.

Is the hum in both channels?

Have you tried to play a signal through it? I mean, will it play music, even with a hum? If it plays music, how loud is the hum compared to the music? Louder, just audible, about the same?

Same hum regardless of which input is selected?

Have you downloaded the service manual from Hi Fi Engine?

 

[mbz]

Do you have an oscilloscope?

60Hz indicates from Ac power supply or ground issue/loop.
Rectifiers convert 60Hz to 120Hz, main filters should only see 120Hz so look elsewhere (could be failed rectifier but unlikely).

Trace the AC supply cables from rear pamel, fuse, to switch then to transformer primary. Then secondary wires to rectifier.
Use chopstick or something else non-conductive and push these AC wires away from other wires noting any change in hum.
Certainly check preamp CND connections.

 

[aatallah]

Yes, the hum is in both channels, feels about the same volume.

It will play music, and as I put the volume louder the hum stays constant volume, but the music will take over. Hum is always audible music or no music.

The music doesn't sound distorted, it sounds really good, just the hum is always there.

Hum is there regardless of input selected, and regardless of tape settings.

I have the service manual, thank you Been using it to try and track down, but so far no luck..

Thank you!!

 

[Semanon]

Welcome to AK. Sorry for the belated welcome.

The description is odd. The volume control does not affect it but pulling the linking does. However, the volume is immediately before the jumpers. Therefore, it's likely arriving via the preamp ground.

I'm kinda looking at the voltage regulation circuit around Qi13 and Qi14. Those guys are dropping 20 volts and run pretty hot. I've read about the heat drying out caps near them. But why would there be 60 (or maybe 120) Hz there?

 

[aatallah]

I just changed Qi13 and Qi14, and no luck, the PCB in that area is quite worn. I put MJE15028 and MJE15029 that I had on hand, and also changed he zener diodes next to the regulator, but no luck

Agreed, it's all DC voltages there, 60 Hz shouldn't be an issue.

Thanks for the welcome

 

[Semanon]

Nice friend, BTW. That is a sweet amp. This should be fixable with a little patience and troubleshooting skills.

 

[aatallah]

Nice friend gave it because of the hum ;P I have a KA-6100 that is my main amp, and did a recap on it a few years back, and it's been signing since. Would love to compare to the 8100 once that hum is take care of

 

[Semanon]

Did you check the four caps near Qi13-14?

 

[Semanon]

Nice friend gave it because of the hum ;P I have a KA-6100 that is my main amp, and did a recap on it a few years back, and it's been signing since. Would love to compare to the 8100 once that hum is take care of

My evil friends only give me broken black plastic crap (BPC). I'm waiting for a friend (or anybody) to give me a broken Yamaha MX-2000.

 

[Semanon]

The (-) side of Ci55/ high side of Ci56 have a direct tie back to the ground of the transformer. If one of those is bad, hum could develop from leakage. I'd check/ replace all four caps on that board.

 

[aatallah]

Desoldered those 4 capacitors, tested them, then put them back, and they are all within range (will order replacements anyway).

My friend bought it from ebay during the pandemic to setup a "cool home office" and then never bothered, LOL! No idea what's the story behind that machine

One thing I did notice is when I unscrew the front panel (where the Control board is), the hum gets much louder than when the front panel is screwed in. Maybe a hint.

 

[mbz]

Measure AC voltage (mV scale) between pin 15 and Power IN RCA GND/Outer (Left then Right).
Jumpers can remain inserted. Repeat for pin 6 (or jumper J4) and L/R RCA GND (outer).
With amp powered off/unplugged maybe try measuring resistance between above pins. Also
measure resistance between above pins and chassis.

Possibly an issue with that 11 pin connector to TAPE JACK?

 

[aatallah]

Hey guys,

Thank you for the tips. An update on the Hum situation.

There were in fact 2 hums at play: a 60Hz and a much smaller 120Hz, combined together.

The 120Hz was caused by he diodes Di1 and Di2 which were burned by the heat of Qi13 and Qi14 I replaced all four (Di1, Di2, Qi13, Qi14), the surrounding resistors, and the surrounding capacitors, and the 120Hz hum went away.

The 60Hz hum is induced by the power transformers. Someone has replaced the original Kenwood transformers as far as I can tell and when I move them around, the hum comes and goes. If I move the transformers outside the case, with longer wires, the hum completely goes away. Cool, but not practical, LOL!!

Two questions:
- Is there a heatsink solution for Qi13 and Qi14 to prevent things from burning up again?
- What are good transformers to put in to avoid the induced Hum (or is there a change I can do to the layout of the wiring to cut it out?)

Thanks!!!

 

[turboyam]

Please, post pictures of the amp. A ground or shielding issue?.

 

[aatallah]

Hey,

Bugged my friend who gave me the amp, and he had kept the original transformers, and warned me that they hum.

Put them back in, and now the amp is a LOT quieter, except the transformers physically hum (sigh) and the amp still hums, but very very faintly. Also, when I crank the volume to maximum, I can hear a radio station.

There is definitely a grounding issue somewhere. I double-checked all the chassis screws, and they are all soldered/tight.

It is the same as previously, when I plug in directly to the power amp's input, very very quiet, and very nice. When I plug in via AUX, the pre-amp adds the Hum (and Radio station).

Attached are various pictures, more than happy to take something more precise

Thanks!
Antoine

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[aatallah]

As far as I can tell, all ground points are correct, unless there are some missing that I am unaware of. Would be awesome if someone had a KA-8100 to compare all the ground points!

 

[aatallah]

Added comments to each picture

 

[turboyam]

What if there is nothing connected to the inputs? still noisy/ hear radio?. Today, there are tons of shitty battery chargers (cordless power tools), that make so much RF noise...

 

[aatallah]

It gets louder when nothing is connected to the inputs.