AMB Laboratories DIY Audio

[Peppe]

Forgot there was a forum just for the CK2III. I posted this on head-fi, but figure I might get better response here.

I built a CK2III and it had been working for many months. Its pretty much the recommended build with different output transistors.

I came home to it the other day not working. The symptom is: it has what i would describe as full volume in the right channel, no volume in the left, and turning the volume knob is not doing anything.

I had it connected to this docking station for my MP3 player (Zune). The only thing odd about the dock is it puts out a low level buzz when there is no zune connected.

Normally I turn the volume knob to zero whenever I take the Zune with me. I think on this occasion I left the volume at a normal level and it may have been amplifying the buzz for a 12+ or so hours. The box was a lot warmer than normal when i returned, but i don't think dangerously warm. I never noticed any heat buildup prior to this with just normal listening.

I have no troubleshooting experience, so are there some things I can test with a multimeter to verify functionality or identify a failed component in the chain?

I'm thinking either the alps volume control or maybe that center IC?

Alps is on the AMB mounting board. Only had time for a quick visual and didn't not see any wires out of place they were all still in their molex connectors.


Thanks for any input/ideas.

[amb]

It is highly unlikely that the pot would "burn out", there shouldn't be any substantial voltage or current at that point. You should check all your wiring to see if anything had come loose, or perhaps there is a bad solder joint or broken wire.

At any rate, it's also trivial to verify that the pot is good or bad by measuring its resistance while you turn the knob.

[Peppe]

thanks for the response.

I'll hopefully have time to test and troubleshoot on Tuesday. Trying to put together ideas before i go blindly fishing.

I'll get it out of the case and check all the wires. If it is a bad solder joint then sounds like I could be in for a hunt. Can these show up months later or cause a break under some stress?

[amb]

If it is a bad solder joint then sounds like I could be in for a hunt. Can these show up months later or cause a break under some stress?

Yes, it could.

But you should also go through the initial setup steps to make sure that all is well with the amp board (i.e., supply voltages, quiescent current, DC offset, etc). Unplug the input Molex connectors on the amp board and listen to the headphones. Do you hear a faint hiss? Does it hum and buzz when you touch the Molex connector pins on each channel? If so, your amp board is probably ok.

[Peppe]

I decided i couldn't wait and took a look today

Ran through the setup checklist and did not find any issue on the amp board. I unplugged the input as you suggested and just had a faint humm/buzz.

I plugged just the left channel back in and was able to adjust the volume normally. Added the right back and it was still full blast and seemed to be leaking into the left channel (or that was my brain).

I then spun the volume knob around a bit and in some spots the right channel cut out completely and there was more static type audio noise while moving than i recall in the past.

I then put the rca inputs directly into the amp board inputs bypassing the volume pot completely and controlled the volume on the mp3 player. Amp sounded normal and both channels were fine. With no source connected there was some light buzzing/noise, but as soon as the mp3 player was connected it went away. Though there was a ton of noise inserting the 3.5mm into the mp3 player (guessing that isn't good for the amp, and i should power off before doing that again).

I pulled the pot board out and took a look at it, but nothing visual looks bad on the solder joints. I forgot today to put the multimeter directly on the pot connections. I'll take a look after work tonight or tomorrow when the wife is gone

[Peppe]

What would be normal readings on the RK27 pin to pin?

I pulled it out of the case so there are no input and no output wires connected. Still on the e27 board.

I get roughly 50k total ohms wiper to ground and input to wiper for each channel.

I think channel to channel should be infinite, but in most spots on the rotation i get about 93k.

Input to input i get 93k. Rotating it can sometime get this to go infinite, but most of the time it is 93k.
A little less often i get 93k wiper to wiper.

[amb]

What would be normal readings on the RK27 pin to pin?

For each channel, you should read ~50K across the input and ground pins regardless of knob position. If the knob is turned to maximum volume, then the resistance across the input and wiper pins should be ~zero ohms, increasing to ~50K as you turn the knob down toward minimum. The resistance across the wiper to ground pins should be just the opposite.

See the following article to see how a pot works and you'll understand why:
http://www.allaboutcircuits.com/vol_6/chpt_3/6.html

I pulled it out of the case so there are no input and no output wires connected. Still on the e27 board.
I think channel to channel should be infinite, but in most spots on the rotation i get about 93k.
Input to input i get 93k. Rotating it can sometime get this to go infinite, but most of the time it is 93k.
A little less often i get 93k wiper to wiper.

When mounted on the ε27 board, both pot sections' ground pins are connected to each other on the ε27. Thus when you measure from input to input (across channels), you'll get the series resistance of both resistive elements, which would be ~100K, but 93K is close enough (it's within 10%). If you turn the knob to maximum, the wiper-to-wiper resistance will also be 93K, reducing to ~zero ohms as you turn the volume to minimum. The ground-to-ground resistance will always be ~zero ohms.

[Peppe]

Thanks for all the info.

Checking it again today in the light everything seems to be good on the alps unit.

Having what I think are bad readings on the output molex pins for the right side, so think my issue is there. Ground to output on the left side gives the right resistance, but the right side is sometime infinite sometimes bounces around and rarely gets to the right reading. Readings on the pins on the alps unit dial right in. So i guess that indicates a bad joint on the output pins.

Thanks for all the help.