New K-12g only one channel working - diyAudio
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Old 12th May 2007, 03:03 AM   #1
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Unhappy New K-12g only one channel working

Hello everybody I have a question for all of you I am new to the tube scene. I just bought and built the K-12g kit from S5 and I am experiencing some difficulties. You see after I got it all hooked up and I finally turned it on I was only getting sound from the left channel. Upon further inspection I found that there is sound coming from the right channel but it is very soft and slightly distorted I do not know what to done about this I have done some research and have found reports of the volume pot arriving damaged but I donít know how to tell if this is the case. If anybody has any suggestions or ideas on how to remedy this problem I would very much appreciate your input. Thank you.
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Old 12th May 2007, 03:32 PM   #2
Sherman is offline Sherman  United States
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You can try a few things, though I'm willing to bet it is a bad connection (solder joint) somewhere, quite possibly in the ground connection on the faulty channel.

1- Check all your solder joints on the bad channel. Start with the aforementioned ground. This is at the "back" of the PCB where your speaker connections are. The through holes in the PCB are quite large and it is easy to have a bad connection there.

2- Check your connections at the volume pot. A bad joint there in the faulty channel can keep the signal from reaching the amp.

3- You can try swapping the tubes from the working channel to the bad channel on the off chance you have a bad tube.

4- Also check the solder joints at the coupling capacitor on the bad channel.

Good luck, when you get both channels working I think you'll enjoy the amp.

Sherman
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Old 12th May 2007, 06:46 PM   #3
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Thank you for the tips Sherman I have already tried swapping the tubes and will check the solder connections latter today. Iím sure I will enjoy it when it is working the left channel sounds great. Thanks again
Matt
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Old 13th May 2007, 02:17 AM   #4
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Well Iíve checked all the solder joints on the board and found one or two that looked suspicious, but even after re soldering those as well as those mentioned above I am still having the same problem no volume out of the right channel. Are there any more suggestions, things to check, I am still suspicious of the volume pot but would like others opinions before I spend more money on this project.
Thanks,
Matt
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Old 13th May 2007, 02:27 AM   #5
nhuwar is offline nhuwar  United States
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If you are suspicious of the pot check it with a analog ohm meter if you have one.
Sweep the pot and which for erratic movement of the needle.
Oh ya diconect the wires from the pot first though.
Do this test with one lead on wiper and the other on one leg of the pot.

Nick
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Old 13th May 2007, 11:08 AM   #6
xaviery is offline xaviery  France
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You may measure the DC voltages of each tube of the last stage the the amplifier: by means of 500 to 1000V voltage meter. Maybe you can find an unbalance in Plate, grid or cathode.
Be carefull when turning on, in a tube ampl, there are high voltage!
Good lucky
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Old 13th May 2007, 03:44 PM   #7
drj759 is offline drj759  United States
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Matt,

Since one channel is working, you can use that to your advantage by comparing the two channels.

Iím using the following schematic as a reference. It is of the older K-12M version but it should be similar to the K-12G version.

http://lib.store.yahoo.net/lib/webtronics/km12sch.jpg

With power off, tubes removed, check the resistances on the bad channel. For example, measure across R7 and R6 or R15 and R16 to verify that they are of the proper value for your K-12G version (I suspect that they will be 470k in the G version as well). Donít rely on reading the resistor color code; use a DMM to confirm the resistance value. You can compare measurements with the working channel if need be. If possible, I prefer to make some of the resistance measurements at the socket pins to rule out a bad socket or bad wiring/circuit trace to the socket. Note that some of the resistors will be in parallel with other resistors such as R9 is in parallel with R11 through the secondary winding of T3.

If the resistances check OK and to add to xaviery's post:

Install the tubes and power up. Compare the DC voltages on all of the plates, cathodes on both channels. The DC voltages for the corresponding tube should be similar for both channels. For example, the DC voltage on pin one on U1a should be similar (within a few volts) to pin one on U3a. Compare DC voltages on other pins such as U1a-pin3 vs. U3a-pin3 and so on. Any large difference between the DC voltages on the good channel vs. bad channel may lead you to the problem on the bad channel. Again, be careful with the high voltages.

If no problems are evident after the above, then the next step is to check the signal path. But we'll save that step for later.

Some other lesser posibilities: bad tube socket, bad output transformer.

Dave
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Old 14th May 2007, 02:54 AM   #8
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Err.... make sure your speaker terminals on the amp are actually connecting to the foil on the PCBoard. Depending on how you install them, you can actually insulate the things.

I built one of these a few months back and had a little trouble there.
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Old 14th May 2007, 07:36 AM   #9
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Well thank you for all the help I finally got the right channel working but there is still a small amount of static only in the right cannel.

I wish I could tell you what the problem was but after much poking around and checking various things suggested previously I plugged it in and the right channel was working fine except for the small amount of static. Is there any thing else I should look at for the static problem?

Also I have found that some sources (my pc for one) I plug in to the k12 are distorted but no distorted when plugged into my Technics amp, any suggestions for this one?
Thank you all for your help.
Matt
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Old 14th May 2007, 02:13 PM   #10
Sherman is offline Sherman  United States
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It certainly sounds like a loose connection. If you haven't re-soldered things and the problem was semi-cured by poking around then whatever the original problem was is still there and it is likely also related to the static you are hearing.

The volume pot should be firmly grounded. With these kits I believe there is a grounding tab on the pot. Make sure that you have this connected.

Also double check the speaker connections as Runco said. When I built my kit I used wires rather than connecting the binding posts directly to the board. Those holes are quite large and it is easy to have a poor solder joint there even with the binding posts.
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