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    Building, troubleshooting and testing of these amplifiers should only be
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Yaqin MC-10L losing volume

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Hello All,

I am having a problem with this amp....the Amp runs fine for four hours then begins to degrade to fuzzy and the bias voltage goes to half. Instead of .35vdc when cold after four hours of play it ends up at .15vdc with the sound getting really fuzzy etc. Any thoughts or advise on where to begin would be appreciated. After looking at the schematics posted here I believe it's a cap to ground problem in the bias circuit ? Or worst is the mains transformer? Just blew V3 several weeks ago and replaced all the EL34 with a fresh quad set last week. Only other mod was replacing the stock fuse with slow blow since my unit died after a couple of hours of play when I first received it two years ago. I love the sound of this amp and really want to repair it. I have worked on troubleshooting cellphones and UHF low wattage two way radios so I have a little experience but nothing high voltage so I am a little concerned with the HT supply etc on this unit.
 
Thanks Sy,

When I powered on the amp this morning it operated sub standard right off the bat making the audio sound garbled. So all I can do is as you requested and take the B+ operating voltage which started off at 400vdc and after an hour warming up it read 386vdc. My system has a white wire for B+ versus a yellow that I noticed in this reference picture:

http://homepage.ntlworld.com/lez/YAQIN MC10/preamp_11_voltages.JPG

thanks again for your interest and help.
 
fter looking at the schematics posted here I believe it's a cap to ground problem in the bias circuit ? Or worst is the mains transformer?
If it is in both channels. Then I would suspect the bias circuit. What are the bias voltages for each of the EL34's? (Where the -21.2 and -19.0) is measured?

Ps... Don't know why the bias voltage should be different for each tube? Or is that just an example of what it sometimes take to balance the output tubes against each other?
 
If it is in both channels. Then I would suspect the bias circuit.

Could be, but why would you suspect that rather than B+? We still don't know if that's OK after the amp gets to its Dark Place.

Jay, what you're measuring is not the bias voltage. Bias voltage is a negative voltage applied to the output tube grids to control the idle (no-signal) current. What you're measuring is the voltage at the output tube cathodes- there's a small resistor there which is used to convert the idle current into a voltage that's easy to measure (no real audio function for those resistors). So, yes, you measure that voltage to set the bias, but you're measuring an effect twice-removed.
 
Found a different schematic.
 

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What should B+ be? 440vdc? Cause my amp started out in its "Dark Place" with no warm up and only reads 386vdc on B+ and the bias measement twice removed is less than half of what I had set it last week.

There's a clue. Measure the voltage on both sides of the 5k resistor. That will indicate whether the electrolytics are dying or whether there's a problem in the signal circuitry that's causing it to draw excessive current.
 
With fixed bia and the bias voltage being way high, I'd look at the voltage across the 1K resistor in the bias circuit as well.

The bias is supposed to be -41.5V acording to the schematic. If it were all the way up at .14 I would expect the tubes to be drawing a lot more current that intended. This could drag down the B+.

But, i'd expect to see red plates as well.
 
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Hello All,

I am having a problem with this amp....the Amp runs fine for four hours then begins to degrade to fuzzy and the bias voltage goes to half. Instead of .35vdc when cold after four hours of play it ends up at .15vdc with the sound getting really fuzzy etc. Any thoughts or advise on where to begin would be appreciated. After looking at the schematics posted here I believe it's a cap to ground problem in the bias circuit ? Or worst is the mains transformer? Just blew V3 several weeks ago and replaced all the EL34 with a fresh quad set last week. Only other mod was replacing the stock fuse with slow blow since my unit died after a couple of hours of play when I first received it two years ago. I love the sound of this amp and really want to repair it. I have worked on troubleshooting cellphones and UHF low wattage two way radios so I have a little experience but nothing high voltage so I am a little concerned with the HT supply etc on this unit.

I had a similar problem.
I bought a new MC-10L about 2 weeks ago.
It sounded fine at first; however, after about a 2 hour warm up, I started getting a lot of intermittent popping and snapping and staticcy noise in the Left channel only.
I tried rotating tubes. this had no effect.
I kept using the amp for 2 weeks, hoping that the problem would go away; but it always came back after the amp was on for 30 minutes to 2 hours. Once I even had a loud pop and lost the Left channel completely.
I even replaced all the stock EL34s with Russian EL34s and all the 6N1s with Russian “6H1N-EB” (Cyrillic). I had planned to do that anyway. It did improve the sound; but did not stop the noise problem in the Left channel.
Finally, I bit the bullet and took the unit apart. I examined every joint and component under magnification. Finally, I found a solder bridge that had formed between 2 adjacent pin sockets on one of the 6N1 preamp sockets. Apparently tube heat had formed it or caused it to expand to complete a intermittent short.
I removed the errant solder with a scalpel, put everything back together; and I now have beautiful music from both channels and wonderful silence from both channels when there is no input.
I'm now listening blissfully to Miles Davis ("Miles Ahead" CD). This amp has sensational sound; and the Price : Sound ratio is worth the risk of having to do self repairs.
 
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