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Old 24th March 2011, 09:06 AM   #21
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Andrew,

Sorry for the slow response, but i have been away for a few days. My original problem was a failed tube that caused a cathode resistor to fail. I replaced the cathode resistors in the left channel and also the valves but still no sound. Thanks to the people on this site I was advised to do a series of checks and eventually I found that is was a small piece of metal rubbish that had lodged in the input selection switch. Two totally unrelated problems I would suggest. Bottom line was that it was a short in the switch. I think that when I turned the amp upside down to make it easier to replace the cathode resistors I have caused the problem in the switch. As part of the process of eliminating likely causes I removed the switch and the metal bit fell onto my bench. My problem was solved. A similar problem with errant metal bits has also been reported.

Regards Mike.
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Old 24th March 2011, 05:06 PM   #22
Helmuth is offline Helmuth  Netherlands
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Quote:
Originally Posted by BogartRed View Post
Andrew,

Sorry for the slow response, but i have been away for a few days. My original problem was a failed tube that caused a cathode resistor to fail. I replaced the cathode resistors in the left channel and also the valves but still no sound. Thanks to the people on this site I was advised to do a series of checks and eventually I found that is was a small piece of metal rubbish that had lodged in the input selection switch. Two totally unrelated problems I would suggest. Bottom line was that it was a short in the switch. I think that when I turned the amp upside down to make it easier to replace the cathode resistors I have caused the problem in the switch. As part of the process of eliminating likely causes I removed the switch and the metal bit fell onto my bench. My problem was solved. A similar problem with errant metal bits has also been reported.

Regards Mike.
How is the sound

The strange values you measure over the cathode resistor can be cause by faulty range it must be DC voltage and current, not ac.
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Last edited by Helmuth; 24th March 2011 at 05:08 PM.
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Old 24th March 2011, 09:54 PM   #23
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Hi Helmuth,
As far as I am concerned the sound is really good. The same as it was before I had my problem with the valve and cathode resistors. I did not replace all 4 valves only the two for the left channel and I used the same Chinese EL 34 B Electron Tube. I have not run the amp for very long since the repairs but it sounds the same to me. I have acquired a set of Electro-Harmonix EL 34 tubes but I am not going to try them until I have used the amp for many more hours. I still like the MC-10L a lot and I am more familiar with its workings now.
Regards Mike.
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Old 9th June 2011, 06:05 PM   #24
Hi_Q is offline Hi_Q  England
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Hi Mike, thought I would give this thread a twirl in the hope your MC10-L is still working fine. Also it is a reminder for you to check R6 to make sure that it cannot short to chassis, as explained on pages 27-28 in my guide,(now updated a little) at

http://homepage.ntlworld.com/lez/YAQ...IN%20MC10L.pdf

This so far un-proven yet potential problem would affect V3 mostly.
Was this the area where you had the original fault or was it around V1?

Regards
Les
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Old 9th June 2011, 11:04 PM   #25
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Les,

Good to hear from you. Looks like you have been busy updating your very valuable information of the Yaqin MC 10 L models. It is very much appreciated. I have not had any problems with my amp since you helped me track down the problem with the shorting in the selection switch. I have probably only run up some 40 + hours since then but everything is OK (touch wood). My original problem was actually with valve 1 going ballistic. While I had the amp apart during that time I was careful to check that nothing was coming into contact with metal as there was not much room in a few places and I thought about shorting.

As I said the amp is again performing beautifully and after a few more hours use I am going to replace the original Electron Tube valves with a set of Electro-Harmonics EL-34 to see what difference it makes.

Regards, Mike.
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Old 8th May 2012, 06:38 PM   #26
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Quote:
Originally Posted by Helmuth View Post
I had it with my yaqin to.

I had the bias at 330mV or was it 350mV? to high I concluded

Now i turn it lower 260mV that solves the problem of drift, sound-wise no change.

In my case it blow the tube fuse and the 10Ohm resistors.
Hello All,

I also had the problem with the Yaqin MC-10L output stage with the original EL34-B (made in China; Electron Tubes) valves and their 10 Ohm cathode resistors only in the right channel, i.e. after one week of playing fine, suddenly a crackling noise in the loudspeaker and then the particular resistor increased its resistance, sometimes also the fuse blown.

Same thing happened again after replacing the resistor. The first thing which I tried was to replace the bias-potis, since somebody gave me this hint, but it didn't help. The next thing, which I would like to try, is to add the (approximately) 750 Ohm resistors "Screen Grid Stoppers/Current Limiters" to the EL34. I think this time I will also replace the EL34 as well. And later I will set the bias to "only" 260mV.

Anyway, if somebody does have something to add, I will appreciate it.

Best Regards
Ralf
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Old 8th May 2012, 07:09 PM   #27
hags is offline hags  United States
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Quote:
Originally Posted by Ralf Biermann View Post
Hello All,

I also had the problem with the Yaqin MC-10L output stage with the original EL34-B (made in China; Electron Tubes) valves and their 10 Ohm cathode resistors only in the right channel, i.e. after one week of playing fine, suddenly a crackling noise in the loudspeaker and then the particular resistor increased its resistance, sometimes also the fuse blown.

Same thing happened again after replacing the resistor. The first thing which I tried was to replace the bias-potis, since somebody gave me this hint, but it didn't help. The next thing, which I would like to try, is to add the (approximately) 750 Ohm resistors "Screen Grid Stoppers/Current Limiters" to the EL34. I think this time I will also replace the EL34 as well. And later I will set the bias to "only" 260mV.

Anyway, if somebody does have something to add, I will appreciate it.

Best Regards
Ralf
I don't mean to be outta line here, and I haven't read the rest of the thread, but if I was having trouble with anything with a Chinese tube I'd look at the tube first.

I've had so many problems with Chinese tubes in the past that if I was to ever own/build/repair or mod another piece of tube equipment I would never consider a Chinese made tube.

From crackling to flashing to shorts and spectacular red hot plates the Chinese tubes I played with, mostly from '95-'02 were not worth a damn.

My $.02.
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Old 8th May 2012, 07:30 PM   #28
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Originally Posted by hags View Post
I don't mean to be outta line here, and I haven't read the rest of the thread, but if I was having trouble with anything with a Chinese tube I'd look at the tube first.

I've had so many problems with Chinese tubes in the past that if I was to ever own/build/repair or mod another piece of tube equipment I would never consider a Chinese made tube.

From crackling to flashing to shorts and spectacular red hot plates the Chinese tubes I played with, mostly from '95-'02 were not worth a damn.

My $.02.
Hello Hags,

thanks for Your input. Yes, I think You are correct. I also have a lot of concerns in the quality of these tubes. Basically these Chinese tubes should have at least a date code to be able to track any manufacturing problem, but they don't.

The only reason why I tried other things is to save some money for buying new tubes. But I think I have to spend it now.

Best Regards
Ralf
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Old 8th May 2012, 09:13 PM   #29
Hi_Q is offline Hi_Q  England
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Hi Ralf, yes you must think about changing the tubes as it is most likely to be the culprit with this fault. Possibly a screen grid to control grid insulation failure.
Les
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Old 10th May 2012, 04:54 PM   #30
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Hello,

now I am using tubes from Electro Harmonix and I like the sound very much.

By the way I built in a volume potentiometer (Marantz spare part) with center tap, to have a loudness poti with the appropriate caps connected to it. I did this for two reasons:
1) I have my appartement in the roof of the house, and for some reason the mid and high tone range is reflected from the 45 angle walls, or in other words I miss some bass, although the loudspeakers (Visaton Solitude) are ok.
2) I hear the music with low volume in most cases.

I like the loudness correction and I don't want to miss it now.

Best Regards
Ralf
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