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Old 9th March 2011, 05:57 AM   #1
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Default Yaqin MC 10l problem

Hi, this is my first post so please excuse me if I make a few mistakes. I have had my MC 10L for about two years and I have been really pleased with it until a few weeks ago. My amp suffered from a similar problem that has been related a number of times on this site, and that was the sparks, smoke and loss of sound from the left channel. I turned it on and while waiting for it to warm up I heard a crackling from the left speaker and then saw smoke from around valve 1. I checked diyAudio site and read up on what was the likely problem and what to check. Well I removed the cover from the amp and sure enough the cathode resistor from valve 1 had burned pretty badly and the valve had a bit of carbon at the base. I replaced the two cathode resistors on the left side and ordered two new valves. I have done checks to confirm the voltages in the empty valve holders and they are negative 35v and 450v as specified. I installed the valves and turned it on but still no sound from the left channel. I have checked the bias on all 4 valves and they are set at .35v or as close to that figure as I can get. While I had the cover off I have closely checked all the wiring connections with a magnifying glass and inspected the capacitors and resistors for any obvious signs of damage. I have not noted any and no "burnt" smells emanate from within.

Any suggestions as to what my problem could be? What further checks can I carry out. I have a terrible feeling that it could be the output transformer although I have no way of checking. Incidentally I have 240v transformers, or at least that was what was ordered and stamped on them. Any suggestions would be greatly appreciated.

Please note that I am also very new to valve amps and have limited knowledge, but I am keen to learn.

Mike.
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Old 9th March 2011, 07:51 AM   #2
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Hi, Mike!
You can check primary coil of output transf. There is two primary coils with middle point, that goes to +U.
1. Check resistance between middle point and every end, that goes to plate of every tube. Results must be the sames.
2. Check all resistance between plates in both channels.
/don't forget to switch off the amplifier!!/.
3. You can put new cathode resistors, switch on the amplifier and measure every cathode potential before some smoke.
4, You can measure every first grid potential of every tube.
5. You can change interstage capacitor to some similar new capacitor, that goes from prestage to bad tube and see what will happen.
6. Or You can disconnect interstage capacitor from first grid of bad tube and measure grid potential.
It must be the same like on another output tubes.
http://pdf1.alldatasheet.com/datashe...LIPS/EL34.html
/It's very hard to burn tube..../

Last edited by azazello; 9th March 2011 at 08:09 AM.
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Old 9th March 2011, 10:17 AM   #3
Helmuth is offline Helmuth  Netherlands
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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.
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Last edited by Helmuth; 9th March 2011 at 10:32 AM.
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Old 9th March 2011, 02:13 PM   #4
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Sorry....p.6: You can measure all grid potentials of tubes and compare, maybe interstage resistor of bad tube is licking.....
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Old 9th March 2011, 04:40 PM   #5
Helmuth is offline Helmuth  Netherlands
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schematics mc10l. right channel
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File Type: jpg mc10l.jpg (200.1 KB, 504 views)
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Last edited by Helmuth; 9th March 2011 at 04:44 PM.
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Old 9th March 2011, 08:36 PM   #6
Hi_Q is offline Hi_Q  England
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Check your speaker lines too as an open circuit will cause flash over in the output valves if you are driving the amp at the time. The presence of a carbon track is one that I have observed on guitar amps brought in for repair here where the speaker(s) were accidentally unplugged whilst the guitarist was playing. Worth a check.
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Old 9th March 2011, 10:01 PM   #7
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Quote:
Originally Posted by azazello View Post
Sorry....p.6: You can measure all grid potentials of tubes and compare, maybe interstage resistor of bad tube is licking.....
Sorry for my bad English and mistake....I mean, interstage capacitor maybe is licking and grid potential is positive, and tube is overloading.
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Old 9th March 2011, 10:13 PM   #8
Hi_Q is offline Hi_Q  England
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Quote:
Originally Posted by Hi-Q View Post
Check your speaker lines too as an open circuit will cause flash over in the output valves if you are driving the amp at the time. The presence of a carbon track is one that I have observed on guitar amps brought in for repair here where the speaker(s) were accidentally unplugged whilst the guitarist was playing. Worth a check.
That's both external and internal lines by the way. You could have a very high resistance connection that was capable of you hearing cracking from the speaker but it was high enough to do damage. If you do not have a multimeter (the best way of checking) you could use a single AA cell in series with a speaker line. i.e. undo one speaker connection from either of the rear terminals, then place a AA cell (either way round) between the wire taken off and the terminal from whence it came. You should hear a healthy crackle as you touch the wire on the cell. A bit of a crude method but should prove circuit continuity through the speaker and secondary winding of the output transformer. Don't do this with the amplifier switched on though .
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Old 10th March 2011, 03:43 AM   #9
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Many thanks to azazello, Helmuth and Hi-Q for your suggestions. I have done some further checks as suggested and it looks like the output transformer is OK. Hi-Q i used your crude battery test and I got a good healthy crackle and my multimeter shows 8 Ohms in both right and left channels. I suppose that means I have an issue with one or more capacitors. As you can probably guess I don't have any capacitor tester so I was considering replacing some or all of the capacitors. Suggestions as to which one/ones are most likely to have failed. Again as a novice could you help me by identifying the capacitors by value. Or alternatively I have copied that handy little document "A LOOK AT THE YAQIN MC-10L" and you could refer to that. As you can see I am very new to this game but I am getting better.

Regards, Mike.
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Old 10th March 2011, 03:31 PM   #10
Hi_Q is offline Hi_Q  England
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Just for interest, if you ever take the base plate off (a mission I know!), take a look at those joints to earth around the output tube sockets. An example is shown in my document you mention. I came across this possible problem on my Amp whilst I was doing the screen resistor modification. I found taking the base plate off was best done after laying the amp down on its top side so that the transformer cans made everything stable.
I made a note of which screws went where as I removed each one so that re-assembly was no problem. Once removed, access to the underside of those EL34 sockets is a cinch. I dread anything happening to the main power board though, I actually managed to get to all of the print once but I would not like to do it again! Mike, only the coupling capacitors should give you any cause for concern but I doubt they are at fault in your case as the negative bias voltage seems OK. Do the voltage checks again on the empty sockets (taking the usual care!) and also check you have 10 Ohms from cathode pin to ground and that pin 1 is connected to pin 8 (pins located either side of the spigot).
I would also suggest moving over the tubes from the working channel to see if that helps, I am still hoping for feedback on owners concerning the slight possibility (and I say slight possibility) of failure of the screen grids due to the absence of a resistor. Controversy reigns here as some say the resistors are to prevent spurious oscillation and improve stability while others say it is there to protect the screen from passing too much current and actually failing. There's probably truth in both arguments. Hope you have some luck with your problem and let us know how you get on.
Les
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