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Old 28th April 2010, 06:04 PM   #1
marley is offline marley  United States
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Default need help with yaqin mc10l

Hello all. I have been using this amp for about two weeks with out a problem and then the line fuse blew and I noticed a flash in one of the tubes. Now the right speakers has lower, somewhat distorted bass. I put the mm on to check the bias and found the rear tube reads 36 volts. I am looking for suggestions, and anyone who has experienced and fixed this type of problem. Any and all help would be greatly appreciated. Marley
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Old 28th April 2010, 08:00 PM   #2
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After the unit has discharged for a while, measure the bias with the ohms resistance setting. You should see 10 ohms at each tube. If not - stop here.

Now pull the big EL34 tubes that are on the channel with the problem (right), and then put the big tubes from the left side into the right hand side.

Replace the fuse with THE SAME RATING of fuse and power it up. If the right hand side now works - you have a tube gone bad. If not - it's something inside.

Before you open the case please read the safety sticky..
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Old 28th April 2010, 08:04 PM   #3
marley is offline marley  United States
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Thank you for the reply. I am still learning so bear with me. If I try another "good tube" in the suspect socket, and there is more wrong than just a bad tube, it will blow that tube as well, correct? What happens if I put the "bad tube" in a good socket? M
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Old 28th April 2010, 08:14 PM   #4
marley is offline marley  United States
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Just did as you said, measured the resistance of the suspect socket and its showing that 10ohm resistor is open. Now, I was also told it could be the coupling caps as well but I dont see any swollen or leaking caps. Do I need to check those caps or just replace the 10ohm resistor? M
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Old 29th April 2010, 12:04 PM   #5
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Quote:
Originally Posted by marley View Post
Just did as you said, measured the resistance of the suspect socket and its showing that 10ohm resistor is open. Now, I was also told it could be the coupling caps as well but I dont see any swollen or leaking caps. Do I need to check those caps or just replace the 10ohm resistor? M
OK so the 10R is blown - mmm. This is a symptom - not the cause.

This could have happened from a) a tube short or b) a coupling capacitor going bad (leakage turning the tube full on). So I would replace the coupling cap (actually I'd do all of them but that's just me), you can test it with a multimeter but it may only leak at high voltage.

Before you do though check out the tube with your multimeter set to 100K or so, and check there is no resistance to anode-cathode and none from anode and cathode to any grid or any heater. Check the tube for any sign of whiteness on the shiny coated bits on the glass.

So to summarize: it's the cap or the tube, test and/or replace one or both!
Replace fuse with the same value.
Re-bias all tubes once you fix it.
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Old 29th April 2010, 01:40 PM   #6
marley is offline marley  United States
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Thanks for the reply. I got my mm on pin 5 of the valve socket and got a reading of -50volts. I believe the problem was a faulty tube, these are the original chinese tubes. I will be replacing the cathode resistor, but I am having a hard time trying to get the value, mine has NEO5, 10ohmJ witten on it but the schematics show a 10ohm 2watt. Clarification and any help with sourcing the resistor would be great! I have never checked a tube before, is there a diagram showing what pins to check? M
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Old 29th April 2010, 02:02 PM   #7
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Quote:
Originally Posted by marley View Post
Thanks for the reply. I got my mm on pin 5 of the valve socket and got a reading of -50volts. I believe the problem was a faulty tube, these are the original chinese tubes. I will be replacing the cathode resistor, but I am having a hard time trying to get the value, mine has NEO5, 10ohmJ witten on it but the schematics show a 10ohm 2watt. Clarification and any help with sourcing the resistor would be great! I have never checked a tube before, is there a diagram showing what pins to check? M
The -50V is the tube bias for the EL34 - that certainly would not cause it to switch on so it sounds more like a faulty tube.

The resistor is a 10 ohm 2W one then - any will be fine of that rating.

Sourcing: Mouser, Digikey, Frys, Circuit City etc. Loads of places in the US!

Last edited by Globulator; 29th April 2010 at 02:04 PM. Reason: Sourcing added
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Old 29th April 2010, 02:08 PM   #8
marley is offline marley  United States
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Awesome! I really do appreciate your help, M.
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Old 29th April 2010, 02:17 PM   #9
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Careful of pin3 (and pin4) when you fix her, the 450Vdc anode voltage hangs around for a while (and comes back a bit too), bleed with a 1K resistor for a few seconds before getting hands on to avoid a life changing belt!

No rings either!
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Old 30th April 2010, 01:26 PM   #10
Ian444 is offline Ian444  Australia
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I had a Chinese amp that blew a few tubes before I wised up, it was an intermittent pot on the bias for one tube, some tubes were stronger than others, but one just turned to white ash inside after a spectacular internal flash. If the pots to adjust the bias are small round gold-looking plastic rubbish, replace them with something decent, before you lose more tubes. I also blew some line fuses too, sounds similar, except my amp was not Yacquin. I have read that the Yacquin is one of the better Chinese amps for sound and build quality.

Ian.
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