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Old 16th January 2008, 12:37 AM   #41
Hi_Q is offline Hi_Q  England
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Quote:
Originally posted by kuroguy
more than likely, it gets hotter because it has less airflow around it.
Cannot agree with that, the pot was originally a sealed box so no air flow at all and the only cooling was by radiation from the transformer to the outer case. The new air vent at least allows the hot air to move more freely to the outside world. I take it Brian that you have mounted your pot onto the top of the chassis rather than thread it through the cut out, thus allowing a nice air gap.

Not familiar with the Yaqin MC 300B but I have heard of problems with it due to overheating, mainly from a 220V transformer being fitted. I have always recommended an auto transformer where the primary windings are 220V. What you need is a transformer with 240/220/200V taps, just feed the mains to the 240V tap and run the amp from the 220V one. My amp has been running now for 2 years with this arrangement and so far no problems. I also use a radio controlled power socket so that the transformer and amp are both switched off when not in use. A 220V model on 240V mains will give higher heater and HT voltage that will add extra stress to other components as well as the mains transformer.

I did spot this website and it may be useful if the time comes for a replacement transformer.

http://www.vt4c.com/shop/program/mai..._id=7&hit_cat=

They do a 240V primary model that also has a bias winding and plenty of poke for the heaters. Not sure on physical size though but if push came to shove I suppose it could be housed in a separate housing. I suppose its a failure we all dread and Brian and myself are just trying to give this hard pressed component a bit more breathing space!

Again with the MC 300B, it is important to keep a check on the cathode currents of the output valves, I believe that this amp uses cathode auto bias and the resistors here may have to be increased in value to keep everything as it should be. I have a gut feeling that these amps may have been designed and set up with 220V supplies and running them from 240V pushes them close to or over their design limits. I was a Tech Officer for a major TV manufacturer here in the UK, we had lots of premature failures until we discovered that they were due to mains variations between the factory and other parts of the country. We told the dealers to reset the power rails when they unpacked the sets and failure rate went dramatically down.

What annoys me most about the Yaqin is lack of space inside the chassis to add the mods I would like to fit. I dare not look at the circuit diagram too much because I get more and more worried at what I see.
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Old 16th January 2008, 12:58 AM   #42
t-head is offline t-head  United States
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Quote:
Originally posted by Hi-Q

Not familiar with the Yaqin MC 300B but I have heard of problems with it due to overheating, mainly from a 220V transformer being fitted. I have always recommended an auto transformer where the primary windings are 220V. What you need is a transformer with 240/220/200V taps, just feed the mains to the 240V tap and run the amp from the 220V one. My amp has been running now for 2 years with this arrangement and so far no problems. I also use a radio controlled power socket so that the transformer and amp are both switched off when not in use. A 220V model on 240V mains will give higher heater and HT voltage that will add extra stress to other components as well as the mains transformer.

I did spot this website and it may be useful if the time comes for a replacement transformer.

http://www.vt4c.com/shop/program/mai..._id=7&hit_cat=

They do a 240V primary model that also has a bias winding and plenty of poke for the heaters. Not sure on physical size though but if push came to shove I suppose it could be housed in a separate housing. I suppose its a failure we all dread and Brian and myself are just trying to give this hard pressed component a bit more breathing space!

Again with the MC 300B, it is important to keep a check on the cathode currents of the output valves, I believe that this amp uses cathode auto bias and the resistors here may have to be increased in value to keep everything as it should be. I have a gut feeling that these amps may have been designed and set up with 220V supplies and running them from 240V pushes them close to or over their design limits. I was a Tech Officer for a major TV manufacturer here in the UK, we had lots of premature failures until we discovered that they were due to mains variations between the factory and other parts of the country. We told the dealers to reset the power rails when they unpacked the sets and failure rate went dramatically down.

What annoys me most about the Yaqin is lack of space inside the chassis to add the mods I would like to fit. I dare not look at the circuit diagram too much because I get more and more worried at what I see.
Thanks. Mine was set for US mains (110v) but we regularly get 118v-120v. Resistors and caps are barely sized and of low quality...I have schematic but is as you say...you get what you pay for...or not what you don't...shame the chassis and remote motorized vol pot is worth more than the parts necessary for sound...

t
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Old 3rd February 2008, 07:40 AM   #43
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Default Yaqin MC-10l fuses??

Hello I am new to this forum, and recently purchased a Yaqin MC-10L. Everything was going along fine, but I recently expereinced the quick off & on blown fuse problem. Ok so here is my real problem...WHERE IS THE BLOODY FUSE LOCATED???. Thanks, hoping for a reply...
TJ
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Old 3rd February 2008, 08:39 AM   #44
Svein_B is offline Svein_B  Norway
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I have not laid my hands on one of these, but I would first look for the fuse in the power-input connector (just below the IEC connector)

SB.
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Old 3rd February 2008, 10:10 AM   #45
gasman is offline gasman  England
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Quote:
Originally posted by Svein_B
I have not laid my hands on one of these, but I would first look for the fuse in the power-input connector (just below the IEC connector)

SB.
Hi, Thats exactly where the fuse is located and on my MC 10L what I did to cure the problem was to fit a slow blow fuse similar value instead of the fast blow that was fitted and I havent had a problem since, Also check the bias on the EL34 valves, Hope this helps Cheers.
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Old 7th March 2008, 08:19 AM   #46
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Default YAQIN TUBES 6N 6H1n or EQUIVALENT ?

Just bought and am listening to my first valve job in 40 years YAQIN MC 10 L - had forgotten how good it was back then. As i live well away from a major town, i want to stock up on some valves etc. Am in ignorance as to the FOUR 6N 6H1n's at the front- a quick google brought up very little- can anyone tell me the equivalent i should look for- any input greatfully received
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Old 7th March 2008, 12:33 PM   #47
Hi_Q is offline Hi_Q  England
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Hi, the smaller valves are 6N1's, you might like to try Svetlana 6n1p as a substitute, I think you will like the difference they will make. You can then keep the original Chinese valves as spares. For EL34's it is again a personal preference choice really, I like the Electro-Harmonix Cryro but am using at present Electro-Harmonix 6CA7EH beam tetrodes.

Most important thing is to set up the bias on each valve so that the idling currents are between 30 - 35mA. The newer models of this amp have terminals at each valve position so you can measure this with a multimeter. Set it to measure DC Volts and adjust the small preset control near the valve for 300 - 350 mV, this will probably appear as 0.3 - 0.35 on the display.
Les
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Old 11th March 2008, 10:15 PM   #48
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Thanks- will now try and overcome my innate fear of sticking something metallic into a device that has power running thru it- ( my father was an electrician and the only thing he taught me about electrics was that it could kill you!- ) watch this space- as i now venture out to buy a multimetre and begin a programme of self discovery- - cheers
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Old 11th March 2008, 10:47 PM   #49
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Have checked the ebay site for Svetlana 6N1P'S-- nothing there- has anybody any leads as to where i could pick up these? thanks
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Old 11th March 2008, 10:52 PM   #50
Hi_Q is offline Hi_Q  England
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Just an idea, but the adjusters are usually fitted with hexagonal holes.
To adjust the trimmers I use a VERY long steel hex allen key with an insulated handle. It may be a wise move to obtain a small hex key, grind a short piece off and set it into a long plastic rod. Just like the giant plastic knitting needles Gran used to have .
But please take care, the bias voltage will give you a tingle but nothing compared to the main HT that could pack you off to the undertakers.
Adjust the trimmers very slowly-carefully and only when you can see exactly what the valve currents are doing using the multimeter.
I noticed in an earlier post that mains fuses blow if you switch the amp off and back on too quickly. A lot of this is due to the bias voltage taking time to come up and control the valves in relation to the HT. You can actually hear a thump from the mains transformer as it has to not only recharge the electrolytics in the PSU but also handle high anode currents of all four output valves. If you do accidentally switch the amp off, leave it for the valves to cool for a while before switching back on, just a few minutes will do the trick.
Les
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