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Old 27th October 2007, 11:40 PM   #11
Hi_Q is offline Hi_Q  England
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Hi Gerry, just a thought, you may not have removed the upper chassis plate before to gain access to the adjustments.
First remove the mains supply!
Remove the four cross head screws on each side that secure the silver alluminium plates, then remove plates.
Next remove the ten small black hexagon head screws, there are two on each side (one front, one rear) and six on top (3 along front edge and 3 along rear edge)
DO NOT ATTEMP TO REMOVE THE LARGER ONES LOCATED AROUND THE TRANSFORMER POTS. It is not necessary, a lock nut will fall into the internals if you try to remove any of these bolts.
The top cover will be free to be removed once the ten small bolts are removed.
It is advisable to use a long reach hexagon tool for adjusting the controls, TAKE CARE! there is around 450V lurking around the boards!
Re-fitting is a reversal of above instructions.
Les
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Old 28th October 2007, 04:20 PM   #12
gasman is offline gasman  England
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When i checked the bias on mine 3 of the valves were set to 0.4v and the last one was 0.9v, have reduced them all to 0.3v,so it does go to show that you cant trust the factory settings on these units so checking does seem to be a must.
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Old 28th October 2007, 07:18 PM   #13
rk-one is offline rk-one  Italy
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Boys you are great (HI-Q u're my hero )
2 last questions because i'm really noob : in which position must be set multimeter and i don't undertansd if i'have to put the red pin on the upper side and the black pin in the lower side of the resistor or the black in other position.

thank YOU!!!!!

Gerry

ps is there any way to check plate voltage? or it's no necessary?
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Old 28th October 2007, 08:01 PM   #14
Hi_Q is offline Hi_Q  England
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Use a digital multimeter set to 2V DC. If you get the meter leads round the wrong way it will still give a correct reading except a minus sign will appear to the left of the readout. If my failing memory serves me right, the control pots are wired correctly such that turning the pots anti-clockwise reduces the valve current and hence the multimeter reading. The controls are fairly course so adjust very carefully. Wow! 0.9 that's high and I am not surprised the anodes were not glowing red.
I just heard today that a friends MC10L has blown its mains transformer! It's a pity that I cannot get access to it (he lives a long way away) as I am sure there must be a fault on the amp other than just blaming the higher UK voltage, which he does! Best to check the heater volts if you can, two yellow twisted wires around V2 on the pre-amp board at the top edge of board. This of course is AC volts and expected to be approximately 6.3V. Take care you do not short these wires out as it will cause loss of heaters on the left hand side and bring forth a burning smell. I know!, how do I know? Well of course I found out the hard way - damned crocodile clips shifted!
It burns out the PCB track on the main board and that is a bit more difficult to get to, but that's another story
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Old 29th October 2007, 11:34 PM   #15
rk-one is offline rk-one  Italy
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Job done!
original bias was not good 33.8 38.9 - 34.4 36.3
Now all at 32.6 thank You Hi-Q

Now i wanna try better tubes (sed or jj kt77)
and new 6n1p, please post your opinions and experiences about tubes.

Gerry
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Old 29th October 2007, 11:39 PM   #16
gasman is offline gasman  England
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Hi All, Some great advice from Les, Tested the heater voltage on my MC10L. came out at 6.5v thats close enough for me but just one thing to note, on my amp which is very new, the heater voltage terminals are on the other side of the board next to V3, Cheers. ps, hope you mean 0.326v RK.
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Old 30th October 2007, 12:38 PM   #17
rk-one is offline rk-one  Italy
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Yes 0.326 v
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Old 30th October 2007, 03:27 PM   #18
gasman is offline gasman  England
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Did you manage to get the heater voltages while you got the lid off Gerry, I did mine, came out at 6.5 v so only just over, near enough for me, Glad you got a result with your biasing, Do you like the sound of the MC10L, Cheers Brian.
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Old 30th October 2007, 05:14 PM   #19
rk-one is offline rk-one  Italy
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No sorry.When i'll open it again i'll check it.
I like mc-10l but i'd like to improve details and make sound better and firmer the bass. I heard the yaqin ms-300b and has better and more accurate highs and larger scene but not so great in the medium-low zone.
I wanna try new tubes but i've to choose which brand.


Gerry
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Old 30th October 2007, 08:51 PM   #20
Hi_Q is offline Hi_Q  England
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Well I am wondering if replacing the 100nF couplers with 220nF Audio grade may help give a bit more bass response. The other components I would like some help with are the NFB, namely 5k1 in parallel with 100pF. Are they worth tweaking with? I am certain the 100pF is only there to stop supersonic oscillation.
Note that the circuit shows the grid stoppers in the right place, at least on my amp, they are directly placed between grid and the 100k resistor at each valve.
Les
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