Bias adjustment C-audio RA501 |
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mmuetst
New Member Joined: 01 July 2020 Status: Offline Points: 8 |
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Posted: 01 July 2020 at 7:43pm |
Hello Forum,
I'am looking for a friend information so I can adjust his amp: C-audio RA501 I can only find other types on hifiengine (RA1001). The amp is not broken, I only want to check the bias settings. That is why I look for a service manual or someone who can give me the information about setting the bias. |
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simonp1100
Young Croc Joined: 01 September 2008 Location: Bristol Status: Offline Points: 1103 |
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For the older RA-500 it's 50ma, BUT the newer RA-501 a is completley different pre-driver circuit and maybe different.
Maybe measure what you got (switch on amplifier and leave for 10 minutes then measure) and see if they are the same or if they are near the older RA-500 bias reading. Hope this helps.
Edited by simonp1100 - 01 July 2020 at 8:01pm |
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mmuetst
New Member Joined: 01 July 2020 Status: Offline Points: 8 |
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I see two measure points A and B on the Gate after the 560 ohm resistors When I measure on these points I get 102 volt not 300mV, so you are probably that this circuit is different. I cannot find any schema of this amp.... |
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simonp1100
Young Croc Joined: 01 September 2008 Location: Bristol Status: Offline Points: 1103 |
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The circuit you have is for the RA1001,RA2001 & RA3001, this is completley different to the RA500 / RA501, so do NOT use this a your reference circuit. Why do you need to measure the bias current anyway ?, this only needs to be done after a repair to any of the amplifier driver / output stage ?.
If you have over 100 volts on the gate resistors of the FET's then there is a serious issue with the amplifier and would need repairing. Edited by simonp1100 - 02 July 2020 at 5:43pm |
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mmuetst
New Member Joined: 01 July 2020 Status: Offline Points: 8 |
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The 100V is between point A and B. I hope some one does have a copy of a manual, or can tell how to check the bias.
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service dept Steve
Old Croc Joined: 30 January 2010 Location: S.W.London Status: Offline Points: 2034 |
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On that drawing the test points are for quiescent voltage which is not what you need, if you are
checking the bias current it is a different procedure which on a c audio
is very easy. warm it up for a while, Remove one of the low tension
fuses and measure the current across the fuse holder. 50ma should be
about right, or you can use a scope to look for crossover distortion but
as was pointed out earlier unless there has been work done you won't
need to adjust the bias. mosfets sound a bit like valves but they don't
drift much.The indicators are mushy highs = too low, easy over heating even without music= too high
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We are not "They", We are "The others"http://www.servicedept.co.uk -
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Monitech
Registered User Joined: 19 February 2008 Location: United Kingdom Status: Offline Points: 78 |
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An RA500 was a 3rd generation rebadge
RA501 was a C Audio design and was different cct To the RA 1,2,3000 The bias will be set by measuring the voltage between the two base's of TR 1 and TR 3. THE Iq will be very low. I have the cct if you require it. I would set up with a triangle wave and look at the crossover distortion at 1 KHz and 10 KHz
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phoenix out of the ashes
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mmuetst
New Member Joined: 01 July 2020 Status: Offline Points: 8 |
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mmuetst
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service dept Steve
Old Croc Joined: 30 January 2010 Location: S.W.London Status: Offline Points: 2034 |
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[/QUOTE]Thanks, I get the answer from Monitech: The bias will be set by measuring the voltage between the two base's of TR 1 and TR 3.[/QUOTE] Good luck with that.
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We are not "They", We are "The others"http://www.servicedept.co.uk -
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Monitech
Registered User Joined: 19 February 2008 Location: United Kingdom Status: Offline Points: 78 |
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Rv1 is the cmrr on the input
Rv2 is the bias I have no notes and it's been 25 years since I looked at one The output stage is a compound configuration R11 & 12 are connected to the base's of tr1/3 The output stage will essentially be off with no signal. You will need a 4 ohm 200 watt load minimum A signal generator & a scope To set up the amp.
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phoenix out of the ashes
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