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Old 2nd July 2013, 09:24 AM   #1
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Default ymaha preamp c60

I purchased the yamaha c60 from ebay.

The problem is that the pre output is too low even with the headphones output
i have the volume between half and 3/4,without cont.loudness.

I tested it with 1 khz 0,8 volt cd in and the output from pre out is 1 volt,
with tonecontrol bypass and continous loudness on flat position.

I measered inside with the 1khz signal cd in from the beginning of the 6 db flat amplifier.
And i noticed that from the njm2041s i only get 200 mv improvement then after 2sb560/2sd468 no voltage gain.

I measured the 2 transitors and proofed good and the voltage around them are good to, but the bias differs one has 14mv the other around 20 mv measered on the 4,7 ohm resistors.

The muting transistors the are good to.

I use the servicemaual from the c80 because its the same only has the inverter
circuit.

So i want to know is the 1 volt without continous loudness and tone control is correct, and how much the opamp njm2041 amplifies?

I hope that you guys can help me out?
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Old 2nd July 2013, 08:32 PM   #2
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Location: Germany
Quote:
Originally Posted by piet21 View Post
I measered inside with the 1khz signal cd in from the beginning of the 6 db flat amplifier.
And i noticed that from the njm2041s i only get 200 mv improvement then after 2sb560/2sd468 no voltage gain.
The latter would be expected... they don't call it a unity-gain buffer for no reason. Still, the whole circuit should give a gain of 6 dB (duh), or a factor of 2. (Gv = 1 + R409/R407.)
From input to output, you should see a maximum total gain of 20 dB, or a factor of 10 (full volume, tone control off, muting off).
Quote:
Originally Posted by piet21 View Post
I measured the 2 transitors and proofed good and the voltage around them are good to, but the bias differs one has 14mv the other around 20 mv measered on the 4,7 ohm resistors.
That's odd. What sort of DC levels do you see on the output? (The circuit employs a DC servo, so there should be very little if it's functioning properly.) Any change if you turn on the subsonic filter?
Quote:
Originally Posted by piet21 View Post
I use the servicemaual from the c80 because its the same only has the inverter
circuit.
That's good to know!
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Old 3rd July 2013, 01:03 PM   #3
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Default yamaha c60

I measered yesterday again and i found out that everything is oke (1.5 volt) until the continous loudness control, there it is weaken again, to 200 mv because it has to go through the continous loudness potmeter.

So the 6 db amplifier have to amplify the 200 mv to 1.5 volt.

So i''m certain it's the 6 db amp.

If you have the service manual to, can you explain to me what the 2sk389 is doing
in the circuit;does it regulate the input for the njm2041?

The problem is in the Netherlands were i live, i can't find spare parts, otherwise
i would replace all transitors and the 2sk389.

Thats why i want to pinpoint the fault in the 6 db circuit.
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Old 3rd July 2013, 01:29 PM   #4
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Quote:
Originally Posted by sgrossklass View Post
The latter would be expected... they don't call it a unity-gain buffer for no reason. Still, the whole circuit should give a gain of 6 dB (duh), or a factor of 2. (Gv = 1 + R409/R407.)
From input to output, you should see a maximum total gain of 20 dB, or a factor of 10 (full volume, tone control off, muting off).

That's odd. What sort of DC levels do you see on the output? (The circuit employs a DC servo, so there should be very little if it's functioning properly.) Any change if you turn on the subsonic filter?

That's good to know!
Thanks for the reply!!

I tried the subsonic filter without luck, dc levels output about 3mv.
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Old 3rd July 2013, 01:38 PM   #5
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[QUOTE=sgrossklass;3547269]The latter would be expected... they don't call it a unity-gain buffer for no reason. Still, the whole circuit should give a gain of 6 dB (duh), or a factor of 2. (Gv = 1 + R409/R407.)

I looked on the internet what unity-gain buffer means and it means that
the voltage stays the same but only provides current.

So in that way the input to the 6db amplifier has to be already 1.5 volt?
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