Mark Levinson No23 repair help

[Handel]

And that is why I am repairing this amp. There is some challenge!

The ripple in the left amp was in both VCCs around 200 mV. That makes 16000uF. Right?
That is clearly better than on the left side.

 

[Hans Polak]

And that is why I am repairing this amp. There is some challenge!

The ripple in the left amp was in both VCCs around 200 mV. That makes 16000uF. Right?
That is clearly better than on the left side.

That's a lot better.
How did you measure, with a scope ?
It might not be a bad idea to replace all four of them.

Hans

 

[Handel]

Now I found the right function from the scope. It is precisely 156mV, Need to measure again from the right channel.

Yes, I have been thinking about that. Quite expensive though.

One guy from the forum contacted me about old non functional ML23 main boards. They might be good boards to get spare components.

 

[Hans Polak]

For €11,- you could buy this universal component tester for semiconductors, caps and inductances up to 100.000uF with high precision.
A fantastic smart instrument

Geekcreit(R) LCR-T4 Mega328 Transistor Tester Diode Triode Capacitance ESR Meter With Shell Sale - Banggood Mobile

Hans

 

[Handel]

Wow! That is impressive. It would save a lot of time...

 

[Handel]

I found great pictures from repairing and measuring ML 23.5. great pictures also from the OL-2 board:

mark levinson no.23.5l

 

[Hans Polak]

I found great pictures from repairing and measuring ML 23.5. great pictures also from the OL-2 board:

mark levinson no.23.5l

Yes nice pictures.
The explanation what he did was also very instructive

Hans

 

[Handel]

You can use google translator

 

[Handel]

Should I test also the capacitors in the VCC prereg line(C301, C302, C305, C306)?
Same lamps, same formula?

 

[Hans Polak]

Should I test also the capacitors in the VCC prereg line(C301, C302, C305, C306)?
Same lamps, same formula?

You didn't measure Vripple of the left amp with lamps in the V unregs did you, because that was not the idea. R20 should be 27.5mV while measuring Vripple.
That makes the total current drawn from Vcc unreg I=0.32

Measuring the other 4 caps is best done by first inserting an amp meter in the resp Vcc Pre Reg lines and making notes of the current in amps drawn.

Then when knowing this current I, you can again calculate C=I*0.01/Vripple

Hans

 

[Handel]

Oh, I see. Then I had the lamp between the VCCs. Now I measured without the lamp 125mV ripple(R20 = 27.5mV), which makes 25600uF.

It is a big differenece between the channels. I will do the same measurement to the right channel when it works again.

 

[Hans Polak]

Handel,
Could you please measure the DC voltage at the Spkr output of the Left amp.
Input is a LM394 dual transistor with 0.5mV max offset.
And since for DC gain of the amp is 1x via R21, I would expect something close to 1mV.

Hans

 

[Handel]

Yes, I measured the DC voltage in the output and that is 15mV fully warmed.

 

[Hans Polak]

Yes, I measured the DC voltage in the output and that is 15mV fully warmed.

Thank you.
I can’t work it out how it comes to this level, but it is absolutely safe and both channels are the same.

Hans

 

[Hans Polak]

I have solved the 15mV output voltage.
Q9 has 1mA collector current, hfe is ca. 300, so Ib is ca. 3,3uA.

For DC, the minus input sees R121 and the plus input R76.
Since R76 is 4k7 smaller, Ib going into Q9 causes ca. 3.3uA*4k7 = 15mV offset voltage.

Hans

 

[Handel]

Finally the MJE transistor have been sent. I really hope to receive it tomorrow.

 

[Hans Polak]

When you would place a 2Meg resistor parallel to R121, the output offset voltage will drop to practically zero mV.

Hans

 

[Handel]

That sounds a big resistor, only to the negative side and nothing to the positive side(with R46)?

 

[Handel]

Sorry, I thought R21...

 

[Hans Polak]

That sounds a big resistor, only to the negative side and nothing to the positive side(with R46)?

Only to the negative side.
Because 2Meg//100k = practically the same as the 95.3k on the positive side.

Hans