Mark Levinson No.27 amplifier,,,NEED HELP

[Hans Polak]

Originally Posted by jc9394

ok, thank you very much@

Did you eventually solve your problem ?
It sounds as if one of the heat sensors on the heatsinks or the cabling to connect them is giving trouble.
Second reason can be DC on the LS output.
Hans

[bakedAlaska]

Thanks for helping, Hans.
I had measured the bias from another 27.5 recently serviced by a ML dealer, it was set to 104mA, which is in line with your value (I measured voltage over R125+R127 only).
Vreg was set to +-64,50V (end value after full warm-up) rather than the specified 65V. I left it at 64,5V, probably goes up 100mV with higher ambient temperature in summer.

BR

[Hans Polak]

The absolute values of Vreg+ and Vreg- are not that important as long they are not too far from 65 Volt.
But what is extremely important is that they are equal in opposite magnitude, preferably within 0.1 Volt.
With 100mV ripple on one side, you will first have to correct this.


Hans

[bakedAlaska]

Yes, will fix the ripple first, and then look at settings again.

BR
Andreas

[bakedAlaska]

While waiting for parts, I further investigated the idle noise issue, mentioned before. It's on both channels, stronger in the right channel (only hear it if I sit 1m from the tweeter).

I shorted driveline + (Pos_Drive: P6, Neg_ P7) to ground via capacitator. The noise is gone. So either it really is dirty VReg, or something else is uncough on AC-8.5 board. We will see.

Seems I'll have to take apart AC-8.5 and AC-8. They are connected to each other with these 'concorde pins'. I had hoped this is some kind of connector, but it seems soldered on either side of the board. That's a bit inconvenient.

BR

[Hans Polak]

Before you take things apart, which is indeed very inconvenient, I strongly advise to first replace the two 680uF caps on the upper board and to make Vreg+ equal to Vreg-.

When these two voltages are not equal in magnitude, the start up circuit interferes with the signal chain, that's why.

Hans

[Mark levinson]

Hello, Hans! Did I understand correctly that the amplifier will not start if the currents from both channels are not the same? I have this problem: I replaced the entire small electrolyte with 4 * Sprague 68uf 25V; 4 * ekz ROE 22uf 63v, 10uf 4 NIC, 6* Bourns trim resistors with resistance setting over the old resistors,all yellow yellow film capacitors 0.1 uf 160v.

[Mark levinson]

The amplifier starts, at the output terminals of 30-40 millivolts,but there is no start mode when the speakers are twitching, so there is no sound... What is the reason I can't understand! The 680uf capacitors are absolutely in perfect condition.

[bakedAlaska]

Originally Posted by Hans Polak

..first replace the two 680uF caps on the upper board and to make Vreg+ equal to Vreg-...

Ok will report back. Will take a moment.

BR

[Hans Polak]

Originally Posted by Mark levinson

The amplifier starts, at the output terminals of 30-40 millivolts,but there is no start mode when the speakers are twitching, so there is no sound... What is the reason I can't understand! The 680uf capacitors are absolutely in perfect condition.

I did say that the regulated voltages have to be of exact the same magnitude for each channel, not at all the currents of both channels.
What do you mean with “there is no start mode ....” ?
What model do you have to 27 or 27.5 ?

Hans