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Classe Six Preamp MC Output not Working

Some of you may recall I decided to switch out two resistors in the MC output section of my Classe Six preamp to boost its gain for a new MC cartridge.

I've been running MM ever since I bought it but have purchased the AT ART9XA MC cartridge and followed the Classe manual directions to replace resistors R-15 and R-16 with 3830F's to boost the MC gain by 8 dB because of the XA's low .2mv output.

My tech friend did the switch yesterday, then found there's no output when the rear panel switch is put in the MC position, while the MM is fine.  I've never used the MC section, so we have no idea if it's been inoperable for years or it's a result of yesterday's resistor switch.  One thought he had is that the MM/MC switch is old and might be suffering from lack of use for 28 years.

Any ideas as to why the MC section has died?  My friend sent me pics of the new resistors in place and they look fine.  We're stumped.
keegiam
9 responses Add your response
keegiam OP
731 posts
 
Thanks all, good points.  We may have inadvertently changed some toggle switches on the front panel in handling the preamp.  I have always kept the bypass switch off and the bypass mode on balanced.

The "tech" is a friend of mine who did this kind of work in his youth and whose skill at switching out the resistors we trusted would be much better than mine.  He didn't expect to run into any problems like this.  I will discuss the front panel switches with him in the AM.  He is concerned about both the MM/MC switch and the age of the electrolytic capacitors in the MC output area.  I do have a pro at a high end shop nearby who can probably diagnose the problem quickly.

Looking back on this, it's true that it wasn't realistic of me to present you all with such an open-ended situation.  I will report back.
lewm
13,030 posts
 
+1 for Dover.
dover
2,243 posts
 
Check the Selector/Bypass/Mode switches are set correctly.
They intereact with each other - reread the manual.
Bypass mode shorts inputs.

Otherwise get another "tech" - any competent tech should be able to figure it out - faulty caps, resistors, relays, latching circuits - this is basic stuff. Go to your local high end store and ask if they know a good tech if you don't. 

totem395
390 posts
 
Although there are several minor differences in Classe 6, DR6, 6 MKII, there are mute and bypass/phono toggles on the
front plate.
I've had this bypass switch in the wrong position more than once.




Post removed 
lewm
13,030 posts
 
I don't know how any of us could hope to give you advice that is as informed as, let alone better than, that of the person who actually did the mod and apparently has your unit in his possession. 


There are any number of shots in the dark that any one of us could take.  Is there a mute switch? Could it be somehow engaged or if that is not the case, has he checked the resistance between hot and ground of either the input side or the output side?  Like Totem said, the MM/MC switch is a viable culprit, because both channels go through it. Could be a short, due to any number of other causes.  Next I would check the gain devices in the MC circuit.  If there is no short to ground, then the likelihood is that one of the gain devices bit the dust, but it would be odd to lose both channels at once that way.  I bet it's something obvious. 
totem395
390 posts
 
Your teck has tested the simple toggle switch used for selecting
MM/MC for continuity?
stereo5's avatar
stereo5
6,385 posts
 
Don’t you mean moving coil input?
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