• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

Feeling a shock from RCA Cables attached to MC240

DrRobocop

DrRobocop

New Member
Hi all,

Last night I was switching between DACs which I had attached directly to my MC240. My finger touched the end of one of the RCA cables attached to the "twin amp" output section of the MC240. I felt a significant shock (no damage done) coming from the end. I'm guessing this isn't normal....
Is this dangerous? Should I not be operating this amp currently? What do you think this could be? How would I fix it?
 
Register to hide this ad
DrRobocop said:
Hi all,

Last night I was switching between DACs which I had attached directly to my MC240. My finger touched the end of one of the RCA cables attached to the "twin amp" output section of the MC240. I felt a significant shock (no damage done) coming from the end. I'm guessing this isn't normal....
Is this dangerous? Should I not be operating this amp currently? What do you think this could be? How would I fix it?
Click to expand...
Probably AC leakage in the '240, not unusual for vintage gear. Has it been rebuilt?
 
Hi,

Thanks for your response. It has been rebuilt. Power supply is new and all capacitors have been replaced. Resistors are largely original Allen Bradleys. However, I admit that I did the work on the amp personally and while I am competent in replacing/soldering components (thanks to this community!!), "real" trouble shooting is somewhat beyond me. Do you know if there is a particular area/capacitor I should be checking to resolve this?

thanks!
 
Make sure that the amplifier doesn't have a capacitor, often called a 'death cap', from one side of the AC line to the chassis. That could cause the slight shock. I personally remove those from transformer powered equipment. If you can reverse the AC cord of the amplifier, and the shock goes away, that may be what is happening. Either that or capacitive coupling between the primary winding of the power transformer and its core may be doing it. Use of a 3 prong grounded power cord would eliminate that.
 
Wow! a "death cap" sounds terrifying. I'll have a look for this and see what I can find.
I had considered swapping out the thin "lamp cord" power cord with a more robust 3 prong grounded power cord. I wonder if this is typically done with these amps. I certainly wouldn't get anything crazy which would necessitate modification of the chassis.
I wonder if I missed grounding something in my prior work on the amp. Is there a vital grounding point(s) in the MC240 I should pay particular attention to? Everything looked intact to my somewhat inexperienced eye.
 
If you have warrantee on the equipment, altering the power cord could void that. If that's not a concern, then either plug the original cord into the outlet in the direction that decreases the tingling effect, or use a 3 wire cord and attach the green (earth ground)wire to the chassis itself. Do not connect the ground to the DC supply common, or signal ground; connect to the chassis directly.
 
Reverse the plug in the outlet, should fix it, but as @audiotemp mentioned you should not plug/unplug inputs on an amplifier when it is powered on.

I would not recommend installing a 3-wire power cord, I prefer to use a 2-wire polarized plug so that it can't be plugged in the wrong way.

<<edit>> Hot side (smaller blade) goes to the fuse.
 
Last edited:
Thanks you all for your help!!

I'll try making sure the plug is in the correct orientation first (I never thought of the plug being polarized!). Then potentially change the cord itself if the issue persists. Also, I'll refrain from changing RCA inputs with the amp on. Hopefully this will remedy the "shock".
 
SaturationPt said:
I still think that the amplifier needs repair if there's enough voltage to feel a poke.
Click to expand...
There's a well-defined test, usually involving a 1.5kOhm resistor connected to ground, that appears in the front of just about every service manual ever printed. It's not about what voltage there is, it's about how much current can be delivered into that resistor (which is a stand-in for "you").
 
AC frame leakage not unusual on vintage power transformers.
There are four ways to deal with it,
(1) reverse power plug for lowest AC offset.
(2) determine polarity for lowest AC offset and wire 3 lead grounded power cord accordingly.
(3) order replacement power transfomer tested for zero leakage from McIntosh.
(4) discard the amp.
 
How about a photo from underneath, giving us a good view of "the "twin amp" output section of the MC240" and of the ac power input side as well?
 
You must log in or register to reply here.
Back
Top Bottom