Howdy All!

I have a CJ MV75a-1 in need of a power supply recap/rebuild. The unit appears to pass a signal thru both channels, but it hums like a temple full of Monks. Internally the unit appears untouched, and i expect a full recapping of the power supply might go a long way toward getting this beast performing as it should once again.

Any advice you folks could offer in the way of obtaining parts, schematic/circuit drawings, and or a fella who specializes in CJ would be greatly appreciated.

Many thanks in advance, Tom

First of all, you must remember that all of the tests on power filter capacitors must be considered hazardous since they may store deadly amounts of voltage and charge even with the amplifier unplugged.

Did you notice in power filter capacitors any signs of bulging, leaking, dents and other mechanical damage?
Also note the condition of any series dropping resistors connected to the capacitors to see if they have been damaged by heat.

Use an ohmmeter to measure the resistance from the (+) terminal of each capacitor to the (-). This should be over 15K ohms, preferably much over that. If you get less than that on any capacitor, unsolder that capacitor and remeasure just the capacitor. Under 15K indicates a dead or dying capacitor; replace it. If the resistance is now much higher with the cap unsoldered, there is a low resistance load pulling current, not a faulty capacitor. Always check all of the power filter capacitors while you're in there. If one is bad, consider replacing them all!

If there is no obvious mechanical problem and the resistance seems high enough, temporarily solder a new, known good capacitor of at least as high a capacitance and voltage across the suspected capacitor or section., then plug in and try the amplifier again. If this fixes the problem, turn the amplifier off, unplug it, drain the filter capacitors again, and replace at least the bad section if not all of the filter capacitors.


Now, there are a few other important factors that can cause hum in an amp, for example:

A defective input jack.

A Faulty tube....

Poor AC grounding.

Induced hum -Placement of the amplifier near other equipment can sometimes cause it to pick up radiated hum from other equipement. Suspect this if the hum changes when you move or turn the amp.

Poor AC Chassis Ground at Power Transformer.

Etc...

Here's a schematic :

http://www.drtube.com/audioamp.htm#CJ

BTW, you can also check some Ground References at :

http://www.sound-thinking.org/index.php?showtopic=583&st=30

Many thanks Clint E, for a very comprehensive reply.

I should have been a little more detailed in my first post, at least to the effect of mentioning that the hum is present equally in both channels whether the inputs are shorted or fed a signal.

A picture might better explain the logistics of accessing the P/S caps, serviceability wasn't high on the list of CJ's design criteria on this one.

My feeling with this amp being at least 25 years old, and considering the access challenge, that i'd be furthest ahead to just do a total replacement of all electrolytics in one shot and hopefully be done with it. The idea of having to remove and replace that cap board more than once really feeds my procrastinator.................. if ya know what i mean.

http://i107.photobucket.com/albums/m283/bo...nderchassis.jpg

You welcome mate.
He looks old but at the same time it's very cool looking amp. Imho, i'll change the resistors - i'd mention before - as well.

Good luck.

Here's a shot of the top side, less the rack mount faceplate and cage for folks who aren't familiar with these. This thing has easily 60lbs of iron bolted to it. I removed the cage and faceplate to try to prevent anymore dings or scratches while it's on the bench. Seems this one has been "unloved" for some time, hopefully i can once again get it looking and sounding at least worthy of it's reputation.

Smashing looks. Congrats.

With nowadays new components - caps and resistors - i'm sure you'll make this baby sounds just fine.

Please keep us posted.