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Zenith help needed

T

travishonn

New Member
Hello all. I am new to the forum. I recently picked up a Zenith MM2605 with a 3L02 chassis. When turning the unit on, there is a strong hum. I plan on replacing the old paper wax filter cap but need some help since I know nothing about electronics. The cap has 3 codes on it. Red 300mfd 200vdc, Green 300mfd 150vdc, and Blue 60mfd 150vdc. What replacement caps do I need to replace this. Any other suggestions would help as well since this is my first project and my lack of electronics knowledge.
 
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It is a multi section capacitor. You can get new units with the same look from http://www.tubesandmore.com or you can get discrete capacitos from Digikey or Mouser, or many places for that matter to replace that unit- the black wire is common to each section as the negative lead. So replacing with discrete caps- all the discretes would be common negative (all common leads tied together).

Consider the line voltages being a little bit higher these days, I would suggest you get two of the 300 microfarads capacitors, with a 200 working voltage rating and a 60 microfarad capacitor with a 200 WVDC rating as well. If you can only find 330 or 280 microfarad caps, those will work fine in that unit as well. If the 60 microfarad is difficult to locate, the 56 or 68 microfarad type will work just as well. The working voltage is important- you want to be equal to or higher (many times best practice) than the original working voltage rating.

It is a "Hot Chassis" type of amp, so keep one hand away from amp on something that is not conductive. That amp can sometimes be at mains power with no isolation whatsoever. If you are not careful- it can kill.

There are a few other threads recently on different aspects of the 3L02 chassis and at least one of them will have the schematic linked as well.

When it comes to wiring the discrete caps, since all the negative leads tie together, they go to the black lead. The red and green lead are not critical if the replacements are the same working voltage, and the blue would just go to the open lead (+)of the 60 microfarad or it's nearest value you are using.
 
Thanks for the help! I read that it was a "hot chassis" but didn't really know what that meant. Am I ok to cut the wires to the old cap without risk of lighting myself up?
 
Check if there's any remaining voltage in them using a voltmeter. Ideally, use a 50 to 100 ohm 10 watts resistor but at worst, short them with insulated wires. Of course, be sure the unit is unplugged.
 
1 last question and I'll leave you guys alone. How do I short the capacitors if one end of the wires are in the paper wax tube? Do I need to tear the tube apart to get to the other ends? I just don't want to get lit up!
 
Make sure it is unplugged and just short the black wire to each color wire. Short the black wire to the red, green, and blue wires. There may not be any residual voltage left over especially after the unit has been turned off and unplugged and being that the capacitor is bad and causing hum but better to be safe.

The hum just means that the capacitor or one of its segments is open and is simply not able to do its job and hold a charge but the other segments could give a jolt.

These series string units (hot chassis) usually don't have enough residual voltage to kill after being turned off and unplugged but sometimes can give a jolt enough to scare you so be safe!
 
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Yes welcome to AK!

There are other caps to replace also for best reliability (you risk transformers and possibly output tubes) such as the coupling caps and bias cap(s). It doesn't hurt to check the resistors for any that have gone high either, clean the tube pins and sockets, clean any switches and maybe pots.

In all it should sound a lot better when totally redone also.
 
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travishonn said:
Thanks for the help! I read that it was a "hot chassis" but didn't really know what that meant. Am I ok to cut the wires to the old cap without risk of lighting myself up?
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If the unit is unplugged- yes. The old failing caps in these types of chassis do not store enough energy to cause any issue. Old TV sets are another matter entirely though.

Start with the power supply caps, and other larger electrolytics on the chassis and give it a listen. You can change the remaining electrolytics and other paper caps at another time. by doing it this way you may hear the improvements as you move through the unit and get a better understanding of what things do in various parts of the circuit.
 
Trio said:
If the unit is unplugged- yes. The old failing caps in these types of chassis do not store enough energy to cause any issue. Old TV sets are another matter entirely though.

Start with the power supply caps, and other larger electrolytics on the chassis and give it a listen. You can change the remaining electrolytics and other paper caps at another time. by doing it this way you may hear the improvements as you move through the unit and get a better understanding of what things do in various parts of the circuit.
Click to expand...


Thanks! This is the answer I was looking for. You guys are all awesome for passing along this advice and I can't wait to do more projects.
 
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