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Powermax PM4-120 amp PSU issue

TimmyTheTorch

TimmyTheTorch

Well-Known Member
Dec 11, 2022
322
252
73
Northeast Wisconsin
More fun in the shack. I am working on connecting the CarlBuilt amp to the PM4-120 PSU. The recommendation from CarlBuilt is to run the amp at 14.5 volts so that is what I am attempting to do. I have the toggle set to fixed output, have a voltmeter connected to the PSU terminals and am using a small, plastic Philips screwdriver to adjust the voltage (this is the same way I adjusted the MegaWatt S-400-12 and it works well) . The fixed mode voltage can range from 13 to 16.5 vdc. With as light a touch as possible, I slowly rotate the voltage adjustment until the voltmeter reads about 14.5, then remove the screwdriver. As soon as I remove even the minuscule pressure being applied to the adjuster (that is just enough to keep the screwdriver in place and move the adjuster), the voltage drops from 14.5 to 13.1 or up to 15.5 or some other random number. The screw is not being moved! I can just set the screwdriver against any part of the adjuster head and apply the slightest pressure to push the screw in and the voltage will change by up to 2 volts, then settle at a different value than it started. Again, without moving the adjuster head. I think I am glad I installed the circuit breaker inline in the amp's positive lead.

I sent in a support request to PowerMax and hopefully they can address this quickly. I just purchased this PSU directly from them in January 2023 and it is supposed to have a two year warranty. I just hope I don't get stuck paying return shipping. If so, I may just ask for a refund and look for a more reliable brand/PSU. Just another very minor speed bump on the road of life.

PM4-120_Top - Copy.jpg


PM4-120_Back - Copy.jpg
 

TimmyTheTorch said:
More fun in the shack. I am working on connecting the CarlBuilt amp to the PM4-120 PSU. The recommendation from CarlBuilt is to run the amp at 14.5 volts so that is what I am attempting to do. I have the toggle set to fixed output, have a voltmeter connected to the PSU terminals and am using a small, plastic Philips screwdriver to adjust the voltage (this is the same way I adjusted the MegaWatt S-400-12 and it works well) . The fixed mode voltage can range from 13 to 16.5 vdc. With as light a touch as possible, I slowly rotate the voltage adjustment until the voltmeter reads about 14.5, then remove the screwdriver. As soon as I remove even the minuscule pressure being applied to the adjuster (that is just enough to keep the screwdriver in place and move the adjuster), the voltage drops from 14.5 to 13.1 or up to 15.5 or some other random number. The screw is not being moved! I can just set the screwdriver against any part of the adjuster head and apply the slightest pressure to push the screw in and the voltage will change by up to 2 volts, then settle at a different value than it started. Again, without moving the adjuster head. I think I am glad I installed the circuit breaker inline in the amp's positive lead.

I sent in a support request to PowerMax and hopefully they can address this quickly. I just purchased this PSU directly from them in January 2023 and it is supposed to have a two year warranty. I just hope I don't get stuck paying return shipping. If so, I may just ask for a refund and look for a more reliable brand/PSU. Just another very minor speed bump on the road of life.

View attachment 63266


View attachment 63265
Click to expand...
These are decent for the most part, sounds like something is wrong with yours, they are touchy to adjust but I adjust both of my 100amp version's with a metal screw driver with no issues.
 
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I was not able to get the spray cleaner last night. Not going to poke the tired bear on that one!

Along with the voltage going pear-shaped when the adjusting screw is touched at all, I noticed that the green light on the back also flickers at the same time.

Some may recognize the name "Red Green" and his saying "If the women don't find you handsome, they should at least find you handy.". My Canadian roots are showing. :) After all, I am working on being more self-sufficient in these types of repairs.

So I took a closer look at how much work would be involved in pulling the board up so I can replace the potentiometer / pot myself. Only a half-dozen or so screws and also some wires to disconnect from the board. I assume I can just use something like an Xacto knife to separate the pot from the goop on the one side of it. Hmmmm.

The pot has 3362 and 1 3 on the front, and X501 and 302B on the top. I went to DigiKey to see if I could find this part there. What are the odds that this one is the correct one?

Here's the one that is in it now:

PotFront.jpgPotTop.jpg
 
Last edited:
I'm not sure the 3362 means anything (but maybe the manufacturers model or lot number of the manufacturer) but the 501 is typically the value of the resistor. 501 meaning 50+1 zero or 500 ohms. 502 would 50+2 zeros or 5000, 503 would be 50+3 zeros or 50000. Should work fine if the leads are laid out the same as the original. Might not be a bad idea to use a 10 turn pot in that position. May be able to get more exact with the output voltage vs a 1 turn (typically 270 degrees-not 360 degrees)

73
David
 
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Why not just take the trimmer pot out and measure it with a DMM to verify what it actually is? Just connecting the DMM to measure from both end terminal connections on the trimmer pot will tell you (not measuring using the wiper and either end of the trimmer pot).

By connecting the DMM to the wiper and either end of the trimmer pot (one at a time), you can check it for a bad spot(s) as you adjust it end to end while watching the DMM resistance reading.

The trimmer pot can always be put back in if you really wanted or have to if you didn't mangle it coming out.

Just wondering if all that goop was put on there due to voltage fluctuations from vibrations (of any kind) to the timmer pot including heat and cold cycling.

If it were me, I would put an external multi-turn (or not) pot on it to make it easier for future changes to the voltage setting.

"Can't become a Red Green if you don't modify it to work better."(y)
Just don't use any Duct Tape!
 
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I am happy to report that the replacement PSU arrived and works as expected. It is now set at a steady 14.5 volts as recommended by the amp builder. The radio is now running off a Megawatt S-400-12 PSU at about 14 volts. A temporary calm has returned to the shack. ;)
 
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