Lonestar screen resistor replacement

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Bill
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Lonestar screen resistor replacement

Post by Bill » Sat Jun 28, 2014 11:43 am

A few observations that may be of interest regarding a bad screen resistor on my Lonestar (standard/classic/Not Special). I recently fired up my MB Lonestar prior to a gig to do some work on my pedal board. Last gig it was fine. I hear it breaking up badly at all volumes, the sound is hard to describe, but it is a cutting out, with a crackling sound, worse from bass notes. The problem is worse in 50 watt mode versus 100 watt. So I proceed to troubleshoot, changing out all the tubes with known good tubes. This included both preamp and power tubes. I ran the rectifier on diode and tube with no difference in the problem. It almost sound like a possible fried coil on the speaker so I ran to another cabinet, but no change. So out of options to resolve this I called Mesa. I didn't get much help on the problem but got an RMA to send it to them. I got an very pleasant customer service guy that tried to help best he could but wasn't able to converse with a tech, they apparently don't allow that.

In my last ditch attempt to figure things out I put on my optivisor (magnifying glasses) and scoped over the entire board. I find the culprit in the screen resistor located adjacent to the outside power tube next to the rectifier. It was swelled and one end was blackened. As I found out from this board (as I'm not a tech, but fix what I can) they clearly blow when a tube misbehaves/overheats or shorts out. So my mission begins to determine value of it, easily found by the color code. It is 1 watt 470 ohm + or - 10%. They can actually ohm out at 10 more or less that there value. I read on the board that they should match with remaining resistors, particularly the neighbor. My replacements (I bought 4 NOS Allen Bradley carbon composite, military spec, on eBay), ohm'd out at 512 each. My remaining resistors in the amp were one at 450 and 2 at 412. Mesa told me it didn't matter if they match, but suggested 2 watt for durability. I installed just one and all seemed fine. No noticable difference due to 10% variance. However, I went ahead and changed the remaining three to have all 4 new resistors, which I would recommend, as it's so easy and cheap.

There are two points here: one is that it would have been nice to have been pointed in the right direction to problem solve this. I had indicated to MB that there had been some issues with the tubes, I had blown a fuse and I had smelled the burning at one point, which was when I mistakenly installed a 6L6 into the rectifier socket, as the 5U4 had fallen out and I though the tube I had grabbed was a 5U4. They are very similar and fit the same socket. This may have caused the resistor to heat up. I described all this and didn't get any suggestions regarding checking the screen resistors.

The second point is that if you get some aberrant behavior such as power cutting out, crackling, unexplained breaking up, and you have smelled something, or you have any suspect power tubes, absolutely take a minute and check those resistors. It's a piece of cake to ohm them out, presuming your comfortable with this level of diagnosis. It would have saved me some time if I had know this. I certainly do now.

Also, regarding installation, even though they are installed with the tips coming in from the bottom, you cannot really do this again without taking the board out. They can be installed from the top very easily and it makes no difference. It also makes no difference which direction the color code goes. As a rule for neatness they are usually installed matching, although my blown one was actually opposite the remaining three.

Hope this is helpful to someone at some point.

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plan-x
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Re: Lonestar screen resistor replacement

Post by plan-x » Sun Jun 29, 2014 2:44 am

Congrats Bill, you are living proof that using the search function is invaluable. I've replaced grid resistors twice now on my 04 LSC. There is an actual pictorial here in the archives that was very informative in identifying these grid resistors.
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Bill
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Re: Lonestar screen resistor replacement

Post by Bill » Sun Jun 29, 2014 10:50 am

You got that right. I've solved MANY MANY problems (amps, cars, home appliances, etc.) with a search. Of course the key is finding the right terms AND to not give up.

Chester
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Re: Lonestar screen resistor replacement

Post by Chester » Sat Jul 05, 2014 1:13 am

Great post - thanks for the heads up :wink:
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stratmunkey
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Re: Lonestar screen resistor replacement

Post by stratmunkey » Mon Dec 15, 2014 1:33 pm

Can anyone point me to the archives?? I've searched but cannot locate. Thank you.

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plan-x
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Re: Lonestar screen resistor replacement

Post by plan-x » Tue Dec 16, 2014 12:01 am

Sorry munkey, I can't find it. I tried for 1/2 hour to no avail. One problem is we used to post in the modern amp section before we got a Lonestar sub forum. Maybe I can help. I've done the procedure a few times.
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stratmunkey
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Re: Lonestar screen resistor replacement

Post by stratmunkey » Tue Dec 23, 2014 5:23 pm

Its all good, X ,,, I just didn't know if there was anything posted that I needed to know. I received my resistors over the weekend. About to change em out. They all measure out between 517, 518. Will that be o.k. or is it too high?

Thanks!!!

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