MG100HDFX Master Volume pot. Is it really REVERSE LOG?

Discussion in 'The Workbench' started by bordonbert, Jan 22, 2020.

  1. bordonbert

    bordonbert Member

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    Hi guys. Just added a cheap MG100HDFX to the other valvers for backup. It has a couple of VERY scratchy pots including the MV. The schematic shows this to be C5k. C is a Reverse Log pot! Looking at the feedback it may be so but I thought I would check if anyone can confirm that that is actually correct before replacing.

    Thanks.
     
  2. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor... Free Mars!
    A "reverse log"pot is the same as audio taper basically.
    But
    Cleaning the pots will usually fix the problem. You don't normally need to replace them.
     
  3. bordonbert

    bordonbert Member

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    Thanks for the reply AMS. I'm afraid I don't agree with your idea that Reverse Log is the same as Audio. As I'm sure you know, Audio is an approximation to true Logarithmic, (which in pots is really only an approximation in itself) and it often has two or three linear sections combined to give an increasing slope. Reverse Audio should do the opposite, give a steadily decreasing slope. Have a look at the plots I have attached. I pulled the pic from an article "The Secret Life Of Pots": http://www.geofex.com/article_folders/potsecrets/potscret.htm.

    I do agree that in most cases the pots only need a spray clean with something like Servisol 10. I will be trying that first but, I promise you, I have never known pots to be so crackly and unreliable. They are literally unusable with constant loss of connection until turned half way up. I wanted to confirm what I would need if they do turn out to be unrepairable. I hope you are spot on though. It would be nice for them to just clean up. Changing out the fan is easy and I'm doing that as a service task, fiddling with pots on the PCB is something I have had too much of for many years and I could do without.
     

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  4. bordonbert

    bordonbert Member

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    Fortunately for me, you were on the money with the pots only needing a clean AMS. I have had time to look into the schematic and worked a little on simple stuff to trim the sound. I'm going to come out and say it, I do like the overall sound of the amp despite the bad press it gets from ditw valveheads. Most important thing was I increased R88 to 2k7 to reduce the gain in the Clean channel Crunch mode to set it to a more usable level about half turn. It was basically OD3 at around 2 to start with. In doing that the pots which were crackly have now bedded themselves in again a lot. I strongly suspect this amp has just not been used for a long while which may be why I picked it up so very cheaply, (even by MG standards). I wouldn't have paid good money for it, only silly money, with a JMP SL100, JCM800 Splitch and JVM205H and others sitting at home. I only needed a small light backup for those occasions when the valves do what they do well, throw a hissy fit.

    I'm waiting for some new Servisol 10 to arrive, you can't just pick it up off the shelf local to me, which should finish the job. Despite this being an easy amp to work on I really couldn't be arsed to fiddle around with too many component changes at the moment. It's a relief to find that your thoughts were on the money and the pots won't need a massive change out even though a good few were disastrous to start with.

    Thanks for getting in there and replying, any chance to chat is a good one.
     

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