Has this 1987X been modded?

Discussion in 'The Workbench' started by Michael Roe, Feb 16, 2020.

  1. Michael Roe

    Michael Roe Well-Known Member

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    I bought this head a few months ago from a local GC.
    I haven't had time to really give it a good play through.
    I did a few times in my small room but it was so loud there was no way to verify it sounded proper.
    I have used it as a power amp with my JMP-1 and the power amp section sounded fine.
    Today I decided to run it with a load box and profile the amp with my Kemper.
    I also have a SV20H that I have profiled recently. So, in theory it should sound some what like the SV but probably a bit more bigger.
    This one does not sound anything like the SV. My SV on the High treble channel has much more bite/Gain and treble to it. In other words, this one has less gain and treble than the SV.
    I swapped in the same exact tubes to see if maybe that could be the difference in sound. No, it still sounds way different. So, I opened it up to take a look and uploaded a pic for you guys to see.
    I can see some areas on the board that look like parts are missing:
    C5, C7, R18 and C2 looks a little suspect.
    What do you guys think is going on here?

    1987x Board.jpg
     
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  2. mickeydg5

    mickeydg5 Well-Known Member

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    The missing C5 is going to drop a lot of high and middle out when at lower volumes.
     
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  3. ampmadscientist

    ampmadscientist Well-Known Member

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    The hum balance hasn't exploded yet?
     
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  4. neikeel

    neikeel Well-Known Member

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    Can you read the values of the Rk and Ck of V1.
    As mentioned the missing cap is the cathode bypass cap of V2a. It was used 68 to 71ish. When installed it sounds best with more NFB than you have (47k/8 vs 100k/4) and with earlier presence (5k/0.1uF vs 25k/0.68uF)
    The 0.022uF cap on V1b is a 1987x feature which in theory makes channel 1 fatter but at the expense of cut in a full band situation.
    Oh and as seems to be traditional now the 4n7 cap on channel 1 pot has been removed.
    Having said that I suspect V1 capacitor skullduggery is more of the issue (and because of the loop board can’t see what is going on at V1 socket to be sure.
     
    Last edited: Feb 17, 2020
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  5. Michael Roe

    Michael Roe Well-Known Member

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    I don't feel confident enough to go poking around inside as of yet.
    So, can I assume that it definitely has been modded?
    I would like to have it put back to factory specs.
    I have a few good techs in my area I am sure could do it.
    Would this be a very expensive repair job? How many Bench hours should it take?
     
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  6. Michael Roe

    Michael Roe Well-Known Member

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    What???
     
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  7. neikeel

    neikeel Well-Known Member

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    I guess an hour, maybe and a half.
    You would need to check values on board, flip it up (pots and jacks loosened completely) desolder and replace then test.
    You should decide whether you want straight 1987x (and which year) or pick a plexi spec (my choice is 1968/69 spec)
    Oh and edit:
    The cheap plastic hum balance pots can’t always cope with heater current and have been known to melt.
     
    Last edited: Feb 17, 2020
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  8. headcrash

    headcrash Active Member

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    The only thing that looks "intenionally modded" to me (and I am not as familiar with the Plexi-type boards and circuits, let alone the varouis incarnations in the past), is the infamous bright cap across the treble channel's volume pot. According to the schematic floating around, it is 4n7.
    C7 is not fitted, according to that schematic. I once installed a switch to a friends 1987 reissue, to let him switch C7 in and out. Basically it adds a good amount of gain, maybe enough for a lead boost.
     
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  9. Michael Roe

    Michael Roe Well-Known Member

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    Thank you for your response. It has been very helpful! So, it would be a good idea to have the tech replace the hum balance pot with a higher end model while they are in there? I would assume that the bias pot is ok since they are dealing with a lower amperage? Or should I have that replaced with a higher quality as well?
    This is a 2016 model. I think I would rather just have it back to stock factory. I'm not looking to hot rod it any but just a simple pure plexi circuit.
    Just for fun, I ran the preamp out of my SV into the 87X's return and it sounded really nice! Even better than the SV. Bigger sounding which it should with the bigger transformers.
    The main reason I bought it was because previously I got a Weber Mass 200 attenuator so I figured if I could find an 1987X for a good price I would just replace the SV.
     
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  10. ampmadscientist

    ampmadscientist Well-Known Member

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    [​IMG]
     
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  11. Michael Roe

    Michael Roe Well-Known Member

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    Oh, not good!
     
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  12. neikeel

    neikeel Well-Known Member

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    The most reliable method is to remove the pot and install a couple of 100R 2w metal oxide (flame proof) resistors instead of the pot and create a virtual centre tap. Any decent tech can do this for you.
    The question re spec rises because a 1987x is not really to a plexi spec. More like early 70s metal panel. I also see odd specs (some have big 0.1uF output couplers like a JTM45 or bass circuit combined with a split cathode lead front end. So bit of a dogs breakfast.
     
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  13. Chris-in-LA

    Chris-in-LA Well-Known Member

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    The hum balance pot on my 2004 1987x hasn’t exploded yet!
     
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  14. neikeel

    neikeel Well-Known Member

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    Is yours an EL34 or 5881 version?
     
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  15. Michael Roe

    Michael Roe Well-Known Member

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    Mine is an EL34 version. I didn't even know there were 5881 versions.
     
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  16. NewReligion

    NewReligion Well-Known Member VIP Member

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    There is a very easy bypass mod posted on the net for the Hum Balance pot.

    More importantly I can’t believe everyone missed ALL of the Mallory 150 Capacitors which have been installed as an apparent upgrade in addition to a couple silver mica’s.

    I am sure there is more but that was just at a glance.

    Nice amp. I have had several of the Marshall’s with the effects loops on my bench. To date 5 had exploded Hum Balance trim pots.

    I caution that the huge fire proof 47k 10 watt resistor next to the bias trim pot drops some 400+ VDC down to 30VDC in order to power the loop.

    This fire proof resistor gets nearly as hot as an output tube. Yes, I suspect this can cause issues in individual amps. In addition the Presence NFB circuit is a little different.

    Presence should be a 5k but I have seen 22k & 25k stock from Marshall on the later model 1987/1959.

    If you do have it serviced I suggest ask the tech to raise the PCB to inspect for any damaged traces and repair as so many components have been changed.

    One last thing. As EL34’s are more prone to oscillation I would add a 5.6k grid resistor to pin 5 on each EL34 socket to help prevent future issues. Really not sure why Marshall omits this practice on its 50 watt models.

    Best Regards, David
     
    Last edited: Feb 20, 2020
  17. neikeel

    neikeel Well-Known Member

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    The Mallorys are all to original values so I did not see any point in worrying about them as they would stay even going back to stock (assuming solder joints are all up to snuff).
    I think the funkiness is around v1 cathodes and putting those back to stock will be helpful.
     
  18. NewReligion

    NewReligion Well-Known Member VIP Member

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    Right I was pointing out the obvious mods.

    Do we know the values of the non stock Silver Mica’s as often they can be trash right out of the bag.

    And let’s not forget to mention how important the output transformer is to tone and the amps individuality.
     

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