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vintage amp mod I may remove

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  • vintage amp mod I may remove

    I have 2 1980's solid state Marlboro Sound Works 1200R amplifiers. I got one a couple years ago, and wanting to try a bit of a stereo rig, I got another identical amp this past week. The one I just got is quite a bit louder on the same settings as the older one. I opened the new one up and discovered a large capacitor had been added to the underside of the circuit you see in the photos. I'm wondering if this cap is what accounts for the beefier volume. I can see no other modifications other than the added 1000uf35v capacitor. I thought maybe the 1st amp I got was just tired and weak....but I think the "new" amp has been hot-rodded a little. Anybody have any ideas what that capacitor is up to in there? I circled in red where the cap is connected, to the board and to the chassis. Can see no other differences between the two amps. Not sure if I want to remove the cap and have the amp sound closer to the other....or add a similar cap to the older amp and beef it up too. I kinda like the older "weaker" sounding one better maybe. Does anybody know if I can snip that big cap outta there, just by looking at the pix? I want both amps to be the same for the stereo rig. (though two different amps can sound good also). The one amp is not only quite a bit louder but has a slightly different tonal quality. Thanks for any help on this. Here are the pix:
    (the reason I know the cap was added to the one and not taken out of the other is because the cardboard circuit board covering cannot possibly fit on, at all, with that big ol' capacitor there.)

    Click image for larger version  Name:	a.JPG Views:	0 Size:	451.9 KB ID:	933644 Click image for larger version  Name:	b.JPG Views:	0 Size:	487.9 KB ID:	933639 Click image for larger version  Name:	c.JPG Views:	0 Size:	399.1 KB ID:	933638 Click image for larger version  Name:	d.JPG Views:	0 Size:	316.7 KB ID:	933640 Click image for larger version  Name:	e.JPG Views:	0 Size:	427.5 KB ID:	933642 Click image for larger version  Name:	eb.jpg Views:	0 Size:	428.1 KB ID:	933643 Click image for larger version  Name:	f.JPG Views:	0 Size:	257.5 KB ID:	933641
    Last edited by kovichcaster; 06-03-2021, 02:40 AM.

  • #2
    That extra cap is a power supply filter cap. My guess is that rather than replacing a bad one, someone just tacked/paralleled another one on the back side of the board. In other words, it's not a mod, but a repair (most likely). Removing it would probably only cause hum since it would negate the repair. I see another axial cap tacked in on amp 2 where a radial goes on your amp 1, which is probably fine but it tells me there are cap problems. My bet would be that coupling caps in the quieter amp have started to fail. It would probably be a good idea to check all of the electrolytic caps in both amps. A quick ESR test of all electro's in circuit would probably guide you to the problem(s).
    Last edited by The Dude; 06-03-2021, 02:54 AM.
    "Yeah, well, you know, that's just, like, your opinion, man."


    • #3
      Click image for larger version  Name:	Amp #1.jpg Views:	0 Size:	398.7 KB ID:	933657 Click image for larger version  Name:	Amp #2.jpg Views:	0 Size:	183.5 KB ID:	933658

      Compare the layout of Amp # 1 and Amp # 2 and you will see that they are not the same
      To get started red arrows on Amp # 2

      Maybe to help in part

      Attached Files
      Last edited by vintagekiki; 06-03-2021, 08:50 AM.
      It's All Over Now


      • #4
        Thanks Dude. Do you mean the added blue cap is taking the place of one of the big, black caps on reverse side of board? Sorry, don't know my axial from my radial. I still can't spot where you mean another axial cap is tacked in. I know very little about amp electronics. I replace tubes and speakers and solder obviously loose wires and points....replace pots, things like that. I need to take the amps in to a good tech and have them gone over. The only guy I know around here, that really knows old amps, has retired for health reasons. Have to hunt down someone else or drive into Ohio....which kinda sucks. Anyway thanks for taking a look. I'll tell whatever amp doctor I find what you thought. later. kk


        • #5
          vintagekiki, thanks for the response. I don't read schematics myself, but it's good to have them for whoever I get to work on amp...thanks. I'm not sure I get what you mean about the amps not being the same.....or the red arrows. What, exactly, is different between the two amps? I checked all the wiring. It all seems to come from and go to the same places....same components etc. ???


          • #6
            The row arrows in figure AMP#2 shows which components AMP#2 does not have, and AMP#1 has.
            That is why they different from each other.
            It's All Over Now


            • #7
              I see what you're saying now with the red arrows. It does look different. I wonder what that's about....changing/removing components. I need to find a good amp-man to unravel and remedy this. I would like the amps to be pretty much identical. We had a great guy around here for years who knew vintage amps etc. up and down. Unfortunately, health issues had him close up shop. Probably road-tripping to Cleveland area is my only option now. Anyway thanks again for the input. all the best.


              • #8
                I believe the 2 amps started out the same. Somebody has obviously been inside amp #2. Some of the differences look like poorly done repairs. Some may be mods.
                "Yeah, well, you know, that's just, like, your opinion, man."