Frankencab needs help. Suggestions?

Discussion in 'Cabinets & Speakers' started by FogDweller, Jul 18, 2019.

  1. FogDweller

    FogDweller Member

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    4152D0E5-4E2E-4D01-BAD6-29D9F84F48C1.jpeg E64EC58B-E0B5-4C22-8B47-2A93E2C57F33.jpeg I bought this used JCM 900 cab that was modified with a birch plywood back and was ported. ☹️ The input jack was also replaced but it doesn’t bother me.

    -Because I don’t want the port, my original thought was to somehow plug/fill the port and tolex the back.

    -I’m now thinking that it would be easier to just replace the back with particle board or MDF and tolex it.

    Any thoughts? Also, did the JCM 900 cabs have MDF or particle board backs?

    In any case, I plan to install the 2x2-inch support (whatever that’s called) that was missing from the cab.
     
  2. ricksconnected

    ricksconnected Well-Known Member

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    tolex the birch back and roll with it.
     
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  3. GIBSON67

    GIBSON67 Well-Known Member

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    You could just cut a new board and paint it black...
     
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  4. FogDweller

    FogDweller Member

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    True. But I do want tolex on it, and applying tolex doesn’t bother me really. Although I like the birch plywood, it’s going to require a little bit of work to fill that hole.
     
  5. Geeze

    Geeze Well-Known Member

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    I'd replace it with another piece of BB or similar plywood - I'm not sure 1/2" is sufficient - I use 3/4" and never needed a post even running 1959/2203's dimed.

    Russ
     
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  6. FogDweller

    FogDweller Member

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    Good advice. I may just do that. Filling that port is going to be a bit of a pain I think. Also good to know you didn’t need the post. Are the stock Marshall backs a 1/2”?
     
  7. Geeze

    Geeze Well-Known Member

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    I've never had one apart so I don't know for sure. I've heard the metric equivalent of 5/8" ish. I always wondered if the post was more to support the baffle than the back.

    Russ
     
  8. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    Wait up. How thick is that panel?

    It does not look thick enough for a 4x12 closed back.
     
  9. FogDweller

    FogDweller Member

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    I think it’s a 1/2”, but I’ll measure it tomorrow and let you know.
     
  10. fifteenohms

    fifteenohms Well-Known Member

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    I would patch it by cutting a plug from scrap wood of any kind of curb-found wood, four cleats strips (like free paint stick stirrers) and a dozen drywall screws. Plus a can of rubberized Black Bedliner Spray, Bondo, 99 cent gloss black & flat black spray paints.

    First, remove back & pull hardware, then transfer hole outline to donor plug board by spraying flat black through port hole.

    Cut plug shape from board, then file or sand edges to fit backboard orifice....

    Uhh... uhuh hihuhuhuhhhuhhh!!

    So, cut two vertical cleats for each outer side of the hole, secured with druwall screws, to level the plug against the outside as the plug has yellow wood glue (or epoxy if want even better) applied to the edges and seated from the inside - until flush with the outside.

    Let dry one day, then remove cleat screws and sticks. These arent glued, you remove them and since theyll crack or splinter w removal, use two fresh ones and this time you DO glue them, right across the horizontal seam and secured with drywall screws.

    Let dry 4 hours and remove screws, scrape and sand smooth plug outer side.

    Now theres 2 choices going forward: skim-coat with Bondo across the entire patch and sand flush, or just sand smooth without Bondo: either because you dont have $10 bucks, or just lazy.

    Still either way if painted at this stage, the hole shadow will show, so to hide it better without Bondo skinned over it, you can brush a few coats of more yellow glue over it plus 2" or more past the seam outer edge. Let dry 1 day.

    Here now you can paint the bare wood:

    Spray a full wet coat of the cheap Depot gloss black in one direction, then 20 mins later another full wet coat 90* degrees criss-crossing the earlier pattern for full coverage. Dry 1 hour. Then repeat in flat black.

    Dry 2 days then give it two similar coats in bedliner. Dry 2 more days before reassembly.

    It will look original like tolex, but not exactly.

    If the board is shy of thickness needed to flush up to the depth of the original backboard, just shim the edges of thecabinet shell with cardboard strips and masking tape.

    You only need a center post for added support of weak particle backboards. I think it sounds best without the post - fewer phase anomalys IMO.
     
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  11. fifteenohms

    fifteenohms Well-Known Member

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    The reason to cover bare wood first with gloss is: higher plasticizers for better sealing of wood grain.

    After its sealed and dry, you can coat it either way, flat black or bedliner.

    You dont have to do both. But this is a good solution for patching holes and recovering rashed-out, peeled or scrored-up tolex or even complete holes.
     
  12. fifteenohms

    fifteenohms Well-Known Member

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    And after glue dries of course, I repeat, pull out the drywall screws.

    If the cleats are glued tight across two edges of the patch, retaining the screws isnt necessary as they may rattle loose.
     
  13. BygoneTones

    BygoneTones Well-Known Member

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    The easiest solution is to sell it and replace it with another in original condition. Unless the reason you bought it is because you like doing woodwork and wanted a project? If you only found out about the mods when your received it, I would be thinking about sending it back to the seller and getting my money back. Thats a lot of work to convert it back to original spec.
     
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  14. FogDweller

    FogDweller Member

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    Thanks for the thorough tutorial, I appreciate it! This is likely the route I’ll go.
     
  15. fifteenohms

    fifteenohms Well-Known Member

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    If you spray Bedliner straight onto bare unsealed wood after the patch, it tends to grind or flake off easily.

    Ive done about 3 backboards this way converting stereo panels to single-input jack plate.
     
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  16. fifteenohms

    fifteenohms Well-Known Member

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    Or he could pull the old "switcheroo" with a cheaper $50-75 Park g412-A when one comes up, or just buy a beater 1960 for $75-150 and sell off parts you dont want.

    Ive done that, too
     
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  17. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    I would not use 1/2" for a back panel in a 4x12 unless it is has gussets and stiffeners up the wazoo.

    What is the normal back panel thickness (wood only) for a 4x12 Marshall cabinet?
     
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  18. fifteenohms

    fifteenohms Well-Known Member

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    The trouble is, we want the look to he original but the whatever-it-is Metric prevailing thickness for this Baltic Birch ship-building plywood is not the prevailing common inches-feet standard dimensional thickness of any plywood stock over here.

    In the Westside L.A. here we have high end plywood dealers like Anderson who stock this stuff for Architectural builders and custom cabinet guys who can pay.

    But you wont find 9-ply birch or whatever it is, 11.5mm particle board sheet, for the cheap at home and lumber stores.

    So if you can find a free sheet of common 1/2" plywood pine cutoff and maybe a buddy with a table saw, just size it for use and cut shim strip for the cab screw-ledge inset.

    Its thinner than stock but so what? Internal pressure is retained.

    Later on as you keep your eyes open for another cheap one second-hand, swap the homemade back for the real back, and just resell it.
     
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  19. FogDweller

    FogDweller Member

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    The current panel is only 1/2 inches, so I have decided not to fill the port.

    Instead, I’m ordering a 5/8” tolexed back panel from Sourmash. They quoted me a price of $60 plus shipping, which is extremely reasonable. If I were to buy the birch ply from Home Depot, it would cost me $55 (they only seem to sell 4’x8’ pieces), another $30 or so for the tolex, and the hassle. I bought a 2x12 can from Sourmash and was thoroughly impressed.

    Thanks for everyone’s advice, I appreciate it very much. You’ve been very helpful.
     
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  20. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    Make sure the center brace is there and has solid contact with both front and back panels.

    11.5mm = 7/16 = standard 1/2" board thickness or about that and
    I myself like 3/4" standard board thickness = 11/16" = 17.5mm or about that for a 4x12 cabinet
    or similar size
     
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