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Friday, April 28, 2017

Beogram 4000: Repair of a Cracked Plexiglass Hood With Laser Cut Patches

The most fragile part of a plexiglass hood of any Beogram is the hinge region. That is where the highest torque is applied to the plexiglass, while it is structurally weakest due to the holes permitting the screws that hold the metal hinge to the hood. The Beogram 4000 that I recently restored had a hood with a large crack through the holes and a broken out piece:
Encouraged by my recent restoration effort on a Beogram 3000 hinge that was broken off I decided to develop a similar fix for the 4000/4002 hoods. Unfortunately, the space one can work with is much less in 400x hoods, since the metal hinge part is bolted to the inside of the hood, i.e. if the plexiglass gets too thick, the metal part does not fit anymore. So my approach here is to use two very thin 20 mil (~0.5 mm) patches that are precision matched to the bolt holes and the hood shape.
But the first step is to try to glue the cracked parts and re-connect the broken out part:
I use Weld-On #4 acrylic glue, which is so liquid that is is drawn into cracks by capillary forces. This is a great feature since one does not have to bend the plexiglass any further to apply it. All one does is squirt a bit on the crack and one can see how it is sucked in. One can then simply press the parts a bit together for ~5 min (play some nice music while you sit there holding the pieces together...;-) and then the next step can be done, which in this case was reattaching the broken out part:
Another 5 min of music listening and then I let it cure for a couple hours. The next step was to attach the laser cut patches. This shows the inner patch:
I used Weld-On #16 for these, since the surface was pretty rough due to the broken out part and previous 'repair' attempts. After curing the patch for ~2 hours (=50% strength of the bond) I did the same for the outer patch:
At this point I let everything cure for 24 hrs to achieve a close to final strength of the bond (~80%). Only after a few days such bonds are fully cured and 100% strength is achieved.
In order to have the metal hinge centered, I also applied an inner patch on the opposite side of the hood. This was a good idea anyway since closer inspection yielded that there already were some minor cracks under development around the screw holes.

Then it was time to bolt the metal hinge in with new stainless steel screws:
The next step was to reattach the aluminum trim to cover the screws. This requires removal of the old glue, which can be done with a razor blade and careful scraping. Take care that the aluminum trim is not bent too much or otherwise damaged. These side parts can break off easily if the bend is flexed too often. I usually use an industrial strength ultra-thin adhesive transfer tape (3M 300LSE) to glue them back on. This allows to generate a very thin and precise adhesive layer, which makes a strong bond. 'Thin' was especially important here to minimize the thickness increase due to the outer patch:
After removal of the tape backing, I pressed the trim on and cleaned some glue splotches from previous repair attempts on the metal surface with Mr. Clean Magic Eraser as good as possible:
This is how this side looks from the front:
It looks pretty good, albeit closer inspection of course will reveal that there is a thin patch underneath. The price for a stabilized hood hinge!

One more thing: Like most Beograms of this vintage, the rubber hood bumpers were broken off:
As usual, I drilled them out with a 2 mm drill bit:
and then glued in snippets of a 2 mm O-ring:
Using a 1 mm template, I cut them to size with a razor blade once the super glue cured:
And here is the end result:
The final step was to polish this hood since it had the usual scratches after about 40 hears in service:
I gave it my usual multi-step polishing procedure that started with 200 grit dry sand paper and then worked it back to a wet 3000 grit level through about 10 sanding steps. I had to be a bit more careful than usual due to the repaired crack. I definitely did not want to break it again...;-). This is how it looks now (still has some black protective tape on the trim):
Time to install it again on this lovely Beogram 4000!






3 comments:

  1. When I ship my Beogram 6000, it will be without the plexiglass hood. Yesterday, I started with installing the new transport lock bushings. Not that easy without dismantling all the internals! I neighbour came over and help me with the one at the front. Still have the one in the righthand corner to do. Most of the cabinet guiding washers were okay, only the one in front needed to be changed. Of course, I replaced the other ones too. Next up is the platter bearing. A friend will make a different one, with a better guiding sideways at the bottom of the bearing. I have also collected a lot of passive components, to replace all electrolytics. The trimpots may also need to replaced as well as some resistors. For the rest, it will probably be up to an expert like Beolover. Checking the AC-motor, the arm transport etc.

    Greetings from Sweden

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  2. It is difficult to ship a Beogram without hood attached...you would need to 'build' something that protects the arms etc...Shipping with hood is generally best, if you put some cut to size foam under it...good luck with your restoration!

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  3. Of course, I intend to cut a piece of foam that holds everything in place. I have an oversized cardboard box, will probably line it with pieces of OSB and put the turntable in the center with foam around it (on the sides, bottom and on topp).

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Comments and suggestions are welcome!