Beogram 4000: Repair of Cracked Hood Hinge

The Beogram 4000 that I recently finished up also had a cracked hood hinge. Sadly, this is a common failure due to the unfortunate way these hoods were designed in the hinge area. The use of countersunk screws in combination with a fairly heavy weight of the hood puts more stress on the plexiglass material than it can handle and often cracks develop over time. This shows how this hood presented:

Fortunately, only one piece broke off making a fairly clean fracture and the rest of the hinge was still in place, albeit with an additional crack between the two bolt holes. I carefully cut through the glue that held the aluminum strip in place using a razor blade. This allowed preserving the strip with little damage:

The next step was cleaning off all the old glue on both sides. This usually works well by soaking the area in isopropyl alcohol for an hour. Then the glue can be scraped off. In this case it worked pretty well:

Then I glued the broken off part back into place with plexiglass glue:

After cleaning up the glue spill out at the crack, I installed Beolover hinge patches. I developed a clamping mechanism that makes use of the two bolt holes to press the patches into the hinge area. The glue usually softens the hinge and the patches can be sunk slightly into the material by tightening the screws tightly:

This is how the outer patch looks like after the glue cured for 24 hrs, and after bolting the hinge back into the hood:

Since the aluminum strip is one of the early 'flimsy' types, the bolt holes need to be 'filled' to make a flush surface with the surrounding plexiglass. Otherwise, the aluminum strip could be damaged if pressure is applied to it from the outside:

Then I applied contact cement to both sides and pressed the aluminum strip back into place after letting the cement dry. Of course, one can see that the hinge has been repaired since the patches add a little to the thickness of the hinge area, and unfortunately, also some glue squirted out towards the front. This is hard to avoid since one wants to make sure the glue really covers the entire patch, i.e. it has to squirt out a bit to  make sure that this happens. But life is mostly a compromise, and so we need to accept that a repaired hood is not as pristine as a new one:

Like pretty much all Beogram 4000 hoods I encountered so far, this one also was missing its rubber bumpers at the front. They usually break off due to degradation of the original rubber material:

I usually carefully (it is easy to crack the thin Plexiglas around the hole) drill them out with a 2mm bit

and then install a 2mm O-ring snippet with some superglue gel:

After letting the gel cure, I cut the snippet to a 1 mm length using a template and a razor blade:

This hood is in business again. I hope it will hold up a while!

Beogram 4004 (5526): Repair of Cracked Hood Hinges

I recently received a Beogram 4004 from Florida for some TLC. The first order of business was the repair of a cracked plexiglass hood. My customer decided to have it fixed since it is in a pretty decent shape except the cracked area. As usual, the crack was around the area where the hinge gets bolted on:

Luckily this was a fairly clean break, and all the fragments were still present:

After I removed the hinge and the aluminum strip side parts it became evident that the other side of the hood also had the same issue, but the parts were still loosely together:

After removing all traces of the old contact cement that held the aluminum strip to the plexiglass, I glued the parts back together with Weld-On #4 acrylic solvent. It can be used if the parts still fit together very well, which allows capillary forces to suck the solvent into the cracks. Once the solvent is in, simply press the parts a bit together and hold for a few seconds. That is it:

On the other side I just bent the cracked off part a bit to enlarge the gap slightly, and that was enough to let the solvent in to mend the parts back together:

Then it was time to install my hinge patches. They are pressed into place with 3D printed clamps that allow the application of significant force on the patches. This basically 'immerses' them into the plexiglass of the hood, as is evident from the dissolved material that gets squeezed out on the fringes:

After letting everything harden for 24 hrs, I removed the clamps:

I drilled out the mounting holes and restored the flat head recesses with a counter sink. Then I was able to bolt the hinge back in:

The final step was to glue the aluminum strip sides back to the plexiglass with contact adhesive. this shows the left side of the hood:

And the right side. Unfortunately, a small amount of solvent was sucked under the blue tape that I used to protect the hood. This left a minor pattern on the plexiglass next to the aluminum strip. If this hood were polished it could probably be removed.

Finally, I replaced the worn rubber pumpers at the front of the hood. Their material degrades over the years. They can be replaced with a small piece of 2mm Viton O-ring. The first step is to drill out the remnants of the decayed material with a 2mm bit:

Then the O-ring bit can be glued in with super glue gel and trimmed to 1mm length:

Beolovely! This hood will close gain with a nice soft 'thump' when the bumpers hit the plinth.

Beogram 400x: Progress Report on the Repair of Cracked Hood Hinges

One sad way for a Beogram 4000/4002/4004 hood to go is that the plexiglass cracks around the mounting holes where the hinge is bolted to the hood. This area of the plexiglass is especially weak since it has to have holes for the hinge, while it also needs to take the highest stress when the hood is opened and closed. So it is no surprise that cracks develop over time, and finally the hood breaks loose from the hinge. 

I have tried a few different ways to fix such cracks over the years with mixed success. More recently, a promising process emerged from my efforts (see here for the original post, which also shows how to deal with the aluminum strip that covers the cracked part of the hinge).

In recent weeks I received a few inquiries for the patches that I used in the above post, which motivated me to come up with a 'hinge repair kit', which would further simplify this repair.

This post gives a progress report on the development of this kit:

For testing I dug out a hood that was damaged when a careless ebay seller sent me a 4002 in skimpy packaging. This shows one of the hinge areas with a 'nice' crack. I salvaged the aluminum strip prior to this experiment for another Beogram, i.e. if you want to see how to get the side parts off, check out the above initial post)

The first step to any successful patching is the removal of the glue remnants, which are always on the plexiglass after removing the aluminum strip:

This glue is soluble in isopropanol if it is soaked for about 20-30 min. I usually dip a wad of cut to size paper towel into isopropanol and then clamp it to the area with the glue:

If the aluminum trim is present on the hood one needs to do it a bit differently (see above post)...anyway, after 30 min the glue came completely off and it was time to try out the hinge kit:

From left to right, there are a clamping block for the inside with the two bolts already installed. Then there is the inside patch, the outside patch and the outside clamping block with the nuts.

The patches are composites of a 3D printed stencil (for precise cutting and hole punching) glued to the foil-coated PETG sheet with double sided adhesive tape. On the picture the stencil is on the bottom, and the top side is the exposed PETG side with a layer of protective foil.

The first step of the installation is to remove this protective foil:

Now the patches can be mounted on the hinge using the two clamping blocks. This shows the inside block and the inside patch:

On the outside, the other patch

and the block are bolted on using the screws and nuts:

At this point the screws should only be hand tight, to leave a small gap to allow the plexiglass glue tp penetrate via capillary forces between the patches and the cracked plexiglass area. Make sure that the outside patch lines up with the plexiglass (the holes in the patch are larger than the bolt, i.e. the bolt does not force it into the proper place like the inner patch)

The next step is to apply some Weld-On#4 glue, which is usually dispensed from a syringe bottle. It is a good idea to squeeze a bit of air out that the bottle has a bit of a vacuum. That makes it much easier to dose the glue between the to be fused parts.

This shows where to apply the glue at the boundary between the patches and the plexiglass:

Apply enough that some of it comes out at the bottom of the patches. It is a great idea to protect the exposed hood areas with a bag or similar. An accidental squirt of the glue can easily ruin the hood (if it happens, do not wipe the glue off. It will only make the damage much worse. Just let it evaporate and accept the fact that the hood needs to be polished now...;-)

After the glue has been applied, the screws need to be tightened well to ensure that the patches bond tightly to the plexiglass:

Weld-On recommends to let the parts harden for 24 hrs or longer for full strength, but after 2-3 hours the mounting blocks can be removed and we can have a look. After unscrewing the bolts the mounting blocks can be removed. The next step is the removal of the 3D printed stencils, which can simply be peeled off from the PETG surface:

and this is the result:

I tried to get the patches off, but it seems they are bonded very well. If you look closely, you can see how some plexiglass got squeezed out at the bottom of the patches, i.e. the glue did make it there. I recommend watching a few YouTube videos about glueing plexiglass sheets etc...it is pretty impressive how the capillary forces do the trick when simply applying a few drops of solvent to the boundary between the materials.

One more note: If the hinge is more deteriorated or if there are small fragments missing, it is better to use Weld-On #16, which has a more viscous consistency. That will be my next experiment with these patches...I am sure, I will soon have a Beogram on the bench with another cracked hood! Stay tuned.

Beogram 4002 (5513): Restoration of a Cracked Hood - Another Process Improvement

The Beogram 4002 that is on my bench right now came with a hood that was cracked around the holes where the hood hinge is attached. In this particular case the left side was completely broken out

and the right side had a crack all the way through around one of the attachment holes:

As usual, the first step was to 'save what could be saved', i.e. I glued the cracked parts back on using Weld-On #4 acrylic solvent. This solvent is extremely low viscosity and can be directly injected into a crack where it gets sucked in due to capillary forces. This shows the broken parts fused back together:

If done right, this step already creates a pretty strong bond between the broken parts. The solvent dissolves the plastic at the crack and after the solvent has evaporated the plastic is (in theory, at least) again a homogeneous part. But of course, nothing is as strong as the original cast material after is gets broken into pieces.

Therefore, the best way to finish this repair is to fuse thin patches onto the cracked areas. Unfortunately, these patches need to be very thin. On the inside due to the tight fit of the metal hinge part, and on the outside for cosmetic reasons since the patch needs to 'hide' underneath the aluminum strip that is glued onto the outside areas to hide the screws.

This time I made the patches manually myself from a 20 mil PETG sheet using 3D printed templates, scissors and a hole punch. PETG was necessary, since apparently there are no thin acrylic sheets available. PETG bonds well with acrylic since it is solvent compatible. This shows the hole punch in action on a cutout patch:

In the past I was not satisfied with my hood repair protocol since it was always difficult to clamp the patches properly to the hood all over their contact area. So this time I improved my method by using 3D printed clamping parts that could be directly squeezed together via bolts through the hinge holes. this shows all the used parts for this repair:

The metal parts are necessary to prevent the 3D printed PLA parts to bond with the plexiglass while the glue is squeezed out at the fringes. This shows the parts in action on the left side:

you can see some dissolved plexiglass and Weld-On #16 (higher viscosity since the glued back together hinges are a bit rough, so #4 does not work that well) being squeezed out on the sides. This is great since it essentially 'sinks' the patch a bit into the hinge bulk. This results in a lesser thickness increase, making the repair less intrusive for the optics of the outside of the hood. This shows the process for the right side:

After doing this for both the inside and outside patches (it is difficult to do both at the same time since the solvent bonds quickly) the squeezed out acrylic/PETG can be trimmed with a razor blade:

After letting it cure for 48 hours I bolted the hinge back in:

Once the bolts are in, the aluminum trim needs to be glued back on. I usually use industrial strength 3M 300LSE laminating tape. This is essentially a glue layer on a piece of wax paper. Difficult to handle, but it does not add any significant thickness to the laminated stack. After trimming the tape with a razor blade along the aluminum trim, I took the paper off and pressed the aluminum trim on. Then I fixated everything in place for a day with locking pliers:

I protected the aluminum trim with a thick piece of cardboard. The red part is one of my templates to mark up the PETG sheet for cutting out the outer patches. It distributes the force of the pliers. 

This shows the hinge areas after removing the clamps:

The patches are almost invisible! The 3D printed blocks really helped flattening the patched areas.

Now it was time to finish the hood up with a polishing workout. It had the usual scratches on the top surface:

This warranted an initial sanding step with 320 grit paper to equalize the surface. Then I polished it back in several steps with ever finer grit paper. This shows the surface after the 400 grit wet polishing step:

This is how it looked after the final step:

Pretty shiny, but of course not perfect. Polishing can only do so much, and small imperfections can be seen under certain lighting conditions. But for the usual presentation of the Beogram on a sideboard or similar, this looks pretty newish! So far so good! The final act on this hood was to install new rubber bumpers to ensure that the hood is level when closed. Rubber bumpers also ensure a good sound signature when the hood meets the plinth. This shows a degraded bumper:

I drilled them out with a 2 mm bit:

Then I superglued sections of a 2 mm O-ring into the cavities:

After the glue had hardened, I cut them to length using a 3D printed template:

They need to be about 1 mm in length that the hood is horizontal when closed:

And that was it! This hood is back in business!