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Saturday, March 11, 2017

Beomaster 8000: Microcomputer Board Reworked

Today was the day to work on the Beomaster 8000 microcomputer board. On an earlier post I noted that this Beomaster is an earlier model unit so it has the first generation Beomaster 8000 microcomputer board. The later model boards are direct replaceable but are definitely different. I will show an example of the late model microcomputer board at the end of this post.

One thing I don't like about these first generation boards is the metal enclosure around the processor ICs. It is necessary for shielding of course but these early boards are difficult to open up. B&O soldered metal tabs to connect the upper and lower sections of the metal shielding box. The problem is the tabs also solder to the ground plane of the the microcomputer board. Because tabs are soldered to the ground plane a lot of heat is required to melt the solder for removal. When people try to open the shield box up to get to the components they usually damage the traces on the board where the shield box mounts to it. This Beomaster has been serviced before and falls into the category of having damage to the mounting points.

The following picture shows the two different Beomaster 8000 microcomputer boards. The board for this project is the bottom board. Notice that the metal shield box on the top board is pressed to fit while the bottom board is soldered together (red circles show some of the solder locations).

Here is the microcomputer board for this project as it was before restoration.

Here are the covers removed.

The early model boards used a metal bar for a heatsink on the two main processor ICs (IC3 & IC4).
The places circled in red show the damaged places on the board where the mounting tabs for the case originally were. The last person to work on this unit was able to re-seal the box with solder but I won't want to do it that way again as I don't trust the grounding with those missing tabs.

This is similar to what B&O used in the metal shield box for the Beogram 8000 and 8002 turntable processors. A strip of special tape is used to keep the messy thermal paste off the ICs themselves.
Here is the component side of the board with the thermal tape removed. The restoration task will be to replace the 22uF electrolytic capacitor to a 105°C, high reliability type. Also the 1uF tantalum. Following Beolover's lead I will also replace the two oscillators for the processor ICs (X1 & X2). Those will require changing two capacitors on each to new ones that match the new oscillators (18pF). Last, I will also reflow the solder joints on all of the connects and board vias as those have been known to have hidden problems.

For the oscillator capacitors, this original board has two 12pF capacitors for each oscillator. One oscillator has its two 12pF capacitors on the component side. The other oscillator has them on the trace side. 

The later model microcomputer boards have mounting holes for the oscillator capacitors on the component side.

Here is the microcomputer board with its updated components.

CAUTION: As Beolover noted in his Beomaster 8000 restoration, messing with the crystal oscillators could be a risk to the two processor ICs (IC3 & IC4). As a safety measure I shorted the two oscillator leads together and removed the processor ICs from their sockets (using an ESD grounding strap of course). With IC3 and IC4 safely out of the way I performed all of the rework to the board.

After doing the tedious solder reflow work it is time to put this board back together. I re-inserted IC3 and IC4, then prepared the heatsink thermal tape and compound so I could remount the top and bottom covers.

Now to solve the broken ground points where the metal covers mount. I have solved this problem before by using copper tape the way metal bands are wrapped around crates for shipping. The copper bands fit perfectly in the board slots where the mounting tabs go. I solder the bands together on the top and bottom of the metal box. I also solder the copper bands to the metal box so the grounding will work as it is supposed to. To tie the box to the board ground planes I run a copper tape strap to the main ground lug (both on the top and bottom side of the board).

The result is not the prettiest thing (rather ugly really) but it is solid and works. The copper straps securely keep the box together and I can measure good ground continuity anywhere on the board. If I have to do this type of repair again I think I will apply strips of copper to the lid pieces first (to use as solder pad anchors if you will), then tie the bands in with those anchors.

Removing the assembly should be pretty easy.  I didn't solder the copper straps on the sides where the top and bottom pieces meet. To open this box up again I only have cut the straps along the sides, de-solder the two main ground straps and the box will open up. In the picture below I marked the cut points with the red arrows. Note the green arrows. Those show the previous repair where someone fixed the broken ground connection by scraping away some board coating at the ground plane and soldered that right to the metal housing. While that should work I think it is risky because it is susceptible to the solder joint cracking if the metal box is stressed. I like the straps better because they provide a much more reliable connection to the ground plane of the board. The grounding strap also has a little give in it so it can withstand any movement.

Now it is time to test this updated board in the Beomaster 8000. 
I am happy to say that works great. Now it needs exercising for a while to make sure there aren't any hidden problems.

For reference - Here is a Beomaster 8000 later model microcomputer board I restored earlier.

These cover plates are easy to remove as they just press-to-fit. Not the rectangular tabs in the top cover plate. Those make contact with the two main processor ICs (IC3 & IC4) to transfer heat away from the ICs and to the metal shield box for disapation.

Here is the component side (restored). Both sets of 18pF capacitors for the new crystal oscillators are on the component side of this board.  Also notice that this board has a frame for the top and bottom covers permanently mounted to the board. This frame provides all of the structure. There is no strain on the board layers and no soldering of the shield box. Way nicer for servicing.

Here is the trace side of the board. All I did on this side was reflow solder joints.

Now I need to get back to the assembly of those Beomaster 8000 display modules so I can complete the last board.

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