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Tuesday, October 23, 2018

Texas Beomaster 8000: Sorting out problems with the microcomputer board

In the blog post where I checked out the Beomaster 8000 power supply I used a spare microcomputer board to run the Beomaster. The reason for that was because the original microcomputer board was not in good shape. During the recapping of that board I could see there were problems with it.

In this post I dig into the original microcomputer board with the goal of cleaning up bad solder joints, investigating some unusual board wiring and making sure the two processor devices are working.

The starting point of this task is with the recapped microcomputer board.

On the trace side the picture below shows this microcomputer board compared with a typical Beomaster 8000 board...in this case the microcomputer board from the Canada Beomaster 8000 unit.

The orange and green connection wires are what I expect to see on these boards. The other wires on the Texas Beomaster 8000 microcomputer board are unusual.

Checking them against a known good board I found that they do connect points that are supposed to be connected. The question is why and if they are necessary.  I started by removing one end of each jumper wire and testing if the connection was still good. In some cases it was so the wire really wasn't necessary. In several wires however the jumper wire was the only thing making the connection.

My suspicion is that board vias (connections between the top and bottom board layers) are not making contact.

Some of the jumper wires appear to have been made as a change to the normal wiring between vias on the component side.

One task on this board restoration is to replace the two crystal oscillators as I did on the Beomaster 8000 from Canada. Starting with that task will give me better access to board vias I want to resolder.

The following pictures show the removal of the two processor devices and the replacement of the crystal oscillator devices. The same care was taken regarding static discharge and the shorting of the oscillator leads while handling the processor devices.

When removing the processor chips I felt the sockets were showing their age so I removed the original sockets and plan to replaced them with modern, tulip style sockets. The pins are sturdier. This picture shows the old sockets (in the rear) and new sockets in the foreground.

While the processor devices and sockets were removed I installed the new crystal oscillators and added three jumper wires (blue, green, orange) between board vias as shown below. That is the normal way the Beomaster 8000 microcomputer board makes those connections. That eliminated three wires that were on the trace side of the board.

I also resoldered the vias involved in all of those connections.

After this first round of work I was left with the following...The green and orange jumpers that should be on the trace side plus three additional jumpers that I found I could not remove yet.
Note that the green and blue jumper wires were white wires earlier. I changed them when I started cleaning up bad solder connections. I did the same thing with the original yellow jumper wire.
I changed it to the white in the photo below.

Testing this state of the board resulted in a partially working Beomaster.

The standby LED dot illuminated and the keypad selection switch put the Beomaster in the power on mode. However the source selection did not appear to function and some displays were not working.
I swapped the two processor devices with some spares I had to make sure the processor chips were not the problem. It doesn't appear to be an issue with those two devices but I need to make some additional checks.

My prime suspect at this point is still connections between board layers. I want to see why those remaining three jumper wires have to be on the trace side.

For the green and blue wires (from two vias next to one of the oscillators) ... to IC7 I discovered that the board vias were the problem.

I removed IC7 to get to the component side of those vias and resoldered them there. I also resoldered the vias on the trace side. That resulted in those connections being good without the jumper wires.

Here is the original state of the microcomputer board trace side compared to the current state.
Note again that I changed the yellow jumper wire to a new, white wire when I cleaned up the ugly solder joints for that.

I am left with just the white wire jumper. That jumper is connecting the board ground between two devices (IC5 and IC6). With the wire removed those two points are not making a good connection.  The wire shouldn't be necessary so there is another via or two I need to check.

In this current state the microcomputer board still isn't functioning correctly. I can control quite a bit of the functions. The volume and tuning dials operate. The channel balance control works as do filter and tone control switches (for the three illuminated lamps below).

There are a couple of things I need to do next.

I want to put the original processor devices in one of my good spare boards to be 100% sure those devices are good.

I want to try and eliminate that grounding layer connection the white jumper wire is trying to do.

If finding and fixing the problem (that requires the white jumper wire) doesn't result in a fully functional microcomputer board then I will have to move forward on this project with one of my spare microcomputer boards and return to this problem board at a later date.

This microcomputer board is the only component in this Beomaster 8000 that is keeping it from moving to functional testing.

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