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Tuesday, October 23, 2018

Texas Beomaster 8000: Microcomputer board fixed

Taking a little break before revisiting the Beomaster 8000 microcomputer board problem resulted in success. I believe this board is functioning properly again.

Two things I wanted to try at the end of my last post were to confirm the two processor devices were good and to try and do away with the extra jumper wire.

I moved the two processor devices (IC3 and IC4) out of this Beomaster's microcomputer board and into one of my good spare boards. Reinserting that board into the Beomaster confirmed the board functioned correctly so the IC3 and IC4 devices are good.  I don't think I have come across a Beomaster 8000 yet that has had any bad processor ICs.

For the extra jumper wire on the trace side, I removed it and resoldered all of the related solder joints. When I was done I confirmed that the grounds on IC5 and IC6 were the same. No need for the jumper wire.

The next step was to try the updated microcomputer board in the Beomaster again. The result was the same.






















I wasn't really expecting the board to be working but I had to try.  IC5 and IC6 didn't make sense as being the cause of the problem above.

Studying the result I realized that everything worked except for the program display segments and the first seven-segment display of the volume indicator.  I started looking for what area of the microcomputer board only affects those specific things. The schematic showed me exactly what I was looking for.






































As you can see per the dotted red line, the IC3 processor enables those specific display segments using IC3 pin 17 (PHASE1). Since known good boards work I knew the problem had to be from the microcomputer board P74 connector pin 4 back to IC3 pin 17.

Checking that path and the components (C31, R30) revealed nothing. Continuity was there. There were no shorts to other paths. C31 and R30 both measured correct values.

The only thing left to do was to desolder that path (including P74) and resolder it. That turned out to solve the problem. When I reinstalled the microcomputer board (for the tenth time) the Beomaster display came to life the way it did with the known good microcomputer boards.
























Here is what the (hopefully) final version of the restored microcomputer board looks like. It looks like a typical Beomaster 8000 microcomputer board.






































What a relief. I haven't had a Beomaster 8000 microcomputer board with this type of problem before.
Now I can proceed to the functional testing with this unit.

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.







Thursday, October 18, 2018

Canada Beomaster 8000: Display Board Segment LED Restoration Part 1

The last board to complete in the restoration of this Beomaster 8000 project is the display board. With power supplied to the Beomaster I could see there were some LED segments that were not illuminating. To really if all of the display segments work or if there is a bad connection I pull the display modules and place them on my home built test jig. Using a bench power supply and the test jig I can turn all of the display segments on at the same time. This will reveal any faulty segments.

Here is the display board before removing the display modules.





















The highlighted, dotted lines mark the display module leads that need to be desoldered for removal of the display modules.






















Here is the board with the display modules removed.

























The test jig reveals that there are four display segment LEDs that have failed.



So it is on to the display module repair.

Canada Beomaster 8000: Microcomputer Board Completed

I completed the electrolytic capacitor replacement on the Beomaster 8000 a few posts back. When I changed the two capacitors on the microcomputer and display boards I said I would return to the two boards later to finish their restoration tasks. I did that because I wanted to test the Beomaster power supply voltages first. Now that the power supply tests are out of the way I returned to the microcomputer board and completed the last task there.

The final microcomputer board task was to replace the two 2 MHz crystal oscillators with new ones. We like to replace the two oscillator devices because the Beomaster 8000 is such a heavy and expensive component to ship. Beolover first ran into a problem with the Beomaster 8000 2 MHz oscillator devices in a 2016 restoration. Since then we decided to make that replacement on all of our Beomaster 8000 restorations that involve shipping. Think of it as insurance.

The replacement is not terribly difficult but care must be taken to not damage the two microcomputer board processor devices.

Here is the board as I left it after the recap. The crystal oscillators and their related capacitors are highlighted.























This is a closer look at the 2 MHz oscillator devices and their 12pF ceramic capacitors.
























Before de-soldering and removing the existing 2 MHz oscillator devices the leads are shorted and the two processor devices (9IC3 & 9IC4) are removed. The red alligator clip connects the board ground to my shop power ground and my electrostatic discharge wristband. It is important to protect against static discharge when working with integrated circuits. Especially when some of these integrated circuits are not available anymore.






















This picture shows the oscillators removed.






















The new crystal oscillator devices are 2 MHz oscillators but their spec sheet specifies that they require 18pF capacitors instead of the 12pF capacitors the old oscillator devices required.






















The metal lids are then reattached to the microcomputer board shield box and I tested that the Beomaster microcomputer board still works.
























The control panel still functions so the microcomputer board looks like it is ready to go.

The display has some failed segments. To verify that I will remove the four segmented display modules and see how they do on the test jig.

Saturday, October 13, 2018

Texas Beomaster 8000: Power Check Milestone

After I finished the capacitor replacement on the final two boards for both the Beomaster 8000 from Canada and Texas my next task was to connect the components up and check out the power supplies.  In my last post I successfully tested the power supplies on the unit from Canada. I am happy to say the Texas Beomaster has also reached that milestone.

There was a slight difference in being able to run the tests on the Texas Beomaster. When I completed the recapping of the microcomputer board I was not pleased with the state of the solder joints on that board. It had been worked on before and there were several places on the board where the soldering was not good. I didn't want to wait until I resoldered that board to check the power supplies. So I used one of my spare Beomaster 8000 microcomputer boards as a substitute in the Texas Beomaster for the power supply checks.

But first I had to reassemble components in order to try powering up the Beomaster.
























Here is my substitute microcomputer board. It is a fully restored and known good board.






















The Beomaster is ready to try powering up.





















With my DMM connected to the +5 VDC regulator I plugged the Beomaster in to an AC outlet.






















Success. The red LED standby dot is illuminated and I have +5 VDC at the power supply +5V regulator output.

I pressed the TP1 (tape 1) button on the control panel and the Beomaster came to life.






















Continuing on the +5V, ±15V and ±55V supplies all checked out.



















































































As on the Beomaster 8000 from Canada this is a big milestone. It's great to know that the supply voltages are all good on this Beomaster.

While the Beomaster was on I checked out the display board. Interestingly there were initially some segments out but later they started working. In the following photo I also turned on the Beomaster Filter button as indicated by the illuminated incandescent lamp.
























I will have to check that out some more later.  The next task on this Beomaster is to get its original microcomputer board resoldered and into shape where it is controlling the receiver.