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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.

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