Texas Beomaster 8000: Preamplifier, Filter & Tone Controls

I finished the rework of the preamplifier board along with the filter & tone control board out of the Beomaster 8000 receiver from Texas. The task was pretty much the same as for the Beomaster 8000 from Canada. The electrolytic capacitors were replaced with new capacitors. The LF353N (or TL 072CP) opamps removed and new LF353N opamps inserted. The tone control sliders and source input volume trimmers were flushed out and cleaned with Deoxit. All of the board connector solder joints were reflowed with solder to insure against any weird problems with hairline cracks in solder joints. It is better to be safe than sorry. Especially with audio equipment this heavy.

The two boards look clean and new again. Soon I will be at the point I can give them a try.

Here is the filter & tone control board before the restoration.

Here are the components I am replacing removed.

There were no real surprises on this board except an area on the trace side that had some suspicious looking solder joints around IC102 and IC202. They just didn't look good to me.

I desoldered the bad looking joints and resoldered them.

After the resoldering I discovered one bad area on the board where most of the trace had broken away.  For that one I wrapped a small wire around the pin that connects to the bad trace pad and soldered everything so it is solid again.

That does it for the filter & tone control board.

Now for the preamplifier board.  Here is how the board came.

This is the preamplifier board after removing components and cleaning. I used Deoxit in the source volume trimmers.

This preamplifier board had TL 072CP instead of LF353N opamps. Both are listed as interchangeable devices in the service manual.

The board cleaned up well and I installed 8-pin sockets for the new LF353N opamps.

Here is the completed preamplifier board.

Beomaster 8000: Finishing up the capacitor replacement

This Beomaster is getting closer to a test run. I completed the capacitor replacement on all of the boards now. There were no real surprises there. I did find a higher than normal number of capacitors that were out of tolerance and quite a few on the border of being out of tolerance. In addition to the capacitor replacement I also replaced some opamp devices and I reflowed the solder joints on all of the board connectors.

Here are the FM and FM Interface boards.

Next, the preamplifier board.

I had seen quite a bit of dirt, grime and rust on this Beomaster in the previous blog posts so I was wary of possible corrosion on the circuit boards. You can see that the six input level trimmers have some sort of grime on the plastic housing.

Sure enough, several of these trimmer knobs would barely turn. I used two types of Deoxit spray on the trimmers. Each trimmer has three slots that expose the inner workings and I was able to flush the grime and get them working smoothly again.

Along with the capacitor replacement there are seven audio opamp devices that get replaced. Beolover just posted a nice description of why we replace these particular opamps. Here is the preamplifier board with the capacitors removed and new eight pin sockets ready for new LF353N opamps.

...and here is the completed preamplifier board.

The Filter & Tone Controls board also has some audio opamps in the signal path that get replaced. There are also a few capacitors to replace. The tone control sliders and switches also received Deoxit treatment so they operate smoothly and have clean contacts.

As I did on the preamplifier board I added sockets for the new opamps. The sockets are nice because it makes it safer to install the new opamps (no solder heat gets applied to the actual device). Having the socket also makes it easy to switch out a different opamp in the future should that become necessary.

The display and microcomputer boards have only two capacitors each (that get replaced). The display board will be revisited later to change out the display segment LEDs and to replace the four indicator lamps.

Here are the two replacement capacitors on the display board.

This is also just a first visit to the microcomputer board. I will come back to it and replace the two oscillators later.

The 22uF (9C85) capacitor on the left has been replaced before. The original capacitor leads were cut and the replacement capacitor soldered to the remaining leads (still soldered to the board). That isn't necessarily bad but I prefer removing the capacitor and properly soldering in a new one. 

An important thing on this microcomputer board is that some of the component leads must be soldered to both sides of the board. The negative lead of 9C85 is one of those type of components.

The capacitor replacements on this Beomaster 8000 are now all done. I can reinstall the preamplifier, FM, FM Interface, Filter & Tone Controls boards in the Beomaster chassis now and start connecting their cables.