Beomaster 8000: Display Repair (III) - Rebuilt Displays Installed on PCB, LED Replacements for the Indicator Lights, and a Broken Transistor

Another 24hrs test of the rebuilt Beomaster 8000 displays passed without incident. So it was finally time to put the displays back on the board and test them in the Beomaster. I also replaced the indicator light bulbs with my latest SMD LED replacement boards. Here is a picture of the boards. They fit directly into the indicator cabinets as a drop-in replacement for the light bulbs:

Some more detail about these boards is given in an earlier blog entry, and a YouTube video.

This photo shows the full glory of the display PCB after everything was put together. The covers of the indicators are off, to show the LEDs:

And here with the red covers on:

There was a bit of an excitement during this effort, since at first the 'clipping' indicator would not work properly after I replaced the light bulb with my LED assemblies. What happened was that the LEDs would not go off if there was no clipping. In effect the LEDs behaved as if I had not removed the collector resistor (R40) from IC4, but I had. So something was fundamentally wrong. The plot thickened when I measured the base voltage at IC4 when clipping was on: I only got 0.6V, while it should be 1.2V since IC4 is a Darlington transistor where two diode drops occur between base and emitter. IC4 is right above the clipping indicator cabinet:

This suggested that one of the transistors in the Darlington had a significant problem. The next test was to connect the base of IC4 to ground. If the transistor is o.k. the LEDs should have gone off, but they only reduced their intensity somewhat. All this suggested that IC4 was damaged. I replaced it, and then everything was fine. 

This leaves replacing the speaker switches in this Beomaster, and then this restoration will be complete.

Beomaster 8000: Intermittent Display Digits - Bad Solder Joints

Oh well...While I was cheerfully listening to an episode of Car Talk on NPR on the Beomaster 8000 that will soon go to Australia, I could not help noticing a couple digits in the freshly restored frequency display fluctuating -on-off-on-off...Back to the bench with this one! It really is a nice workout, carrying the 45 pound behemoth to my electronics lab...;-).

The digits that blinked suggested a problem with the Phase 3 strobe that governs two digits in the frequency display, as well as the balance digit. I quickly found the problem: A broken solder joint at P74 that, among other signals, connects Phase 3 to pin 15 of 9IC3 (the master microprocessor). Here is a picture of the solder joints of the connector:

If you look carefully, you will notice the shadow around the leftmost joint...this indicates a broken out joint.

I re-soldered all jacks and plugs on the two joined PCBs and now all is fine. These two large, but poorly mechanically supported boards are really the most problematic area of the 8000s...whenever there is erratic behavior, it is usually some solder joints or vias on #8 or #9. I will add the re-soldering of the board-to-board connectors to my standard Beomaster 8000 restoration process. Live and learn!

Back to the living room for some more in the field testing before the long journey.

Beomaster 8000: Display Restoration with SMD LEDs - Reassembling the Display Units

The SMD LEDs that I put into the Beomaster 8000 displays survived the 48 hrs test without problems. Time to put the display units back together. I applied the method I outlined earlier in this blog:

http://beolover.blogspot.com/2012/09/beomaster-8000-display-repair-hopefully.html

After I glued the transparency foil pieces to the back of the displays, I fixed them with carpenter clamps:

And after extracting them from the clamps:

Then I replaced R14 and R16 with 3.3kOhm and 1.5kOhm resistors to adjust the display voltage to reduce the intensity of the LEDs. See this blog entry for details about the adjustment procedure:

http://beolover.blogspot.com/2012/05/beomaster-8000-display-brightness.html

Then it was time to solder the displays back in. The insertion of the displays back into the board is tricky since one needs to line up all the pins at once...patience and calm music pays here...;-). Here is the display board after installation:

I adjusted trimmer R15 to get about 3.1V keyboard strobe (yellow) and about 4V on the LED rail (blue). It is important that the keyboard strobe does not go below 2.7V (see above blog entry for a discussion):

Looks like we are almost done. The last step before I put it back together is to check on the processor board.

Beomaster 8000: Rebuilding the Displays with SMD LEDs

I saved the final restoration task until the end. The Beomaster 8000 displays are one of the major trouble spots, and this Beomaster needs to survive a long journey to Australia. So we decided to rebuild all four display units with surface mount (SMD) LEDs to ensure that the displays will still work post-shipping.

As usual I took out the display board for easier access to the solder points of the displays:

It is advised to use a desolder gun for taking the displays out, since all display pins need to be loose before removal of the displays. Trying to do this with copper braid is not advised. So I took my trusted Hakko 808 to it and removed the displays:

Then I removed them from their plastic alignment supports:

And opened them up. this is done by cutting off the plastic tabs protruding out of the bottom of the circuit boards with a sharp knife (take care to not damage the traces...):

The next steps were to scratch off the old LEDs (here also caution is advised to not damage the traces). And then, after a thorough cleaning with ethanol it was time to solder the SMD LEDs on:

It is imperative that the LEDs are well-centered on the pads that the white reflector masks can fit on without straining the LEDs and without gaps between the masks and the circuit boards (otherwise, light will penetrate from below into adjacent, potentially off segments and make them light up partially...an ugly effect).

After I completed the job I set the displays up for testing on a breadboard. I usually run them for 48 hrs before putting the displays back together. I also test them for stability by trying to wiggle them with a toothpick left and right to see if the solder joints are good. One of the issues when soldering the LEDs is that the boards do not have a solder mask, hence the solder tends to flow away from the components, which can cause problems. This can be counteracted somewhat by heating the part first, then the pad, but nonetheless, this must be kept in mind during this procedure.

So far so good! I'll be back in a couple of days and let you know how this test went...

Beomaster 8000: Replacing the Reservoir Capacitors

Today was the day for the reservoir capacitors of the Beomaster 8000 on my bench. Always a pleasure to replace these big 10000 uF cans! After all it all begins with them! I replaced them with the 105C ECES1JA103EZ Panasonic type. They have almost the same size (35 mm diameter vs. original ones at 40 mm and same 80 mm height) and clock in with a solid ESR of 10-11 mOhm at 100Hz for an unimpeded flow of power. They are also rated for 105C, i.e. promise to last for a while, even if it gets a bit warm back there. Unfortunately, this type was discontinued not too long ago...it is getting difficult to find this form factor...electronics parts continue to get smaller. The 5 mm diameter difference was corrected with a wrap of thin corrugated cardboard. That way they fit snugly in their compartments.

Here are some pictures:

Before:

Pretty dusty back there...(sorry for the slight blur...)

And after replacing them:

Almost sexy!!...if you know what I mean...;-)

And here a detail shot of the left channel:

On to the speaker switches, while the heat sink cover is off...

Beomaster 8000: Occasionally Missing Display Segments - PCB #8 Rebuild

Today I worked on the display PCB (#8) of the Beomaster 8000 that I am currently restoring. While I worked on the power supply board I noticed that the display segments flickered occasionally when touching the display board. The contacts between the processor board (#9) and the display board is really one of the weaker points of the 8000 internal design (no weak points on the outside, in my opinion!) and prone to issues. So I took the display board off and cleaned the contacts with a fiberglass pen and then coated them with DeoxiT D100. I also bent every second pin of the headers slightly to increase the pressure between the male and female contacts. This usually does the trick. Here is a pic of the contacts - it really does not take much bending:

I also replaced the two electrolytic capacitors on the board (there are only two - I'll let you find them...;-).

Before:

And after:

Detail (C7):

After I put the board back in, wiggling it would not produce missing segments anymore...hopefully this is now stable!

It is noteworthy that this display board features fiberglass PCBs for the volume, input and frequency displays - note the green color on the display in the upper right corner of the photo - previously, I only saw these boards used for the balance display, and these usually then had no missing segments. Maybe this explains that this Beomaster has not had display issues so far.

On to the inner sanctum, the processor board (#9)...problematic vias may need some attention there!

Beomaster 8000 Microcomputer PCB Damage

Back to the Beomaster 8000. Today I looked at the missing display segments. First, I thought it is the usual dead LEDs on the boards. The display looked like this:

So I removed the three main displays. Luckily, I decided to plug them into the display 'harness' I made with a breadboard for the last display repairs. Powering them up yielded this:

This is the first Beomaster 8000 display I see that fully works. This told me the problem is elsewhere. I soldered the displays back in and looked at the circuit diagram. The particular LED segments that were out suggested a problem in the drain path to the SN74247 7-segment driver responsible for these segments (IC 2). I checked the signals on plug P-75 and indeed the pins responsible for the c and d segments were at 5V and did not show any strobe signal. This suggested a contact problem. I removed the display board and cleaned the plugs and resoldered the p-75 plug and socket. To no avail. The segments remained dead.

I followed the current path towards the SN74247 and found that the plug pins did not connect to the respective current limiting resistors R21 and R22. This meant that the PCB traces between P-75 and these resistors were broken somewhere. 

A closer look revealed that there had been the same problem at some point in the past with pin 3 as indicated by the white jumper soldered between capacitor and resistor:

I did the same for pins 7 and 8:

This brought the segments back:

The question now is whether to refurbish the displays anyway or leave them as they are.