Beomaster 8000: New uProcessor Crystals, Improved Decoupling of the uProcessors, and New IC Sockets

A Beomaster 8000 that I restored in 2014 recently returned to my bench due to some erratic behavior. Apparently it turns itself on spontaneously once in a while. While I was not able to reproduce this behavior for several weeks, I experienced it once with one of my own Beomaster 8000s, and there are some sporadic reports on the internet about similar issues.

Since I was not able to find anything wrong with this Beomaster, I am left with a hypothesis, namely that the uProcessor once in a while suffers from a power fluctuation or that one of the clock crystals is going bad, and that puts it into an unexpected state.

So I decided to replace the crystals and add 100nF decoupling capacitors to the power supply pins of the uProcessor IC, which only carry 1 uF from the factory. These days most of the time one finds 1nF in parallel with 100nF, which allows filtering a broader frequency range.

This shows the inside of the uProcessor can in original condition:

I removed the precious processor ICs from their sockets to ensure that they would not get damaged during the surgery. The crystals and also their capacitors can be charged with a high voltage out of the box, and that can damage the ICs. Therefore, I normally discharge all components against GND and I remove the ICs before working on this board. 

This shows one of the crystal setups in more detail:

I removed the crystal and their two 22pF oscillator capacitors and implanted modern crystals with their specified 18pF capacitors:

I also replaced the IC sockets, while I was in there:

This shows the an original and a new 2 MHz crystal in comparison:

When putting in the new 18pF capacitors, one needs to remember to solder one of the pins from both sides, since it serves as a via.

The final task was to add the 100nF capacitors. This is an easy thing to do since the 1uF cap is soldered between the 5V and GND pins on the back side of the PCB. This shows the original setup for IC3:

I simply soldered the 100uF cap in parallel across the 1uF cap:

After this step I put the board back in, and the Beomaster still worked...let's hope the new parts help suppressing the sudden-on phenomenon. Only the future will show...

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: Final four capacitors replaced

Like the Beomaster 8000 from Canada I replaced the last four capacitors on the Texas Beomaster.  My original plan was to run some power supply checks with the recapped Beomasters. Unfortunately this Beomaster 8000 from Texas has a few additional problems with the last two boards. The problems aren't anything that can't be fixed but rather some things that must be reworked. This Beomaster from Texas was worked on before. Whoever put a soldering iron to this receiver did a poor job. I already have had to fix some bad solder joints on other boards in this unit. The microcomputer and display boards also have some ugly solder joints.

I started with the microcomputer board.

Removing the top and bottom covers of the metal shield box reveal the microcomputer components.
This Beomaster is missing a paper insulator piece that fits on the inside of the bottom cover. I will make a replacement for that.

There are quite a few jumper wires on this board compared to the pristine board from the Canada Beomaster unit.  That doesn't mean the wiring is incorrect.  I have seen several other Beomaster 8000 units where updates to circuit boards were made in service centers after the units had left the factory.

The problem with this wiring though is that it looks pretty sloppy.  Here is a closer look at some of the soldering on the wiring.

There is no way that was done by a professional B&O technician.

I will go ahead and get the two capacitors on this board replaced but will have to do a bunch of rework on the sloppy solder joints before I begin using this board.

Here are the two capacitors being replaced.

Here are the replacements installed.

The display board is next. It is in decent shape.

This board had a bad soldering pad for one of the capacitors being recapped and I found a diode lead that was barely attached to its pad.  I fixed both problems.

I still need to reflow all of the solder joints on this board before using it.

Beomaster 8000: Step Six - Rebuilding the uProcessor Board

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ACHTUNG: Please, note that the crystal exchange procedure described here can damage the microprocessor chips on the uProcessor board. It is recommended to remove the chips before replacing the crystals and the capacitors. Due to the inherent capacitance of these devices, high voltages can be present between their terminals, which upon release, can burn out the gate that forms the oscillator together with the crystal. Make sure you short circuit the leads of the parts before installing them.

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After completing the display board rebuild with new SMD LEDs in the displays themselves, as well as replacing the incandescent light bulbs in the indicator cabinets with SMD LED based assemblies, it was time to do the uProcessor board.

The uProcessor PCB is the main source of operational trouble due to a few shortcomings in its design. In the 80s double sided PCBs were only beginning being standard in consumer electronics, and the earlier 8000s have boards without through plated vias. This made it necessary to insert metal plugs into the vias and solder them. Many 8000s have startup trouble due to these solder joints cracking. Luckily this Beomaster 8000 already had an updated board where the via plugs were replaced by soldered in colorful wire jumpers. This shows the processors, the jumpers and the crystals:

Another source of trouble are crystals that stop working reliably. This can cause severe malfunctions in the start-up circuit potentially damaging the main transformer. I started replacing them as a standard restoration item since I had a malfunction in one of my own Beomaster 8000s that caused the uninterruptible power supply (UPS) I run this Beomaster on to go in protection mode. This indicated that very high currents were flowing in the main transformer while the malfunction occurred. More here about this phenomenon. When the exact same condition happened in another Beomaster that I restored a few years ago, I took it as a sign that this is an issue that may be prevalent in many 8000s.

So here we go. This shows the old big crystals replaced with smaller modern 2MHz units. 

You can pretty much use any 2MHz crystal, as long as you also install the correct capacitors. The original crystals use 12pF capacitors to resonate, while modern units usually have 18-22pF. If you look closely, you will see that I also exchanged the two small capacitors directly underneath the crystals. They are now 18pF. 

I also replaced the two electrolytic capacitors on the board with high quality 105C grade Japanese units.

Here are some more photos: This is a detail shot of crystal X1

and a picture showing the solder side:

This shows the solder side of X2 for reference:

After this I plugged everything back together, and it was time for a test run! This shows the display board in all its glory:

Beautiful! Very Beolovely! By the way: One can 'provoke' the clipping light (left most) to come on, too, by setting the tuner to a station and then cranking up the volume. Above about 5.0 the clipping light usually comes on (make sure you do not have speakers connected during this test...;-).

On to replacing the speaker switches!

Beomaster 8000: Inputs Not Working

A Beomaster 8000 that I rebuilt last year arrived a few days ago for trouble shooting. The owner told me that he suddenly lost all the inputs. No matter what input button was pressed nothing was audible, while the display would show the right input etc...This indicated to me that the uProcessor was working properly. So I opened it up with the plan to look into the chain of command between the uProcessor and the input selector chips. Since both channels of any input did not work, I hoped I could rule out a failure of the input selector chips. 

Once I had the unit open, I turned it on. Everything worked. That is of course frustrating if the problem is intermittent. I started methodically wiggling the connections between the uProcessor and the input chips. Nothing came up, everything was stable. 

Upon further reflection (while listening to some nice NPR jazz on the frustratingly working Beomaster) I determined that an interruption of the power supply of the preamp board would have a similar effect. So I checked the connections between the power supply board and the preamp. And indeed, when wiggling P46 that carries the power rails to the preamp board, I was able to get the inputs to turn off occasionally. So there was a bad contact somewhere. Here is a picture of the area:

The colorfully wired vertical plug is the one in question. This area is also prone to cracked traces on the circuit board due to the multitude of connectors that torque the board in that corner. A bit of a design weakness.

I took the board out and inspected for cracks and the like, but was not able to find anything obvious. So I resoldered all the headers and cleaned them and put the board back in hoping that a hairline crack in one of the header pin solder points was the root cause. After reconnecting all plugs, I fired the Beomaster up and wiggled the plugs in that area again, and nothing happened anymore. Seems everything is stable now. So it seems this issue may be fixed...but of course with intermittent stuff, one never really knows until one knows.