Beomaster 8000: Sudden On or Switching to FM Preset Phenomenon - The Story Continues

I am still in head scratching mode regarding the Beomaster 8000 that occasionally turns itself on to a FM preset, usually P1. Despite improving the decoupling of the processors from the power supply by adding 100nF capacitors to their power pins, this Beomaster finally turned itself on to P1 while I was present, and a few days later it switched itself to P1 while it was playing from my iPad on Tape 1. While this was frustrating news, it also meant that I was finally able to reproduce the issue, and that the phenomenon happens independent from where the Beomaster is plugged in (one of my theories in absence of being able to reproduce the phenomenon was that maybe the location of my customer's house is prone to a different level of line voltage fluctuations etc...I was grasping for straws, I know...;-). 

Anyway after it switched itself spontaneously from Tape to P1 I thought that the issue must be originating from the P1 input. A look at the circuit diagram showed that the only way P1 could accidentally trigger itself would be the absence of the 4.7k pull down resistor R1 (as long as no key is pressed on the keypad, the pins are electrically only connected to ground via the pull downs and to the input pins of the uProcessor). The interesting thing about the pull down resistors for the keypad inputs is that they are part of an integrated 7 resistor array with a common pin, i.e. the entire array has only 8 pins. This conveniently allows connecting all 7 resistors to ground with just one solder point. My hypothesis was that the array may have some intermittent issues, permitting some of the keypad inputs to float (which is known to cause erratic input conditions in microprocessors) hence potentially explaining the sudden switching to P1. 

I opened the Beomaster up again and took the uProcessor board out. Before unsoldering the array I removed the slave processor IC3 where it is connected to the input pins for the keypad. I usually do this when I work around the processor ICs since new parts may carry a charge which may accidentally fry some pins on the processor while implanting the part. The black 8 pin flat package in the center of the photo right in front of the IC socket is the array.

I extracted the array. This shows it together with the new part (yellow, Newark 62J2860):

While taking the original array out it occurred to me that the common pin (on the left in the first picture) is soldered into a via. And that made me think that maybe not the array is the problem, but the via, whose intermittency is a common problem on this board. The via is needed since the ground connection of the common pin is made on the composite side of the board, but they only wave soldered the board only from the solder side. This means they had to put vias around such pins that capillary forces would draw the solder into the via and through to the component side of the board, making the connection to the top copper layer. In the 1980s they were apparently not yet able to through-plate vias with electroless copper deposition technology, like it is done today.

I soldered the new array in paying attention to soldering it also on the top side. This shows the final result:

I put the board back in and fired the Beomaster up, and it still worked. So far so good...only in a few weeks we will know if this finally cured the problem.

Beomaster 8000: Exchanging the Opamps in the Signal Path and Test

After updating the uProcessor board in the Beomaster 8000 that I am working on right now, it was decided to also update the opamps while the unit was in service position. We recently noticed that the opamps in the signal path of the Beomaster 8000 can degrade resulting in increased distortions (THD), i.e. it is a good idea to also replace the opamps when the boards are out for restoration. Here we go:

This shows the control panel PCB before the upgrade:

Most of the 8-pin ICs on this board are signal path opamps. This shows the board with new socketed LF535 opamps installed:

On to the preamp/input board. I forgot to take a picture of the original condition of the board. Here are a couple shots of the board after replacing the opamps with socketed LF353 units (except the phono input, which was replaced with a low noise LM833 type):

A detail photo of the phono pre-amp section:

After implanting the boards I characterized the performance of the unit with my QA400 audio analyzer. The bandwidth curve yielded the spec -1 dBV drop between 100 and 20,000Hz, and the total harmonic distortion (THD) values at volume 5.0 (just below clipping) were 0.008% on both channels, which is consistent with other Beomaster 8000s we measured. See here for a detailed discussion of such measurements. So far so good...the unit went on into our living room to see if the performance of this Beomaster 8000 is consistent in day-to-day operation.

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...