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Friday, October 7, 2016

Beomaster 8000: Rattling Main Relays and a Broken 2MHz Oscillator Crystal

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.

Oh well, nothing lasts for ever!...The fully restored (2011) Beomaster 8000 that I am using in our living room recently broke with a rather strange fault. I woke up at 4 am in the morning to the beeping of an uninterruptible power supply (UPS) in distress. First I thought we had a power outage, but it turned out that the UPS turned itself off due to a malfunction in the connected Beomaster 8000 (I run all my B&O equipment on UPSs to reduce the risk of damage from grid voltage spikes and the like). I restarted the UPS and immediately the main transformer power relays of the 8000 started going on/off in rapid sequence. And then the UPS caved in again, turned itself off and started beeping again. This immediately pointed to a severe fault in the 8000 since a UPS only shuts down if the current drawn exceeds the rated 10 or 16 amps. This suggested to not try running the Beomaster again before having a look inside.

I shut everything down and went back to bed. The next morning I swapped the 8000 out with one of the other 8000s I have around the house in less important locations. A few days later I opened the malfunctioning unit up and ran it. First everything was normal but after a while it started to rattle again. I switched it off immediately and started wondering what might cause such behavior. First I thought there is a problem in one of the outputs triggering the protection circuit. The reason was that I initially detected 15V at the output of the protection circuit (collector of 6TR15) during relay rattling phases. The 15V pull up the base of 6TR11 via 6D12, which then causes the relays to go off, cutting power to the main transformer. This is usually caused by malfunctioning output transistors or overheating of the outputs. However, in this case the 15V were rather a consequence of an entirely different root cause.

After some playing around with the unit and enjoying a few more UPS shutdowns and relay rattling events, I finally figured out that the crystal oscillator of 9IC4 on the main processor board had some issues. I figured this out due to two phenomena:

1) After one of the rattling episodes I was not able to turn the Beomaster back on. The standby LED was lit, but no more reaction to the keypad. Panic ensued since I thought I had accidentally fried the processor or some other disastrous event occurred. So I did a processor self-test by pressing the Monitor key first and then additionally the on/off bar on the keypad. This initiates a self-test sequence. And I finally saw my first error code on a 8000: TE8!
The manual yields a rather cryptic cause for this error:

"Error TE8:    Defect IC:9IC4(RAM)      Or short pin to chassis: 10-11"

Initially I thought I had really fried the slave processor that is responsible for the relays...but then ,after my initial panic subsided and normal brain functions kicked back in, I thought that my maxim "silicon usually dies last" should apply here too, since even a complete main power supply failure and dramatic short circuit etc...would have a hard time killing anything on the processor board. There is simply no direct connection. Furthermore, when I tried to turn the unit on again after an hour or so, it worked normally again indicating a healthy state of affairs on the processor end of things (silicon either is alive or dead...rarely there is an intermittent state in my experience). Also, once it worked again, the error code went away and I got this from the self-test:
A happy Test Passed (TP).

2) The second indication toward the oscillator issue came from an oscilloscope measurement I made after the error code episode:
The probe was hooked up to the base of 6TR11, which is connected to Pin 16 of 9IC4 via some resistors. At this point the unit was working and there was no rattling from the relays at all. But you see some type of digital random oscillation of the signal that normally should be just near-0V when the unit is on. The digital character of the signal already suggested to me that there might be a processor issue...I watched this for some time and then it changed to a more aggressive behavior:
A much stronger deviation from 0V and still pretty random and close to switching 6TR11! And then it went completely crazy and the relay clicking started again as 6TR11 did its job. I did not save the measurement at that point since I was busy turning off the unit as fast as I could...;-). This finally put me on the right track, however, and I started looking into 9IC4. The same random signals were also directly visible at Pin 16 of 9IC4, just on a 0-5V scale since at Pin 16 we are directly at the source of the signal and there are no resistors over which the voltage drops. This told me that indeed the processor was going 'crazy' occasionally. This combined with the error information above that alternatively to a processor malfunction also Pins 10-11 could be 'grounded' finally made me see the light since the oscillator crystal is connected to Pins 10 and 11!

I wiggled the crystal a bit and indeed, I was able to cause the Pin 16 signal to change from completely quiet 0V to the above shown random signals and to 'crazy' causing relay rattling. So I guess what happened was that the clock of the processor started to have random hiccups causing timing issues while processing its firmware, which then caused the normally constant output at Pin 16 to become random triggering the relay circuitry.

I ordered a few 2MHz crystals from Newark (21M6819) along with matching 18pF resonator capacitors (46P6436) - the original crystals run on 12 pF capacitors, i.e. they need to be exchanged along with the crystal to get a proper oscillator signal. After a few days I received the parts and put them in. This shows the original crystal with the two capacitors (brown, right below):

This shows the board with the parts removed:
This is the new smaller crystal in comparison to the original one:
Like most components modern ones are considerably smaller than the original ones...and this finally shows the new units implanted (I also exchanged the 9IC3 components assuming both the original crystals were from the same batch...;-):
The exchange is straight forward, but the right capacitors are difficult to remove since one of their legs is fed through a hollow via, which is a bit of a pain to unsolder due to the very small space between lead and via insert. One has to hold the solder tip to the via to liquefy the solder and then pull the cap out...three hands would be great for this process...;-)

One more thing: It is crucial to cut the leads on the solder side of the board very short to prevent short circuits to the EMI can that encloses the processors:
I had them too long initially, and I got some really spectacular readings on the displays when I turned the unit on. Essentially, it became completely unresponsive showing some random zeros on all of the displays. Another near-heart attack...;-). Cutting the leads solved that problem...live and learn...;-) 
I am running the unit now for a couple days back in the living room and it seems the issue has gone away. Also wiggling the crystal did not cause the issue anymore. So I am assuming we are back in business with this lovely Beomaster 8000!

Remarkably, a few days after this happened to my 8000 I received an email from an Australian customer whose 8000 I restored a couple years ago telling me that his Beomaster developed the exact same symptoms (I am sending him a couple crystals and the capacitors plus some instructions...this is Beolove!).
So I am thinking that exchanging the crystals should become a standard part of any Beomaster 8000 restoration...two at the same time is a rather strange coincidence, which points to a systematic issue with these crystals. Probably the leads are delaminating from the crystal material after 30-35 years...
Time to get back to my other restoration projects!

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