Beomaster 8000: More Output Trouble - Broken TR207 Collector Trace

After I fixed the broken PTC thermistor lead in the left output of the Beomaster 8000 that I am restoring right now, I thought I was finally done with the outputs, but no: When I tested it for a while it ran happily, and then out of a sudden the fault switch triggered again a shutdown of the output supply. This one was difficult to find since it turned out to be intermittent. After a while I finally figured out that the trace that connects the collector of TR207 (voltage gain stage) to the positive 55V rail was cracked. After removing the delaminated trace, I fixed it with magnet wire:

I like to use magnet wire for fixing traces, since the polyurethane insulation of the wire can conveniently be burned off with the soldering iron, resulting in short circuit proof connections that are insulated right up to the solder point.

Unfortunately, this did not fully fix the output issue. While the 8000 came on again reliably, I realized that my repair efforts must have caused some other problems. More trouble shooting finally yielded another broken off lead to the heat sink. It must have broken off while I lifted the board up to do fix the trace. The many alterations on this board and also the left channel suggest that the previous owner had a hard time to pinpoint the issue with the intermittent trace and the fault switch issue, and therefore the board was lifted a few times too many for replacing components finally compromising the integrity of the leads that go to the heatsink. This time the (green) lead to the base of IC205 broke at the solder terminal on the PCB. This caused this Darlington to be turned off. It was remarkable that the output still amplified under this condition. I only noticed the issue since after repairing the trace I ran the output with external power supplies while measuring the voltage drop across the R236/7 emitter resistors, which could not be raised anymore above 3 mV. This indicated that there was something wrong. After I reconnected the base of IC205 everything was finally good again. Live and learn. No Beolove without some degree of pain, I guess...;-)

Beomaster 8000: Fault Switch Gets Triggered by Disconnected PTC Thermistor

Oh well...I guess I was a bit too enthusiastic when I declared victory in my last post about the output amplifiers of the Beomaster 8000 that I am restoring right now. It turned out that there was an intermittent issue that triggered the 'fault switch' and that in turn turned off the relays that enable power to the output stages. Here is the relevant circuit as shown in the Beomaster 8000 'Technical Product Information' booklet:

Here is how it works: TR16/17 form a latch that is triggered by a >~1.5V voltage coming from the "fault output" of the output stages between R254 and R253. When the latch is set it turns on TR15, and 15V apply to R68. That pulls up the base of TR11 and it turns on, which in turn turns on TR12. This pulls up the base of TR18 and the relay looses power. This cuts the power to the output boards, and the 8000 goes silent. All other circuits remain on, i.e. the display are functioning normally etc...

The two failure modes that can trigger the latch are:

1. A DC voltage at the 'AF OUTPUT' (this usually means that one or more TIP transistors burned out due to overload or failing quiescent current trimmers
2. The heatsinks get too hot. This increases the resistance of the positive temperature coefficient (PTC) thermistor that is connected to the junction between R353 and R252 and GND. When it is cool it has about 50 Ohms which pulls the fault output to GND. If the resistance increases, a DC voltage develops due to the pull-up to 55V and the latch is triggered.

In my particular case, the receiver came on, played for about 30 sec on the left channel and then it went silent. One could also hear three clicks in rapid succession right after starting it up, which is one too many. Normal are two clicks, which correspond to the start-up sequence during which the two relays that control the current into the toroidal transformer get switched in succession to limit the inrush current into the transformer. The third click meant that the fault mechanism turned the relays off again. In the beginning I was a bit confounded by the fact that the left output was continuing playing for half a minute. It turned out that this is possible due to the charge that is in the reservoir capacitors, i.e. it shut down when the voltage dropped close to zero in these capacitors. Why it did not do that on the right side, comes from the fact that the right supply also powers the preamplifier via an attached ±30V supply. This causes the charge on the reservoir caps on the right side to dissipate more quickly.

Since the output stages showed perfect current values during my test with external power supplies, I first thought that the fault switch itself was at fault. So I tested it by grounding the fault output from the output stages, and this caused the issue to disappear.

Note: This test needs to be performed from a cold start, since the latch in the fault switch only resets when the 15V rail turns off, i.e. the Beomaster needs to be turned off for that. However, the Beomaster runs a turn-off sequence after pressing the off bar on the keypad, where the 15V remain alive for another 60-90 sec. This means that one needs to wait a couple minutes before turning it on again to see if the grounded fault output disables the fault condition.

In my case this test yielded a functioning fault switch, and therefore the issue had to be in one of the outputs! It turned out that the left stage had a PTC that was intermittently disconnected from R253/R252, which caused it to loose its ability to pull the fault output to zero at normal temperatures. The picture shows the broken white-blue striped lead that connects the PTC (which is mounted on the heatsink) to the fault switch voltage divider. The tricky part here was that the lead looked perfectly o.k. since it was attached by the insulation to the solder terminal and the lead was only broken off in the inside.

After restoring this connection, the Beomaster started working normally.

Beomaster 8000: Standby Issue - Beomaster Does Not Turn Back On During a 50 sec Window

Today I looked into a strange phenomenon I came across in the last Beomaster 8000 I restored. When no speakers are connected, or the speaker switches are in 'off' position, then the Beomaster would not turn on the output stages (i.e. no relay-click audible) if a turn-on occurs in a time window of 10-50 sec after turning it off.

This is not much of an issue in 'normal' operation, where one listens to some music, then turns it off, and comes back an hour later for more. But on the bench, when one tries out a few things and the Beomaster gets turned on and off in quick succession, and often without speakers attached, this phenomenon became quickly apparent.

This is the first Beomaster I encountered with this problem.
So I thought I would try to get to the bottom of this. The first thing is always to do some Googling these days. What came up was a Beoworld.org 'Vintage Products Forum' entry by Dillen/Martin where he states that:

 "It's not uncommon to see that a Beomaster 8000 won't switch the amplifiers back on right after being put into standby. It will light up the displays but not power on the amplifiers.
Put back into standby for 15-20 seconds and it will power up fine again. This is quite normal."

In the particular case discussed here the Beomaster does come on during the first ~10-15 sec, and then not until about 50 sec have passed. After 50 sec it starts up again. The phenomenon is completely reproducible, but the speakers must be disconnected/switched off to experience it. When the speakers are on, everything is fine.

Anyway, I opened it up again today and did a few measurements with my oscilloscope (since this is a transient phenomenon, we need a time dependent measurements...). I fairly quickly identified the cause for the phenomenon: Pretty much the only way to prevent the outputs from turning on in a reproducible fashion is via the fault circuit:

Both right and left channels have a 'fault output' that triggers the 'Fault Switch' on the power supply board to cause a shutdown of the outputs in case a DC voltage is present at the speaker output, or if the temperature in the heat sinks gets too high. In either case a non-zero voltage occurs at the voltage divider between 5R24 and 23 (on the left...that part of the circuit is on the #5 PCBs (output amps), which is used as 'fault output'

I measured the voltage at that R23/24 point depending on the time after shut down with speakers connected. The yellow trace in the scope shot below is the positive 54 V rail which was used to trigger the measurement at 48.8V, i.e. a short while after turning off the Beomaster as the rail started to decay. The blue trace is the voltage at the fault output. So we see that there is a ~600-700 mV negative voltage spike that decays within about 10 sec to zero. This is not enough to trigger the fault switch because of  6R62, and everything is o.k.

An entirely different picture presented itself after disconnecting the speaker from that channel:

Note the now larger 1V scale for the blue trace, i.e. the initially still present small negative voltage spike seen with speakers connected in the scope shot above was completely overwhelmed by a ~4V positive potential when the speaker was disconnected. This potential boost was reset after about 45 sec and zero voltage remained at the fault switch at that point. This corresponds nicely with the ~50sec I measured that I had to wait to get the Beomaster to turn on again. It seems that after 50sec everything resets. This correlates with the delay when the microprocessor turns off the 15 V rails after shut down has been initiated.

It may be that there is a potential build up due to an inconsistency in the decays of the positive and negative 54V rails after turning the Beomaster off. This is no problem as long as there is a speaker connected, which provides a path to ground if there is some DC transient on the output during shutdown. Without speaker, the potential builds up, and the fault switch is triggered.

The real question is: Why do only some Beomasters exhibit this phenomenon, while others do not. I am aiming to do similar measurements on one that does not show this phenomenon, maybe we can learn something from that...