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Monday, October 12, 2015

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.

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