13-Nov-2003
Background:
A small number of B50 Super Varios have exhibited a problem with the audio after some time in service. The symptoms are a large increase in audio volume (usually sounding distorted) which is not controllable with the volume control knob. In some cases the sound then stops due to the speaker having been damaged.
Cause:
The cause has been identified as an open circuit between the display board in the B50, which also has the volume control potentiometer mounted on it and the board containing the audio amplifier in the main board stack. It is suspected that the open circuit is caused by the ground track on the display board cracking due to loads imposed by the mounting in the panel. The first instruments to show this had all been involved in accidents but a few are now showing up with the same problem after normal use.
The Fix:
Since this was first brought to our attention and the cause identified, we have been fitting an extra redundant ground wire between the ground connection of the volume control potentiometer and the ground pin of the audio amplifier(after serial number 240). This direct connection prevents any board problems from having the effect of causing an open circuit. In B50s with the problem, fitting this wire has cured the problem. If the speaker then produces no sound replace it with any 8 ohm speaker. The larger the speaker, the better the sound quality.
Accompanying this text are photos showing the extra ground wire.(multi strand hook up wire)
While the photos show the entire B50 circuit board assembly removed from the chassis it is not necessary to remove the front control/display board from the front of the chassis. The volume control potentiometer is accessible with the control/display board in place.
You will need to remove the B50 cover, then the DB15(two screws) from the rear end of the chassis after removing the XCB. Remove the six screws holding the board stack in place and swing the stack out to get to the boards.
Solder the approximately 5 inch (12.5 cm) wire in place to the top contact pad on the volume potentiometer and under the board to pin 4 of the LM 386N-8 audio amplifier IC.
When re-installing make sure you don't crush or pinch the pneumatic tubes to the sensors. Have a look at how they are arranged before dis-assembly.
Use a good electronics assembly or repair, temperature controlled soldering iron. Any competent electronics technician should be able to perform this work.
We can also do this at the factory if you desire.
Mike Borgelt
13-Nov-2003