The thermistor in my Vocal Master failed. Does Shure still have these parts?
What is the problem?
In the Vocal Master, part number R23, a thermistor located in the ac power supply, is likely to fail after a period of time. This thermistor is no longer available from Shure.
Where can a substitute part be purchased?
We recommend a CL-90 thermistor manufactured by Keystone Carbon Company. It is described as a negative temperature coefficient (NTC) inrush current limiter. It can be purchased from either Mouser Electronics or Digi-Key Electronics Catalogs.
- Mouser Electronics-1-800-346-6873--Mouser Part No. 527-CL90
- Digi-Key Electronics-1-800-344-4539--Digi-Key Part No. KC009L-ND
What are the basic specifications of the thermistor?
The "cold" resistance is 120 ohms at 75°F. The steady state current capacity is 2 amps.
What is the purpose of the thermistor in the Vocal Master?
When the unit is first powered up, the thermistor limits the current that can flow in the power supply. This prevents possible damage to the loudspeakers by eliminating a turn-on transient (loud "pop").
Can the thermistor be eliminated from the Vocal Master?
Yes, if the user keeps the Vocal Master Master volume control set to zero when powering up the unit. This will prevent any turn-on transient from damaging the loudspeakers. Replace R23 with a piece of 20 gauge wire.