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NuTone Intercom Photo Gallery > NuTone Stereo Music Intercom Model SM428 > NuTone SM428 preamp board

NuTone SM428 preamp board

The Pre-Amp / Intercom board is the largest module in the SM428.  A common problem with this board is failing or failed transistors in the input circuit.  This can cause a imbalance in the input circuit, which will cause an imbalance in the output of the SM428.

Transistors should be check if this problem is observed when the operation of a SM428 is checked.