Thursday, August 09, 2007

PNP or NPN TR1 in the Quad FM2?



We recently had an enquiry from one of our customers in Turkey, a Quad fan who was trying to make sense of TR1 in his Quad FM2 valve tuner. TR1 controls the stereo lamp which in his unit was off although he was definitely hearing FM stereo. He was confused because TR1 is shown as a PNP device in the circuit diagram but listed as a Mulllard AC176 which is listed in the datasheet as an NPN device.

The answer seems to be that the Quad FM2 circuit diagram is wrong and the device TR1 should indeed be shown as NPN to agree with the datasheet and the electronics. TR1 has a collector voltage more positive than its base voltage, and receives a positive base voltage to turn it on, so it is clearly an NPN device.

Possibly the mistake arose in the drawing stage because the AC187 is a germanium transistor, which are usually PNP.

However we advised our customer that the fault was most probably in the lamp itself rather than the transistor. The lamp is a standard type still available today, or can be replaced with a yellow LED in series with a small resistor.

Dada Electronics Australia specializes in the Quad FM tuners, for repairs, renovations, and RF/IF/MPX alignments. See the Dada Australia web-shop http://stores.ebay.com.au/dada-australia.
The complete FM2-schematic can be obtained for free by sending an e-mail to info@dadaelectronics.com.
Esmond Pitt
Dada Electronics Australia

2 Comments:

Blogger Jos said...

AC187-188 is a complementary pair for output stages in small radios, AC187 is NPN, AC188 PNP

Jos Van Dyck

4:16 PM  
Blogger Jos said...

AC187-188 are a complementary pair for small output stages.
AC187 is NPN, AC188 is PNP.

4:23 PM  

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