Replacing NKT452 with AD149
 

Some confusion here, I have a Dynatron HFC-20A, which I can't find a circuit. I am replacing the output transistors with AD149s, I am sure I have used these before and they were a straight swap. Towers say they are the same, two of the transistors I can read on the Peak tester say the case on the NKT452 is E. The AD149 the case is C.

Are they a straight swap, have I just got a false reading?

Thanks

Re: Replacing NKT452 with AD149
 

The case of a TO3 transistor is always the collector.

The Peak Atlas semiconductor analyser can get confused by power transistors. Basically it tests for diode junctions; then seeing two diodes, tries to measure the gain with the possible-collector and possible-emitter both ways around to decide which is which. At the test currents it is using, the gain "right way round" will be very low, and it might even have better gain "wrong way round".

Re: Replacing NKT452 with AD149
 

Thank you for the info, I did think it was straight swap, I also thought C was on the case. Thanks

Re: Replacing NKT452 with AD149
 

If you have a multimeter with a hFE function, you could try bodging some leads from your NKT452 and AD149 transistors to the test socket, and measuring the gain in both configurations.

Some Worcester gas boilers have a BC547 mis-inserted on the ignition control board. The base is in the middle, and in this application it had just enough gain with C-E transposed to pass end-of-line testing .....