In the circuit above, the transmitter portion consists of a blocking oscillator incorporating Q1, and produces a 500 cycle train of pulses that each ring at 67 kilocycles. Whiteâ€™s had a TR (transmitter-receiver) design like this in the late 1960s. It is interesting that this design was made as late as 1981. I too had a look at your splendid page for this detector. It seems easier to think a somewhat odd circuit arrangement and an incorrect replacement guide thanĪ schematic with two errors that balance it out. But that's only if the transistor is a PNP, which then means two mistakes in the schematic. If it was supposed to be a PNP in that output stage, then the emitter would be where the collector is now. The schematic looks fine, though I somewhat wonder why the speaker is in the emitter, which is a less common arrangement, but still viable here.

I'm not sure what these '9000' numbers represent, they almost sound like replacement transistor numbers to begin with. My memory of the Radio Shack replacement guide was that it wasn't comprehensive, and there were errors (though that's with a guide from the seventies). NPN need their collectors to go to + and their emitters to -, while PNP the reverse. Not just the symbol for NPN or PNP, but how they are connected. If any of the transistors change to their opposite, some of the schematic would have to change. Note that Q4 is also the only transistor with it's collector to the negative side of the batter. The circuit as shown seems to match how it's drawn.Īs shown, all the transistors are NPN, except for Q4. My life is loosely based on a true story. Thanks for any insight from the circuit gurus!Ħ3_3001schem.jpg I also asked the guy if he knows the part number in his unit or he's just pointing out the problem in the factory schem.

Hoping to resolve the question without having to drag it out and open it up to see what's actually in there. I have the detector in question boxed up and stored.

I'll assume for now the connections are correct as drawn, hopefully it's either just the symbol or the part number that's in error. I guess my question is if the circuit is correct for an NPN in the Q6 position and the number 9012 is incorrect, or should the symbol for Q6 be changed to reflect it being PNP as this man states. Using my 1984 RS substitution guide, the 9011 is RS part # 276-2058 and the 9014 is 276-2009 which are both NPN, while 90 both cross to 273-2023, a PNP device. The issue seems to be Q6, a 9012 transistor, crosses to a PNP, while the symbol shown is NPN. Today a guy emails me to say the schematic is wrong and I should correct it before someone referring to it gets confused. I have the manual for the Radio Shack Model 3001 that includes the factory-published schematic. I host some info on vintage metal detectors.

Radio shack discovery 3000