Early Garrett BFO oscillator question

[Ken H.]

I have a late 60's? Garrett Treasure Hunter BFO detector with the 5.5 inch search coil.

The reference oscillator is crystal controlled, and runs at 400khz with a 1.5 v p-p signal on the emitter of the PNP transistor

The oscillator for the search coil also uses a PNP transistor. Garrett designed the tank circuit (search coil and 13-320 pfd var. cap)in the collector leg of the transistor. The transistor base is biased by 4.7K ohms to +, and 6.8K ohms to ground (base voltage shows 3.6v). The emitter connects to + through a 4.7K ohm resistor, and shows 4.3 v.

There is no feedback loop from the collector to emitter, and I don't see how this oscillator should work. (It is not working now)

I have checked all the resistors and caps in this transistor's circuit (disconnected one end), have checked the variable cap for shorts (none), and the single loop of wire to the search coil shows 2 ohms resistance. I have even tried a test coil in place of the search coil. The transistor was tested, and also a substitute transistor tried.

What am I missing??

[Beachcomber]

Re: Early Garrett BFO oscillator question

The Crystal may be bad.

HH

Beachcomber

[Sean Goddard]

Re: Early Garrett BFO oscillator question

OR the transistor in the tank oscillator MAY be a unijunction type.

I've seen some wierd oscillator types using these and thought WTF ('scuse the language)is going on there?

[Ken H.]

Re: Early Garrett BFO oscillator question

Thanks for the suggestions. The crystal oscillator is running....just the search coil oscillator is dead. The search coil transistor is a 2N3638a, which is listed as a PNP. Even so, the unijunction transistor suggestion was a good tip.

I connected the search coil to a breadboard Colpitts oscillator last night, and it ran fine at 375 khz....so I don't think the search coil is the problem either.

[Beachcomber]

Re: Early Garrett BFO oscillator question

Kevin said it was a PNP transistor and that he had tried substituing it. ALso the Garrett BFO's were back in the 1960's and early 1970's. I don't think we even had UJT's back then as the 60's were the early years of transistors. Heck they didn't even use transistors in TV's until the mid 70's they still used vacuum tubes We sure have come along way in 30 years From single transistors to LSI's with millions of transistors in them to smd's. Geez I am starting to show my age

HH

Beachcomber

[Russ_NY]

Re: Early Garrett BFO oscillator question

With something of that vintage I would be very suspicious of any electrolytic capacitors. sometimes it's simply easier to change them all rather than try to troubleshoot. Even if they are not shorted the leakage current may be quite high.

Russ

[Robert S.]

Re: Early Garrett BFO oscillator question

I have an old garrett, which seems to have a problem with thermal intermittents. When I warm it up it runs great till I store it for a month or two than it's dead again, Haven't been able to isolate the problem component yet. You might try warming the board with a hair dryer
GOOD LUCK