Fisher Scientific T-30 Metal Detector - Circuit schematic

[jimmyboy111]

Looking for service manual for this old thing or a very similar detector it is from 1965 with germanium transistors .. I didn't think there was any real use for it until someone asked

.. some photos

https://imgur.com/a/0fV72w5

.. board photos

https://imgur.com/a/BlseVfS

[eccerr0r]

Re: Fisher Scientific T-30 Metal Detector - Circuit schematic

yay, imgur is now on my ban list. Could you upload to bcn directly?

[jimmyboy111]

Re: Fisher Scientific T-30 Metal Detector - Circuit schematic

sure .. uploaded

[momaka]

Re: Fisher Scientific T-30 Metal Detector - Circuit schematic

Looks like someone soldered two "modern" resistors in there at some point. I wonder why that was.

Unfortunately, I don't have much knowledge in the way of metal detector theory, but if bad comes to worse and you can't find a service manual on any of the popular websites online (Elektrotanya, eserviceinfo, and etc.), then perhaps you can try to draw a diagram of the circuit. The PCB doesn't seem too complicated or with too many components, so should be do-able.

[jimmyboy111]

Re: Fisher Scientific T-30 Metal Detector - Circuit schematic

I found a circuit that looks similar to the one here and shows the basic function blocks of these old detectors .. I may look around and see if I can find a schematic from another detector circa 1960-1970

https://www.nutsvolts.com/magazine/a...metal_detector

.. i scoped out those transistors and resistors .. the old carbon resistors drifted .. that is why the knew ones were piggybacked

[eccerr0r]

Re: Fisher Scientific T-30 Metal Detector - Circuit schematic

These old metal detectors (along with even new ones) are still working the same way, they should be implementing an oscillator which changes frequencies when you pass over metal objects. Yeah I suspect the resistors indeed drifted but shocked they weren't simply replaced instead of parallelized...

What's the problem with it currently other than the apparent cut wire? Any function at all?

[jimmyboy111]

Re: Fisher Scientific T-30 Metal Detector - Circuit schematic

It is running and oscillating and I can hear a very quiet low hum putting on some regular headphones .. it was probably meant for some piezo headphones with high impedance since the output is so quiet

.. I will try to check each oscillator and look at the signal closer with a scope tomorrow and maybe trace out the circuit a little now that I have a rough detector circuit to draw from and look for some parallels

.. I reconnected the cut wire to the variable plate capacitor to the control knob but it does not change much to the sound