Seben 5002 Metal detector

Hi I'm testing for a friend a Seben 5002 Metal detector that the threshold tuning sound does not hold
and you have to keep on pressing the re-tune button to get the starting sound but dies very quickly
there for you loose a lot of depth and sensitivity I know it's not worth the time to fix etc
But has anyone had the same problem and fix it?
I checked all the IC'S and transistors and diodes and they are all seem OK  
Also he has two coils and both are OK
Not going any further for the meantime like capacitors etc
What is the more likely problem like I said if anyone had experience with this problem let us know
Detector only gets a good signal from large metal objects or very close small items
But the main obvious fault is re-tune threshold fades away loosing most power and depth
Please no criticism on the brand or make or is good or bad etc We know all that
Just facts of knowing what components could be the culprit
Thanks  

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Hi and thanks for most of your reply-As you say it's not worth it but that's life
The owner is a friend and Favors are not dealt in Dollar values
Getting back to the subject the detector was working fine I have actually used it before when working properly
And it was very sensitive and powerful for what it is But the only fault now is the Tuning does not hold
We know you have to press and hold and tune dial to get the ultimate starting sound
But in this case now it fades very quickly until you press and hold re-tune button but fades straight back
I will have to check with more detail some Capacitors that would hold the tune for a lot longer that may be faulty
Without a proper schematic as you say it can be very time consuming
That's why if anybody has or had the same problem and was rectified could save a lot of time
But because of the non popularity this model has it could be a stab in the dark
It must be a low quality control issue since some are really good and most are Faulty and unstable
They have enough IC's and transistors to perform properly but no Tuning Module like the older Garrett's
But like I said that is another dilemma that is not going to fix the problem
Thanks 
   

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If you want to look into it a little deeper, you probably have a bad or malfunctioning PLL circuit. That's a Phase Locked Loop which is what keeps the machine tuned rock solid. If you can read the chip part numbers, look for one marked NE602. That's a common chip in metal detectors which is actually a double balanced mixer that can be used as a PLL. Also, an NE567 tone decoder is used as a PLL circuit. If you see either of them (both are 8 Pin DIP) those might be the problem.

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		Quote:			Posted by conan
	Hi again in relation to NE555 comment Thanks yes you are right the pic of the board is the same one but not the actual one Since yours does not have the Two Pots (Tuning and Discrimination Pots) Since there are slight difference of the same model If I have more time to take photos I will remove yours and display mine Both are the same as mention for description purpose

i can not guess what you mean. yup two pots were removed from board - for more clear pics.
i always delete all wire and pots that obscure a bord for better view at camera shoting.
indeed on your first pic i see an unit with TWO pots at panel, NOT four pots.
while you maybe mean a chinese with 4 pots aside.

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		Quote:			Posted by Mudflap
	Ne567! They are still using those? The time constant RC values were temp and voltage fluctuations would cause the decoder to drift off frequency. Used to use those in home made DTMF decoders.

I used them in a design for a wide band rf decoder for remote garage door openers 25 or so years ago. I also used them in the remote for a PLL circuit to tune the device. Versatile beast and if you do the design correctly, rock solid stability. Yes, still in use although the NE602 has a newer descendant. If memory serves, it is the NE607 but I may be wrong. I pretty much stay away from analog these days.

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