Help needed with Garret XL 500

[kt315]

I have done the checks and I have put the results on the picture ...what do you mean by check to see if it is firing?(I forgot to add the LM 555 figures I will add that in the next picture

very strangely is you got positive voltage on NEGATIVE side of capacitor...
be more accurate in time you drawn the signs...

[homefire]

I'm thinking your 78L05 may have gone tits up.

Your reading are not good according to this.

[Micheal]

Hey thanks guys ...I am doing all the checks at the moment..
By the way this board could be later model and has some changes..i will put the changes aswell to make things more clear shortly

[Micheal]

1...Do what Homefire says put your volt meter positive lead on pin 8 and the negative lead on pin 1 of the 555. It should read about -4.9v. If not check the LM79L05 regulator. Is it backwards? Are any of the other ICs or transistors backwards? You said you replaced all of them.

-the result for the lm555 pin 8 & 1 test was +4.95 volts
-the LM79LO5 is the correct way around there are no ICs or transistors backwards ..yes I replaced all the transistors and they are all in the correct way

2...Using your finger, do any of the black parts feel hot after running for a few minutes?
NO all the parts feel cool


3...Using your bare finger again. Disconnect the coil and turn on the unit. Touch the solder connections under the LT1037, does the audio or meter change? Do the same thing with the CA3140.

...with the coil on and off when I touch the LT1037 as you have described the audio increases and the signal on the meter increases immediately
....with coil on and off when I touch the CA3140 as described the audio does not increase

3..Next with the coil connected turn on the unit and place the coil on or agaist an AM radio(power on). Do you hear a buzz from the radio speaker?

...I doent have an AM rdaio at the moment (amy thing else I can use )

4.....On that version of the XL500, Garrett changed the preamp from a LM709 to a OPA37GP. You have an LT1037, is that the original? They also removed the offset null trim pot. It was replaced by a 10M resistor and 2 22K resistor. You can see these parts on your last photo on the far left side by the yellow wire. The 10M is brown,black,blue,gold see it? The 22Ks, red,red,orange,gold. Those 22K resistors are where the offset adjustment was on early versions. The reason for telling you all this is if that LT1037 is out of spec or your PCB is dirty. The output of the preamp could be fully latched on or off, which would keep it from detecting.


The LT1037 is the original when I received and the machine was working faultlessy and none of the resistors you have mentioned have been altered or tampered with

5..I'm thinking your 79L05 may have gone tits up.

Your reading are not good according to this.




...I have replaced the 79l05 already with a new one and still no go( by the way it is 79L05 not 78LO5)


Im thinking well it might be an idea to get new LT1037 and CA3140 It might work but the problem is I hope they keep them in stock down here

And all the sockets by the way were put in by me after having this problem so I can easily remove or change parts if needed

[Altra]

Micheal,

The AM radio test is to see if the circuit is actually transmitting a pulse. If you don't have an AM radio, try a cordless phone or cell phone. The pulses from the coil will induce a tone into the speaker coil in many cases.

There is a LM78L05 on the circuit board, its over by the cd4538s, it should read about 5 volts also. Use homefire's photo and measure between the output pin and the gnd pin.

The finger test under the lt1037 and ca3140 tell me that you probably have the correct voltages and the demodulator is functiong. Otherwise the opamps and tone oscillator would not function without the +/- 5volts.

That leaves you with the FET, LM555, LT1037,coil and connections. As homefire stated you need to see if its pulsing. Without test equipment you are running out of options.

Mark

[homefire]

OOOPS! Your right! 79L05 it is.

Oh well all regulators needs checkec any ways.

No AM radio?

A old piezo element across the coil wire should squawk if it's firing.

[Micheal]

heres the result for the79LO5

[Qiaozhi]

The 79L05 has a different pinout to the 78L05:

78L05 has input-gnd-output
79L05 has gnd-input-output

Therefore the 4V you measured is the difference between the input and output voltages. That is: 9-5.

[Micheal]

Hi guys...I still cant find the problem damm

[techobg]

Hi, I have the same metal detector, which for many years catching dust on the ceiling.
The problem is that no sound is heard (only humming).
It was burnt by the charger. Did not want to be included.
I changed two 2n3904 transistors and nsdu45 and then just buzzing!

I know someone is very difficult for someone to help, but still try!

And will upload pictures of what I remove .........

Greetings!