Help needed with Garret XL 500

[Micheal]

Hi guys Im back again I really need your help to fix this thing,I will be greatfull if you help...
The machine has not been working for a few months but I finally have time to look at it..

Heres the problem ....I was using it at the beach the last time and everything was fine ,when I turned it on the next day wouldnt work...when I say it wouldnt work it turns on ok like normal ,it does the battery check normally ,the tone is there and works fine everything works fine except it wont detect anything (nothing at all)I have replaced all the caps and the op amp and fets and things like that and it is still the same ...the only thing I havent changed is the resistors and diodes and the little things....I have measured at the coil and it is showing 0.3volts and it is constant (not changing if I turn the sensitivity up or down)
I will add the pictures shortly
Any clues guys will help greatly...thank you

[Micheal]

heres some pictures ...I hope it helps

[Micheal]

some more photos

[homefire]

Sounds like it's not firing!

Pulsing!

Check every thing associated with the transmit pulse.

[Altra]

Micheal,

A couple of things to check.

Disconnect the coil and measure the resistance. It should be between 0.65 to 1.2 ohms. Any higher or lower would indicate the coil is shorted or open. If you see corrosion on the shielding of the coil cable that usually indicates water has enter and wicked up the full length. The shielding becomes brittle and breaks inside the insulation. The coil becomes irratic or open.

Measure the DC voltage across pins 4 and 7 on the LM741 op amp. It should read about 10 volts, if lower one of the ICs or transistors may be shorted. Or the DC to DC convertor may not be working.

With out an oscilliscope its hard to go much further.

Do a search, if you have not already, there is a schematic diagram of the XL500 on this site.

Good Luck
Mark

[nakky]

Micheal,

A couple of things to check.

Disconnect the coil and measure the resistance. It should be between 0.65 to 1.2 ohms. Any higher or lower would indicate the coil is shorted or open. If you see corrosion on the shielding of the coil cable that usually indicates water has enter and wicked up the full length. The shielding becomes brittle and breaks inside the insulation. The coil becomes irratic or open.

Measure the DC voltage across pins 4 and 7 on the LM741 op amp. It should read about 10 volts, if lower one of the ICs or transistors may be shorted. Or the DC to DC convertor may not be working.


Do a search, if you have not already, there is a schematic diagram of the XL500 on this site.

Good Luck
Mark

First thing ,clean up your Pcb and try again.Grt Nakky.

[Micheal]

Thank you greatly for your replies guys...I am currently doing the checks at the moment and will post the results shortly

[Micheal]

ok I have done the checks and the coil is fine showing 1.2 ohms and I have 2 coils they both show the same and are fine ...
the LM741 pin4 is around the 5volts constantly...and the pin 7 is showing from about -3volts to +3 volts as I increase or decrease the sensitivity pot.

[homefire]

Check these!

See if it's firing.

[Micheal]

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

[Micheal]

readings from the LM555

[homefire]

The 555 is the trigger source for every thing.

Is it triping? Firing?

Put your meter in AC and see if it's working.

Pin 3 is the out put I think.

You show Pin 4 and 8 as 0 volts!!!

If all tracks look good , replace the 555!

You must have a early version. All chips are in sockets!



There should be + potetial should have source on it. Your not getting power to the 555!

Follow the power to the LM8L05!!!

looks like you lost your + battery on it.

Also check the Diode feeding trigger on Pin 2!

[Micheal]

I have put in a new LM 555...and the results are exactly the same.Hmmm very tricky one .I wonder if there is a service manual for this machine ..it really is difficult to work it out without it.Like I said everything is working and sounding exactly the way it should ,but no detection at all somehow no power is getting to the coil

[homefire]

According to your checks, the 555 is not getting power.

pin 4 and 8 should be 5 or so volts pos.

[Altra]

Homefire, this is like directing a blind man along a cliff using a telephone!



Micheal,

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.

lets get scientific,

Using your finger, do any of the black parts feel hot after running for a few minutes?

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.

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?

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.



Good luck