XL500 sample timing

[homefire]

Here you go!

http://www.tpub.com/content/neets/14...s/14179_45.htm

http://www.tpub.com/content/neets/14...s/14179_45.htm

[porkluvr]

No im not sure how to check diodes and transistors with a multimeter ...is it the same as checking resistors?

Sorry I couldn't answer sooner. My PC is having issues and I can do only a few things at one time.

What kind of meter do you have?? You may have an analog meter, the type with pointer and dial which is harder to use and has no diode check function. In contrast, many digital multimeters have a diode check function that displays the voltage across a PN junction under test, and maybe also sound a bell.

If there is no diode check mode, then set whatever meter you have on the MOHM range. For diodes, you put the red, or "+" lead on the opposite end from where the arrow points, and the arrow points at the black lead. If there is no arrow on the diode then a bar usually marks the "-" end.
A good diode should give a low reading with electron flow against the arrow and a high resistance reading with electron flow with the arrow. If you have an analog meter you may get better readings on one of the lower scales. Try with a known good diode and see what happens.

BJT transistors have two PN junctions. If it is PNP, when the "-" meter lead is on the base you get a conduction with the "+" lead on the other two pins. One pin pair should conduct a little more than the other and the lower resistance (or lower voltage) reading is usually the EB junction. With NPN transistors, with the "+" lead on the base then the other two leads will conduct as you touch them with the "-" lead.

[Micheal]

No im not sure how to check diodes and transistors with a multimeter ...is it the same as checking resistors?

[porkluvr]

Just a guess but probably a semiconductor is at fault, not a filter cap or switch. Diode or transistor most likely, maybe resistor but less likely to be only a resistor. Please don't start yanking out parts willy-nilly.

Do you have a digital multimeter, and does it have a diode test function? It is more difficult to (or impossible) to accurately check components in-circuit but you can often locate open or shorted semiconductors using the diode test mode of a digital multimeter.

Do you know how to check diodes and transistors with a multimeter?

A large capacitor (or other component) connected to a diode make it impossible to get a good reading so you may need to unsolder ONE lead of the diode (or cap). That is an often used technique for in-circuit troubleshooting - to unsolder one component's lead, which effectively isolates that component (if it's a two lead component.

There is always the possiblity of one part taking out another so if you think you've found the fault be sure and trace the board and check other connected components.

[Micheal]

more pictures

[Micheal]

I am not that experienced at that but like I said none of the wires that go to the main board have power so the culprit has to be on the little board as far as tracing it Im using my multimetre to pinpoint the problem and it looks like it could be the 470uf 35v cap(althuogh it looks healthy and there was no pop when the ACCIDENT occured and other than that I dont know how to check caps with a multimeter) but im not sure and I also think it might be the on /off switch because there is no voltage coming out of it but once again Im not sure and I hope it isnt the switch because I cant find the same one anywhere on the net .the other parts that I am suspicious of is 2N3904 along with the NSDU45 because these have show+9V and a few times unexplanably dropped to +2.2v and I dont know why ,so I will replace these an see how it goes and the wierd thing is no part looks like it is fried but the smell is there...
But logic tells me it has to be one of the parts on the little board (as in the picture)

[Qiaozhi]

there is no power on anything on the main board...however on the little board which has the on /off discrimation and threshold pots on it has power from the battery ...because this is where the power from the battery leads into but any wire that connects from this little board to the main board carries no power (no pwer going to main board),so I am thinking that the damage might be restricted to this area (little board) and there isnt many parts on on the little board so I will change what ever parts there is and hope fully this will fix it...I will post some pics so it helps.

If the "little board" is that small, why don't you trace it out, and debug the fault properly, rather than blindly replacing everything?

[Micheal]

there is no power on anything on the main board...however on the little board which has the on /off discrimation and threshold pots on it has power from the battery ...because this is where the power from the battery leads into but any wire that connects from this little board to the main board carries no power (no pwer going to main board),so I am thinking that the damage might be restricted to this area (little board) and there isnt many parts on on the little board so I will change what ever parts there is and hope fully this will fix it...I will post some pics so it helps.

[homefire]

Bummer!

Yep! Check your power rails.

Smell your regulators.

[porkluvr]

Uh-oh. What did you short, and to what? Do you have either of the power supply voltages (+5, -5)V?

Use the voltmeter function of your multimeter, black probe on ground and red probe to what needs checked.

[Micheal]

somehow I shorted the xl500 today smoke came out and it looks like its dead.
...Problem Damn..

when i turn it on I have nothing ...I am going through everything to find out what has burnt out hmmm.all the tracks are ok ,all the wires and joins look ok..any ideas could it be the voltage regulators..capacitors..ic..fets etc ...at this stage I have NO CLUE...any ideas are welcome

[Micheal]

sorry I havnt had time in the last few days to work on the vco .I will be working on it in the next few days to see if I can get it going...will keep you posted

[porkluvr]

"VCO output should be at the end of the 47k resistor that connects to the LM358-7. You would need to disconnect the LM358-7 because that WAS your sound oscillator output but is no longer."

I just want to confirm that I understand this correct please correct me if I am wrong...ON the LM358 pin number 7 is not to be connected so I am assuming i will bend or remove that pin (leg) so that it doesnt connect into the saddle therefore it doesnt make contact...

Sorry I missed this post or I would have answered sooner.

The simplest way to disable the factory sound while you try to get the VCO running is to unsolder the end of the 47k resistor where it connects to LM358-7.

But, you installed a replaced the LM358 with a LM1458 in a socket, correct? Then it would not hurt to just pop the chip out temporarily because it is unnecessary during VCO trials. All it is doing was providing the sound, but you are trying to replace the sound, so either remove it temporarily or lift the end of the 47k resistor from pin-7. The resistor end is where you would solder the VCO output. No, don't go bending pins or you'll soon need a replacement IC.

Saddle is socket in Australian, right? (Language barriers can be tough to overcome.)

edit: Give me some time to digest your final post. Meanwhile, you may need to turn the pots a little to get the VCO to come into range. "zzzzz" may be a good sign. You did already try turning the pots, right???? You didn't say.

Turn one pot, then another. Try all all pot position combinations. If that doesn't work, last resort is to try shorting the "trimset" resistors - the resistors on the ends of the potentiometers that I put there to limit the range of the trimmers. If you short across them with a wire soldered from one end to the other then the trimpots can sweep the entire range from +5V to -5V.

But don't short them all at once. Short one resistor at a time, turn the pots. If nothing happens then short across another resistor, turn the pots, etc.

[Micheal]

I have connected the vco today ...results are that there was sound but no vco ...the sound is a zzzzzzzzzzzzzzzzzzzzzzz type of tone ,the detector is working and detecting but like I said with a zzzzzzzzz tone and the output volume is weak or maybe I should describe it as low...I have gone through everything a few times and checking .W2 W3 W4 were connected to the right spots which was not difficult but w1 and w5 were a bit difficult because the is circuit around the LM741 and CA3140 are different on my machine,it is not the same as the older models (I cant find the schematics anywhere on the net for this thing)...the 5 volt power is actually +&- 5.69volts power and it is consistant at all the points you mentioned. Nonetheless I have gained a little more knowledge this time around.Things are starting to become more clear...
Thanks for your help porkluvr I appreciate your help very much .
While I was doing all this and the machine was open I added a switch to the 5k resistor mod so I can switch between 5k and 12k and it works nice.
I also added a switch for the tone so now I can switch between mono tone and the original pulse tone ,also works nicely...and the switches are inside the machine so no wholes are drilled on the waterproof housing.
Back to the VCO I am leaning towards the Idea of...where the headphones are connected there is a + & - terminal ,from this terminal feeds the speaker (headphones) and also a red LED ...this red LED gets brighter as you get closer to the target(same as the volume at the same time) which is a nice little feature ...when I connect my multimeter to this +& - terminal I have discovered that the xl 500 is actually a lot more sensitive than it is able to put out audibly.On a simple 20 cent ozzy coin I can see that the voltage increases on the multimeter noticably a good 5cm before it is audibly hearable and on bigger objects like a coke can It is much more.So the audio amplification is a weak part of this machine really hiding its true sensitivity...
I am thinking that,going back to my idea earlier a vco power from this source is from 0volts when there is no target to a bit over 3volts when a target is right near the coil.Do you think it would be easier to incorporate a vco from this +&- terminal...I am looking around on the web and I have found a few simple looking shematics but nothing in a pcb form ,and at this stage I dont have enough knowledge to turn schematics into pcb design so I am hitting into brick walls...any ideas or should I end this VCO segment.

[Micheal]

"VCO output should be at the end of the 47k resistor that connects to the LM358-7. You would need to disconnect the LM358-7 because that WAS your sound oscillator output but is no longer."

I just want to confirm that I understand this correct please correct me if I am wrong...ON the LM358 pin number 7 is not to be connected so I am assuming i will bend or remove that pin (leg) so that it doesnt connect into the saddle therefore it doesnt make contact...