Fixing CZ6a. Can You ID This Diode?

[Critterhunter]

A friend had his brother solder in a new pin point switch to replace a broken one on his CZ6a. The pin point switch now works and so does the machine as normal, but the meter no longer moves to ID targets or to show depth in PP mode. I suspected his brother might have bumped loose a wire to the meter, so I opened her up and inspected things. Can't find any bad solder joints or loose wires, but then I noticed that the PP switch and meter share a common ground lead, and right near where he soldered the new PP switch onto this lead is a diode that isolates them both from the ground trace.

His brother is a goldsmith by trade, so I suspected right away he used a very hot soldering iron meant for doing jewelry soldering work to solder in the new PP switch and he cooked the diode. So I used the conductivity mode on my meter that will sound a tone if the circuit is complete. Flipping the leads of the meter to alternative sides of the diode back and fourth with the diode in between them (of course), I get no tone to indicate an intact pathway. Obviously diodes are meant to only allow current to flow in one direction, so I should be getting tone here from it with the leads the proper way polarity wise to allow the current of the meter to flow between them.

This diode is grey in color and fairly small, with the black band at one end to indicate the direction of allowed current flow. The diode says "Motorola K", and then below that it says "624", and then below that it says I believe 5B (reading this # backwards with a small mirror as it's under the thing, but I believe I got it right flipping things in my head here).

I've googled the part # and can find nothing to indicate what type of diode it is. My knowledge of diodes is a bit weak (been a while since I played with them in the past), but if I remember right once you know the type of diode it is the specs don't really matter, so long as it's rated to handle at least up to the voltage and current it's going to see in the circuit.

So what I'm trying to figure out is what kind of diode this is. For one, I suspect it's either a signal diode or zener type, and so long as I get one of those (whichever type it is) that can handle say up to 9V and say a current draw of 100 or 200ma I should be fine here? All detectors I ever checked the amp draw on were very low, such as in the case of my current Sovereign GT of around 50 to 70ma depending on if it's sounding off to a target or not.

Still, been a long while since I played with diodes, so I need a refresher here and any input as to type and suitable replacement I might find off the shelf at the local Radio Shack. Thanks.

PS- Before removing the old one, I plan to just jumper a new one across the circuit to bypass it and see if the meter comes back to life before committing to soldering a new one in. I also checked the meter with a slight bumping of it to make sure the needle wasn't froze and it moves easily. Thanks in advance for any help here....

 

[pan4au]

does your meter has a diode function on it? Some do and some don't... the meter has to be able to foward bias the diode to allow current to flow through the diode

 

[pan4au]

to rule out or verify whether it's a zener diode or not check the polarity. a zener diode is a reverse bias device and the marking band(neg) would be soldered to the pos and not the ground portion of the circuit board.

since it physically damaged, I'd desolder it and take it to radio shack and see if you can match it up. after you desolder it... check to see if you can bias the diode with the meter

 

[Critterhunter]

Yes, the meter I used has a diode testing function, but funny you should say that...As I was suspicious after I made the post and did some further inspection. Turns out the diode is fine and the meter just wasn't pushing the current past the diode. I pulled out another diode and saw that the meter wasn't even sounding off with a known good one, so then I checked the diode another way and can see that it is working fine.

I dug further into the machine to be sure no other wires or such were loose anywhere. Back traced the meter again and can't find any shorts anywhere. I suspect the meter is fine but the driving circuit for it is bad. The CZ6a also uses it's ID meter as a battery voltage check, so I turned that function on and nothing. Pin point mode is also supposed to indicate depth. Turned that on/off and tried waving a coin in front of it to show depth. Nothing.

Not sure if this meter is using a simple voltage output to indicate target conductivity/depth/battery level, but I figured perhaps it does and the meter was just bad, so I hooked a volt meter up at a low voltage scale to the pin outs to the meter and tried all the above things again. Still nothing.

I even checked all the above steps with a volt meter hooked up in both DC and AC modes, just in case it was putting out some funky alternating current type deal that DC mode wouldn't see. I suspect the meter isn't bad though and that it's whatever on the board drives it.

Beyond my skill level now to go further with it. I can start checking a few resistors and stuff on the board but don't really want to dive that far into it. Recommending he sends it back to Fisher to get fixed. Better yet, going to recommend he sends it to a guy who tunes CZ models for more depth if the guy can also handle fixing the meter problem as well.

The detector works perfect. Just no meter movement. I didn't check with headphones on to see if his PP switch is working as it should but I suspect it does since he mentioned only the meter didn't work but otherwise he could hunt with it. He did mention he still has the 3 tones though, so I suspect if PP wasn't working he'd mention that too. I should probably call and ask on that. Never plugged in headphones while I was messing with it.

 

[pan4au]

. Recommending he sends it back to Fisher to get fixed. Better yet, going to recommend he sends it to a guy who tunes CZ models for more depth if the guy can also handle fixing the meter problem as well.

That sounds like the best option . There's so many factors to consider. a trace could be damaged from the excessive heat.. Some component could be damaged from ESD (Electrostatic discharge) CMOS are extremely sensitive to ESD. Best of luck and I hope to problem is soon resolved.

 

[Critterhunter]

Yep, without a schematic now to go further I'd be lost as to what to check function wise. I can check individual components on the board, but without knowing what should be doing what at various points of function it's going to be like trying to find a place without a road map.

I've found many problems on circuit boards in the past that were due to just simple cold solder joints, but since this machine worked flawlessly for years, up until his brother soldered in that new PP switch, has to be he either cooked something or zapped a part with static electricity I would guess. Just too much of a coincidence that the meter stopped working right after he put in the new PP switch.

I also noticed that it would be very easy to short something out by bumping the board to the RF shielding if he didn't make sure the removed board wasn't laying on the shield partly when fired up for a test run after doing the job. Maybe he still had the board pulled out of the control box and figured he'd fire it up before putting it back together to make sure the PP switch was working and the RF shield shorted it.

This thing has two boards stacked face first to each other like a sandwich. I just don't feel like removing those to I can inspect further for any more possibilities. Not without knowing what pathways should be doing what without a schematic to guide me.

 

[jimmyjiver]

Hi, run the battery test to confirm if the meter is still functioning right.....Pictures would also help out... I will see if I can find my schematic......JJ

 

[Critterhunter]

Did that before. No dice. Even hooked a volt meter up to output to the meter to see if any juice was showing up, or perhaps if the meter is using simple voltage levels to indicate target ID or depth it'd show something. Used both the AC and DC function. No power I can see going to the meter. The meter's needle moves if I bump the housing so it's not froze either. If you can dig up a schematic I may go further with it if I can see some obvious check points on components to check, but my background in component level parts is spotty. Been a number of years since I've read up on various things. I currently work in an electronics related field troubleshooting various components both usually this is on a much larger scale (large capacitors, contactors, relays, safety controls, pressure switches, etc). Would need to do some reading to fill in the blanks on certain board level components to check and refresh my memory on.

 

[jimmyjiver]

Hi, Did he put a factory replacement pinpoint switch in? The problem should be related to what he done while installing the new switch...You should not have to remove any boards to replace this momentary switch....On the meters plus side there is one resistor and one grey wire going to it...They are tied into a potentiometer...You should be able to set your ohm meter on low ohms and touch the meter connections very briefly and see the meter deflect.....JJ

 

[Critterhunter]

Yes, factory PP switch. They had to order 5 at a time several years back to be able to get them from Fisher. Didn't think about the ohm meter mode to juice the meter and see if the needle moves. I'll try that here. Yes, appears a resistor coming off both the PP switch and meter if memory serves here. Also one going from meter/PP to a diode and then to ground I would guess (unless this thing is laid out with the positive being the main trace for stuff).

I had suspected that diode was cooked where he soldered in the new PP switch since it was close to that particular solder point, but realized later my diode meter on my meter wasn't working right, after I used a brand new diode I had laying around to test that on the meter and got the same impression via meter reaction that it was bad too. Flipped the meter leads both ways to be sure polarity wise as it's been a while since I messed with diodes and couldn't remember which polarity direction of current flow the band indicated.

Now that I'm thinking of it, I should do an ohm reading on that PP switch and see if it's even working (non-energized of course when doing an ohm reading). Might be the PP switch is funky and it isn't switching modes back and fourth and making the machine not act right that way perhaps here...