Surf PI Pro

[Tepco]

Voltage change is good but lack of response is not. Assuming for now timing circuit is good, check pin 6 of 5534 input amplifier, and response to offset adjustment, should be around 0V and responding, with some very small change for large metal near the coil.

[sentella4d]

Hello Tepco;
Voltage at pin 6 of 5524 is at 0.01 volts offset,no response to metal,voltage stays the same.

[Tepco]

Forget to mention (not sure if you posted details earlier): First verify TX part of detector is working. Measure current consumption with and without coil connected, (around or less than 20mA without, typically 50-60 or so with coil depending of type), no change and low current consumption means TX failure or coil not connected, weak buzz from coil, or in magnetic headphone placed nearby can be heard if this is OK. Check if offset adjustment trimmer can change 5534 output voltage, also, if you have some permanent magnet at hand, move it across the coil to see if there is some response. In absence of TX pulse, but coil connected, detector will respond to this but fail do respond to metal.

[sentella4d]

Hello Tepco;
I can change the 5534 offset voltage.If I wave a magnet in front of the coil, there is no response.I tried measuring the consumption,but it seems old my meter is defective, not measuring milliamps.

[Tepco]

From this point troubleshooting without oscilloscope is very problematic (with it less than 45 sec.) but there are few more things you can try without equipment. First figure out if TX is working. Measuring consumption is easiest way (replace fuse in multimeter), headphone or even beer can near the coil will produce weak but audible buzz, but more things you can check this way: Disconnect piezo, connect one end to GND and with other end make some probing. Briefly touch coil output, you must get loud signal if TX is present. (555 and surrounding parts are good, you have audio, just MOSFET or connection can be problematic). Then probe U4 (4093) pins 3,4, for equal weak signals. This is first and second sample pulse, (inter-sample pulse is OK, audio is generated from it) same signal must be on 4066 pins 6 and 12, just to check proper connection\PCB. Probe 5534 pin 6, also some weak signal must be present. Lack of it indicate some failure around input amp, check for misplaced parts (1K and 1M reversed, input diodes wrong oriented etc). Unfortunately chip alone (may be faulty despite offset control works) , also 4066 chip cannot be tested this way, you can only try to replace.

[Tepco]

And forgot to mention, before all this, most ridiculous failure: check with ohmmeter integrity of coil\cable connection to PCB, actually to MOSFET drain (tab) and battery -. Behavior described here is about what you can expect from detector without coil connected.

[sukri01]

Hello , anybody here have osciloscoppe troubleshooting guide for the smpi pro? ive made the surfxx and james owen design and it works, it detects coins to 11" loud when threshold is minimum, Delay dont make much difference but as long it get good depth , ill be ok with that. but somehow when i go out with it test it for couple hours it started to make humm sound and detection become bad, both design became like that i wonder why??

[Harp]

I tried measuring the consumption,but it seems old my meter is defective, not measuring milliamps.

hello sentella4d,
it could mean you're blown fast fuse in the unit, usually of 315mA or 500mA. The area intended for the measurement of amps (not miliamps!), has no removable fuse.

[sentella4d]

Hello Tepco;
I performed all the test you requested with the piezo buzzer.On pin 6 5534 i get weak signal.On both IC's 4093(pins 3,4) and i get equal weak signals,tested on pins 6,12 of 4066,equal weak signals.I have no TX sound from coil connection.

Thanks sentella4d

[Tepco]

This is strange, actually very common on surf 1.2 version but in this version Q1 and Q2 are connected differently. 555 circuit and Q1 is good, MOSFET failure this way is very improbable (you have some signal from 5534 output). Check with piezo for signal directly at MOSFET drain connection (metal tab) and gate (goes to 555 pin 3) and use ohmmeter to check physical connection from drain to coil connector (for possible PCB track or connector problem), also from source to B+. With coil connected you must have only few ohms between drain and GND.

[sentella4d]

Hello Tepco;
Thanks for all the help.I am new to electronics,I am learning as I go along.You mentioned to "Check with piezo for signal directly at MOSFET drain connection (metal tab) and gate (goes to 555 pin 3)",Are you talking about Q1?. Also,I am not sure which pin is the drain pin for the Q1 mosfet.

[Tepco]

Some confusion with all this is unavoidable but let me clarify: First, I always trying to write any instruction purely “technical” as you can expect in most equipment service manuals, clear and unambiguous, only sometimes harder to follow but more detailed explanation can be added later. Also, some confusion started to grow here already, there are few different schematics and PCB layouts in circulation with somewhat different component markings.


In schematics on first page of this thread MOSFET is marked Q2, large IRF 9640 transistor with metal tab, Q1 on schematics i'm looking at (marked Q3 in original) is small plastic device (and everything seems to be fine with it). MOSFET orientation is as follow: imagine it lying on metal tab with a hole in it, plastic part of enclosure with marking facing upward, left pin is Gate, middle (also connected to metal tab) is Drain, right is Source. So with piezo touching this metal tab or middle pin (another end at GND) you must get loud tone.


With detector turned off, no power, and coil connected, measure resistance between this metal tab (drain) and GND (battery-), must be almost short circuit, only few Ohms, coil and cable DC resistance. By all chance coil is not electrically connected to this point, maybe due to PCB track broken, faulty connector, open circuit somewhere in cable\ coil connection etc, verify integrity of this connection first (this was the purpose of current consumption measurement, to see if coil is supplied with pulses at all). If this is good, check other physical connections around MOSFET, also there is some possibility of very atypical MOSFET failure (usually fail differently producing different symptoms).

[sentella4d]

Hello Tepco;
I went ahead and tested Q2 or 9640,against the drain pin or metal tab,and i got a loud tone.But when i tested between the drain and the ground, I got 390 ohms ,the same as the coil dumping resistor.
Thanks Sentella4d

[Tepco]

Exactly what I pointed in last few posts, coil electrical connection to detector is broken at some point. Just use ohmmeter to test every junction, from coil to cable, cable to connector, eventually PCB track from connector to MOSFET to find open circuit point.

[sentella4d]

Hello Tepco;
Since i tested the mosfet the coil connector at the PCB,what should I try next?