Coinmaster 6/DB detector troubleshooting

[Croustafa]

Hello all
I’m brand new to the forum and I’ve already found some valuable info on the site. I have a Coinmaster 6/DB Detector. The last time I had it out was ≈ 15 yrs ago and it worked fine; it has been stored in a garage ever since. I have found the schematic on this site…but I wonder if anyone has any detailed troubleshooting tips/measurements.
The only thing that seems to work is the battery check for 9V & 12V which registers good on the meter. When I go to perform the “G.E.B. Tuning Indoors” procedure per the manual, I get nothing after the battery check, no tone at all, despite adjusting the tuning & volume. I have pulled the knobs and used Deoxit on the 2 switches and WD40 on the 4 pots. Everything is a lot smoother except the Discriminate Adjust potentiometer; it is really hard to adjust and the inside case nut was not right, so the shaft barely sticks out. I’m going to de-solder that potentiometer and see if I can free it up or replace it. I would like to test the detector loop, it feeds the case with a 5-pin connector. Can anybody provide the correct values at this connector or any other troubleshooting tips?
The circuit board looks clean, the meter, speaker and battery pack connections are all soldered secure. Thanks in advance for any information that might get this classic to work again.

[Carl-NC]

Below is the service manual for the 6DB. A major problem with the old White's models was the rotary switches used for mode selection and battery check. They eventually go bad and there is no replacement. I assume you sprayed cleaner inside the switch housing? It could be the coil but I don't know what the parameters should be. The best thing is to get an oscope and probe around to see what is and isn't working.

COINMASTER 6DB COINMASTER 6DB HIPMOUNT SERVICE MANUAL.pdf

[dbanner]

Does the meter respond to any metal target? Try plugging in an earphone.

Further checks:

Check for power supply voltages(test points 1 and 2 on schematic). check for +7V and -6V respectively.

[Croustafa]

1). Thank you for the service manual link!! Yes, I sprayed Deoxit in both rotary switches & worked em’ back and forth. The Deoxit and WD40 treatment really loosened up everything as it’s been sitting, unused for 15 yrs.

2). The meter does not respond to any metal. The meter only deflects when in battery test mode. No tone or sound at all. No deflection of meter in any setting, no response to metal target, even w/ headphones

I will check the PS voltages.

I’ve never felt a potentiometer so tight as the Discriminate Adjust. T-shooting that now. Again, thanx for your inputs

[Skippy]

The usual problem components include the electrolytic capacitors. They dry out, resulting in low capacitance, high internal resistance, and general poor performance. Consider replacing all of them. Any that are clearly power-supply decoupling ones could likely benefit from being replaced by ones with larger capacitance / lower ESR . The smaller ones may be value-critical, so they need to be the correct capacitance value, though they could be higher voltage-rated ones.

[Croustafa]

Update:
I was able to desolder the Centralab pot and repair it; it now turns freely. The White’s repair facility I spoke to said don’t ever use WD40 on pots because it’s oil-based and won’t evaporate. So I used Deoxit to clean out the four pots. Detector still has the same symptoms but I now suspect the detector loop is bad. I used an ohmmeter to check the 5-pin connector.
pin 1&2 is the RX coil, pins 4&5 are the TX coil. Pin 3 is ground/shield. 1-2 is 20 ohms, 4-5 is 20 ohms. 3 is not shorted to ground but it is shorted to all 4 other pins and the individual coils are shorted to each other. The detector loop is in real good shape so I’m not sure why it went bad. There are several electrolytic caps that I will probably replace if I can get a replacement loop.

[Skippy]

Coils can suffer from long-term shrinkage of the resins used internally. This can move the copper wire coils, disturbing the induction balance, enough to stop the detector working. Sometimes it's possible to use ferrite and/or small non-ferrous objects near the coil to disturb the induction balance back into 'normal' operating range.
The other obvious problems with coils relate to the use they get : flexing / strain can fracture wires, cause wires to short together, break solder joints on the connector.

[Croustafa]

Thank you Skippy
I’m looking for a used loop at this point, I just do t know how interchangeable they are among the multitude of White’s detectors.
I wonder if I can break the seal on my loop and see if there is damage, especially where the 5-wire connector enters the loop.

[ivconic]

Post #2 on this page: https://www.geotech1.com/forums/foru...coil#post91380

[Qiaozhi]

Update:
I was able to desolder the Centralab pot and repair it; it now turns freely. The White’s repair facility I spoke to said don’t ever use WD40 on pots because it’s oil-based and won’t evaporate. So I used Deoxit to clean out the four pots. Detector still has the same symptoms but I now suspect the detector loop is bad. I used an ohmmeter to check the 5-pin connector.
pin 1&2 is the RX coil, pins 4&5 are the TX coil. Pin 3 is ground/shield. 1-2 is 20 ohms, 4-5 is 20 ohms. 3 is not shorted to ground but it is shorted to all 4 other pins and the individual coils are shorted to each other. The detector loop is in real good shape so I’m not sure why it went bad. There are several electrolytic caps that I will probably replace if I can get a replacement loop.

You should check the coil windings with an inductance meter, not just an ohmmeter. Otherwise you might be misleading yourself with the measurements.

[Carl-NC]

Another test trick is to disconnect the coil, turn on the detector, lick your finger, and poke your finger into the coil connector on the main box. This will cause direct coupling between the TX and RX pins and the detector should respond with some random audio burps and belches. If it does, then the coil is likely bad. If it does not, then the problem is likely in the main circuit. For this test it's best to have the detector in an all-metal mode like GEB Normal.

As I'm writing this, it occurs to me that even with the coil disconnected you should be able to adjust the Tuner to get an idle threshold audio. The Tuner knob is actually a Threshold knob. If you can't get a threshold audio then the circuit is faulty.

[dbanner]

the shield should be connected to ground inside the loop. if it is shorted to all other pins( beside the rx ground pin) then the loop is likely bad. Loop surgery is required.

you should be able to get a constant threshold sound by adjusting the tuner pot even without the loop connected. could it be that your loop shorted and took out some component(s) on the board.

Any +7V and -6V?? A catastrophic failure of Q4 could take out the positive rail.

[Aleks85]

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[Croustafa]

Thanks for all your suggestions. I’ve tried dbanner’s suggestion and there was still no tone. But one of the selector switches is kinda flaky as well. I will test voltage if I can get the coil working.
thanx again.

[Gwil]

I think I'd check all the supply voltages first, it's quicker.