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White's 6000/DI Pro SL Adjustment/Repairs?
Recently acquired a 6000/DI Pro SL on local flea marked place for very cheap (~$60), that seems to have spend the last 25 years in the basement/cellar, without much or any use.
Basically original packaging with no sign of usage. Not sure, whether it was just bought and forgotten or if the previous owner stashed it away, after it didn't work as expected.....
Since I don't have any C-Type batteries at the time (box of 20x C-Type NiMH batteries and charger is ordered), I powered it with linear power supply, about 200mA current draw @6V, which matches what I read elsewhere.
Also found the schematics for it ;-)
Now the problems, GEB/NORM Mode with everything else on preset sees to work fine.
Once I press the trigger switch on the handle, it starts beeping, then needs a slight adjustment of SIG-BALANCE pot in clockwise/upwards direction for the beep to stop.
With adjustment, it detects coins up to 30cm in air test, depending how carefully the SIG-BALANCE is done.
Press trigger again, it starts beeping again, the same SIG-BALANCE adjustment is required again.
For each trigger press you need to turn SIG-BALANCE slightly more upwards for beep to stop, unless you turn down SIG-BALANCE and press trigger.
I.e. you always need to turn SIG-BALANCE slightly more upwards, each time you press trigger switch, independently of SIG-BALANCE position.
If balanced in GEB/NORM Mode and switched to GEB/DISC Mode, it beeps wildly once the trigger is pressed, meter needle swings around on the left side and generally neither adjustments or trigger switch can stop it from beeping.
Only stops beeping, once you return to GEB/NORM, balance it and return to GEB/DISC.
Sensitivity in GEB/DISC seems poor, too, after switching from GEB/NORM.
Auto-Balance switch is in preset/GND position.
Would really like to fix that thing, to hunt some lost brass on local shooting range.
Some adjustments are obvious, like R170 for +/- split voltage supply adjustment around virtual GND provided by R172/R173, U14/LM324 and C90.
But other adjustments like R5 are not that obvious, some kind of receiver coil gain adjustment, along with phase lag adjustment, because of the 47nF/C1 in the path towards GND.
Capacitors so far mostly check out fine, that constant need for adjustment of SIG-BALANCE in GEB/NORM mode seems more like that a resistor has degraded out of spec.
Like a constant error.
Or too large offset voltage in some OpAmp?
Is there a service manual somewhere out there or perhaps some of the former engineers can tell me a bit more about the function of the different parts in the schematic?
And what to look for, adjustment procedures?
The whole auto balance stuff in GEB/DISC seems to be based on CA3140 with guarded traces for low leakage and CD4016/CD4053 that also need to be equal low leakage.
Circuit board seems clean, with some kind of conformal coating, so I'm not ready to clean it in IPA and dishwasher, yet.
Maybe also a CD4016/CD4053 has become leaky, which might be rather hard find.....
Basically original packaging with no sign of usage. Not sure, whether it was just bought and forgotten or if the previous owner stashed it away, after it didn't work as expected.....
Since I don't have any C-Type batteries at the time (box of 20x C-Type NiMH batteries and charger is ordered), I powered it with linear power supply, about 200mA current draw @6V, which matches what I read elsewhere.
Also found the schematics for it ;-)
Now the problems, GEB/NORM Mode with everything else on preset sees to work fine.
Once I press the trigger switch on the handle, it starts beeping, then needs a slight adjustment of SIG-BALANCE pot in clockwise/upwards direction for the beep to stop.
With adjustment, it detects coins up to 30cm in air test, depending how carefully the SIG-BALANCE is done.
Press trigger again, it starts beeping again, the same SIG-BALANCE adjustment is required again.
For each trigger press you need to turn SIG-BALANCE slightly more upwards for beep to stop, unless you turn down SIG-BALANCE and press trigger.
I.e. you always need to turn SIG-BALANCE slightly more upwards, each time you press trigger switch, independently of SIG-BALANCE position.
If balanced in GEB/NORM Mode and switched to GEB/DISC Mode, it beeps wildly once the trigger is pressed, meter needle swings around on the left side and generally neither adjustments or trigger switch can stop it from beeping.
Only stops beeping, once you return to GEB/NORM, balance it and return to GEB/DISC.
Sensitivity in GEB/DISC seems poor, too, after switching from GEB/NORM.
Auto-Balance switch is in preset/GND position.
Would really like to fix that thing, to hunt some lost brass on local shooting range.
Some adjustments are obvious, like R170 for +/- split voltage supply adjustment around virtual GND provided by R172/R173, U14/LM324 and C90.
But other adjustments like R5 are not that obvious, some kind of receiver coil gain adjustment, along with phase lag adjustment, because of the 47nF/C1 in the path towards GND.
Capacitors so far mostly check out fine, that constant need for adjustment of SIG-BALANCE in GEB/NORM mode seems more like that a resistor has degraded out of spec.
Like a constant error.
Or too large offset voltage in some OpAmp?
Is there a service manual somewhere out there or perhaps some of the former engineers can tell me a bit more about the function of the different parts in the schematic?
And what to look for, adjustment procedures?
The whole auto balance stuff in GEB/DISC seems to be based on CA3140 with guarded traces for low leakage and CD4016/CD4053 that also need to be equal low leakage.
Circuit board seems clean, with some kind of conformal coating, so I'm not ready to clean it in IPA and dishwasher, yet.
Maybe also a CD4016/CD4053 has become leaky, which might be rather hard find.....
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