Help repairing Fisher 1265x

[Koala]

kind of getting somewhere.


if I swing a copper coin over the coil while adjusting R329 at one point I get the same dirty broken sound.


Tried with fresh batteries still the same.


guess the disc bias is drifting over time ?

[dbanner]

Just a thought, but could it be that the discrim bias pot is faulty, or some other pot maybe the culprit. I've had experience with bad/ dirty tracks with pots which misbehaved in circuit causing all sorts of weird things to happen.

[dbanner]

Just another thought, I hope you don't mind, but 5 hours of operation followed by problem points to slow breakdown of IC or some other active component. What causes this to happen, high current. What causes high current? Bad capacitors among other things. Since caps do breakdown over time, and seeing this is older Fisher, It wouldn't hurt to change for new caps.

[kt315]

but 5 hours of operation followed by problem points to slow breakdown of IC or
--
Koala has to check all ICs and comparators. i did esspecial board for the quick checking job.

[Koala]

but 5 hours of operation followed by problem points to slow breakdown of IC or
--
Koala has to check all ICs and comparators. i did esspecial board for the quick checking job.

Doesn't fail completely just a slight drift of the Discrimination Bias

[Koala]

Just another thought, I hope you don't mind, but 5 hours of operation followed by problem points to slow breakdown of IC or some other active component. What causes this to happen, high current. What causes high current? Bad capacitors among other things. Since caps do breakdown over time, and seeing this is older Fisher, It wouldn't hurt to change for new caps.

its a strong possibility as I have already found one on this board that was stopping the static mode from working

[Koala]

Just a thought, but could it be that the discrim bias pot is faulty, or some other pot maybe the culprit. I've had experience with bad/ dirty tracks with pots which misbehaved in circuit causing all sorts of weird things to happen.

possible. Quck check and they seem okay.

given a quick spray of contact cleaner. Will have another proper look on Monday. See how it goes as detecting all day Sunday. but will take a spare machine.

[Koala]

inital settings as per

http://www.geotech1.com/forums/showt...0720#post80720

I get a tone instead of quite ?????

[dbanner]

Fully clockwise and anti clockwise pot setting could reveal a fault with pots/trimmers but especially pots, as some tend to go open(wiper) at end limits when they are old. Backing them off a bit from end limit puts them back on good track.

[Koala]

Fully clockwise and anti clockwise pot setting could reveal a fault with pots/trimmers but especially pots, as some tend to go open(wiper) at end limits when they are old. Backing them off a bit from end limit puts them back on good track.

POT appears fine 8v on one side and the centre pin smoothly increases as you turn it.

[dbanner]

Hi K, haven't you got it working by now, that pesky problem? I would refuse to accept defeat from the little circuit. Even if I had to leave it running for five hours, with an oscilloscope and three multimeters hooked up to the thing. Just to monitor any significant changes that stands out such as voltage changes or increased current consumption.

[dbanner]

Or it could be just the speaker
It's not unheard of.

[Koala]

Changed C310 and took it for a spin

No different


Field was full of foil. Stopped find the foil. Found by accident that if I work in all metal mode for awhile it start to work properly again.


Guess its going to be one of the larger capacitors as sometimes there is a delay of about 2 seconds switching from disc 1 to disc 2. Starts of crackly and then sharpens up. Was going to probe and keep alternating between each disc

Except the silly machine decided to stop doing that


Changed C306 on whim. Take it for another test drive on Sunday.

[dbanner]

It just occurred to me something. The 4066 is in the signal path. As a matter of fact, it's in the signal path of both the discrim 1 and 2. Just because it's switching properly from one to the next, doesn't mean that it isn't affecting the signal itself. It could very well be the problem.

[dbanner]

Yes, this could indeed be the problem.