Defective Compass 77b detector..........

[Melbeta]

Okay, bought a Compass Yuknon 77b detector on eBay, The older one that tunes in metal and minerals, seller said it worked and he found a quarter, then the "batterys went dead" on him. It has a taped up coil coax connector, so I think it maybe experienced a short.

He adjusted the sale, real quick, so I think he knows more than he said...

Anyway, I replaced the lousy battery packs with new battery packs. He had brand new 1.5VDC batterys in it, each measures out good, the packs measure out 6.1+ VDC, so no problem with the batterys/packs.

The battery check, when the switch is on, measures each battery pack, so that appears to be good. The meter displays that good reading. However, the adjust meter needle in center, does not appear to work.

I have cleaned out each pot and switch with TV tuner spray, including cleaning out the mercury speaker disable switch. The mercury switch seems to work okay. I may jump it tomorrow to take it out of the circuit. I will resolder every solder connection, to remove any cold joints from the repair.

The unit appears to not see any metal with the coil. It has no sound, no audio, no buzz, except for the battery readouts on the meter, it appears to be dead. I have the 10 turn fine tune tuner, set at the mid point, 5 turns in from the right side. I have the coase tuner set in the middle, others are set where they should be.

Have pulled the entire guts out of the case, and will begin to resolder every solder joint tomorrow, and read the pots, as there seems to be no indication of ohms on the pots.

The Compass 77b schematic is not accurate, either the person that made it, made mistakes on it, or he had a different model machine. So I am working without a good schematic.

Anyone have a good schematic? One with voltage readings? One with ohm readings? One with voltage drop readings? If you do, I sure can use it.

Or any suggestions, that are good suggestions. Anyone have a similiar repair problem? Would save me a lot of time you know...
Melbeta

[Qiaozhi]

Anyone have a good schematic? One with voltage readings? One with ohm readings? One with voltage drop readings? If you do, I sure can use it.

Here's the schematic I have in my collection. It looks like an original, and not a back-engineered one, so hopefully it's correct.

[technos]

seller said it worked and he found a quarter, then the "batterys went dead" on him.

I think that we found the problem. Now the question is what caused it. It's within the realm of possibility that this detector did, in fact, work. But powering it up took out some components domino style. I think many of the detectors from this era (especially compasses) sold really are not working, or working very poorly. Moisture and leaky batteries seem to be some of the more obvious problems.

The schematic that was posted was for the "77b AT" (automatic tuning). There are different models of this detector, and you could have the simpler straight "77b". But there should be a lot of commonality.

[Melbeta]

My unit is much older "straight" Yukon 77b machine....

I think that we found the problem. Now the question is what caused it. It's within the realm of possibility that this detector did, in fact, work. But powering it up took out some components domino style. I think many of the detectors from this era (especially compasses) sold really are not working, or working very poorly. Moisture and leaky batteries seem to be some of the more obvious problems.

The schematic that was posted was for the "77b AT" (automatic tuning). There are different models of this detector, and you could have the simpler straight "77b". But there should be a lot of commonality.

Yes, that is the same schematic I have, and it indeed is for the last revision of that model (which ended before 1985 and they considered reviving in 1985 if they had sufficient orders from dealers, which did not happen), which had a automatric tuning built into it, but mine differs from that machine. The machine I have, is marked Copyright 1973, and it has a 0014 main board number, the power supply section does not have any number. Yes, it is possible that this machine really did find a quarter by the seller on eBay, anything is possible.

I have even removed the ribbon cable connector, that is on the main board, wire brushed it with a brass wire brush, so the contacts are shiny, still does not work. Do not see any signs of any part being burned out anywere. Is pretty much simple analog discrete parts, so the fault can be anywhere. Mine is the straight old 77b Yukon model.

[Qiaozhi]

Yes, that is the same schematic I have, and it indeed is for the last revision of that model (which ended before 1985 and they considered reviving in 1985 if they had sufficient orders from dealers, which did not happen), which had a automatric tuning built into it, but mine differs from that machine. The machine I have, is marked Copyright 1973, and it has a 0014 main board number, the power supply section does not have any number. Yes, it is possible that this machine really did find a quarter by the seller on eBay, anything is possible.

I have even removed the ribbon cable connector, that is on the main board, wire brushed it with a brass wire brush, so the contacts are shiny, still does not work. Do not see any signs of any part being burned out anywere. Is pretty much simple analog discrete parts, so the fault can be anywhere. Mine is the straight old 77b Yukon model.

Probably your best solution would be to trace out the schematic from the PCB. Then it should be relatively straightforward to identify the problem.

[Melbeta]

Schematic software...

Probably your best solution would be to trace out the schematic from the PCB. Then it should be relatively straightforward to identify the problem.

I have spent the morning, identifying the parts, on the component side, and now am ready to begin making a schematic from the trace side.

Is there a easy to use software, that I can obtain from a download, to begin the schematic? The power supply seems to be more complex to make a schematic then the motherboard PCB, which should be pretty easy.

What do you suggest?

[Qiaozhi]

I have spent the morning, identifying the parts, on the component side, and now am ready to begin making a schematic from the trace side.

Is there a easy to use software, that I can obtain from a download, to begin the schematic? The power supply seems to be more complex to make a schematic then the motherboard PCB, which should be pretty easy.

What do you suggest?

Everyone has their favorite editor, but I would recommend using Eagle. It can be downloaded as freeware from here -> http://www.cadsoftusa.com/download.htm
There are Windows, Linux and Mac versions available.
The only limitations of the freeware version are:

• The usable PCB board area is limited to 100 x 80 mm (4 x 3.2 inches).
• Only two signal layers can be used (Top and Bottom).
• The schematic editor can only create one sheet.

For back-engineering of the Compass there should not be a problem.