BOUNTY HUNTER III

[Dr Vel]

Just got one on ebay, not working due to a bad coil. Also missing the gold anodized battery cover. Does anyone have a junked one they would sell the battery cover from, and if anyone has a schematic this would be useful. The cover looks identical to the one on my BH Rustler except it is gold not the Rustler's blue, meaning maybe several old BH models could provide a cover if anyone has one to sell. I was going to use my 12 inch Jetco coil to make a coil for the HH PI but it works on this BH III just great by altering the on board adjustable inductor for the frequency range of the old Jetco coil. In fact so well the distance is way better than the BH III specs using the stock 6 inch BH coil (which is defective anyway). Right at the moment this BH III using the 12 inch Jetco coil detects deeper for reasonably large metal items than my 20 odd other detectors. Not bad for my first BFO MD! Guess that puts me back to building a coil for my in progress HH PI project (bought the lone Jetco coil to use in the HH endeavor). Anyway a schematic would be nice, if no one has one I'll just end up tracing. My problem here is the 8 pin dip IC does not come up in searches. It has two rows of print:

RC1870N (the 1 could be an I and the 0 could be a D, severely worn print)
RAY7228

Any info on this IC would be invaluable in my reverse engineering of the circuit. The JFET TIS58 and the NPN 2N2925 I found data for. All I need is the IC data to draw the schematic.

The gold anodized battery cover however is a major problem to replicate. I would take one that fits of any color, blue being to most common I am guessing.

[Dr Vel]

Too late to edit last post, after looking with a magnifying glass the number for the 8 pin dip is RC107DN. This MD model was built in the 1970's so it is an older part number, but my searching has been in vain so far. If anyone at least knows what this IC is I would be happy as I am still in the dark. My best guess is an opamp similar to the LM741 but the number has not shown up in my searches other than dead end links to various discrete transistors which I know is incorrect. If anyone has a really old databook maybe they could scan and post any pages on this IC.

[porkluvr]

Sounds like possibly a Raytheon part similar to LM107 (extended temperature range LM307) which is indeed very close to LM741 (originally the Fairchild uA741).

Different manufacturers were solving the same problems with slightly different techniques at around the same time. So, LM741 and LM107 are not exactly alike but have reasonably similar specs - the main difference being in the offset null connections.

RC107 = LM107 is a possibility but I have no historical reference to be sure.

[Dr Vel]

Thank you for your reply. I agree, so far drawing the circuit seems to indicate this is correct. Only other question I would have is does anyone know what frequency this model operated at. The search coil is open circuit so I have no inductance measurement to work with. The local oscillator is running at 1.117 MHZ, not well versed in BFO types but this seems high for a metal detector. Considering both oscillators feed pins 2 and 3 of the opamp I had doubts about what old number would run well up to 1.2 MHZ. Seems high for an LM741 but I need to get a PDF of the 741 and 107 and look at them. Still in desperate need of a battery cover.

[Ed in SoDak]

I have a few substution books from the early 80's and the RC107DN is not listed. The LM107 crosses to an RCA part SK3596, an 8-pin single opamp.

I have old Bounty Hunter manuals here: http://www.whiteriverprep.com/vintag...rdetector.html

The BH-III manual is here: http://www.whiteriverprep.com/vintag...hI_II_III.html

-Ed

[porkluvr]

Ahhh. Thank you Ed.


edit: retracted my "told you so" because there is no direct reference to RC107 anywhere.

This may not have any relevance but just for info, the LM101 is same as LM107 except for having no internal compensation... meaning inherently higher slew rate but the amplifier will not usually be unity gain stable. These are "ancient" devices (late '60s) but still very common today.

[porkluvr]

The opamp has no negative FB and so is acting as a comparator. Should be well within the capability of a '307 to output an audio tone at
the difference signal.

Main problem is, how did they keep the oscillators from synchronizing to each other?

Supposing they used a 100uH coil with about 180pF parallel capacitance, that would make the transmit frequency to be around 550kHz and
probably do a lot to keep the oscillators from locking to each other.

This simulation has the lo running at about 1.11Mhz and the tx is running at about half that.

BFO "tuning" is kind of time consuming so I won't try to pin this down any closer.

[Dr Vel]

"Main problem is, how did they keep the oscillators from synchronizing to each other?"

If I had to guess I would say they attempted to do so using the 15K and 30 pF SM in series on each signal source?

I was running it today doing tests and I can tell you there is a locking issue. It is very strange and annoying at the same time. Symptom is right at zero beat, (+- 2 on scale due to high gearing of the tuner), it appears to go dead. Nearing highly conductive metal of relatively small size such as a silver dime there is a very small but noticeable oscillation of much higher frequency. I can only describe it in terms of regenerative receivers I used to play with back in the 60's. Actually I am not quite sure how to describe it other than oscillation way beyond the tone you hear nearing larger metal objects at this threshold (where it seems to come back to life detecting again). In this window of insensitivity it does not detect well if at all other than if you listen closely you can tell it sees metal but does not act like it is detecting properly. Not a problem if you are looking for items slightly larger than say a quarter. I wish I had a way to determine the inductance of the original 6 inch coil. A circular ring PCB with a spiral foil pattern where I can only see the turns edge on where it was cracked by someone trying to pry the coil apart. I would need to spend hours with a dremel as this ring is encased in a solid block (ring) of white plastic just wider than the pcb and a half inch thick. I need to expose a section of the board to count the turns. Imagine the pcb ring suspended centrally in a mold form which was filled with molten plastic then cooled. I cannot tell if a capacitor exists inside where the leads entered (the lead in wires were ripped out) but I do not think so. My 12 inch Jetco coil is 70 uH with a compression tubular trimmer in parallel and for all I know I have it working on a much different harmonic than the original design. Virtually useless for dime sized objects but uncanny depth for larger items. No way to alter the 12 inch coil but if I can determine the L of the original coil possibly I can alter the LO to get back to the original design harmonic ratio even if it ends up operating at a different frequency than stock design. At least I think this is why I have this strange dead zone at zero beat. If you get used to a slight tone all the time with low volume (said tone on either metal or mineral side of zero beat depending upon how you are running it) combined with good ears for pitch differences this thing really is impressive for distance. Vastly superior to my BH Pioneer 101 with 8 inch coil for things a silver dollar or larger.

Looking at this post I think I did not do well describing what I think this locking effect exhibits. A narrow dead zone where it seems to turn into a radio receiver of the Super-regen type weakly and sporadically as it nears small conductive metals items or the ground depending upon which side of zero you are running, more pronounced on the conductive pitch rising side.

Still looking for a battery cover.

[Dr Vel]

"Main problem is, how did they keep the oscillators from synchronizing to each other?"

If I had to guess I would say they attempted to do so using the 15K and 30 pF SM in series on each signal source?

It was late, forgot to mention; the decoupling plus choice of harmonic.

[Dr Vel]

"Supposing they used a 100uH coil with about 180pF parallel capacitance, that would make the transmit frequency to be around 550kHz and probably do a lot to keep the oscillators from locking to each other."

If you look at the two 680's in series this would be 340 pF to ground. This in parallel with the 300 pF giving 640 pF at the BNC jack on the box which connected to a cable that went into the old coil (had been ripped out of the coil). In effect between the BNC hot to ground is 640 pF, these caps are OEM on the board so that value is not a variable. The only unknown I can see is the coil inductance. I used a few feet on RG-174 sized coax which is pure silver stranded center conductor and pure silver shield braid, all insulation being teflon. No idea what this coax cost in it's day but I love this stuff for projects such as my BH III. Anyway the point I wanted to make is the inductance of the OEM coil must have been well below 100 uH. The 1,117 MHZ I measured for the local was before I touched any adjustments. I removed the coil and tuning capacitor running them through their range using an LCR meter to find the values, first noticing the factory set the coil to 58 uH after removing it from the PCB and measuring without touching the adjustment. The tuning C is mid range at 50 on the dial scale.

Note to Admin: I would like it better if the 60 minute edit time was more like 24 hours to avoid multiple posts.

[Qiaozhi]

Note to Admin: I would like it better if the 60 minute edit time was more like 24 hours to avoid multiple posts.

The 60 minute edit time is already more than generous. In fact, it used to be much shorter (something like 10 minutes) before it was extended.

If the edit time is made too long, then several members may have posted replies. During which time you could have gone back and edited your original post, thus causing some discontinuity in the discussion.