Eric Foster Coil Design

[cristiano.sar]

I really like Eric Foster's coil designs, although I am in doubt about the rod support he always used, do you know of what material is it made? I'm in doubt in building something like that or if I should make the coil with the suppport everything in a single piece (vacuum formed) - I am not sure if there is some reason why eric foster always done it in as a separate part.
My main goal is to use this kind of coil in the beach.



Thanks.

[Carl-NC]

I assume the piece he used is Delrin, and I assume he did it this way because it's difficult to make good ears with vacuum forming.For beach/surf, I much prefer a center mount, like this:

[cristiano.sar]

Thank you, I also like that format with the center mount. Maybe in the future I design one similar.

[Goggy]

Hi gentlemen, visited the forum after a long time away and saw your post. Thought I would reply with an FYI as I worked at Pulse Technology (and PPD) with Eric.

We used PVC rod turned / milled into swivels. PVC would solvent weld to the vac form more efficiently.

I have an old SuperScan swivel (museum piece now ) which is a hybrid Delrin / PVC swivel. The holes in the vac form connection are so the the potting resin have a purchase for more strength with the heavier search coils

Hope this is useful. All the best, Chris.

[cristiano.sar]

Thank you for sharing all the details !

[GeoMax]

Hi Goggy,

thanks for sharing this informations with us. From the information you provided, we can conclude that Eric also manufactured his coil housings himself, correct?

[Goggy]

Hi GeoMax, yes, that is correct. Pulse Technology (PTL) and PPD (Eric) shared a building. One of the perks, was that the PTL workshop made his vac-forms for him while making the ones for the Goldscan and SuperScan (up to 12 inch). Over this size were made for us by a outside fabricator.

[Goggy]

Hi All, not sure if you have ever taken one of the search coils apart to check the construction? The Minipulse, SuperScan, GoldScan and EuroDec all used a two pot build regardless of size right up to the 21 inch. 'Ultralight' weight resin on a 1st pot and a skim 2nd pot of high density resin (Usually black) for wear protection.

[Altra]

Hi Goggy, Do you recall which coils had electrostatic shielding? If so how was it done?

Thanks for the information!

[Goggy]

Hi Altra,

The only coils which PTL had routinely electrostatic shielded were for the Goldscan and AquaPulse. I think Eric (PPD) also did this for his range of machines like the Beachscan, DeepStar, Aquastar, etc.. The early method for the coils used a conductive coating on the inside of the 'vacform''. It was a graphite loaded coating called 'Aquadag' (More commonly used to coat the back of Cathode-Ray Tubes for screening back in the day!!). It painted on like water and then dried to a conductive matt-black finish. It was quite a low resistance once dried. A piece of
insulated 16/0.2 stranded wire was then fanned out in contact with the coating and overpainted. This was connected to the coax screen once dried.

An improved method was used later on from this for the GS2 & GS4 (and probably the PPD search coils). This used a combination of the aquadag coating method and a fine gauged ribbon-cable wrapped around the coil winding and wrapped in tape once assembled. One end of the ribbon winding had all of its cores stripped back and shorted together, while the other end was left open and insulated. The shorted cores were connected to the coax screen by a short link wire. The soldered joints were kept as small and precise as they could. This was then mounted in the assembled vacform and 2-part potted like the others. It was important to scrape the coating or mask with tape around the lip of the assembled vacform to allow the 2nd pot to seal to the edges of the coil former with resin. Water ingress can occur if this step is missed out, creating issues of its own!.

As I see it, the aquadag coating method could be applied to any vacform and coil assembly to offer some electrostatic screening. However, the ribbon cable method, if I remember correctly can effect the search coils in certain scenarios, and ultimately, sensitivity and range.

Hope this helps.

[Altra]

Thanks for the detailed information. Sorry to bother you, but you are the last link to Eric's work. Do you know about what year the Aquasport and Aquapulse 1 were in production? I used to repair these in 1990's and have the old pcb's for these models. The Aquapulse 1 had three round pcb's stacked inside the pvc tube.

[GeoMax]

Hello Altra,

Please check the label on this circuit board. Eric usually uses the first two digits of the serial number to indicate the manufacturing year of his boards/detectors. Sometimes he adds a model series note between the year/month and the actual serialnumber. In the case of the AquaSport, the manufacturing year is 1984 and the serial number of the PCB/detector is 003 (see PCB/picture).

For example, for a GoldQuest SS detector. If the serial number on the device is 0307SS138, the manufacturing year would be 2003, the manufacturing month 07, the model series SS, and the serial number 138. Eric was pretty consistance with labeling serialnumbers of his detectors.

[Altra]

Thanks, I see the 84003 on one of the Aquasport pcb the other boards that I have the labels are missing. The date codes on the ICs for AP1 are 1979, 80 etc

[Altra]

While we are on the topic here is a photo of a coil I have. It came from a Goldquest. I added the 5pin connector for my TDI. It is roughly 8.5" or 21cm outer diameter. Inductance is 306uH 5.25ohm

[Orbit]

I had one coil of goldsacan 4 that had teeth so that the angle of the cheek could be moved and the other one was with the bracket from the picture made of pvc but this bracket entered the bracket that was on the coil, I opened the coil of gs 4 because as mentioned the thin layer of resin on which there was electrostatic protection the conductor somehow broke off from the first application and then I made a new bracket like the gs 5 bracket which was usually red in color myself on the milling machine, he made the same support from PVC, but in black color, so that he could touch these carbon lower handles.