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Originally posted by Qiaozhi
First you have to unsolder and remove the GEB and SENS presets from the PCB, and replace these with potentiometers on the front panel. The DISC control (labelled as Reject on the the original front panel) also acts as the on-off switch. It may be advisable to remove the DISC control and the headphone socket to allow room to attach the other controls. All front panel controls should be connected using flying leads. Although the GEB and SENS controls were originally multi-turn presets, there does not appear to be any problem replacing these with normal linear potentiometers. A switch is also required to allow selection between ALL METAL and DISC modes of operation. The coil was hardwired to the PCB on the SMART2, but the turbo version now boasts a chassis mounted connector to allow easy swapping of coils.
Here are the necessary circuit modifications:
1. Add a 10nF capacitor (C1) across C26 - near U7 (LM555N). This is used to lower the audio tone slightly as it's too high and rather annoying.
2. Add C8 (100nF) between the base of Q1 (audio driver) and SCR to provide a more stable output.
3. Disconnect the anode of D1 from U8B pin 5 and reconnect via S1 (see attached schematic) to Q1 base. This small modification greatly increases the sensitivity while improving discrimination. The detector has less chatter, and the threshold can be adjusted by the SENS control to allow easier identification of deep targets.
4. Add a R10 (15K) resistor between S1 (see schematic) and pin 2 of U7. The addition of this resistor provides a stable 2-tone audio output for ferrous/non-ferrous target identification. S1 is wired such that the 2-tone mode of operation only works in the ALL METAL setting. It would be possible to fit another switch to independently enable/disable the 2-tone mode if desired.
5. The turbo version was re-housed in an aluminium case, and it was necessary to connect the case to SCR to prevent instability. The original metal stem and plastic mounting was reused to mount the new case. The rivets first had to be driiled out and replaced with appropriate sized bolts.
The result is a lightweight sensitive detector that outperforms many other more expensive models. With the external GEB control it's easy to find the ground balance point, and the SENS control needs virtually no adjustment when in use. Overall the turbo'ed Rimatron is extremely stable and is surprisingly good in the field.
Air Test Results (with conservative measurements to lowest 0.5"/1cm)
Victorian penny = 10" (25cm)
American dime = 7" (18cm)
Coca Cola can = 17" (43cm)
Small hammered silver = 6" (15cm)
Large Roman coin = 10" (25cm)
In field tests the depth is still maintained. Signals from aluminium tent hooks buried vertically in the ground were easily detected at 10" (25cm). i.e. only the top of the hook was within range. The detector was also tested in an iron-infested area where identification of non-ferrous targets was good with the 8" coil. If any targets were difficult to identify, switching to ALL METAL mode allowed easier identification with the new 2-tone feature. It was also possible to distinguish between ferrous and non-ferrous objects that were very close together.
Conclusions
Although I'm not convinced by the use of OTAs instead of analog switches, and the use of a fixed DISC sample point, the detector does work well in practise, and that's what counts.
Enjoy!
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