SIGMA TEK (ARC) 300, 400, 800 SERIES TRANSPONDERS Types RT-359A, RT-459A, RT-859A. Service Letter No. 21. External Suppression (EXT SUP) Modification

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1 SERVICE LETTER DOC. NO. 100L021.DOC SIGMA TEK (ARC) 300, 400, 800 SERIES TRANSPONDERS Types RT-359A, RT-459A, RT-859A Service Letter No. 21 External Suppression (EXT SUP) Modification RT-359A Mod 6 RT-459A Mod 6 RT-859A Mod 7 EFFECTIVITY PURPOSE This Service Letter applies to the following ARC Transponders: RT-359A P/N (all dash numbers) RT-459A P/N (all dash numbers) RT-859A P/N (all dash numbers) The purpose of this Service Letter is to describe the procedures necessary to incorporate External Suppression (EXT SUP) capability into the RT-359A/459A/859A Transponder. This service letter contains all the necessary information for the modification. It will be helpful, however, for the service shop performing the modification to have a copy of the 300, 400, 800 Series Transponders Service/Parts Manual (P/N ). THEORY OF OPERATION EXT SUP connects to the existing mutual suppression (MUT SUP) bus in an aircraft. MUT SUP equipment (DME or ATC Transponder) sends a suppression pulse to this bus during a transmit. In units without suppression capability, false interrogations can occur due to RF overloading of one's receiver by another's transmitter. In an EXT SUP Transponder circuit, the suppression pulse deactivates the gate that normally triggers the Modulator Multivibrator circuit. Consequently, no waveform reaches the transmitter and the EXT SUP Transponder does not reply. EXT SUP is a uni-directional bus and does not generate a pulse for the MUT SUP bus. Sigma Tek, Inc Industrial Road Augusta, KS TEL (316) FAX (316)

2 TOOLS AND EQUIPMENT REQUIRED Installation of the External Suppression Mod requires no special tools or equipment. Standard equipment required is: NEW PARTS REQUIRED Screwdriver -- small standard Soldering Iron -- approximately 30 watts Solder -- 60/40 Tin/Lead Wire Cutters Wire Strippers -- for #22 AWG Installation of External Suppression requires the following parts: Qty Item ARC P/N Ref Des 6" (150 mm) Wire, #22 AWG, Brown Diode, Zener CR18 1 Resistor, 2.0k, 1%, 1/4 W R129A 1 Resistor, 4.32k, 1%, 1/4 W R129B AR Tubing, Heat Shrink REFERENCE INFORMATION 300, 400, 800 Series Transponders Type RT-359A, RT-459A, RT-859A Service/Parts Manual - P/N , Change 1 or later. DISASSEMBLY Reference Figure 7-1 of the Service/Parts Manual. Remove the six screws (item 8) retaining the top cover (item 7) and then remove the top cover. Remove two screws (items 13) and remove the protective plastic (item 14). COMPONENT PLACEMENT Cut resistor leads of R129A approximately 1/2" (13 mm) from the body of the resistor. Cut leads of CR18 and R129B approximately 1/4" (6 mm) from the body of each component. Solder components in place and cut trace as shown in Figures 3-1 and 3-2 on the following page. Ensure component leads do not short to any traces. PAGE 2

3 Figure 3-1 Back of A4 Board (component placement) Figure 3-2 Detailed view of component placement PAGE 3

4 FIGURE 4-1 Modified schematic ASSEMBLY Replace protective plastic, screws, and top cover. Tighten retaining screws FINAL PROCEDURES Reference section 5-11 of the Service/Parts Manual and perform the Decoder Multivibrators test. Next, test the unit to the additional steps: 17. Apply a 7 Vp-p EXT SUP pulse to pin J1-4. The pulse should coincide with the interrogations (Standard/Backup Mode A and Mode C). Check that zero replies are generated. 18. Repeat step 17 with a 15 Vp-p EXT SUP pulse. Check that zero replies are generated. Perform other tests as required to determine conformance to applicable standards. Stamp out mod level "6" for RT-359A/459A or mod level "7" for RT- 859A on the mod sticker located on the rear of the unit. Fill out attached Modification Record Form and return to STI. INTERCONNECTION Connect a single shielded wire (ARC P/N , or P/N , or equivalent) from the mutual suppression bus to pin J1-4 of the RT-( )59A Transponder. PAGE 4

5 MODIFICATION RECORD FORM INSTRUCTIONS: Complete information below and return to Sigma Tek, Inc., Attention: Manager Quality Assurance. This form can be returned by mail or by fax. UNIT MODEL NO. / PART NO. UNIT SERIAL NO. MOD NUMBER INCORPORATED MOD INCORPORATED BY: SERVICE SHOP NAME ADDRESS TECHNICIAN DATE INCORPORATED FORM NO. E1003C Sigma Tek, Inc Industrial Road Augusta, KS TEL (316) FAX (316)