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Barry Mishkind - The Eclectic Engineer
Last updated 8/10/05


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Tom Osenkowsky shares:
Here are some notes I have compiled after many years experience with MRC-1600's at various stations:


  1. Remove Telco Input & Output boards from transmitter site unit and bring to studio.
  2. Label all boards. Place transmitter site Telco Output board into Studio unit.
  3. Connect a Freq Counter to TP-1 (U1-pin 11) on Telco Output board. Place S1-1 and S1-2 to OFF. Place S1-3 ON, S1-4 OFF. Normally, S1-1 and S1-2 are ON, S1-3 and S1-4 are OFF.
  4. Adjust LOW FREQ pot (R2) for 1070 Hz. (2025 Hz for Studio Telco Output board)
  5. Place S1-4 ON. S1-3 stays ON. Adjust HI FREQ pot (R7) for 1270 Hz (2225 Hz for Studio Telco Output board). Remove Freq Counter.
  6. Adjust OUTPUT LEVEL (R12) for 0 dbm (.775v rms) across L1/L2 on Telco Input board.
  7. Place S1-1 and S1-2 ON, S1-3 and S1-4 OFF on Telco Output board.
  8. Connect a Freq Counter to TP-1 (U1-pin 3) on Telco Input board. Place S1-1 and S1-2 to OFF. Place S1-3 ON, S1-4 OFF. Normally, S1-1 and S1-2 are ON, S1-3 and S1-4 are OFF.
  9. Adjust VCO FREQ pot (R5) for 1250 Hz (2125 Hz).
  10. Place S1-1 and S1-2 to ON. Place S1-3 OFF, S1-4 OFF. On Telco Output board, set S1-1, S1-2 and S1-4 OFF, S1-3 and S1-5 ON. Verify with scope connected to RX DATA (U1 pin 7), readjust R5 for symmetrical square wave.
  11. Return both boards S1-1 and S1-2 ON, remainder OFF. Now do next set of boards.


MRC-1600 was totally inoperative upon arrival at transmitter site.

Investigation began with power supply measurements. +5 volt supply read zero volts. +5 volt load (red wire) was found to be shorted (about 2 ohms to gnd). +5 volt supply is folding back. Problem traced to Analog/Status/Command board in rear of inside of MRC-1600. 

Cutting PC board traces revealed problem in U14 (74LS244) Status buffer. Replacing this chip alleviated +5 volt short. MRC-1600 now activates, but has question mark (?) like readings in window, status lights all lit, Failsafe, Setup, etc lights all lit. Unit does not respond to commands or resets. Problem traced to U11 (74LS245) on CPU board. Replacing this chip cures the strange window characters. 

Next problem is lack of communication with studio unit. Telemetry present at rear terminals of MRC-1600 with transmitter terminal shut off. Tracing FSK tones on Telco Input board on 4136 chip shows no output at VCO chip input (pin 2). Problem is actually not a problem because the FSK present at the input to the bandpass filter is the FSK from the Telco Output board. Problem traced to I/O board mounted on rear of MRC-1600.

Both protection diodes blown apart, traces on PC board blown up. Breaks occurred right at terminal posts where they meet the PC board clads. Jumpered the broken traces. Communications now OK.

Problem is now that the MRC-1600 is "scanning" and remaining in Maintenance mode. Channels are decrementing as though someone is leaning on the channel decrement arrow yellow button. The only way to stop the scanning is to hold down the channel increment arrow yellow key. Channel 17 reads 0. It should read 2048, indicating that the D/A is working. Replaced U7 U6, U12 on Analog/Status/Command board. No effect. Check U8 for pull-down on front panel PC board. Pin 3 of U8 stays logic low when A/C/S board connected (blue ribbon cable connected). Scanning stops when A/C/S board is disconnected. Problem traced to U13 (74LS244) on A/C/S rear board. Removing this chip solved scanning problem. Replacing chip restored normal operation. 

XR-2211 failed in another MRC-1600 causing no communication between studio and transmitter unit. If communication path is OK, but channel 18 reads bad one way, be sure to first check VCO pot R5 for correct adjustment.

Tom Osenkowsky, CPBE



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