Modifications for the Kenwood TR-7600

Alternator Whine, TR-7600/7625

Author: Trio-Kenwood Communication, inc.

Service bulletin no. 39 (9-7-1979)

In some mobile installations, alternator whine may be heard during transmit. External B+ filtering does not have sufficient effect.

In some instances, using the cigarette lighter will cause this problem. Running off the fuse block, and even directly from the battery may still not eliminate the noise.

First, check power line routing in the engine compartment. It should be kept clear of all secondary HV harness wiring, and clear of the alternator, regulator, and ignition primary circuits. Careful routing should reduce or eliminate noise coming in through the power line.

Check for clean ground connections at the antenna and radio.
Check for clean and tight battery connections.

If all of these measures fail, replace L22 on the VCO unit X50-1580-10 from the original green coil, to either an orange or black coil. No adjustments should be necessary.

Old part number L40-1021-03 1mH
New part number L40-1021-43 1mH

Please notate your Service Manual.

Correcting Frequency Difference Between RM-76 and TR-7600/TR-7625

Author: Trio-Kenwood Communication, inc.

Service bulletin no. 33 (19-3-1979)

The RM-76 will be about 500Hz higher in frequency then the TR-7600/TR-7625. This will not present eny problem most of the time. Out specified tolerance is 750Hz.
The operating frequency may be out of the receiver input of a repeater if the transceiver is turned slightly high however.
The frequency difference may be corrected by adding additional diodes in series with both the 02 and 52 lines in the RM-76. This is shown in the figure below.

The foil paths from the connector to diodes D18 and D19 are cut and the new diodes soldered on the foil side of the board. The cathode and of the diodes is soldered to the connector side. We recommend using either 1S2076 or 1S1555 diodes.
TR-7600/7625 Audio tone on TX

Author: Trio-Kenwood Communication, inc.

Service bulletin no. 45 (10-8-1979)

Some TR-7600/7625 users report a 0w background tone during TX, especially noticable at full quieting by the receiving station.

On the PLL unit X50-1580-00, changes:

New Part L18 33uH L40-3301-03

TR-7600/7625 Optional ceramic filter

Author: Trio-Kenwood Communication, inc.

Service Bulletin no. 50 (9-8-1979)

Adjacent channel rejection may be improved by changing one capacitor & installing an optional narrow ceramic filter. On the TX RX unit X44-1320-10, change;

Old Part L41 L72-0309-05 CFT-455 F2
New Part L41 L72-0306-05 CFL-455 F

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