Kenwood TS-120S wont TX on 15m & 10m

[AC2RF]

I am unsure if this is the right place for this, but the TS-120S is a 30 year old rig so it should fall under the "classic" title. If not Mods feel free to move this to where its suppose to be.

When ever I key up the PTT button in SSB on 15m & 10m the needle on the S-meter maxes out (as in the needle hits the side of the meter) and stays there. It doesn't transmit like the protection circuits kick in. Originally I thought it was my antenna so I hooked up a 100 watt dummy load just to be sure and the radio does the same thing. This only happens in SSB mode. CW will still transmit as it works as I use the CW carrier to use the transmatch on my antenna.

Any thoughts on what might be wrong with the radio?

[WB2WIK]

Seems like it's oscillating on those bands.

Possibly a failure in whatever is switching in the PA low pass filters.

[W5RKL]

Try tapping on the IF Unit relay with a pencil or plastic rod when the problem occurs. The IF Unit board relay controls a number of circuits for transmit and receive and this relay is noted for causing problems.

The Filter Unit also has the same relay. Tap on it using a plastic rod in the same manner as the IF Unit relay.

If tapping on one of the relays causes the symptoms to change, replace the relay, don't waste your time trying to clean the relay.

Both boards, IF Unit and Filter Unit boards must be removed to replace the relays. The original relay is no longer available but Ken, K4EAA, sells new drop in replacement relays on his website, www.k4eaa.com .

If the Filter Unit relay is found to be the source of the problem, removing the Filter Unit board is not an easy task. There are 10 small RG-174 coax cables, 5 on each side, that must be removed. The pins for the coax cables are labeled on the board but the cables themselves are NOT labeled. There are 4 plugs that also must be removed along with 6 Phillips head screws that hold the board to the metal enclosure. There also is a 2 pin plug on the Antenna Change Over relay board and a coax cable that plugs into the RF Unit board that also must be pulled. You do NOT have to disconnect the Final Unit from the radio to remove the Filter Unit board. Remove the 6 screws that hold the Final Unit and Filter unit from the main chassis. Carefully slide/swing the enclosure 90 degrees to gain access to the plugs and screws. I recently replaced the relay in the Filter Unit in a friends TS-120. The problem in the 120 I repaired was a receiver recovery issue (low receiver sensitivity when changing from transmit to receive). Tapping on the relay caused the symptoms to change and replacing the relay eliminated the problem.


73s
Mike

[WA7PRC]

Hi Ron,

I've owned a TS120S since the mid 80s. I haven't experienced that problem but there's a service bulletin (SB-844) that addresses intermittent oscillation on the 15M, 10M, and/or 20M bands. See the attached PDF.


The IF Unit board relay controls a number of circuits for transmit and receive and this relay is noted for causing problems.
But not this problem.

The Filter Unit also has the same relay. Tap on it using a plastic rod in the same manner as the IF Unit relay.
If tapping on one of the relays causes the symptoms to change, replace the relay, don't waste your time trying to clean the relay.
Both boards, IF Unit and Filter Unit boards must be removed to replace the relays. The original relay is no longer available but Ken, K4EAA, sells new drop in replacement relays on his website, www.k4eaa.com .

The original Omron pn LZN403 is no longer manufactured. This one, identified by EA4RJ (http://www.eham.net/reviews/detail/703), is the one I used on my TS130S (same relays):

Manufacturer: Panasonic
Manufacturer Part #: NF4EB-12V
Mouser Part #: 769-NF4EB-12V (http://www.mouser.com/Search/Refine.aspx?Keyword=769-NF4EB-12V) ($12.46 each)


If the Filter Unit relay is found to be the source of the problem, removing the Filter Unit board is not an easy task.
Agreed! Take your time, mark all the unmarked cables, and be very careful removing/reinstalling the cables.

[VE7DQ]

Check the antenna connector. If it's the least bit loose, it'll allow these oscillations. I've also been known to solder the slip-on coax connectors on the RF board. Make sure the six screws retaining the RF board (the board with the bandswitch wafers) are really tight. The screws holding the little relay board above and to the rear of the RF board should be snugged too.

Performing the modifications in the service bulletin is recommended.

External issues in the antenna system, like loose connectors, can cause this. Does the radio oscillate when connected to a dummy load through a known-good cable? All the shields in your PL-259s soldered? Gotta dummy load?

I've not seen a relay issue cause this oscillation problem.

73
Tom

[AC2RF]

Check the antenna connector. If it's the least bit loose, it'll allow these oscillations. I've also been known to solder the slip-on coax connectors on the RF board. Make sure the six screws retaining the RF board (the board with the bandswitch wafers) are really tight. The screws holding the little relay board above and to the rear of the RF board should be snugged too.

Performing the modifications in the service bulletin is recommended.

External issues in the antenna system, like loose connectors, can cause this. Does the radio oscillate when connected to a dummy load through a known-good cable? All the shields in your PL-259s soldered? Gotta dummy load?

I've not seen a relay issue cause this oscillation problem.

73
Tom

I've already ruled out the antenna. All connections were good and I've hooked up a dummy load a few times with different cables. The problem is the radio.

Thanks everyone for the advice, this weekend I'm putting some time aside to check all the screws on the RF board to make sure nothing is loose. If nothing is loose I'm going to find the parts needed for the service bulletin.

[K2KHA]

I have what seems to be the same problem from 24-30Mhz. It pulls 30+ amps when I key up with a carrier, and outputs absolutely nothing. Doesn't even tickle the external wattmeter.

I've diagnosed it to R11 and R12 relays and possibly the filter unit for 24-30Mhz which looks easy to diagnose as I've built a high pass filter before.

So check the relays and the filter circuit for that band.