Kenwood/Trio TS-120S help needed please - Page 1
Forum section: electronics / repair

author: christopher iles
author_rank: Regular Contributor
timestamp: April 11, 2020, 08:46:14 am
content: Hello,I have a TS-120s with no receive, just a faint hiss from the speaker.Checks so far:VFO is good, digital readout on all bands is fine.I have checked the voltages on every pcb and they are all correct.I have scoped the oscillators and they all are present.When I connect a signal generator to the antenna socket, I see it on the input of the rf board but not on the rif out.So, I injected 8.83MHz on to the rif input on the IF board..still nothing.If I pull the 2 pin connector (+14v and RB) off the IF board I get white noise out of the speaker.Obviously, I have checked that the rf gain pot and all other controls are set as they should be so the radio SHOULD WORK!I've checked all the devices on the IF pcb and the board for dry joints, broken tracks etc and all appears ok.It's starting to baffle me now; any help would be GREATLY appreciated, thank you.73, Chris G0VOE, Somerset, Southwest England.

author: m3vuv
author_rank: Super Contributor
timestamp: April 11, 2020, 11:13:47 am
content: hi chris,have you tried hooking a sig gen to the antenna connector?,maybe dirty relays in the filter?is it the same on all bands?

author: Manul
author_rank: Super Contributor
timestamp: April 11, 2020, 11:48:28 am
content: What is prehistory of this fault? How it happened? Was it working and failed suddenly? In operation, or while off?So by your observation it seems that both RF section and IF section is failing? It then should be something common between both. Like power, control signals. Maybe relay control or AGC?Do you have a service manual of good scan quality?

author: M0HZH
author_rank: Regular Contributor
timestamp: April 11, 2020, 12:09:14 pm
content: Does it transmit ? If so, do you get full power on all bands ?I think I've seen the same problem in a TS-120 before (certainly on TS-130 / TS-430 and other radios of the 80s & 90s): PIN switching diodes fail with high reverse leakage, which brings the whole control line up/down and might also impact other (apparently unrelated) areas (LPF control, TX/RX switching etc). Check BPF control voltages and PIN diode forward/reverse bias both on RX/TX and on each band, compare to service manual values (if TS-120 has them).Cheers,Razvan (M0HZH).

author: coromonadalix
author_rank: Super Contributor
timestamp: April 11, 2020, 01:56:19 pm
content: Service manualhttps://w5rkl.com/wp-content/uploads/2015/02/TS120S_serv.pdf

author: christopher iles
author_rank: Regular Contributor
timestamp: April 12, 2020, 06:23:40 am
content: Hello,Yes I connected a signal generator to the antenna socket and can see the signal on the input of the rf board.However, I can't see the signal on the output of the rf board that goes to the IF.Kind regards,Thank you for your kind reply,Chris.

author: christopher iles
author_rank: Regular Contributor
timestamp: April 12, 2020, 06:29:25 am
content: Hi Manul and thank you for your kind reply.The history is this.When I got the radio, it had the dreaded dots on 14MHz and above.3.5 and 7MHz were receiving fine (I never got round to testing the transmitter.I replaced tr24 and tr25 on the pll board and now I have frequency readout on all bands BUT no receive.The finger test on the af gain pot produces a healthy hum from the speaker so it's not a faulty phone or external speaker jack.Kind regards,Chris.

author: christopher iles
author_rank: Regular Contributor
timestamp: April 12, 2020, 06:34:22 am
content: Hi Razvan,Thank you for you kind reply.It must be something I've done when I changed two transistors on the pll board.Prior to that, the radio was receiving on 3.5 and 7MHz but had the"dreaded dots" on all bands above.Replacing tr24 and tr25 on the pll has brought the digital readout back on all bands but I have lost receive.Finger test on the af gain pot produces a hum out of the loudspeaker so I've ruled out a faulty phone or ext. speaker jack.Kind regards,Chris.

author: christopher iles
author_rank: Regular Contributor
timestamp: April 12, 2020, 06:35:35 am
content: Thank you for the link, you're very kind.Regards,Chris.