Tuesday, August 28, 2012

TS-120 Tribulations

I recently picked up a Kenwood TS-120S at a hamfest for what I thought was a fair price. I asked the seller if there was anything wrong with it and he showed my how sometimes the PLL would unlock and pushing on a particular connector would 'fix' it. "No biggie" I thought, "loose connection, a little NoAlOx will fix it". Also there was no mic. I brought the rig home with a CB mic I found for a dollar.

Once home, the rig's true nature quickly became clear. USB reception was only possible with the IF shift knob was turned fully clockwise. USB transmission was impossible. LSB transmission sounded like crap. Turning the power higher than a watt or two caused the finals to oscillate with full 100W power forwards and reflected. The audio was crackly and the PLL lockup issue was a significant annoyance. Sometimes after TX the RX sensitivity would dive.

After way way too much time on the bench, I finally have these issues resolved to my satisfaction. 10m TX audio is still distorted, but I get good audio reports on other bands and have made several contacts on 20 and 40 meters.

I suspect this radio will eventually find it's way into my Jeep.

Here is a full list of the work I did to resurrect this rig.

PA Board

Distorted TX audio & CW tone

  • Exciter output sounded great but PA output sounded like garbage. Properly adjusted base currents; still sounds bad on 10m, other bands OK

Preventative Maintenance

  • Resoldered all transistors.
  • Applied fresh heatsink compound.


RF Board

Preventative Maintenance

  • Replaced D2,3,4,5 and 8 on RF board with new 1N914 parts. D8 showed high reverse leakage in circuit, but that may have been caused by an alternate path to ground.

Finals Oscillating

  • Applied SB-844 to resolve PA oscillation; R47, R50, and C61 already had the updated values, from the factory I think; Did NOT add series diodes due to possible RX IMD degradation; Did add AVB bypass cap.


IF Board

Loss of RX sensitivity after TX

  • I found that sometimes after TX, RX sensitivity would be lost, and a bonking the rig would bring it back. Replaced IF board relay RL1 with new part. See also filter board repair.


Filter Board

Loss of RX sensitivity after TX

  • I found that sometimes after TX, RX sensitivity would be lost, and a bonking the rig would bring it back. Replaced Filter board relay RL1 with new part. See also IF board repair.

Preventative Maintenance

  • Applied SB-051 Final Protection mod


AF Gen Board

Symptom: Crackly RX audio, PLL unlock (dots on display)

  • Re-soldered broken joints on Q3 & Q7 on AF board; solder cracked due to thermal cycling and insufficient solder from the factory; I would check Q2 also.

Preventative Maintenance

  • Replaced C10, C11, C13 and C16


CAR Board

Symptom: USB BFO frequency low

  • At first, USB reception was only possible with the IF shift knob cranked fully clockwise. During alignment it became clear that the USB BFO frequency could not be brought into spec. It's supposed to be 8.8315MHz, and the highest I could get was 8.8314MHz, which is a big problem. This is probably due to the crystal aging. I got the CAR board set up so I could play with it in vivo and experiment with adding additional capacitors in parallel with C22. I found a 1pf cap lowered the resonant frequency just a bit. I then put the 1pf cap in series with C2 22pf USB CAR osc fixed cap. That brought up the frequency into the useful adjustment range of the trimcap, making it possible to complete the carrier point step of the alignment procedure.



  • Drilled holes in sides of front panel to access front assembly screws without removing panel; this makes removing the RF board much easier; loosen shaft coupler front set screws; remove front panel screws; pull front panel forward to free shaft from coupler; remove rf board screws; angle front panel down, remove RF board by sliding up and back off the shaft.



  1. Checked PSU voltages, OK
  2. Adjusted USB & LSB car outputs; OK
  3. Adjusted CW CAR; NOT OK! Behaves oddly; rx freq = usb freq; tx freq is offset lower; procedure calls to adjust VR2 to set CW car osc rx freq but adjusts tx freq instead (sigh). Need to review TS-130 manual for potentially improved procedure.
  4. TX/RX IF shift; OK
  5. Center RIT; OK
  6. Carrier suppression; nulled by ear with a second receiver
  7. Carrier point; adjusted first using factory procedure, then by ear listening to RX signals
  8. Adjusted final PA base current; OK

Twiddled RF board output balanced mixer balance trimpot; oops; how to fix? Not documented in svc manual.

Still To Do:
  1. Apply SB-037 VOX Operation Pick-up Time
  2. Apply SB-816 Band Pass Filter Change
  3. Adjust CW CAR
  4. Add speech proc
  5. Add DSP (or a notch filter at least)


Anonymous said...

What was causing the dotd and no lock?

Anonymous said...


I also had the problem with unlocking PLL, it showed only dots on the display. On my ts-120 this was caused by the local oscillators for the different bands ( board with the bandswitch on it ). they were way off to be usefull, so i had to retune them. check the service manual to see the schematic.
It has been a while since I did this, therefore my explanation is not very clear, but hopefully you'll get on the right track.
Best 73', Kevin ON5DRE.

Anonymous said...

I have the same problem with a 120s I just bought. When I start the rig the 2 dots are on every band but 14 and WWV. In my case after about 30 min The other bands come in? After warm up If I switch to 21 with the VFO under the middle part of the band I get the 2 dots but after tuning to the higher part of the band the whole band will come in? Ive tightened the shaft on the band switch and cleaned the switch. Ive tightened all the screws on all the boards I can get to. Has any one seen this problem? Thanks

N1YWB said...

I think the main issue causing the PLL unlock in my rig was broken solder joints. The rig was festooned with with them. I suggesting removing and very carefully inspecting each board, wiggling the components, and touching up every questionable solder joint, especially on heat-sinked components, they were the worst. I would just touch up all of those heatsinked components regardless of their appearance.