Kenwood TS-120S repair and restoration

[xrunner]

I got this from a friend of a silent key. It's apparently been sitting on his desk unused for many years. I am probably the first to power it up in that long.

It works OK on 80, 40, and 20 meters - both transmit and receive. It does not work at all on 15, and 10 meters. It will not receive or transmit, nor does the digital display show any numbers, just two decimal dots.

That is what I will be trying to repair, and I think I have a good idea (after some initial research) what is wrong. 

[xrunner]

I think I found the problem. There is a 10 MHz signal and a 20 MHz signal that come into the PLL board. Both have to pass through an input stage, which one depends on the band selected. The 10 MHz signal is of the correct amplitude, but not the 20 MHz signal. It should be the same or even larger than the 10 MHz sig. This can be caused by weak 2SC460 transistors gain falling off. The remedy is to replace them. I have some on order so hopefully that is the remedy.

[GreyWoolfe]

Good luck with the replacement parts. It'll be nice to see an old radio back on the air.

[xrunner]

Good luck with the replacement parts. It'll be nice to see an old radio back on the air.

Well, I tried a trick and it worked, but I still want to replace some transistors.

The PLL board uses quite a few 2SC460s, so I unsoldered the two that didn't seem to want to play with 20 MHz, and swapped them for two others on the board. Lo and behold - that fixed the issue. Now 15 and 10 meters recives and transmits and shows on the display. Luckily the ones with the lower gain worked OK in the part of the circuit where they were swapped into.

The new replacements will be here next week.

[xrunner]

Replaced the bulbs with white LEDs w/associated current limiting resistors today. Running off of the same supply that the bulbs used - a 14V supply. Came out OK. Also worked on the cleanup of the external parts and knobs. Case parts go in the sink - warm soapy water. Knobs all go into the ultrasonic cleaner. I love to see all that dirt blasted off when the thing is turned on. 

[German_EE]

White LEDs

Ummm, no

Give me that warm glow anytime.

[xrunner]

Give me that warm glow anytime.

Not even blue?

[VK5RC]

+1 for the 'warmer' colour lighting, maybe a mixture of a 3 colour LED?
Nice to see old gear getting another run

[German_EE]

Definitely not blue, this guy has the right idea

Take some warm white LEDs and sand the surface to defuse the light, add series resistors so that they're not so bright. The aim is to provide something that looks just like a standard incandescent bulb but runs cooler and takes less current.

[David Hess]

Not even blue?

Especially not blue.

[xrunner]

Guys ...

I know all about LEDS and how to pick colors and diffusing and so on. However, in this case the white LED "color" is the one I want to use right now, because not only am I restoring the radio, I'm also making this change so it matches the "color" (white) of my other radios such as the FTM-100 and the FTDX-3000. If I want to or need to change the LEDs to sell it or for some other reason, it's a very simple matter to do so. So thanks for the opinions, but the white look is exactly what I want at this time. If you owned the radio then you would do something else, perhaps. For my application and taste at this time, it's what I want it to look like. Besides, the main thing is that it works as it should, and all that is coming together nicely so far.

Onwards! 

[voltz]

Quite right Xrunner. You can make it any colour you like since its YOU who's going to be looking at it
Its all a matter of opinion and taste. Some like cool white (towards blue), others warm (towards orange).

But i think if i were restoring this radio for a customer, and they did not express a specific colour of choice, i would default to warm white. Basically replacing the lamp with an LED of close Kelvin. Around 2500-3000K for warm white.  https://en.wikipedia.org/wiki/Color_temperature

Good job on the radio - yes, thats much more important than a lamp colour. Agreed. These are just the finer points.

[GreyWoolfe]

Xrunner, I like the white also.  A bulb burned out in my Astron VS-35M so I replaced all the bulbs with 110 degree wide angle white LEDs.  Much better than the original yellowish light.  I liked it enough that I did it to another RS-35M that I had that I ended up selling to a fellow club member when I won the VS-35M.

[xrunner]

I'm restoring the radio, but improving it as well, as in adding LEDs, because not only are they reliable, they reduce the heat build up inside. Now, some perfectionists would say that it isn't a "correct" restore if I add LEDs, because it 's not original. Well, in that sense it's true. But so be it. However, I did add more diffusing material in front of the white LED, so will post a pic of it later.

Left the radio powered up all day yesterday, and noticed the display randomly flickering with random digits in the +/- 50 Hz range later in the day. Went back into it. But the problem went away for whatever reason, so it's back together again, and no problems. It's a 37 year old radio and they can be very finicky.

Went back through a few of the alignment procedures one more time, and it looks OK now. Next I plan on running it using JT65, so I have a Signalink USB on order. I also will need to monitor the output power because you need to run something like 20W and less for JT65 - not 100W. So I'll have to back the power off either by using the ALC pin on the rear (a negative DC V input), and/or using a very low level of modulation. I do not yet have a microphone for it.

[xrunner]

Did some testing on the air this morning. Made two JT65 contacts at about 20W. Good to go!
I reduced the power by making the audio input very, very low, but I don't want to have a low (noisy) audio input to accomplish reducing P_out. What I'm going to do is run a negative V into the ALC pin at the back, which will let me apply a decent level audio input while reducing the P_out to the level needed for JT65 (5 - 20W or so).

(no, the white LEDS are not nearly as bright as they look in the picture)

[voltz]

Have you tried turning down the carrier level control? Does that reduce power to 15 watts?

And there is apparently an ALC pot within the radio that can be adjusted too.

Reading this discussion:
http://www.eham.net/ehamforum/smf/index.php?topic=54011.0;wap2

[xrunner]

Have you tried turning down the carrier level control? Does that reduce power to 15 watts?

The carrier level control on this radio only works in the CW mode. 

And there is apparently an ALC pot within the radio that can be adjusted too.

Reading this discussion:
http://www.eham.net/ehamforum/smf/index.php?topic=54011.0;wap2

I will take a look at that thread, thanks.

[xrunner]

And there is apparently an ALC pot within the radio that can be adjusted too.

OK, yes there is that adjustment, and it would work if you wanted to set a specific power and leave it alone. However, I want to have some (easy) control over a small range of power level, and using that pot for that would not be convenient. So I think the way to go is to hook up an external negative referenced supply and use that for P_out control via the rear ALC pin. I've tried it and it works, takes around -7V to -8 V to get the output down to 5 to 20 W.

[xrunner]

Picked up the matching VFO-120 from a local ham who didn't want it anymore. Needed a good cleanup but it works just fine. It also had a burnt out bulb so did the LED thing to it.

I also got a TS-120 "carcass", meaning a broken radio just for parts, from another local ham. I needed a coil slug because there was a cracked one in my IF board. So I swapped it out and got that IF tweaked up real good. The parts radio also had the CW filter, so I got that off the IF board in the parts radio and put it in mine.

Now I have a complete system. And - it works great! 

[voltz]

Nice!. Looking sweet

[xrunner]

Here's the parts carcass unit I got. Why is it sitting outside? Because it reeks of nicotine/smoke smell. A smoker had it and it's basically ruined because of the smoke residue that is all over every wire and board (not to mention the hacks that were done to the thing). Individual components are OK once cleaned, but it just smells so bad I had to sit it outside. Every part of it is yellow!

If you are a smoker, please do not smoke around your equipment (or other people). Just imagine that going into your lungs! 

[xrunner]

Parts salvaged and cleaned. Each  major part unit will be bagged and stored. Main radio carcass will be bagged and stored in a box with the other parts.

[xrunner]

I've got several PMs this year as to how to fix the issue with the unit not working on 10m and 15m, and the display showing "dots". The fix has been described elsewhere on the internets, but since people Googling end up in this thread I'll describe it here also.

You want to check the frequency doubler in the PLL unit, specifically Q24  & Q25. The amplification of the transistors may be low; should be between 60 and 120 or so. If it's too low the board function will fail. Just swap them out if you can't test them or observe the signal levels. Hopefully that will fix your problem.