Kenwood TM-241 – No TX

One of our club members recently requested me to take a look at a Kenwood TM-241 he bought a couple of years ago, but never used it. He plans to use it just for some monitoring of a couple of frequencies for local traffic.

In any case, RX is just fine, however, when clicking the PTT-button, nothing happens. No TX.

These transceivers are known to be – let’s say – “difficult” when it comes to the PA stage. They use a monolithic module which houses the complete PA. They tend to fail.

Upon inspection, someone had already been in there, and there was cooling paste between the heatsink and the module. The PA wasn’t dead either. It just wasn’t TX’ing. I remembered that the OM said that he tested the mic on another Kenwood transceiver, and it was working fine.

Could there be an issue in the front panel? I disassembled it a bit more, and yep, there’s the issue.

Kenwood used some kind of glue to tack down SMD components before soldering (I suppose they used wave soldering, hence tacking components down). Nothing wrong with that actually, however, the glue they used, absorbs moisture with time. This in turn, turns it acidic.

The trace damage can be seen in the middle – follow the trace of the center pin of the connector down, and you’ll see it. The part of the trace that goes up was also starting to go bad.

The ground trace for the mic was completely eaten away. After recreating the traces with some enameled wire, everything was fine again. Happy days.

73 de ON8AD

Kenwood TM-V71 low power

…Or not? Here’s a short post about often overlooked issues with supplying power to your mobile rigs (or any rig for what it’s worth..); Here’s the story I told to the owner of a Kenwood TM-V71…

First of all; let’s start with some basics:

  • Ohm’s law
  • Resistivity of a wire
  • Contact resistance

We all know, that the higher the resistance, the higher the voltage across the component. Ohm’s law right? Imagine your power supply wire is a component.

Every gauge of wire has a certain resistance for a certain length. The thicker the wire, the lower the resistance. Choice of material plays a great deal in this as well.

Contact resistance. Often overlooked. A picture is worth more than words:

This is what contact resistance does. The fuse itself survives; but due to lack of good contact between it’s blades and holder – be it due to oxidation, bad design, or lack of pressure – the resistance will cause heat to be formed in the holder. Eventually degrading it further and further.

In short, the ‘repair’ of this TM-V71 was one of refreshing some basics to the user.. Lengths of thin wire, with cheap fuse holders and a plug that shouldn’t be there is a big no no.

Ham radio operators: keep power runs short, with thick wires, and premium fuse holders. Always. No exceptions.

ICOM IC-910h – Preamp bias troubles

Recently I was asked to take a look at a IC-910h VHF/UHF all-mode transceiver which had developed an issue where the bias kept being enabled. Even on transmit and when the preamp wasn’t enabled. It luckily didn’t cause any havoc as the operator’s antenna wasn’t DC shorted at the time..

If you look at one of my previous posts – you’ll know how a basic bias circuit works (check it out here – SEA remote ATU bias & mods).

The problem here, was that there was no way of turning it off and on. Nothing seemed to be burnt out or shorted on first sight. So what does this look like on the schematic?…

I’ve traced some lines to show you how things work. Q702 is a PNP transistor – a Rohm 2SB1132 T100R. This is the main switching transistor. Rated 500mA. Q703 is a DTC114EUA T106. A bit of a special beast. It’s a “digital transistor”. It has built-in bias resistors to enable the configuration of an inverter circuit without connecting external input resistors.

So to troubleshoot, I first checked if VPRI was being switched from the main processor with reference to ground. It was. Pfew – no main processor issue. Next step was to check if Q703 actually did any switching: it did as well. Hmm, odd.

Checked Q702; and it wasn’t shorted when checking in diode test from Base to Collector, or Base to Emitter. Maybe C703 or R702 failed? Nope, not that either. Any tin whiskers around due to lead-free solder? Nope.

I still suspect Q702, as for some reason, it conducts, without anything applied to the Base. (Otherwise known as a “Leaky” transistor). To confirm, I replaced it temporarily with a bog standard BC560. Works like a charm. Leaky transistor confirmed. Here’s the testing bodge in all it’s glory.

I quickly ordered a couple of Rohm 2SB1132 T100R from https://www.arrow.com/ (I kind of like their fast service and clear website to be honest..). Delivery took a little longer than usual due to the fact that it was in stock in a HK warehouse and I’m in the EU.

Remember to really clean up the old pads when working with SMD components. It’s just much nicer. I used Chemtronics fluxed solder wick here. Pads clean up nicely with it.

Here’s the new part installed. Looking great.

And confirmation it works and can be switched on/off..

The UHF part is similar; so if you run into issues where bias is kept on with your IC-910h; make sure to take a look at these components. The UHF part is a bit trickier, as you need to disassemble the entire PA unit..

73 de ON3AD – Jeffrey

FT-2000 – Deaf as well?

I recently had a look under the hood of an FT-2000; and just as I’m done repairing two FT-897; I notice that the FT-2000 should be a suspect as well! Yaesu: what gives?!

You can see the same white oxide leaking out of the porous plactic of those filters.

A warning to FT-2000 users: you should check them periodically.

Yaesu FT-897 – Deaf on receive

In our local ham radio club ( UBA-ARA ) – two of our club members had a Yaesu FT-897 who seemed deaf in certain modes. I took both transceivers home with me and took a closer look.

Upon opening the transceivers, I quickly saw what looked like water damage on the IF crystal filters:

You can see some white deposit on those filters. My guess is that after re-flow/wave soldering these boards, Yaesu probably washes these PCB’s. However, the plastic used in these filter either is brittle, porous, or the bottom isn’t hermetically sealed.

Removing these filters is a pain in the ass. And I’m not overreacting here. The ground plane in this PCB is quite good and you can really struggle to get the solder out. (lead-free as well!). Here’s the PCB with the filters removed:

Looking at the flux in other places on this board, I suppose this transceivers has had work done before…

I opened up one of these filters to get an idea of the damage done. A sharp blade does the trick:

You can clearly see the copper oxide. No wonder this thing was deaf!

Anyway, it’s worth opening up your transceiver if you have an FT-897 to check and see if there’s any of that white “powder” on those filters. Big red flag if you see it.
Here’s a picture with already one new filter in place. Make sure to put the right filter in the right place…

Yaesu FT-450 – deaf on higher bands

I’ve always wondered why my FT-450 I bought second hand had a lower RX level on the higher bands, starting around the 20m band. I took a look at the schematic, and the input on the RX side is very classic: a DC blocking cap and protection diodes.

Protection diodes, wait – those aren’t RF protection diodes; they are bog standard SMD red LED’s! What was Yaesu thinking here? (“Let’s save 2 cents?”). Only reason they might have used these (and the only reason I can think of) – is that they maybe have a low capacitance. But seriously, Yaesu.. ?

Anyway, I set my multimeter to the diode setting and quickly realized that one of them was shorted. Really, who tries to use LED’s as protection diodes? Really not up to the task. Here’s one that was still working. The picture gives a good indication where you can find them on the PCB (underside of the transceiver).

The diodes are placed anti-parallel to clamp any over-voltage/ESD. But there are better parts to be found. I came across the Infineon ESD0P4RFL. It’s quite a bit smaller, but we can move the part a bit. It’s actually made for these purposes:

  • Very low line capacitance: 0.4 pF @ 1 GHz ( 0.2 pF per diode)
  • Very low clamping voltage
  • ESD protection of RF antenna / interfaces or ultra high speed data lines acc. to:
    • IEC61000-4-2 (ESD): ± 15 KV (air / contact)
    • IEC61000-4-4 (EPT): 40 A (5/50 ns)
    • IEC61000-4-5 (surge): 5 A (8/20 μs)
  • Ultra small leadless package:1.2 x 0.8 x 0.39 mm³

Read that last line, yes – THAT small

I cleaned up the original location of the LED’s with some solder wick; also cleaning any flux residue on the board with IPA as flux does have a capacitive effect. I removed a bit of solder mask up the trace a bit as to fit the component correctly and applied some solder paste before re-flowing the part. Measures like a standard diode with a voltage drop of about 0.6 V – so easy to verify. It’s really small compared to the original LED’s – check it out at the bottom right of the trace.

Overall if you have a FT-450 – check that these aren’t already blown. I see *some* complaints about bad RX on these transceivers, but that’s really not the case *if* these diodes aren’t blown. So check up front, and replace “just in case” would be my advice.