26 October 2008

Kenwood TS-790 revisited


Well, I needed to get back to my recently repaired Kenwood TS-790 as it developed a new problem... first ATV label (yes, there is an ATV label !) blinked from time to time, with no apparent reason and, sometimes, it kept lit for a long period. I checked schematics and got lost when trying to find the origin of the signal...

But eventually it got worse: yesterday, while talking in a local QSO in 2m FM with the 790, I found a very curious thing. My 790 was SLOW to react to key presses, frequency change and even PTT!. Yes, there was a variable delay, from zero to some seconds, for the unit to execute what it was asked to do... Having worked for years in digital electronics, I thought the CPU was receiving some kind of interruption which made it work slow and even lose some commands or inputs to it. Immediately I thought of a backup battery leak damaging the circuit...

Here you have a low quality video of what happened. Sometimes dial is not updated but frequency changes. Also, F key is erratic:


So I got the 790 back to the workbench and, yes, there it was!. Battery, which measured a perfect 3.32V, had leaked and damaged some PCB traces below it. I don't know how I missed this when I fixed the unit!

This is what was below the battery...

I got the bad battery out, cleaned the acid residue and added a battery socket and a 2032 3V battery which is smaller capacity than the original one but lots easier to locate and, with the socket, a breeze to replace next time. I used Renata swiss made battery and holder as they are IMHO one of the best batteries available.

Note that the battery holder was not a perfect fit so I needed to add some wire to connect one of the poles to the PCB. But, all in all, it is lots better than the original soldered battery.

I tested the rig and it worked almost flawlessly. Yes, there is now a different fault which I need to further investigate... Now ATV icon does not light and rig reacts fastly to keypresses, PTT and such. But now ON AIR LED is sometimes behaving strangely: it sometimes does not light and sometimes keeps on even when PTT is not active (but rigs go to TX and RX as it should). So I need to check carefully how ON AIR LED is activated and check the unit to locate the fault. I will do it when I install the UT-10 1.2GHz module I recently bought... so keep tuned for a further update :-)!



11 October 2008

Yaesu FL-2100Z restoration (I)

Well, here I am again :-)

This time I am working on a Yaesu FL-2100Z linear amplifier. I am currently working on it so this post, I hope, will be more interactive than my first one.

I got the unit in a pretty sad condition (yes, it seems all units I get lately are that way!). Price was right, though, so I ended buying it.

Here you have some pictures of the unit so you can see the condition.

Trust me, it looks better in the pictures than in the real world... cabinet has several dents as it has in the front panel top (not seen in these upside down pictures)

Well, let's see the inner top side in detail:

Besides the dirt, it can be seen tubes are Cetron and, probably, original ones. I got a set of chinese National tubes... they were dirty so they had been inside the unit for some time. Why were they removed?... I found it easily, as the top cap snapped in one of the tubes (it seemed burnt!) just when carefully cleaning it and in the other tube, it was loose...

I tried to resolder it but it seems they use some special soldering alloy... I guess it has perhaps some silver. Does anybody know what they use?

Well, let's see the lower side of the unit. I found several things which needed some care. First, there is a relay which is used to modify tube polarization when it is or not transmitting and it was flooded with oil. Not a good thing!

As I had some spares for my FT-102, I decided to replace it:

Once the relay was replaced, I checked one of the problems I knew in advance the unit had: it didn't read SWR... do you guess why?. Check pictures...

I got all the faulty items, from completely dark resistors to dead 1SS97 diodes. I also cleaned the PCB and finally got this:

So I need to replace all the parts and, specially, 1SS97 diodes. I have thought on using 1N5711 Schottky diodes. Any opinion regarding this?

See you on Part II !


05 October 2008

Kenwood TS-790 repair

This is my very first BLOG entry... I hope you find it interesting.

I got this unit, an European TS-790E, in a somewhat sad condition, as you can see in the first two pictures.

It seems packaging material was dirty and so the unit got some dirt all over it... well, anyway, I like to clean the rigs thoroughly so this was not a big problem. So I opened it and vacumm-cleaned the internals. It was not too dusty and result was good.

Once cleaned internally, I proceeded to clean the front panel. As there was some dust in the dial glass internal side, I decided to completely dismatle front panel, including display glass, keypad and so on. Beware if you do this: there are very tiny parts which are prone to get lost.

Once front panel was clean and everything was back, I did a first general check of the unit. Sensitivity was fine in both bands but I found some problems with the power output in VHF. I use a nice Rohde & Schwartz CMT-84 Communications Test Set and it measured from a maximum of about 50W to a minimum of about 33W... something was clearly at fault. So I carefully checked VHF power amplifier assembly and found that someone had resoldered the power module (probably an attempt to fix the problem). I noticed also that there was no thermal paste in the power module... nor in the dual UHF modules!. This seemed to me the same Kenwood fault that they had with some TM-241s. So I took out the complete VHF PCB.

As you can see. there was no thermal paste... and several soldering points were cracked. This could explain the power output odd behaviour so I resoldered all the suspicious solder joints and added thermal paste both to the power module and to the temperature sensor.

I did the same with the UHF modules (this rig uses one as a driver and another larger as final). UHF PCB had no cracked soldering joints but, just in case, I resoldered the power area.

After that, I measured power again and adjusted it to 50W (VHF) and 40W (UHF). Power modules are 60W maximum so it was a safe setting.

And, what is better, power output kept constant!

Only thing the rig needed to be perfect was an slight touch of the reference oscillator. Once that was done, I put back covers.

And then put the rig on its operating position.