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 :-)!




David Moisan said...

How did you fix the traces under the battery?

Jose EB5AGV said...

Hi David,
Well, luckily they were only a bit darkened but there was no cut in them so I just carefully cleaned them of acid, so problems were gone (except for the ON AIR thing which I need still to check...)

Anonymous said...

Hi Jose- my ts790 wont tune in the CH.Q mode- the solenoid goes in but no step tuning- ever seen this problem?

Jose EB5AGV said...

Hi Joe,

No, I am sorry, I have not dealt with that particular problem. Do you feel the steps on the encoder when you turn the VFO knob?. I would check for a plainly mechanical problem first.


Anthony G7LRQ said...

I have a 790G with 23cm.
The output power on 23cm is either 1w or 8w max. It seems to change between these 2 levels at random, and will stay like this for many uses and then just change by itself for no apparent reason. Any ideas on where to look for this problem.

Anthony G7LRQ

Anonymous said...

Hi Jose !
I have a Problem with my TS-790E which I use since many years :
When I press the PTT on 2m USB the Output-meter shows full power, and the ALC-indicator also is on the right side...On a Monitor Receiver the Modulation sounds "scratched" and has much distortion. The same Problem is on LSB.
In the service-Manual I found the measure-points for the inductivities L25 and L30, and I tuned the ac-voltage to the right value-the difference to the value in the service-Manual was about 0,2 V. So the Problem was solved, but unfortunately after 2 years it is back...
Did you hear from a problem like this or do you have an idea how I can fix it ?
Klaus, DL4FCY

Jose EB5AGV said...

Hi Klaus,

I have not found that particular problem on a TS-790 but I found it on an Icom IC-9100. On that case, the problem was unwanted RF feedback due to a faulty receiver attenuator which works only on transmit. So I would look for a similar circuit on the TS-790 and would check if it works properly.

73 Jose EB5AGV

Klaus, DL4FCY said...

Hi Jose !
Thanks for the Infos you gave me, but the problem was much easier to fix than I thought : I dismantled the radio and made a visual check of the 2m-PA, because on the lowest power stages the signal was ok. All things looked to be ok. So I unsoldered the connections from the PA-Unit 144 MHz to the PCB, cleaned the "Soldering areas" with a little alcool (isoprop.-YES, Alcool can solve some problems...;-))), and soldered them new. Then I realigned L25 and L30 (VCO 144MHz MAIN CAR. + 144 MHz SUB CAR.)to the correct voltages, and that was it. Since then I never had the problem. Maybe this will help some TS-790-Users who have the same problem, and their radio dealer want to change the complete PA-Unit (about 200 € incl. all) - IT IS NOT NECCESSARY IN THE MOST CASES !!!
But now I have another problem:
I changed the original battery, which is now about 23 years on duty, to the holder and a cr2032-battery, and since then the memories are gone when the radio's power supply is off for more than 3 or 4 hours. The battery's voltage is ok. This is a fault I want to solve next-I think that there is a lead damaged and the battery-voltage will not reach the point where it is neccessary...
As a next step of modifications of my radio I switched my "E"-Version into a "A"-Version by cutting 3 diodes on the control-unit (D21, 23 and 24) and set a diode (1N4148 or similar) on the D22-position (It is a modification published by M5POO). So it is possible to encode (BUT NO DECODE !!!)CTCSS-tones when transmitting on FM - Some repeaters in germany acually need a ctcss-tone to use them - and every day it will be more, so this modification makes sense. After the modification the 1750Hz-Tone will not work anymore, and you have to set the offset (on 430MHz) manually, but the 2m-Band will be extended to the US-2m-Band-specifications-So be careful with the ARS-Function !!!). I was searching a long time, until I found this kind of modification which is very easy to realize. I think there is no easier way to bring the TS-790 up-to-date .
Klaus, DL4FCY