#18 Kenwood TS 940 DOT on display Chasing down the rabbit hole Pt 1 - Page 1
Forum section: electronics / general

author: Radio Tech
author_rank: Frequent Contributor
timestamp: December 08, 2015, 02:36:35 am
content: #18 Kenwood TS 940 DOT on display Chasing down the rabbit hole Pt 1In this video I have a TS-940 that has the dreaded dots on the display. After doing the regular checks for signals and voltages on the PLL's I decided to start looking for other issues that can cause this problem. So which is causing this problem? Bad wire connectors, cracked solder joints plus PLL unlocked. We will look at the boards and connectors in this part one series. Just some basic visual checks with our eyes. In part 2 we will go to the schematic and start checking signals and voltages.So, if anyone objects to seeing my videos on the forum please speak up, let me know.

author: German_EE
author_rank: Super Contributor
timestamp: December 11, 2015, 09:10:15 pm
content: This section of the forum is supposed to alert members to other blogs, so please keep posting updates in case others miss them. As for your current task I am NOT impressed by the serviceability of the TS-940 and if I had one I would dread the moment I saw those dots on the display.Good luck in the repair!

author: Radio Tech
author_rank: Frequent Contributor
timestamp: December 11, 2015, 11:27:32 pm
content: Quote from: German_EE on December 11, 2015, 09:10:15 pmThanks,I noticed at times folks post here and do not see it in the normal "new post" button.But I mostly check here every time I log in.The TS-940 can be very intimidating when the covers are removed and one looks in. But instead of having a main board and a few other boards the 940 has many boards, 6 PLL circuits. All these are tied together with many wire harness. So once you locate the problem and narrow it down to one board it is not really that hard to repair.However age comes into the equation. Many pin connectors become corroded, insulation breaks down. Then the night mare beginsThanks for your reply

author: German_EE
author_rank: Super Contributor
timestamp: December 12, 2015, 08:59:33 am
content: One other thing. I have two pieces of commercial equipment in my shack (a scope and a Ten-Tec transceiver) and both have a piece of paper glued on the inside of the top cover. On the paper are listed any modifications I have made so that any service tech will have a head start if something goes really bad.

author: Radio Tech
author_rank: Frequent Contributor
timestamp: December 12, 2015, 01:15:46 pm
content: Quote from: German_EE on December 12, 2015, 08:59:33 amI have seen this and think it is a very good idea. Had a rig come in for repair and found a 3x5 card taped to the inside cover listing parts that have been changed. Saw a TS-520 with an actual circuit schematic draw in a very fine sharpie on the inside on the top cover. Some mod that was done to the audio circuit.As you said it gives future techs a heads up when working on equipment. Thanks for sharing that.

author: SeanB
author_rank: Super Contributor
timestamp: December 13, 2015, 10:40:02 am
content: There is a lift running with my hand drawn note on a sheet of cardboard ( a part of a carton the one contactor came in) laminated, and stuck in the service manual. Was a simple modification I suggested to reduce the occurrence of a nuisance trip that only would occur because the thermal trip time of a breaker might be exceeded on a run from bottom to top because they had originally wired it incorrectly on the master drawing. 5A 3 phase breaker used as an indicating breaker, by using the one phase as a plain contact, and routing all the current through one of the other contacts to drive 2 solenoids. I simply put a jumper wire and separated the load, as each coil drew 3A of current energised. Also added 2 110VAC MOV units on the output of the breakers ( 110VAC as the tech had a lot in the install bag, used on the 110VDC parts of the lift) to the common rail, so as to clamp the spike.Had to train the lift techs that there was a difference between 110VAC and 240VAC MOV units, and that, while the one can be used in place of the other, and will work for a while in one case, it not really is good. Use the 240VAC one on 110VAC and it is almost useless, but will last forever. Use the other way round and it will fail in a few weeks and blow apart.

author: voltz
author_rank: Frequent Contributor
timestamp: December 30, 2015, 02:20:06 pm
content: Hi, just following your thread. So it looks like you got power back to the displays which has stopped the flashing digits problem. You repaired many dry joints (cold joints) on the board but still have an 'unlock' situation showing as decimal dots.Those voltages you measured from the various PLLs, are they correct? If its only one of them showing wrong then that suggests its that PLL VCV control voltage at fault. Maybe check the VCO is actually oscillating and if so, at the correct frequency. After that it could be elsewhere in the control loop, like a divide or prescaler problem.Good luck with it.

author: Radio Tech
author_rank: Frequent Contributor
timestamp: December 30, 2015, 03:29:48 pm
content: Quote from: voltz on December 30, 2015, 02:20:06 pmThanks, I really going to need some luck LOL.Everything previous check in the video, throw it out.Yesterday I decided to start back on this one. I put it on the bench and powered it up so I could check the PLL's again. To my surprise, no display, none of the front panel buttons responded. Hmm I said, so recycled power and just about everything on the front was on. Every LED all characters on the display. At this point I scratched my head and recycled power again. But this time all functions were are they were designed to be. But the frequency readout was counting up. I could grab the VFO and turn it and it would start counting down. Recycled power and no display and no functions worked.In this state I just decided to check the PLL's again. Each one was below one volt. I pulled connector number 2 and checked pin 5 of connector 5 and had 4.6 volts. This is an indication that the PLL's on this board are working and the problem lies in the carrier board PLL.So until I can find what is causing the radio to be so unstable it is useless to continue trouble shooting the PLL issues.The owner before me used this radio for 3 years and said it had issues from the time he got it but would go to work after it set a spell. Till it finally quit all together.I really believe there are still issues in some of the connectors although I have pulled every one and cleaned, tighten the clips in them.I have reflowed all joints on the PLL board, digital board, and the carrier board.I seen previous repairs done to the radio, some not so pretty.This is going to be a tough one.

author: voltz
author_rank: Frequent Contributor
timestamp: December 30, 2015, 03:40:07 pm
content: Would be looking closely at all power rails. Voltage and noise checks.. It sounds almost random in nature, bad power rails could do that and are easily checked. I agree you need to at least get things stable before chasing PLL issues. VFO counting up by itself? might need to call in an exorcist! ha.

author: Radio Tech
author_rank: Frequent Contributor
timestamp: December 30, 2015, 03:54:44 pm
content: Quote from: voltz on December 30, 2015, 03:40:07 pmYou got it. That is the plan now. Go through all power rails and check that correct voltages are there from beginning to end and also ripple. Just because the rail shows good does not mean that the other end is good. I already found two connectors with bad pins. They looked good but no continuity. The wire had broken and was only held on by the insulation crimp. No telling how many more are like this.QuoteYeah. At another time the display started flashing random characters. Different LED's started turning off and on and I could hear relays clicking in and out.So while I am on the phone calling the exorcist I may also need to look up the number to the guy from ancient aliens.Something very strange with this one.

author: AF6LJ
author_rank: Unknown
timestamp: December 30, 2015, 04:05:22 pm
content: Keep us posted, Sadly this radio has been mishandled.Oh Well....Good Luck.

author: Radio Tech
author_rank: Frequent Contributor
timestamp: December 30, 2015, 04:17:26 pm
content: Quote from: AF6LJ on December 30, 2015, 04:05:22 pmSure will Sue. And I agree about the being mishandled.But I always love a good challenge

author: GreyWoolfe
author_rank: Unknown
timestamp: January 03, 2016, 10:28:58 pm
content: Sad to see a radio like the 940 abused. I have one here and have no plans of ever selling it. It is now relegated to backup radio since a Yeasu FT-1000 MP came to me in a rather interesting trade.

author: Radio Tech
author_rank: Frequent Contributor
timestamp: January 03, 2016, 10:51:00 pm
content: Quote from: GreyWoolfe on January 03, 2016, 10:28:58 pmThat is true. Seems I get all the good ones lol. Have not had time to get back on this one due to so much going on ion the shop. That 1000 MP is a nice radio btw.

author: AF6LJ
author_rank: Unknown
timestamp: January 05, 2016, 12:53:46 am
content: Quote from: GreyWoolfe on January 03, 2016, 10:28:58 pmA friend bought a pair of 1000MPs paid less than $100.00 for the pair.Both broke, one is up and running with a minimum of effort.Nice buy.