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The TS-850S Repair Page

The Kenwood TS850S Amateur Radio Transceiver is a very popular radio. It was in production from 1989 until around 1996 when the TS870 replaced it. This page is dedicated to helping 850 owners keep their radios on the air.  This is starting to become a bigger and bigger challenge as time goes on.

I would like to thank all of those who have provided information used on this page. If you have other information that would be of interest, please pass it along to me and I will add it to the page.  My e-mail address is tree@kkn.net.

One of my personal frustrations has been the reliability of some of the DDS chips (you can find them on the CAR board, underneath the metal cover behind where the speaker rests). These are expensive little buggers to purchase ($28 each) and I have had all of them fail on three different radios!! I have also received enough mail from other people to believe that there is a component reliability problem with them (perhaps limited to the parts made '90 and '91). Kenwood hasn't owned up to there being any kind of reliability issue.  Recently, I replaced a YM6331 on a board that Kenwood had worked on before. They had replaced the other 3 of them. I don't like it when a manufacturer obviously knows there is a reliability problem and they don't own up to it.

It appears that the early chips with the part number 6631 on them have the problem and the newer chips, with 66312 on them are fixed. I would suggest if you have an 850 with the 6631 chips in them, that you consider swapping them all out with new ones at the first sign of trouble.  I am not aware of any failures with the 66312 parts yet.  

If you need to have one of these reworked but don't have the equipment, send me a note and maybe we can work something out. I am willing to replace the parts for $25 each, or $75 for a board plus shipping. If you send me your board, leave it mounted on the metal plate.

Also - if you know of any junker 850s that could be used for parts - please let me know.  

Here are some links that you might find useful:

Enjoy!

73 Tree N6TR
tree@kkn.net


Common Problems with the 850 and Possible Cures

Reduced receiver sensitivity and transmitter output as you increase frequency. This is the typical symptom from the most common component failure I have heard of in the 850. The cause is that some of the band pass filters on the RF board are always enabled by the 74LS145 decoder chip. You can verify this by using a voltmeter and seeing if one of the outputs from the chip is always "low" (near zero volts).

I would suggest putting a socket down on the board for this IC - so if it ever fails again, you can replace the part quickly. This will require removing the RF board (which is the one underneath the radio with the shiny shield on it). The IC is under this shield.

I have received a comment that Kenwood often replaces a MI303 diode when replacing this chip (thanks K5TSQ). There is mention of a modification on the Kenwood ftp site that might also help eliminate this failure. Check out page one and page two of the service bulletin (thanks VE3PN for updated URLs).

I believe this failure might be a result of a high voltage being presented on the receiver input. I do think one or two of the failures I have seen might have been caused by transmitting with full power with a different radio with antennas that were close. It has also been suggested the 47K ohm DC input resistance of the receiver could easily allow nearby lightening hits to produce a high voltage. Perhaps putting an inductor on the RX antenna input to ground would help as explained as follows (thanks K0ZL):

cure by removing the surge absorber tube on the small pc board connected to the antenna connector, (remove top cover of filter unit, top of rig, rear-center, to get to it). obtain a 1-2 milli-henry pi-wound choke (one of the big oldies-but-goodies), and tack solder the surge absorber across it. reinstall the assemble where the surge absorber was. that's it. you now have 2 ohms to ground dc, and much harder to develop high static voltage...

Frequency display is all dots - and the radio sends "UL" in morse code over the speaker (indicating an unlocked condition with the frequency synthesizer):  This failure might be temperature related - and possibly show up more often when tuning the VFO in specific places (around 1800 - 1850 kHz is one place I have seen it fail). This is probably the DDS chip on the carrier board that produces the VFO frequency. The CAR board is under the bracket that would hold the voice memory board if you had it (under that door on the top of your radio). Remove the four screws that hold the bracket and find the CAR board with the four big chips on it. You can put a scope on the PLL-1-DL01 output and watch the output voltage as you tune around. On a functioning board, the output level should be pretty constant as you tune the VFO through a 500 kHz segment. If the output changes a lot when the VFO jumps from one frequency extreme to the other, then the failure is probably the IC1 DDS chip. They cost $28 and will require you to have the right equipment to work with surface mount VLSI ICs.

One step you should take first is to make sure all of the connections to the CAR board are good.  Remove all of the cables, and reseat them.  

Raspy signals on some bands - maybe no display or all dots:  I had this problem with one of my radios after taking it to Louisiana for the Sweepstakes.  The signals on 20 meters all sounded raspy - and broad.  At first, I thought it was something wrong with the transmitter of the other station, but when all of the signals on the band had the same problem, I knew it was my receiver.  I believe my transmitted signal sounded as bad.  It seemed to be temperature related and in extreme cases, the LO1 output from the PLL board was going away.  And then Dave, NR1DX, hit the nail on the head with this:

I just finished fixing the second TS-850 in my career with this problem...particularly symptomatic is the raspy sound which gets worse as the unit heats up. In both cases TC1 on the PLL board needed only to be adjusted so that TP2 reads 5.0v. (See the service manual page 98, step 8).  The little trimmer cap TC1 inside the VCO-2 can has been documented as getting flaky with time. On the first radio the problem occurred at about year 7 resetting the cap cured the problem and the radio ran fine for another year without incident before I sold it and upgraded to a TS-950SDX.

I recently came on a bargain ($550) TS-850SAT (10 years old) which was too good to pass up (including the a 500HZ and 270 Hz CW filters and a DRU ).  It had this same problem only worse in that after setting the cap the unit would play fine... for a while, then drift back into the same old problem.  I then replaced the cheap Kenwood trimmer cap with a glass piston trimmer cap which so far after four hours is stable as a rock.. Ill send you a picture of the mod ( if you want it) as I had to drill a hole in the side of the VCO-2 can to mount the new trimmer cap.

So check your problem radio's PLL - TP2 when it gets raspy, I'll bet it is no where near 5Volts, the setting is quite critical.

When I checked the voltage on TP2 (located near the front of the board right next to the smaller shiny box), it was around 4 volts.  I adjusted it up to 5V, and things sounded worse...  that was until I removed the lead to the DVM (it was acting like an antenna and picking up all sorts of junk).  After that, the radio sounded like new.  When I thanked Dave, I got this in response:

Don't thank me ... thank Cliff at AVVID he is the one that clued me in the first time three years ago. The guy is amazing

Here is the picture of the modification and the PLL board afterwards.

You will see the little piston cap on the left of the VCO2 can. The little melt from the soldering iron on the edge of the little blue plastic thingy was an "aw- $#!^", in trying to clear the holes in the board. Didn't hurt it electrically...

To do this you have to remove the whole VCO2 enclosure there are about 5 places where the can is soldered to PLL board plus three wires. If you have never used solder wick now is a good time to learn. Removing the can is almost as difficult as brain surgery but still not as bad as changing one of the DDS chips.

Passband tuning is erratic while on SSB, but seems okay on CW. People also tell me I am off frequency. This happened to one of my two TS850Ss. The culprit was IC4 on the CAR UNIT (DDS chip) that generates the CAR signal (on the bottom of the CAR UNIT schematic page). They cost $28 and will require you to have the right equipment to work with surface mount VLSI ICs.

The SSB and/or CW monitor functions do not work - or is very distorted on SSB. Another DDS chip on the CAR unit (IC3) produces both the RF frequency used to demodulate the SSB for the monitor function (MCAR) and also the audio tone used for the sidetone (STON). If this chip isn't working correctly, it would explain these failures. They cost $28 and will require you to have the right equipment to work with surface mount VLSI ICs.

Receiver overloaded by AM BC stations. This could be the 74LS145 failure mentioned above. It could also be the diode mentioned in the Service Bulletins (thanks KS4XG). I also had this problem in spades on one of my radios - but it turned out to be a pair of back-to-back diodes I had installed when adding the external RX antenna modification. Removing this cured the problem.

ALC voltage problem (from NB1B): The problem manifested itself as varying power output; if you close the key and watch the power meter, it will go from zero to full output and then back to zero output on about a 5 second cycle. What is happening is easy to see by using an external wattmeter and putting the 850 meter on ALC; the ALC goes from zero to max as the power goes from max to zero. This was explained to me by the guy that diagnosed and fixed the problem. The circuit that is bad is one that has resistors, transistors, and capacitors all on a multi-legged component. It is a multivibrator that develops a voltage that is then rectified and amplified and is used as the ALC voltage.

Something goes bad and it causes a very slow oscillation, and the ALC voltage swings from zero to max and back again, causing the power to oscillate in step. I don't know how much the part cost, but the guy spent 8 hours on the radio (not a Kenwood Factory Rep) before he could figure out what was going on. Since then, I have seen probably a dozen postings from hams that have had the identical problem that have sought an explanation for it.

Also, K0BX reports the same problem: The DC-DC board X59-1100-00 fails and causes the power output to go up and down. First time I had Kenwood replace it for a total cost of $150.00. This past December I replaced it my self for a cost of about $14.00 with shipping. I have a detailed description on how to do this repair on my webpage and then go to TS850S ALC/TX PROBLEM.

And this from K2UT:   One of my 850s had the DC-DC board replaced about 18 months ago after experiencing the usual problem: output power oscillating back and forth.  Had it repaired and the rig was working great.  Then, about 4 months ago, a new
problem appeared. I had a couple DC-DC boards on-hand for repairs, but this didn't seem to fit the usual pattern.

When the rig was on for a while but not transmitting, the first SSB transmission would peg the ALC meter for a couple seconds with no output. It would then drift back down to a normal level, and output power would rise.  Transmitting
again within 10 seconds would work fine; no ALC problem. After about 15 seconds, the cycle would repeat.

I called Kenwood and suggested the DC-DC board, but the repair tech didn't think it could be the problem. Rig went back to Kenwood for repair. The problem? The f***ing DC-DC board! $135 and six weeks spent when I could
have repaired it myself.

Low Receiver Sensitivity (from K7FR):  The only failure that I had on either of my 850's was that one of the switching transistors that muted the receiver during xmit went sour and wouldn't go all the way to cut-off in receive. This had the effect of putting about 20dB of pad into the receiver. This happened on both. The first one went out to be fixed the second one I did myself. 

Low receiver sensitivity could also be related to the first item on this list. Also, I noticed an interesting problem while at W5WMU last time. One of the 850s that lives there was down in sensitivity from the one I took with me. I am not sure what the problem is, yet. It could be this switching transistor problem.

No receive except on FM (from W9PL/7):  My 850 was purchased new in 1993. It was repaired twice under warranty for the same symptoms. The symptoms (to make a long story short) involved an inoperative receiver in all modes other than FM. The initial failure occurred after a few months of use, in 1993. The second failure occurred after only 5 hours of operation following the first repair action. However, since the second repair action, it has worked just fine and I am very happy with the radio. The first time they repaired it, Kenwood replaced a VMI308 diode and a 3SK131(M) FET (Q15) on the IF Unit. Q15 is the last of stage of IF AMP preceeding the SSB Demod. The second time they repaired it, they replaced the same FET (Q15) on the IF Unit. I suppose they had a "bad" run of Q15 FET devices at that time. 

Poor solder connections. It is possible that one of my DDS chip failures was really due to a poor solder connection. I noticed when replacing the chip that there wasn't a lot of solder on the IC pins. I was able to reuse this chip on another board and it came up and worked fine. The following comments are from an ex Kenwood service person: The history we were given regarding the solder quality is that amateur gear is a very low priority, low quantity production run at Kenwood. The PCB blanks are all made at a plant at one time for the estimated life production run of the rig. Then they are shipped to a plant where maybe 6 months or even years later the PCBs are used for a small run of the units. This plant is on a coastline and the boards suffer from salt air corrosion problems, so the solder does not wet properly. I can believe that story, but they still suffer from very poor QC with regard to insufficients, cold solder, and lack of eyelet properly done before soldering on the older PCBs. Kenwood has service bulletins out on some boards, where they point out 30 to 60 feed through for shotgun resoldering of eyelets with a bare wire fed through the eyelet hole when resoldering and then clipped off.

CT2HMX sent me the following - radio fails in the FM transmit mode - FM microphone amplifier not working: I saw the Board X59-3000-03, FM mic amp and all components were okay. I put a 1 kHz tone, 5 mv, into the the mic input on the front of the radio and saw the signal on C630 on the RF board (X44-3120-00). I did not see the signal on the collector of Q609 however. The hfe (beta) of this transistor was very low and I replaced Q609 (2SC2912A) on RF board and the problem was solved. The old transistor wasn't bad - but the hfe gain was very low and caused the failure.

DJ3TZ offers the following story about repairing the rotary encode:  I recently had a problem with my 850 which I could fix myself and would like to share this information. It concerns the rotary encoder that transforms any movement of the main frequency dial into digital pulses. The part in question is a COPAL RMS20 encoder.

For a long time, my Kenwood TS-850 had the problem that while turning the main frequency dial, the frequency would stop changing with the last digit jittering. For several years, the problem occurred so rarely that it did not really hinder operation, and I saw no chance to locate the problem.  Recently, however, the problem became more serious, making the radio almost useless. Using the 850's service manual, I was able to locate the encoder and its connection to the radio's main data bus.

The encoder has two data outputs, A and B. Both carry digital signals at 5V level. Its frequency depends on the rotation speed and the signals differ in phase to allow for clockwise/counterclockwise detection. Together with my friend Theo, DJ9PK, I observed these signals with an oscilloscope. One of them vanished whenever the the frequency stopped changing.

Taking a look into the encoder, we tried to locate any kind of mechanical problem, perhaps a little bit of dust, but there was none.  The encoder consists of a magnetized wheel and an magneto resistive Hall sensor, in addition with some electronics. Checking the signals from the sensor revealed that is analogous. It consists of a voltage of approximately 2.5 V DC plus approximately 80 mV AC when turning the shaft.

We found that the amplitude of the AC voltage strongly differs while turning the shaft. The reason is perhaps that the magnetic field produced by the wheel is not constant, but seems to depend on its position towards the Hall sensor. The amplitude seems also to depend on other, unknown factors.

The PCB inside the encoder has an IC that obviously works as a comparator and digitizes that input. Checking its 8 pins revealed that two carry the input coming from the sensor, two carry the digital output, and two pins are connected to reference voltages. These reference voltages can be adjusted with two variable resistors on the left and the right of the IC.

Turning one of the resistors fully into one direction causes the digital output to become constantly low or high, respectively, because the reference voltage then is always higher or lower then the sensor signal. Within a range of 10 or 20 degrees, the comparator works as intended. I suspect that the reason for the occasional failure was that the amplitude of the AC signal was sometimes below the comparator's reference voltage, thus causing the output to stay high or low.

Rotating the shaft by hand but at a constant speed and observing the digital output on the scope allowed us to adjust both resistors so that encoding now works fine.

Here is the pin layout of the IC:

1 Output B
2 Reference voltage B
3 Input B from sensor
4 Gnd
5 Input A from sensor
6 Reference voltage A
7 Output
8 + 5

I hope this information will be helpful to you and others.

This note came from PA0RLS:  Beware of C1 on the CAR-UNIT: If it starts leaking (I mean fluid coming out of it) then you may expect all sorts of crazy things.  Most common are: RX is deaf and/or the TX led is on during receive. Monitor the TXB line during receive: It MUST be 0V. If not the problem above may be the cause, as the + from this cap. leaks to TXB ! Replace C1 by a normal (not SMD/metal can) electrolytic capacitor: 100 uF / 25V will do. You may see that the fluid from the old C1 is etching the PCB and in one case I had a hole burnt in the PCB just under C1.
Note that there may be other causes to make TXB not 0V during receive such as D30 on the RF-unit.

TS-850 CARRIER SET POINT ADJUSTMENT (without instruments) submitted by K6LL:

1. Open the little hatch on top of the radio. Locate VR501, VR502.
2. While depressing F. LOCK, turn the transceiver ON.
3. Click the M. CH. control until MENU 00 is displayed.
4. Select LSB
5. Turn the power control all the way down.
6. Push the SEND switch.
7. Make a slight adjustment to VR501 to make your voice higher or lower while speaking into the mic, listening to yourself in the monitor.
8. Put the radio in USB.
9. Make a slight adjustment to VR502 to make your voice sound higher or lower while speaking into the mic, listening to yourself in the monitor.
10. Put the SEND/RCV switch back to RCV, and then press CLR.
11. Listen to on-the-air signals on both USB and LSB to see if additional adjustment is required.

TS-850 Carrier Balance Adjustment submitted by K6LL:

1. Open the bottom hatch of the TS-850.
2. On the IF unit, locate VR8 and VR9 (near the little Murata filter).
3. On the TS850: PROCESSOR OFF, MIKE GAIN MIN, FREQ 14.2, MODE USB, POWER MIN.
4. Construct an RF voltmeter by sticking a silicon diode, such as 1N914 into the antenna socket. Connect a sensitive DC voltmeter between the diode and ground.
5. Push the SEND switch on the TS850 and adjust VR8 and VR9 for minimum indication on the voltmeter.
6. Make sure that BOTH of these controls are going through minimums. It is possible to get so far off on VR8, that VR9 has no effect.
7. Check that LSB is ok also.
8. For the best final measurement, take a piece of insulated wire, bare one end, and stick that end into the antenna socket of the 850. Bring the insulated wire NEAR the receive antenna connection of a second receiver tuned to the carrier frequency, and make fine adjustments to the null, with the POWER control set at MAX. Of course, do not connect the second receiver directly, in case your hand slips while you are making the adjustment! I just wrap the insulated wire around the center pin of a PL-259 connected to the second receiver.

Receiver is dead - no audio comes out - except on FM.  Seems to work okay on transmit (added September 2002).

I had a friend send me his radio with this symptom.  I noticed when I turned the radio on, there was an oscillation that I could hear coming from the RF board.  I found it on a bunch of signals - and measured it around 11 kHz.  I looked at the 3rd mixer of the receiver (which is on the IF board) and saw the 455 kHz signal going into it just fine - and nothing coming out of it.  You could inject an audio signal there if you wanted to see if the audio chain was working okay.  I also didn't see anything going into the mixer. 

My attention shifted back to the front end - where I saw all of the strange oscillations.  I looked for the signal coming into the first mixer - which is a pretty high frequency from the LO1 signal - buffered by Q28.  I couldn't find it.  I checked the two diodes that steer the output of Q28 to either the receiver or the transmitter - and saw it working FB for the transmit condition, but not so good for the receive condition.  The bias for the diode comes from the signal RXB - which should be around 8 volts when in receive.  It wasn't.  This is generated on the IF board on one of those little modules that stand vertical on the board.  Without this voltage - there is no way the receiver is going to work.  I could jumper the RXB signal to 8 volts and make the oscillation go away.  My next step is to see if doing this makes the receiver function again.  I will probably replace the components on the little pc board to fix the problem as Kenwood probably wants too much money for the replacement board.  Three is only about 10 components on the board - and they should be cheap to replace from DigiKey or Mouser.

Well, it turns out the little board is probably okay.  I found that the problem was the TXB signal was not going all the way to ground when in receive - which kept the RXB signal from turning on all of the way.   If I grounded the TXB signal, the receiver came to life.  I removed the module - and found that both RXB and TXB were at ground.  However, if I pulled RXB up to 8 volts, TXB came up to about two volts...  NOT GOOD!  There is obviously something leaking RXB voltage over to the TXB signal.  My next step is to try and find where the leakage is taking place.  Probably a diode somewhere has turned into a short circuit. 

I swapped RF and IF boards with a good radio and isolated the failure to the RF board.

I checked all of the diodes on the RF board with a meter. There were a few that had funny readings, but they were the same as my good RF board.  I started removing various inductors and resistors to isolate the TXB and RXB signals and eventually found that removing the series inductor (L5) that biases D3 during receive stopped the leakage.

This narrowed it down to either D3 or D31 (the path back to TXB) - and it was D31 that was bad (RLS135).


Hope this information was useful. You can reach me at tree@kkn.net if you had any additions.

73 Tree N6TR / K7RAT



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