Kenwood TS-440 repair

This page describes some repair tasks related to the Kenwood TS-440 transceiver. You will be guided through the repair proces step-by-step followed by descriptions and pictures. Any picture on this page can be enlarged by clicking on it.

I recommend reading this document written by Mike Freeman KC8QNO: TS-440S Repair FAQ. There is an excellent Yahoo-group for TS-440 repair: TS-440. 

Rebuilding VCO1 on the RF-unit

VCO1 filled with goop.
VCO1 filled with goop.

The problem 
Kenwood TS-440 is designed as a mobile transceiver. To withstand mobile use, some of the circuit boards are partly covered with "goop" (glue) in order to prevent components from vibrating and causing frequency instability.

As time goes by, the goop becomes slightly conductive and may cause leak currents flowing between components. This is very bad in VCO circuits, where the varicap diode is extremely sensitive to any voltage change.   

A TS-440 with conductive goop in the VCO circuits can -as in my case- sound distorted when receiving a signal.

I measured 1-2 V DC between ground and the goop in VCO1. I used a digital voltmeter with 11 Mohm input impedance.   

 


Goop halfway removed.
Goop halfway removed.

Removing the goop

Make notes on how cables etc. are connected to the RF-unit, or take a close-up picture. Now disconnect the cables, remove the screws, and extract the RF-unit. 

The goop in VCO1 is removed using hand tools. Use a stanley knife for cutting out pieces of goop and for cleaning (scraping) the component side of the circuit board. A screwdriver is handy for lifting components when unsoldering. A long-nosed plier is used for removing components and pieces of goop. This work requires several hours of intensive labour. I recommend you take several breaks in between.

Some people say that components dirty with goop can be removed, cleaned, and reused. I could not do that. The goop removal proces will probably destroy many components. Furthermore, it is easier to clean the circuit board when everything on it has been removed.  

Note: Do not destroy the original 2SC2668(Y) transistors. Unsolder them, clean them carefully, and reuse them. I offer this advice because it was difficult for me to obtain new transistors with the same properties as the old ones. 


VCO1 cleaned.
VCO1 fully cleaned.
Rebuilding VCO1

I ordered brand new components from East Coast Transistor in New York, USA.

The rebuild work should be divided into steps. Build one voltage controlled oscillator and test it. Then continue with the next one. 

Each VCO is powered by applying 7.7 V to L97 and to the 100 ohm resistor in the collector of Q21, Q22, Q23, or Q24. The voltage, which controls the varicaps, comes from a 10 kohm potmeter temporarily connected to the power supply and the slider connected to R99. A suitable connection point for a frequency counter is "PLL VCO" on the side of the PCB. I put a 22 pF capacitor between "PLL VCO" and the center conductor of the counter cable.

I could not make transistor Q24 oscillate in the proper range. It would only oscillate down to about 49 MHz, then it died. Replacing Q24 and the varicap D50 did not help. So I ended up changing the value of two components:

ComponentOriginal valueNew value
C19468 pF68 pF + 15 pF
C1955 pF33 pF

Q24 now oscillates in the specified range (45.08 - 52.55 Mhz).

The original varicap ITT310TE (D44, D46, D48, D50) can be substituted by BB139. That worked in my case. 

Dave (WB7DFV) used NTE614 as varicap replacement. He wrote: "It turns out that the NTE614 was a good replacement. I could not get the oscillator with T21 to work and had to reduce the 39 pF cap to a 22 pF cap. Now the radio works perfectly."


VCO1 finished
VCO1 rebuild finished.
VCO1 rebuild finished
The rebuildt VCO1 is pictured on the left. Some of the new components have been placed on the trace side of the PCB.

Larry (KA5ZWY) told me that he had problems with T22. However, when he applied pressure to the PCB near T22, the problem disappeared. Resoldering T22 and the components nearby will solve that problem.

VCO1 adjustment 
Remount the RF-unit and reconnect all cables. Follow the Service Manual p. 92 item 13. Please note the typo at the 3rd step. The correct value is 14.4999 MHz, not 14.9999 MHz.

Main dialTP10Remarks
29.9999 MHz2.49 VAdjust T21
22.0000 MHz5.94 VOK value
21.9999 MHz2.51 VAdjust T22
14.5000 MHz5.86 VOK value
14.4999 MHz2.50 VAdjust T23
7.5000 MHz5.93 VOK value
7.4999 MHz2.38 VAdjust T24
30 kHz6.62 VOK value - the PLL remains locked within the range
Note: TP10 is the top of R138 on the PLL-unit and can be accessed from the side.

Rebuilding VCO5 on the PLL-unit

The position of C178 and R144 as given in the Service Manual or printed on the PCB is not correct. This table shows the right position:

ComponentPosition
C178R144
R144C178

Click this link to view a picture showing the correct position of the 2 components.

VCO5 before goop removal
VCO5 filled with goop.

The problem 
Some of the components in VCO5 are covered with "goop" (glue) in order to prevent components from vibrating and causing frequency instability.

As time goes by, the goop becomes slightly conductive and may cause leak currents flowing between components. This is very bad in VCO circuits, where the varicap diode is extremely sensitive to any voltage change.   

I measured about 300 mV DC between ground and the goop in VCO5 using a digital voltmeter with 11 Mohm input impedance. This voltage level may seem low, but it is poison to a voltage controlled circuit!   

VCO5 cleaned.
VCO5 fully cleaned.

Removing the goop
Make notes on how cables etc. are connected to the PLL-unit, or take a close-up picture. Disconnect the cables, remove the screws, and extract the unit. 

Remove the lid of the shielded box. The goop in VCO5 is removed using hand tools. Use a stanley knife for cutting out pieces of goop and for cleaning (scraping) the component side of the circuit board. A screwdriver is handy for lifting components when unsoldering. A long-nosed plier is used for removing components and pieces of goop.

Eventually I used propyl alcohol on a cotton bud to clean the area.             

VCO5 rebuilt finished
VCO5 rebuild finished.
VCO5 rebuild 
The finished VCO5 is pictured on the left. C186 and C192 were placed on the trace side of the PCB.

According to the circuit diagram, Q36 is 2SK192A, but this FET is not available any more. I substituted a BF256B, which works just fine.

I divided the work into steps. I first build the amplifier Q34 and Q35, and tested the DC levels. Then I continued with the oscillator part Q36 and Q37. I checked DC levels, and measured the output with a diode probe. Finally, I connected a frequency counter to R154, applied 5.0 V DC to TP 11, and adjusted T20 until the counter showed 36.22 MHz.

Please note that the Service Manual has swapped the DC readings at Q36's gate and source. The gate should read 0.0 V and the source should read 1.0 V. The drain level is correct as you should read 7.4 V here.  
VCO5 adjustment
Remount the PLL-unit and reconnect all cables. Follow the Service Manual p.91 item 3. Connect your voltmeter to TP11 (top of L40) and turn the T20 slug slowly until 5.0 V is measured. Finally, put back the lid on the VCO shield box.

Help - there are still dots-in-display...

After having rebuilt VCO1 and VCO5 as described above, I still had dots-in-display. This error turned up some time ago. It happened at any frequency, and often right after I touched some wires or cables. At other times, the TS-440 behaved normally. What to do now?

Step 1: Check the band select circuit (RF-unit).
Service Manual page 23. My advice is to follow the frequency values in table 13 and check B0-B3 at connector 15. Check also the control signals to ATU, filter unit, and band-pass filters.

Step 2 : Check diodes in the band pass filters (RF-unit).
The diodes in the band pass filters on the RF-unit were checked by following the procedure described in TS-440S Repair FAQ. All diodes were OK.

Step 3: Provoke the dots-in-display error and make it stable.
The idea is to poke cables and wires and slightly move components until the error becomes permanent. As I was doing this on the RF-unit, I discovered that one of the wires in the flat-cable between VCO1 and Q41 was unstable. When I touched the wire, dots-in-display turned up. Touching the wire again could make the error go away.

Loose wire (RF-unit)
Loose wire on RF-unit (click picture to enlarge).

The wire in question carries DC to the buffer circuit in VCO1. When the wire becomes loose, the output of VCO1 disappears completely (!) and causes the dots-in-display error. The solution was simple: I soldered a replacement wire under the PCB. The display problem is now resolved. I have not seen dots-in-display on my TS-440 since then.

Excessive hiss from the loudspeaker

When the AF control is turned fully down (fully CCW), a rather loud hiss is heard from the loudspeaker. There is something wrong with the AF amplifier circuit on the IF-unit. The electrolytic capacitors around IC7 have probably dried out.

Step 1:
Replace C173 (IC7 feed-back). No change, still excessive hiss.

Step 2:
Replace C170 (IC7 input), C172 (IC7 decoupling), and C175 (IC7 output).

The problem is apparently solved, no hiss anymore - at least until the next day - then the hiss came back! I dismounted the 4-hole molex header at J2 which connects the AF potmeter. The loudspeaker became silent immediately. My conclusion: The hiss comes into the AF amplifier IC7 via J2. As I remounted the molex header onto J2, the sound suddenly became right. I discovered that the ground pin at J2 had a bad connection. 

Step 3: J2 has to be cleaned. I apply Contact 60 (cleaner agent) to J2 in order to reestablish the ground connection. Problem solved.

If the AF potmeter has no ground connection (as was the case here), it is placed in series with the input of IC7. The noise can never be fully turned down. When the ground connection of the potmeter is working, the pot is connected in parallel with IC7's input. The potmeter can now completely silence the loudspeaker when the AF pot is fully CCW.

Hint: By tilting the IF-unit 135 degrees, you can access the solder side as seen in this picture.

Written by OZ1BXM Lars Petersen 10-October-2009. Latest revision: 26-Nov-2011.

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