[Kenwood] TS-940S - Digital A board. These mods OK and should be left in PLACE

[Dr. David Kirkby]

On 06/19/11 02:57 AM, jaking wrote:
> Hi David,

Hi Jaking

> You are starting your first steps on the Kenwood 101 class repair learning.

Yep.

> The digital A boards look completely normal and in good condition. Kenwood
> made more than 17 versions [I.e. up Mark 17] of the TS-940. They made these
> changes by tacking components on the back of the boards. All 940s have
> components on the boards.  They later the 940 the more components. Kenwood
> never issued revised boards. Why should Kenwood when components were put on
> by human beings and it was easy enough to have them tack extra components on
> the back.

Neater, easier to service, less confusing to someone trying to service one like 
myself.

> To use your analysis skills
> - From each end of the tacked on component on the board drawing out with a
> pencil and paper the circuit, going at least two components backwards up
> each circuit.
>
> - Then down load the service manual from the web [www.mods.dk] which I
> believe is one of the latest but not necessarily version 17.

I have downloaded a copy from the web. I don't know if its the same though. Mine 
has a file size of 70444057 bytes. The md5 checksum of the file is 
d9b2c6e66044e698bae83062c029c7e7

I also have an original (paper) version of the manual, which came with the rig. 
It says on the front "Revised edition". I've no idea of the exact revision of 
it, though it says on the back

1985-2/85-9/87-3/87-4/87-10/98-8/90-3/91-7/92-10/94-10/95-4 PRINTED IN JAPAN 
B51-2058-10 (N) 150.

I'm guessing that might mean its dated April 1995, though I don't know for sure.

> - Compare the circuits you have drawn with circuit diagrams [NOT THE FULL
> SCALE DRAWINGS OF THE BOARDS] in the  service manual. You will find that
> Kenwood has drawn the tacked on components in the circuits.
>
> - down load all Kenwood's 15 or so service bulletins and do the same
> verification.

I have originals of most, if not all of them. I have the following, listed in 
chronological order.

1) TS-940S LCD CLOCK DISPLAY ERRATIC OPERATION 9/14/85 (896)
(replacing IC2)

2) TS-940S PLL UNLOCK 10/18/85 (900)
(readjustment of L22, L23 and L24 in PLL)

3) TS-940S ANTENNA TUNER RELAYS 4/4/86 (907)
(reduce power when tuning)

4) TS-940S PLL UNLOCK 4-8-86 (908)
(This one is different from the other PLL unlock note #900, as this involves
changing a 33 pF cap to a 15 pF cap)

5) TS-940S AVR UNIT CAPACITOR CHANGE 4-4-86 (909)
(increases voltage rating of a cap)

6) TS-940S AGC CIRCUIT IMPROVEMENT 6-3-86 (910)

7) TS-940S TRANSMITTER HUM IN SSB 9-15-86 (912)

8) TS-940S SIGNAL TO NOISE RATIO IMPROVEMENT 8-28-86 (913)

9) TS-940S VCO CARRIER TO NOISE RATIO IMPROVEMENT 3-2-87 (917)

10) TS-940S SQUELCH SWITCHING NOISE 7-21-87 (918)

11) TS-940S SSB TALK POWER IMPROVEMENT 7/22/87 (921)

12) TS-940S ERRATIC DISPLAY 01/20/89 (SB-951)
(removal of ROM socket)

13) TS-940S MRF-485 Driver Transistor notes January 16, 1992. (SB-988)
(addresses stability problems with higher gain transistors)

14) TS-940S FSK Shift Width Selection April 14,1994 (AAN-0007)

> Even if you do not find components in Kenwood's Documents DO NOT REMOVE
> COMPONENTS. There were several other well recognised modifications that
> could have been done, and you should enquire about each and very tack on
> separately.

There also seems to be some rather dubious ones too. Since I know nothing of the 
history of this unit, I have no idea if any of the boards have dubious mods on 
them.

The AVR unit has clearly blown up at some point, as someone has removed 
connector #1 and filed out a semi-circular hole. Several wires leading to that 
board have been cut and joined. C6 looks as though it has been overstressed, as 
the top is bulging. I was hoping to get another AVR board, but it looks like I 
might have to replace C6 now.

> If you do not like how Kenwood modifies boards you are not impressed by the
> fact that Kenwood practices radio engineering in a manner completely
> accepted by radio engineers. Try reading work from W7ZOI.
>
> In another 8 years after you publish recognised Kenwood repair work you may
> be able to gain a PhD in Kenwood 101 repair.

Hopefully it wont take me 8 years to get this back in order. If I can't get it 
working right, I'll consider sending it to Clif Holland KA5IPF, but given he is 
on the other side of the Atlantic, I'm not keen on that idea.

I bought the rig from a ham in the US who assures me it was working OK when it 
left him. I do believe him.

The PLL unlock issue is just intermittent, which makes problems more difficult 
to solve, though leads me to believe there's no major component at fault.


> Best of luck with the repair. If you are consistent, methodical, and logical
> your will create a superb TS-940. Seriously consider the LT1028 modification
> for substantial receiver improvement.

I've not seen the LT1028 modification mentioned anywhere. I'd appreciate knowing 
what that is. I know that's a very low-noise amp, so perhaps someone has 
replaced a higher noise amplifier for this lower noise one. But a Google does 
not throw any light on it for me.

> Yours sincerely
> Jeff King ZL4AI


Dave