[Kenwood] TS-940S - How to identify Genuine Kenwood Modifications

[Dr. David Kirkby]

On 06/21/11 08:12 AM, jaking wrote:
>
> Hi David Kirkby,
>
> Im not sure what you mean by dubious mods.

There is one for example of replacing the hard to get driver transistors with 12 
V ones. My guess is the linearity would be poorer and it would be better to use 
28 V transistors. The person describing the mod had not verified the performance 
of the transmitter with a spectrum analyser. That's what I'd consider dubious.

Most receiver mods seem to be based on S-meter readings, not with signal fed 
from signal generators and viewed on a spectrum analyser.

In any case, there's a lower limit on sensitivity beyond which there seems no 
point in trying to achieve due to the atmospheric noise.


> If you will accept Kenwood's circuits then what I am advising you to do is
> to draw out
> the full circuits including what you see as being modifications.
> Then compare your circuits with Kenwood's and you will find that most of
> what you maybe assuming are dubious modifications are in fact Kenwood
> modifications.

Yes, I got that.

> What I am suggesting you do is verify each and every tack on as being a
> Kenwood circuit or not a Kenwood Circuit.
> Your original question was as to whether you had valid authorised
> Modifications expected on Kenwood radio or done by someone else for an
> unknown reason.
> The modifications by someone outside maybe dubious.

A lot of people appeared to have made a lot of mods, but I'm not convinced many 
of them have been properly investigated.

> It took me along time to realise the easiest way to read circuit differences
> was to manually draw the modified circuits out by hand. [I.e. Reverse
> engineering]

I can see that.

> Kenwood made a huge number of what appeared to be micky mouse modifications
> and it took me along time to understand that. I thought like you that
> someone had been hacking around with radio I had brought. Im trying to
> explain to you how you can verify the tacked on modifications you find as
> being valid Kenwood designed modifications.

I get what you are saying, but as I note below, there seems problems with this too.

> The Service Manual dated 95-4  is not the last one, but it will have almost
> all the modifications because production of the 940 ceased in about July
> 1992. Your lucky to have obtained that manual.

It was a bonus I got when buying the radio. It was not advertised with any 
manuals, nor was it advertised with the AM filter, but it came with user and 
service manuals along with an AM filter.

What I find a bit odd is that in the AVR unit, a Kenwood mod (dated 4-8-86) says 
C12 should be changed from 6800 uF cap to a 5600 uF 25 V cap due to the risk of 
the 6800 cap getting too much voltage in areas of high line voltage. That sounds 
very logical.

Yet my manual, dated almost 8 years after this Kenwood mod was released, shows 
C12 as being 6800 uF 16 V, not 5600 uF 25 V. I've not actually checked my 
circuit. The AVR unit in my board has suffered some damage in the past and I'd 
like to get another if possible. But it is working. I need to change two caps 
which is bulging - one is bulging quite a bit, the other only very slightly.

In fact, I intend changing all the electrolytics on the AVR board to new caps 
rated for use at 105 deg C which have a long lifetime. Depending on value, I can 
get caps with rated lifetimes of between 5000 and 10000 hours at 105 deg C, 
whereas the cheapest caps are rated for 1000 hours at 85 deg C.

> Kenwood did not design their radios to be as you put it:
> "Neater, easier to service, less confusing to someone trying to service one
> like myself."
> Because Kenwood never expected wanted them to last so long. Most
> manufacturers in fact do not want products to last.

I know the Yaesu FT-ONE had at least a new IF board. The changes were not tacked 
on. There seems some rather serious design issues with the Yaesu FT-ONE, but it 
is assembled very well. I'd much rather work on that than a TS-940S, but the 
FT-ONE has some serious design flaws.

> What I was pointing out
> was this radio was made before automation, and when everything was assembled
> by hand labour. It was much easier and less expensive to issue a new
> instruction to the laborers than to rework a major component.

Yes, but making a change to the layout of the PCB would be a relatively minor task.

> For the LT1028 reduced phase noise modification and the PLL unlock repair
> are found on read about it at:
> http://homepages.ihug.co.nz/~jaking/TS-940_02.htm

I find that page, which I believe is yours, hard to follow. The areas in gray 
are almost impossible to read. The links don't work either. At the top you have

RECEIVER 6: PHASE NOISE. 25

6.1 Observation of the Phase Noise problem by Thomas Hohlfeld DF5KF in 2005. 26

6.2 Extensive History of the Phase Noise Issues upto about June 1987. 26

6.3 Data Sheet for Problem BA718 OpAmp. 27

6.4 What is Phase Noise and how to measure it by John Grebenkemper, KI6WX.. 27

6.5  Phase Noise measurements on Elecraft K2 by John Grebenkemper, KI6WX.. 49

6.6 Significant Phase noise improvements developed by Thomas Hohlfeld DF5KF.. 50

6.7 Information on Ultra Low Noise OpAmps available in 2008. 56

6.8 Further assessment and Phase noise improvement evaluations planned by Jeff 
King ZL4AI 58


But clicking any of those links does not lead anywhere.

I think I found it with Google

http://www.jking.kol.co.nz/TS-940_02_part2_1.htm

It would be good if you could provide a summary page of what you feel are worth 
doing on the TS-940S, and avoid discussing the things you don't feel are worth 
doing.

> If your systematic and record everything it will not take long to fix the
> 940.

The problem I have is the only major problem (PLL unlock) is intermittent. Those 
sorts of faults are hard to track down.

I'm pretty sure I know the cause of the low audio output. The cap in series 
between the output pin of the audio IC and the speaker is bulging and I suspect 
has dropped in capacitance. I have a new cap on order which will be here today. 
I'm 99% sure I've found the cause of that issue.

> If you just want a radio that works with similar performance and cannot be
> bothered doing the repairs then you would be better to buy a TS-480SAT.
> Buying a TS-480 would less expensive than sending back to the USA. Cliff
> will repair individual boards so that maybe the way to get each board
> validated. My opinion is you will find it less expensive to verify each
> modification you find than to send the components to Cliff to have him
> verify the modifications.

I was not aware Cliff would verify boards. That's useful to know. But I hope I 
can sort this out myself.

> Best of Luck from
> Jeff King

Thank you Jeff.

Dave, G8WRB.