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Vintage Amateur and Military Radio Amateur/military receivers and transmitters, morse, and any other related vintage comms equipment.

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Old 4th Aug 2021, 3:02 pm   #1
Wendymott
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Default Kenwood TM421E Sick

Hi Peeps.... Now I am totally against black boxes.."Usually" but at this junk sale last night a Kenwood TM421 was offered at 5...... well you cannot look a gift horse in the mouth.. so to heck with my principles..5 changed hands.... its complete and with service data.
On the bench, it receives but is profoundly deaf " I SAID... oh dont bother Typically 10 m/v before anything is heard ..... Transmit is ok. Thus obviously a RX front end issue, I cannot see a forum on this radio..... I just wondered if any of you out there have knowledge of this Black box.
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Old 4th Aug 2021, 3:12 pm   #2
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Default Re: Kenwood TM421E Sick

https://www.radioamatore.info/attach...ice_manual.pdf

Good luck. It's PIN diode switched for TX/RX see D2 and D3 in final unit.
You could try measuring sensitivity again on the cable to RX front end which departs final unit after D3.

Last edited by Jon_G4MDC; 4th Aug 2021 at 3:17 pm.
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Old 4th Aug 2021, 6:56 pm   #3
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Default Re: Kenwood TM421E Sick

Hi Jon. Time to hold my hands up..... was me..... the Receiver is ok..... however, I now know why it was a "fiver" its the European version as in "E". Thus has no CTCSS tones, just a single 1750 Hz tone.
So for 5... I get a box of modern bits which are very useful... a PA IC that is quite valuable.... so if I don't get it modified to access the repeaters..I am not too bothered.
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Old 4th Aug 2021, 7:11 pm   #4
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Default Re: Kenwood TM421E Sick

I wouldn't write it off completely Wendy, quite a few repeaters still support 1750Hz tone as well as CTCSS.

As it's in working order, it's probably worth a bit more than you paid for it to someone else, try offering it around. Its biggest handicap is probably that it is 70cms only, I very rarely hear or work people on that band now.
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Old 4th Aug 2021, 8:01 pm   #5
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Default Re: Kenwood TM421E Sick

I think UK radios of that era were all E, you might well find that CTCSS was an option board. I have a Kenwood 'E' h/h of that era that has this "feature". I rather think I have a Kenwood mobile, again of that era that has CTCSS on transmit only.

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Old 4th Aug 2021, 9:33 pm   #6
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Default Re: Kenwood TM421E Sick

There are cheap and simple ctcss kits out there as well as old PMR boards that can be squeezed into a rig such as yours. Unless you have plans for extensive /M, usually just one or two sub tones will do you. Attempting a mod like this can be quite educational as well as adding value to your 5 investment.
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Old 5th Aug 2021, 6:50 am   #7
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Default Re: Kenwood TM421E Sick

CTCSS is on page 55. It is serially loaded though the data is changed to parallel for presentation to the CTCSS IC itself.
I have used simple opamp circuits for CTCSS generation. They were tunable and perfectly serviceable.
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Old 5th Aug 2021, 9:37 am   #8
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Default Re: Kenwood TM421E Sick

Hi peeps. Thanks for the comments....I am not an operator as such, mobile is out of the question..but I will look on eBay for modules.... I hadnt thought of that....when I have either made something or got to work.. once its "air tested" I loose interest and start the next project. But at least its "radio"
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Old 5th Aug 2021, 9:51 am   #9
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Default Re: Kenwood TM421E Sick

It might be fun to build it a CTCSS.

Finding their board or even chip is probably not going to happen but there are ICs by CML and many ex PMR boards based on them which turn up from time to time.

FX365 was a common one. I bet the Kenwood serial data latch could be copied and used to program one of those ( they could take serial as well as parallel IIRC but Murphy says the format will be different). The tones might be all over the place compared to the original design but would that matter so long as you could find the 8 or so which are needed. ( Format is not that different - both are 8 bit serial shift registers)
Attached Files
File Type: pdf fx365ds.pdf (289.4 KB, 11 views)

Last edited by Jon_G4MDC; 5th Aug 2021 at 10:00 am. Reason: add data for FX365
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Old 5th Aug 2021, 12:02 pm   #10
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Default Re: Kenwood TM421E Sick

Or build a PIC (or Arduino or some other micro) based one.

One example https://pe1grl.khds.nl/ctcss/ctcss.htm
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Old 5th Aug 2021, 12:26 pm   #11
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Default Re: Kenwood TM421E Sick

Haa Haa.... my one erring fault is that I tend to knee jerk and do things right away..so I ordered a CTCSS module from eBay...but I thank you both for the data and I will look at them both seriously, whilst waiting for my delivery...
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Old 18th Sep 2021, 2:06 pm   #12
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Default Re: Kenwood TM421E Sick

Have a look here for a simple CTCSS homebrew encoder:-

http://www.fenlandrepeater.org.uk/ctcss.htm
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