[TenTec] The amazing 253 ATU

[G3JAG]

Easily my finest purchase .... (until I hock the XYL and buy an Orion) 
It came from bankrupt/demo stock here in UKand I made an overnight 500 mile 
trip to get it before anyone else did. OK; it needed a trivial mechanical 
repair, but it replaced a home brew wide range ATU and it is absolute magic.

My 253 came complete with a socket for the eeprom, thank goodness. It is a 
fairly old memory chip design and it has a finite life, after which it 
becomes increasingly difficult to re-write/store data. Replacement is the 
only sure fire fix. 

I have had to swap the 253 eeprom out only once in many years. The ATU gets 
used every day, but I have most band settings stored (mid-band SSB, CW and 
WARC) so I do not need to run the "tune" function too often. The antenna 
selector switch comes in handy for this. Position 1 is CW, position 2 is SSB 
and position 3 is for the WARC bands. Three cables go to an old Heathkit coax 
switch and the output of that switch goes to a second switch, so I can then 
select antenna or dummy load.   Messy but extremely convenient for band 
hopping. 

There is only one official mod I know of for the 253 and that relates to the 
SWR bridge grounding. It is possible to set/adjust the null more accurately, 
but you have to make a minor change to the bridge.

But there was also more than one firmware eprom for the 253. I have the second 
(latest ?) in mine now and it has an occasionally very annoying feature If 
you play around too much with the manual push buttons, it suddenly decides 
that you do not know what you are doing and it simply sends the roller 
coaster to the end stop and does a full reset.. You have wait for this 
routine to finish before you can start over, but it does not wipe any other 
stored settings, thank goodness.

When you turn the rig and 253 on, the SWR may be way out from the last time 
you used it. I find that if you simply swap bands and then swap back, it 
usually goes back to where it was when you switched off.  

All other problems here are entirely my fault.  Rain badly affects my ancient 
wire antenna. It is a 40m dipole used on all bands, but due to operator error 
there are no end insulators - just plastic rope. When first I put the thing 
up, its main job was to stop the mast falling down and I never got around to 
adding the insulators.  Ooops. I always need to check the WX and then the 
SWR.
But in practice, I can often null the SWR by manual tweaking using the push 
buttons, but sometimes I have to use the tune function. How heavy is the rain 
today ?? 

Never mind.  I keep a bag of those eeproms, because there is also one hard 
soldered into the Omni-V.9 and after a few years that too gets very tired. 
The symptoms are the rig getting very confused about the band/mode etc. 
Resetting the CPU does not cure it, so the chances are that the eeprom has 
passed its sell by date and will not store new data correctly. TenTec 
asuggested that when I replaced the Omni eeprom I should really save myself 
some grief by also fitting a socket, so I did, using a gold plated turned pin 
socket. An awkward job, but well worth it.

Should TT start making automatic ATUs again ? Good question, but not another 
253.  The Elecraft KAT 100 shows the way to go, tight integration with the 
radio, more memories, more flexibility and no moving parts (except latching 
relays.) 
  
Now to make myself a proper antenna with insulators - if it will stop raining 
long enough.

John  G3JAG