How to hack a commercial VHF rig to good use.
The aim of this page is to write up a series of emails between myself and Rob Denton, G4YRZ. Rob saw my reference to the modification of my Tait VHF radio to 2m use and emailed me for details. To make this process easier for other hams I aim to give the same information here that I gave him. Although the emails concerned only the Tait radio the procedure for other synthesized rigs should be similar.
So lets introduce the man who is responsible for this write up as he introduced himself.
From: "Rob Denton"
To:
Subject: tait t500 mk2
Date sent: Sun, 23 Jul 2000 22:02:48 +0100
Dear friend,
I saw your page on QSL.net. I have recently acquired the tait t500 series2
on 170MHz and I wish to modify it to 2mtrs. It says on your page to write
to you if I need help so please could you send me any mod info you have. I
would be very grateful if you could, many thanks. Rob Denton G4YRZ
Now before I totally confuse the OM (or seem a bit condescending) on the other side of the keyboard lets gauge his capabilities.
Hello Pieter,
You asked what equipment I have, well basic really, e.g. frequency
counter, multimeter, soldering gear etc, but I may have access to a scope
and sig generator from a friend. I have not done this before, but I have
built a wxsat rx and some other projects, so I am willing to try. I
appreciate you are busy, just fit me in when you can. Many Thanks Rob
G4YRZ
----- Original Message -----
From:
To: Rob Denton
Sent: Monday, July 24, 2000 7:17 AM
Subject: Re: tait t500 mk2
> Received your email, will have to take a peek at exactly how I did
> it. What type of equipment do you have? I had access to some
> handy RF equipment at the lab here which made things a lot
> easier. Also have you done this kind of thing before. I would like to
> give you the instructions in such a way that I can put them on the web
> when we are finished. Then more hams can use it. At the moment I have
> a bit of a backlog in my work, but we will just have to make time, HI.
Rob seems to be wide awake on the other side so lets hit him with some jargon.
Ok
I am going to send only a short message at a time rather than not
sending anything at all.
If you open the radio, take off the bottom cover which holds
speaker. You will probably see two plug in boards. The small
almost square one at the bottom right is a board with a couple of
links soldered to it. (I am assuming that the controls of the radio is
facing towards you.) The longer one to the right is the CTCSS board (if it
is fitted, all the radios I had had these).
I removed the CTCSS board just by unplugging it, maybe there was
a retaining screw. The CTCSS board used CTCSS on both
transmit and receive, the quickest way to get on the air was just to
remove it completely. Later on I adjusted it to the 88.5 Hz CTCSS tone
used by some amateurs. I also had to short out a link do disable the
required CTCSS tone on receive.
If you look around the main board you will see a couple of
interesting chips and components. The power control trimpot is
clearly marked on the main pcb. (I turned the power control a bit
down during my tests but this is not critical.) Just below and to the
right is the MC3357 FM demodulater IC. A bit further down and almost at
the left edge of the board is the Deviation set trimpot. The parallel
synthesizer IC is mounted below the small square plug in board with the
links soldered on.
Anyway enough for now, let me know if you are still following.
73
Pieter
And Rob's response to this inside peek of his radio?
Hi Pieter,
Ok, I follow you so far no problem, ready for the next bit!
Rob G4YRZ
As you will see in the next email I hoped he would give me more time to prepare for the next step. I realised Rob was keen to get his radio on the air so I just decided to give him a quick overview of all the steps involved in modifying a radio for a different frequency.
Oh no, I hoped you would have a lot of questions. ;-)
OK here we go, I have done this quite some time ago so I have to
work through it again as we go along. To summarize what we are
trying to do:
The main thing we need to know is how to reprogram the
synthesizer. Once we have done that it (the synthesizer) will try to
force the RX and TC VCOs to the ham frequencies we set the synthesizer up
to do. The two VCOs usually have trimcaps to pull the center (unforced)
frequencies to the relevant band we are trying to use. The radio I had
was operating at about 155MHz and I had to retune both VCOs for 145MHz
(BTW do you also have only 144- 146MHz in the UK? I know the USA goes up
to 148MHz).
After the VCO is running at 145MHz you should 'align' the receive
bandpass filters to let the max 145MHz signals through and min.
other commercial signals. Again mine was somewhere around 155
and had to be retuned to 145. How do you do this? Well I had a
HP comms analyzer. So I injected a 145 MHz signal and
measured the SINAD of the receive audio from the speaker. I just
tuned the filters for max SINAD. I guess you don't have a way of
measuring SINAD. You probably can get away with measuring the
RMS audio output. And tune your filters for max. reading. With
the comms analyzer I start with a very strong signal about -70dBm
or more to get trough the RF filters while they are still at 155MHz. As I
get better and better readings I lower the input signal until I get 12dB
SINAD for a -120dBm input signal.
Another question are you comfortable with dB dBm etc? I do not
want to scare you off, I am sure we can get this radio up and
running with minimum test equipment!
After you are happy that you can receive properly you can also
adjust your power output. The radio was probably tuned to give
max. RF power output at the commercial frequency. A simple way
of doing this is hooking up a SWR/power meter and a dummy load.
Press PTT and tune the trimmer caps on the power output stage
for max. RF power. You should be careful here, watch the current
you are drawing from your power supply. You can do this
adjustment with the power control trimpot turned down a bit, to
avoid damaging something.
Well that is the whole process outlined. Now I have to go and
search for the details of how to program the synth. The synth is a
motorola MC145152P2. There is a little math involved in getting
from the transmit frequency you want to the programming bits you
have to set up on the board with the links.
Any questions thus far? Just ask, comments on my English is
also welcome, it is not my first language, how is your Afrikaans?
73
Pieter
Ah and finally I managed to get too fast for Rob. ;-)
Me Again :o)
Well, some of that went right over my head! I guess if I do it slowly a bit
at a time I will get there.
In answer to your questions: Yes, we only have 144-146 MHz over here., the
actual freq I would like to put it on is 144.625. As for your English,
well.....it's first class! I can speak some Bosnian and German but not
Afrikaan. I am not spot on with db's but understand the basics (I think)
:o)
The synth on this one is a MC145152P1 and not 2 is that the same?
So, my friend, where do I start? I found it a bit "heavy" :o) sorry to be a
pain but i know I will get there in the end.
Rob G4YRZ
So in the next email I looked up the specifications for the synth IC Rob's radio contained, I made a bash at the Americans and jumped right in and explained how the division ratios for synthesizer works. Straight from the datasheets didn't figure it out myself. Oh and this time I got carried away and typed a monster lengthed email.
All right, I will go a bit slower. All these things are really straight
forward, maybe some of the buzz words can sound intimidating.
Ok so its again the American who are bigger and better with their
extra 2MHz of spectrum on 2m.
My mother is Dutch and I can copy when she speaks with her side
of the family. I cannot really speak Dutch because I never had to.
All the family understand Afrikaans perfectly as they have been
here since the late 1940s. German I will be able to understand bits
of, had one year of it at school. But maybe only in writing following
it when someone speaks is a bit too much.
Ok the MC145152P1 is not a problem if I remember correctly this
just refers to the package style. Let me check...
From the databook: "The MC145152-2 is an improved-performance
drop-in replacement for the MC145152-1. Power consumption has
decreased and ESD and latch-up performance have improved."
SD is probably Electro-static discharge. Not really a problem down
in the Cape area as we are close to the sea. In Gauteng ex-
Transvaal area working with electronic devices you have to be
careful with static discharge as the air there is a lot drier.
The 'P' suffix tells you it is a plastic DIP (dual inline package)
package. A 'DW' suffix is the surface mount version.
I now have the synth ic datasheets in PDF format. Downloaded it
from motorolas website. It is 423K big can I email it to you?
The best way to explain how to connect the links for a certain
frequency is via an example: My radio is a 2 channel model at the
moment I have a 145.300 simplex frequency and a 144.550
simplex packet frequency programmed.
Check pins 4,5 and 6 on the MC145152 synth IC. All three mine
are approx. 8.75V. This is a logic 1 for the IC as it uses a 8.75V
supply and not the normal 5V supply. All three 1's indicate that
the crystal frequency used for the synth reference is divided by the
reference divider and by a factor of 2048. The 2048 number I get
from the datasheets.
Ok so if you know the frequency of the reference crystal you can
get the reference frequency used by the synth.
How to get the frequency the reference crystal is running at? Well
mine does not have it written on the crystal itself :-(
Pin 27 is the OSC in pin and Pin 28 is the OSCout pin. You can
hook up your frequency counter to the OSCout pin it should be
amplified. I am going to try and measure it with a scope, hang on.
I measure about 12.8 MHz with scope on the OSCin pin! The
OSCout pin output seems to small. Did someone switch the pins
around? I will try and get a more accurate measurement with a
frequency counter, hang on:
No useful output from the HP frequency counter, the poor thing is
older than me. Get a second opinion from a small Thandor
frequency counter: it says about 12.7 MHz. Funny thing is I get a
more stable measurement from the OSCout pin this time round.
Black magic these ICs.
If I remember correctly I did not actually measure this when I
converted the set, I checked the original transmit frequency and
worked back from it until I got the reference. Anyway 12.8 MHz
should be the correct frequency because I remember that the
channel spacing is 6.25kHz on this set. To check 2048 (from the
reference values measured on pins 4, 5 and 6) multiplied by 6.25
kHz gives us the reference crystal frequency of 12.8 MHz!
Interrupt me if I am confusing you.
Now how do we get to the actual transmit frequency? Let's check
my 144.550 frequency.
Start with transmit as this is easier (receive is 21.4 MHz lower on
my set due to the IF being 21.4 MHz).
The links soldered on my set is as follows from N9 to N0 and from
A5 to A0 as follows (hope your binary math is OK).
Read an open link as a one and a short as a zero:
N9-N0: 1001000010
A5-A0: 001000
Now change that to decimal (my calculator can do this, do you
have something, otherwise you can do this manually?)
The lowest bit (the on furthest to the right is a 0 or a 1 the next bit
position to the left 'adds' 0 or 2 the next bit to the right 'adds' 4 or 0
and it goes up by a factor of 2 each time, maybe a calculator is
easier than my explanation)
Anyway the N number above in decimal is (using the procedure
just explained: 2+64+512=578)
The A number is (8=8)
The following equation is used for this synth IC (it is used by most
synth ICs):
Ntotal=N*P+A
where
N is the total division value
N the N9-N0 number just calculated
A the A5-A0 number just calculated
and P something called the prescaler divide values (something I
forgotten to tell you about! The small 8pin chip just to the left of the
cast iron box with the RX/TX letters on top is the prescaler IC, mine
is a MC12061. The prescaler has a division ration of 40/41 use the
lower one thus P=40 )
Now the big moment lets calculate the total division value and
multiply that with our reference frequency.
Ntotal=578*40+8=23128
Multiply this by 6.26 kHz and you get: 144.550 MHz !
It works!
Lets check receive, but this time remember that our VCO runs at
the receive frequency minus the IF frequency. For my set this IF is
21.4 MHz. If you want I can explain on all the IF stages used later
(there is actually a second IF at 455kHz at which the FM
demodular IC operates).
Lets calculate the receive frequency this time, but this time we
start with the receiver frequency we want and work towards the
open and shorted connections we want in the radio itself!
OK I want to receive at 144.550 -21.4 = 123.150 MHz.
This is what the receive VCO runs at. Divide this by 6.25 kHz and
you get 19704 for the total divide value. Now to get to the A and N
values. Divide 19704 by 40 and you get 492.6 A=492 and N equals
0.6 times 40 or 24. (Just check that
Ntotal=N*P+A=492*40+24=19704). Convert N and A to binary: Use
a calculator or I can explain the manual method.
N = 0111101100
A= 011000
(remember to have the same number of bits each time you may
add 0s to the front of your number, I had to add one 0 to both the N
value and the A value to make up the 10 and 6 bits respectively).
A quick check with my RX1 soldered links in the radio (where a c
is a closed link and a o is an open link, remember c is a 0 and o is
a 1):
N coooocoocc
A cooccc
It checks out perfectly. Now you can trust my calculations,
If your radio has the same reference and prescaler as mine you
can follow my calculations exactly. I wrote a small program to
check my calculations but it runs under a mathematical package
called Matlab. One day I will have to port it to something that runs
on its own.
Once you have changed the soldered links you radio may not rx or
tx yet. Connect a dummy load and a wattmeter to your radio.
Press PTT and turn the trim cap with a tuning tool or preferabbly
non-metallic screwdriver in the TX hole of the diecast metal box.
You should see the RF power jump up for a certain trimmer cap
position. You can get a feel for how far you can turn the trimmer
before the power drops again. Set the trimmer to the center
position between the two position where the RF power drops.
To get the radio to receive is a bit more difficult because you need
to adjust the RX trimmer cap as well as the input RF filters. Start
with a strong input signal say about -50 to -70dBm at your RX
frequency. Turn the RX trimmer cap until you hear something.
Again try to find the centre of the trimmer cap position.
Now you have to start tuning your input filters. These are tuned by
the four metal boxes at the back of the radio. (I actually do not
remember if all 4 have an effect I can check but not tonight) Three
of them have the same colour core lining, gray, the fourth one has
a blueish core lining. This blue one might be for a different band
i.e. the IF so maybe leave it for the moment.
Decrease the signal power from the signal generator until you just
start loosing reception of the test signal. Start turning the leftmost
metal box (called a tuning coil) until you hear the signal clearly.
Decrease the signal again until you just start loosing reception and
tackle the next tuning coil. Once you have worked through all
three decrease the signal again and tweak them again. You can
repeat this a couple of times. If you decrease the input signal to
about -120dBm and you can still hear your test signal with some
clicks (splatter) appearing in the output audio, you have done a
good job. (A test signal can be only a carrier, but most people use
a carrier with a 1kHz audio tone FM modulated ontop of it. I do not
know if you have facilities to generate this signal. The first part of
this process can probably be done by monitoring a local repeater
as a signal source, and more distant signals from fellow hams as
you start tuning the RF filters, but this is a bit of a 'cowboy'
approach.)
Good luck, this is probably the longest most technical email I have
ever written. Feel free to ask if I left something out. I am sure that
you will jump through the roof if you get this going. I am still
excited each time I manage to get a rig onto amateur bands!)
73
Pieter
Was Rob intimidated by this monster email? Nope, he was getting worried about his roof.
But then Rob got quiet and I was getting worried, somewhere out there a perfectly good Tait radio wasn't converted to the good cause yet. I emailed Rob fearing the worst and so it was, ligthning struck.
Hello Pieter,
Sorry for the delay, my phone line was struck by lightning and took out the
phonebox AND my new modem (boo hoo) :o) I have just got back online today.
I will be taking the rigs to the local radio club on thurs eve, they are
converting one and I am doing the other. I feel confident enough now to have
a go and finally understand the instructions. I will let you know the
results. Sorry for the delay ut it was out of my hands, Rob G4YRZ
----- Original Message -----
From:
To: Rob Denton
Sent: Saturday, September 02, 2000 5:56 PM
Subject: Re: tait t500 mk2
> Hallo Rob, you have been quiet. I also have been quite busy, one
> of my colleagues was on leave this term and I had to take up some
> of his responsibilities. How is that conversion of the Tait going?
> Just shout if you need any help or if you are stuck somewhere...
>
> 73
> Pieter
But Rob was back in full force and with the help of his local club started asking difficult questions.
Hi Pieter,
Just to let you know the outcome from last night. Firstly we got the diode
matrix board sorted out on frequency then went to adjust the trimmers to
lock the chip on freq but the voltage hung around 0.8v!!!! I believe it
needs to be around 4v or something like that? My friend said it needs
either a core in the tuner or a cap placing across the bottom of the board
as the frequency is lower than it's set for. What is your feedback on this
please?
Rob G4YRZ
The question was too tough and I had to think up a really good excuse not to seem to stupid
Hi Pieter,
The voltage is measured on TP2 (between the diode matrix and the screened
metal box with tx/rx trimmers enc) it would be interesting to see what
voltage you get, I think he used the middle of the three pins but I don't
know if it matters.
Rob G4YRZ
----- Original Message -----
From:
To: Robert Denton
Sent: Friday, September 15, 2000 12:53 PM
Subject: Re: Mods
> Hey Rob sorry for the delay. Went to a three day conference and
> have a bit of catching up to do. Where exactly do you measure
> that voltage. I assume it is the VCO control voltage?
>
> 73
> Pieter
I was feeling a bit out of my league but hey I have been teaching for a couple of months now and if you don't know the answer try to stall...
Hmm maybe we have a different model
I measured at TP2 there are three pins
With radio facing towards me:
. . .
pin 1 2 3
I measure for a 145.300 receive frequency
7.06V 6.65V 0V
and for 144.550 receive frequency
7.70V 5.62V 0V
These voltages seem a bit off from the values you have quoted. It
might be that you have a model that is not a VHF model? Maybe a
70MHz model ?
73
Pieter
Rob was undaunted he was on a mission to get this radio on the air. Just a quick verification of the synthesizer frequencies and we are on schedule again.
Hi Pieter,
With the front of the radio facing me the voltages are:
l to r 0v 0.8v 8.8v! The radio was originally on 173Mhz. I put the freq
counter on it to check. the diode board has been set for 144.625 i think!
Can you confirm for me it is like this:
T1 occoccccoc/cococc
R1 coooocoocc/occocc
is this correct?
Rob G4YRZ
Rob's calculation checked out perfectly as you will see in the next email.
Ooh now let me go and have a look at the explanation I gave you a
couple of emails ago. I have to check if an open is a 1 or a 0,
standby...
Open is a 1 according to my archive. Lets start with your T1 as
ther is no IF to consider.
10 0100 0010 and 010100
578 and 20 according to Matlab
Ntotal = N*P+A (P is 40)
= 578*50 + 20
=23140
multiply by reference 6.25 khz
144.625 MHz
Yes, you are right!
Quickly check RX
0111101100 and 100100
492 and 27
Ntotal=40*492 + 36
=19716
*6.25 khz reference
123.225MHz
what was that IF again...
21.4 MHz according to records
I get
144.625MHz let me just check the typing
yes your RX is correct also
I assume your rig uses the same 6.25 kHz reference, the same
21.4 MHz IF, and the same divide by 40 prescaler.
Then your VCO's should work. I had to tweak the trimmer caps in
throught the holes in the die-cast shielded box to get the vco's up
and running and the 144.550 frequency I used.
I never checked the voltages at test point 2 until you asked me...
I can't remember if we missed any steps in the process. Even if
your receive and transmit filters aren't retuned yet your VCO's
should be able to lock.
We have to get this radio going, ask me more questions I will also
think a bit on this.
73
Pieter
And then a breaktrough on about the 19 September...
From: "Robert Denton"
To:
Subject: Re: Mods
Date sent: Tue, 19 Sep 2000 13:22:13 +0100
Hi Pieter,
Good News!!!!!!! You know how keen I am to get this thing going, well I got
up this morning and decided to take another look at it.
I put the voltage meter to the centre pin and still it's on 0.8v. So, I
decided to think about it and have a look around. Anyway, not knowing much I
remember there is the tx side as well as rx and wondered if it would make
any difference altering the tx coil. Whilst still keeping the voltage meter
on TP2 centre pin I adjusted the tx side also, in fact both of them. Bingo.
The voltage shot up to 8v on the centre pin as well as the right one. I have
so far fine tuned it by firstly setting it to 4v (it seemed a good place to
start) :o) then talking on another 2m tcvr until I could hear my voice. I am
now receiving :o) Now I have to sort out the tx side maybe I will have to
visit the club again as they have more knowledge than me there. Do you have
any ideas and what do you think of it so far? I think we will get there
Peter :o)
Rob G4YRZ
Rob was so excited he got in a second email before I could reply
Me again,
How stupid of me, it says in the info to tweak rx, only someone at the club says it wouldn't affect it on rx, how wrong they were. Awaiting your reply.
Rob G4YRZ
Rob was still so excited he even got a third email in before I could reply.
Hi Pieter,
Just to let you know the rig is now up and running :o) It was all there in the instructions thanks to YOU.
One thing, I would like to put the second channel on 144.725 but can't get the maths right, can you help,
Rob G4YRZ.
p.s. We raised the roof!
Finally I managed to get in a word edge wise. I was glad we got the radio up and running, but I wanted to make sure that we didn't do a sloppy job.
> Good news, I am glad you got it up and running. I do not know
> how much tx power you can output yet. Or if you have tuned your
> receive filter for maximum sensitivity yet. I think I described these
> to you already. They rx filters might be a bit tricky without the
> relevant test equipment.
My flat response to Rob's excitement didn't put him off at all.
Hi Pieter,
I did it as you said, left to right for rx until I got a good signal, I was
tuning it on a friends transmission 20 miles away! Thanks for the maths, do
you have it for receive too on the same frequency ( 144.725) the first freq
is our local chat cahannel when not in use for raynet skeds. Oh, i'm so
happy it's running.
Rob G4YRZ
By this time I had some leave planned and warned Rob about it, but first I checked the frequencies he requested.
Glad hear about all your successes. I am going on a loooong
weekend until about Wednesday. Visiting my parents in the
Northern Cape on a farm near Niewoudtville. If you can find that on
a map you are really good or you have good maps. Will try to
email from there. Everything runs from Solar panels and a Lister
Diesel engine and generator.
73
Pieter
Oh the RX frequency forgot all about it, lets see...
144.725-21.4 for the IF
/6.25 kHz
19732
/40
N=493
A=12
N=111101101
A=1100
Very quick calculations hope they are correct!
Enjoy
So, finally, after several emails across (or below?) the oceans out there, somewhere, is another little Tait radio that could. Doing good work in the Amateur service. It took a couple of weeks, a bolt of lightning and finally an earthquake but the little rig is up and running.
Hello Pieter,
Just would like to say a big "thank you" for all your help. The two tait
t500 rigs are now up and running on two channels. I appreciate your
patience and all your help you have given me to get through this. My
friend (who has the other rig) is over the moon that they are working. He
gave me them on the condition that if I got them converted I could have
one and he the other, and so it is.
Let me tell you a little more about myself, my other hobbies are weather
satellites, and I am just getting interested in amateur sats, I once tried
it a year ago RS12/13 and was well impressed, my next rig will be a
vhf/uhf for working the birds. I noticed SO-35 at the end of your email,
are you something to do with sats? My other spare time occupation is
voluntary aid worker (14 trips to Bosnia to date) you can see my webpage
at www.g4yrz.freeserve.co.uk
Anyway Pieter, I hope you had a good holiday, I could do with one myself,
I sponsor a child in India and have been invited over in the future so
maybe one day I will make that trip to the Himalayas where she lives. I
love to travel.
We had an earthquake here this morning, maybe you saw it on tv , it
measured 5 on the Richter scale, very disturbing, but nothing drastic
thank goodness.
Thanks again Pieta for everything, take care my friend.
Rob G4YRZ
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