CTCSS on an IC-48A

[VK3ZIE]

I find it hard to believe that Icom would produce the IC-48A (to supercede the IC47E, which had full access to its internal CTCSS tone generator, as a standard feature) WITH its own CTCSS tone generator built-in as standard on the main circuit board, and it appears NOT to access the generator.
Icom Australia tell me that I need the UT29 tone access accessory board - which, of course is no longer available from Icom - in order to let the CTCSS tone through to modulate the FM carrier.
Piexx do still have their equivalent to the UT29 for sale, but it would cost more than I paid for the whole IC-48A to buy it and pay the postage from USA.
Does anyone have any clues on getting an IC-48A to produce CTCSS, in order to access repeaters?

[VK3ZGC]

I find it hard to believe that Icom would produce the IC-48A (to supercede the IC47E, which had full access to its internal CTCSS tone generator, as a standard feature) WITH its own CTCSS tone generator built-in as standard on the main circuit board, and it appears NOT to access the generator.

Not surprising really, as this is idiosyncratic of the design specs as applied to varying band plans and marketing requirements on a region specific basis.

Does anyone have any clues on getting an IC-48A to produce CTCSS, in order to access repeaters?

This dilemma got me thinking about my IC-228H which is a vintage later then the IC-x8 A/H series, the internal design concepts are similar if not identical.
The CTCSS components and connection to the CPU are however.
I bought my radio new in 1990. At this time there were no repeaters in VK5 that required CTCSS and most of the radios marketed in oz had CTCSS as an option. ( If you could get the option board that is ! )

I opened the 228H tonight and had a look to see if the CTCSS generator chip was installed. It was indeed as per your 48A. I solved my CTCSS generation a few years back by installing a UT-40 CTCSS encode/decode option board not realising that the required components were already populated.

Referring to the IC-228H manual it states that the CTCSS is operational for the US version only. This is also true for the IC-x8 A/H USA versions.
It would then follow, that if the radios are modified to be US versions then the CTCSS should work as the chips are driven directly from the CPU.
The mod would be accomplished by altering the diode matrix that sets the CPU start up condititon. The service manual for the IC-28A shows the various configurations for each region.

Having said that, the US version of the IC-48A covers from 440-450 Mhz which is not real useful in VK land. It would need to be adjusted to include what would be classed as extended coverage from 420-450 Mhz. I am not sure what diode combination would be needed for this to happen as I don't have a copy of the 48A service manual.

I cant guarantee the accuracy of the information but it should give you somewhere to start!

Now i need to have another look at my 228H to see if I can also make the on board CTCSS work.

Gary VK3ZGC ex VK5ZGC

[VK3ZIE]

Gary, Thanks for that reply. It really mirrors what I have been thinking about my rig. Not having a more recent VHF Icom rig, I was in some doubt about Icom's direction of design, but you have confirmed most of what I was thinking.
I do have the IC-48A service manual, so a diode matrix change is a good start - only 2 diodes to change, by the look of it - but as you say, that may just change the frequency band to the USA range, which is of no use here for repeaters.
There appears to be no region-specific "software" embedded in an EPROM area in the CPU, as there are no specific versions of the CPU for any given region.
If I change the diode matrix, and the CTCSS tone selection becomes a visible thing on the display, then I'll know I'm on the right track, and only have to find out how to open up the frequency range to 420 to450 MHz.
The user manual for the 48A says exactly the same as your rig's manual - only the USA version has CTCSS available.
Thanks again.

[VK3ZIE]

Gary, What I forgot to include was my work on an IC-27A. That rig had USA style frequency steps, and I wanted the Oz-style steps, so I looked at its diode matrix, too. But, at least, the band limits are the same on 2 metres in Oz and USA.
You can't get a maintenance manual for the 27A off the internet, so I used the 47A manual, and extrapolated to the 27A - with some changes. It took about 6 months of effort, but I finally cracked it.
The 27A has the CTCSS tone generator built-in, and can be readily set to transmit them. I couldn't see how Icom would take such a backward step with their next generation radios - the IC-x8A/H. That's Icom, I guess.

[VK3ZGC]

Glad to be of assistance!

You have actually raised another pertinent point with regards to frequency steps.

Changing the region settings can also have other undesirable effects such as introducing the somewhat odd frequency steps used in other areas.
With the IC-x8 A/H and IC-228 A/H this should not be a problem as the UK, VK and Asia specific models will handle 5,10,15,20 and 25 KHz steps.
It is only the European model which will only do 12.5 or 25 KHz steps.

"There appears to be no region-specific "software" embedded in an EPROM"

Dead right there, the function of the CPU is determined a startup by an external diode matrix. This did change around the time of the IC-x71 A/H series where the complete software/firmware was loaded into a battery backed Prom. ( Anyone remember the flat battery syndrome? and subsequent return to ICOM for reprogramming )
The diode matrix has since returned thank goodness!

You also mention the tone selection not being visible on the display. That gets me thinking about the function of the 228. From a somewhat rusty memory the 228 tone display has always functioned and the tone frequency was selectable from the setup menu. Having said that, I performed the RX expansion mod early on in the life of the radio and I am wondering whether I may have unwittingly turned on the CTCSS function by doing that. I may have a crack at reversing the mod or at least pulling out the UT-40 to see if the CTCSS is still active.

Gary VK3ZGC

[VK3ZGC]

I may have a crack at reversing the mod or at least pulling out the UT-40 to see if the CTCSS is still active

Done that! Well, pulled out the tone board that is, which by the way turns out not to be a UT-40. oops!

Discovered that the CTCSS still functions, follows the Tone switch (on/off) and the TX range matches the US version. it would seem that the theory holds that to enable the CTCSS you need to tell the radio that it is a US version.

I may have a go at reversing the expansion mod when I get some time to see if the tone button stops functioning.

Gary VK3ZGC

[VK3ZIE]

Gary, Wow, you are awaking a few Icom secrets with your prying. I'll be having a shot at the 48A becoming a USA model, to see if the tones are then available. If that happens, then I'll look for a possible mod being available to open up its band coverage to 420 to 450, and whether the tones are then still available. Thanks for the shoulder to lean on.

[VK3ZGC]

Extended TX mod apparently goes as follows:

1 LOCATE R55 ON EF UNIT.
2 MOVE R55 FROM A TO B.
3 LOCATE AND CUT D15 ON THE MAIN UNIT.

should give you 430 - 450

[VK3ZIE]

Gary, New thread started - now have CTCSS, but not in VK repeater band. Freq. mod is NQR..

[VK3ZGC]

Gary, New thread started - now have CTCSS, but not in VK repeater band. Freq. mod is NQR..

Hmm, not as desired, but not surprising.

Gary

[VK3ZGC]

Looks like there may have been conflicting information.

The service manual at :
http://www.ko4bb.com/Manuals/ICOM/IC-38 ... Manual.pdf

seems to indicate that the USA range is 440-450 MHz.

It would also appear that the mod information may also be misleading as the A & B points on the EF board do not make sense.
R55 is connected between CPU pin 67 and ground.

[VK3ZIE]

Gary,
MANY thanks for your forbearance on this topic - I have finally achieved what I initially set out to do.
First, I finally found the diodes which are the "initialising" matrix, and removed the one diode that changes the rig from Australian to USA - one half of D14. That changes the rig from 430 to 440MHZ into a USA rig which then covers 440 to 450 MHz and enables the CTCSS tone encoder, exactly as per the manual.
Then I tried to find the "wide band RF" mod. bits and pieces. The first part is relatively easy - instead of cutting D15, you extract one of the sockets from P3 which plugs onto the main board (and that in the same as cutting the diode).
Then the "far from easy" thing is to find R55 - it's reasonably close to the CPU. By reference to the IC-28A circuit, the equivalent of R55 - a pull-down resistor on pin 67 of the CPU - is removed for the full frequency coverage of the IC-28 (it goes from 144 to 148 MHZ, into a rig which covers 136 to 174 MHz), and leaves a 100 K ohm resistor as pin 67's pull-up to the Vdd supply rail.
I removed the 4.7 K R55 pull down resistor on my IC-48A - small chip resistor - and put a 100 K small pig-tail pull-up resistor on the alternate pads alongside where I had removed R55, and BINGO I have a rig which has CTCSS, and a frequency range of 420 to 470 MHz (just have to be careful not to transmit on frequencies outside our 430 to 450 Mhz band).
Thankyou so much for your ideas and prompting - all much appreciated.
My IC-48A is now able to access all the 70 cm repeaters with CTCSS - most of them around Melbourne seem to need that facility.

[VK3ZGC]

Great Stuff - Glad you finally achieved your aim.

Thanks for the insight into R55.

The service manual on the K04BB website unfortunately has poor scans of the board overlays and identifying some components is far from easy.

Gary

[VK3ZIE]

Gary,
I have not found ONE web site where the PCB overlays are easy to read - the screen display of them is usually slightly better than a printed version. You can just "imagine" some of the component identifiers. I just identified pin 67 of the CPU, then traced the copper tracks to where there were components, then measured a few components to make sure I had the right value.
Thanks once again for your input.