Totally uneconomic job today - converting an old (mid 70's era) UHF CB radio from the the 1980's 40 channel system to the current 80 channel allocation. The radio's owner boss had given us a lot of "real" work over the last couple of years and when we mentioned we owed him a big favor, instead of the usual alcohol / chocolate / shopping vouchers he asked "Can you convert an old 40 channel radio to 80 channels" so we said "sure, but it'll be a strictly spare time job". He then gives us the radio (actually three identical radios, two like new and other well used but functional), and says that as far as he knows such a conversion has never been done before. Challenge accepted, worst case would be we'd say it's impossible and give him the usual Jack Daniels and a box of Roses.
Radio was a Phillips FM320, designed in the mid 70's but our examples were dated somewhere near 1982.
Three radios were supplied - the well used one was used for development / testing, the other two were done afterwards. The owner only wanted the two in nice condition returned.
These are Australian made and were the first synthesized UHF radio for mass consumer usage. There was also an amateur radio version made (FM321) which was pretty much the same except for the frequency range covered. The radios used two PLLs - one analog using discrete digital logic (no microcontrollers or programmable devices), somewhat archaic by today's standards but this would have been state of the art when it was designed - all other UHF commercial and amateur service radios back then used a pair of crystals per channel, not really practical / economic for a 40 channel design.
The radio uses discrete CMOS logic, which is limited to 5 or 10 MHz, nowhere near the 476 MHz required. A "HF loop" is used to generate a signal from 4 to 5 MHz in 25 KHz steps (i.e. 1MHz coverage, 25KHz x 40 channels = 1 MHz).
A pair of crystals - one for receive and one for transmit - is also used, with several multiplier stages, to create a signal near 471 MHz, which is then fed into one side of a mixer. The other input to the mixer comes from the UHF local oscillator, and the difference signal (4 to 5 MHz) is filtered out and in turn fed to one of the inputs of the analog PLL. The other input of the analog PLL comes from the HF loop.
The analog PLL then creates an error signal which is used to control the frequency of the UHF oscillator, and therefore the operating frequency of the radio. By setting the HF synthesizer to a frequency of 4.025 to 5.000 MHz, the operating frequency can be set to any of the then 40 channel allocations that ranged from 476.425 (channel 1) to 477.400 (channel 40). In other words, the radio operates exactly 472.400 MHz above whatever frequency the HF loop is set to.
A decade after it was made, repeaters were allowed on CB frequencies, which dramatically increased the operating range, especially when operating from a vehicle. This required a 750KHz transmit offset (i.e. 30 channels) to access them and operated from channels 1 to 8 Rx, so therefore 31 to 38 Tx. New radios included this as standard, and the manufacturers also sold a "repeater kit" that could be retrofitted to existing sets.
A decade later again, the ACMA increased the channel allocation to 80, but to conserve the bandwidth needed, they didn't just add another 1 MHz going up. Instead, they offset by half a channel and started at the bottom again, so 41 is half way between 1 and 2, 42 is is half way between 2 and 3, and so on. By doing this, they doubled the number of channels for only an additional 12.5KHz of band allocation (as 80 is half a channel above 40).
In addition, they designated channels 22 and 23 as data only for applications such as signalling and telemetry, and prohibited normal voice transmission on them. Channels 61, 62, and 63 are also locked out for transmission as using these could interfere with 25KHz wide digital signals of devices operating on channels 22 and 23.
Also, the number of channels allowed for repeaters was doubled, with channels 41 to 48 Rx (and therefore71 to 78 Tx) added.
Step 1 - Get radio working correctly as a stock unitAny noisy switches / pots were cleaned, and any bad components replaced, mostly electrolytic capacitors as these are in the 40 to 50 year old range. Any repeater offset addons were removed, which meant "uncutting" two cut tracks (only 1 cut trace in later rev boards). Check radio operates as a normal 40 channel (ignore "NOM"/"RPT" switch positions for now) was done, and a basic alignment done. Radio now operates correctly as a standard 40 channel unit.
Step 2 - Get the radio to operate on all of the 80 allocation channelsDid this first, as if this couldn't be done it would be pointless doing any display or control changes.
This turned out to be the easiest step, disabling the HF loop and replacing it with an AD9833 DDS synthesizer module that can be had from eBay or Ali Express for under $10. These need only 3 control wires plus 5V and ground to generate any frequency from 10Khz to 10 MHz in 1 Hz steps.
Components in red were removed and the output of the AD9833 module fed ino the analog PLL "Phase Det.".
Resistor linking the "HF VCO" to the "Phase Det." removed
Temporary switch fitted to switch between the original HF loop and DDS control. DDS temporarily set to 5 MHz using a dev unit. Both work, so original HF VCO components removed to permanantly disable it.
To make room on top for the module, 3 x 22nF caps and two resistors (1K and 1K8) were removed and replaced with 0805 surface mount types on the bottom side of the radio PCB.
AD9833 module stuck to top of analog PLL "Phase Det." (IC2) with double sided tape. Output connects as below:
- Remove R63 (15K)
- Wire AGND to pin 14 IC2
- Wire OUT to C51 side of removed R63 (C51 is coupling cap to pin 1 of IC2)
- Wire VCC, DGND, SDATA, SCLK, and FSYNC back to uC or dev unit
- Program AD9833 for 0.7V sine output / 5.000 MHz
- Check radio operates at 477.400 MHz on all channels
Disable HF PLL/LD/VCO/Reference
- Remove IC3 (MC1469B) and IC4 (MC1468B)
- Pin 12 is LD, no need to link as open=locked, grounded=unlocked)
- Remove Q15 (BF494)
- Link pins 1 and 2 of IC1
- Remove XL3 (2.500 MHz)
- Link pins 1 and 2 of IC6
- Link pins 5 and 6 of IC6
Reconfirm radio operates correctly on 477.400 MHz.
Edit 16/06/2024 07:49 - fixed minor spelling / typos
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