VK7RHF 10M repeater project
20 October, 2015 - Reading time: 9 minutes
For a couple of years now I have been contemplating putting in a 10M repeater in Hobart for use when the band is open and to encourage more activity on the 10 metre band.
This is the lowest band that has FM repeaters, and can present quite a challenge.
Isolation at 29MHz is difficult to achieve. Cavity filters if manufactured would be expensive, and long, too large for possibly most sites. Physically longer antennas present problems too. In most cases, they cannot be vertically separated very well. 10 metres of vertical separation at 30MHz is only about 25dB isolation. Horizontally you would need over 3km of separation to get near to 70dB.
This is well documented here - Isolation
Repeater splits are only 100KHz on 29MHz. This represents a 0.34% difference between RX & TX. To give some comparison with 6 metres we have a 1MHz split, which is 2%. 2 metres is 600KHz or 0.4%. 70cm is 5MHz or 1.15%. So we can see what we are working with.
So in reality two separate sites are needed linked together over a low power UHF link or similar.
Next.... who has low band radios? There are plenty of over the counter radios that have 10 metres in them, but frankly are a waste and not suited for repeater use.
Thanks to Pete VK3TBN a GE MLS 1 has been obtained. This is a 60W mobile radio with a 20% duty cycle. The PA and combined VCO/Control board was remounted to a Unilab KL heatsink for better duty cycle, and the receiver removed for another purpose.
Picture above is the MLS as stock standard.
Mods were done to bypass the antenna switch as we don't have a receiver anymore, and SO239 removed as it doesn't fit anymore. Permanent power modding was also done as detailed here - GE MLS Repeater Mods
PA board and receiver removed below.
The EEPROM is a 28C16 or 2816 type. Fairly rare. The MLS 1 could only be programmed via a suitcase programmer, which again are rare. Thanks to Bob WA1MIK who managed to rewrite the EEPROM data for me to reburn.
The 10 pin ribbon cables are a nuisance... Pull them out and pull the connector carefully off the board and solder wires. 2 x 2 of the pins are connected together, and another is unused so you only really need 5 wires to connect the control board to the exciter/PA stage.
Above is the VCO circuitry. RX coil is the white plastic former and the metal can is the TX coil. The RX coil may come in handy later, it has not been removed yet though.
Just a standard empty KL heatsink... nothing much to see here.
This is one of the bandpass filters in the MLS low band front end. There is not much data on the beast, but one would assume it covers the whole band split of 29-42MHz. I have yet to sweep it. May be handy to put into a KL29 front end. On the right hand side one of those is a RF high pass filter unit.
Mounted and running. Did 30 mins TX time. The heatsink started to warm up which is what we wanted to see, and the pre-packed smoke stayed inside which is always a bonus. Output power measured is 60W at 12A. It dropped 2 watts in 30 mins of TX time.
The yellow blob is a thermistor. Apparently it gives thermal protection to the unit when reaching 90 degrees, throttling back the output power. I have yet to see it work, which is probably a good thing.
Next job... test the transmitter some more then onto the receiver. In hindsight I should of left a KL UHF receiver in there and done some 7DB Low Power mods to it. Then it would of been a nice neat package, but I don't want to go and re-drill and tap 30 plus holes.