Some years ago I got a couple of radio boards on the flea market in Eskilstuna. I don't remeber exactly from who or what I paid, but they were dirt cheap. The seller claimed that the only thing possible to reuse was the power amplifier. It should give 50W output. He was incorrect.
Two years ago our 2m repeater broke and we needed to repair it. As both the 2m and 70cm units used the same logic we had to disconnect the 70cm repeater while repairing the 2m and when we got the 2m repeater back we did not manage to connect the 70cm again. They were actually soldered together so we to the old 70cm down to fix some connectors and while at it, change it to 2MHz input/output spacing. The new European standard. When all those boxes ended up on my livingroom floor I remembered those old RS-4000 cards stuffed away somewhere. Hmmm, lets do a net-search and see if someone else has got anything on them. And, yes someone had.
The RS-4000 is a essentially a complete radio on a single PCB. Everything from the receiver frontend to the detectors on the RX side and modulator to HF on the TX side. Power output is low, perhaps 1mW, but in the same casing there is a PA capable of delivering around 20W on 432MHz. The RX section is dubled so that diversity can be implemented, but the cards I have are only half populated. That is, only one of the receivers have components mounted.
Frequency is controlled through two separate synthesizers for the RX and TX section, which is of cource an absolute requirement if you are to use them for full duplex, which they are designed for.
The Receivers have a first IF of 71.2MHz and second on 455kHz.
In order to make this radio work on the ham band you need to do a few modifications.
The following descriptions will describe why and how each step can be done.
The operating range for the VCO in this radio is adapted to the NMT use. For NMT operation the raceiver needs to cover ? to ? and the transmitter ? to ?. This is a lot over what we need for ham radio operation. The swedish 70cm band covers the range 432-438MHz. The two units I have did work on these frequencies without any modifications at all, but on a few occations the PLLs has not locked on power on without having to power down/up the card a few times. For my experiments this is ok I guess but for a 24/7 operation as a repeater on top of a hill it is NOT acceptable.
In order to move the frequency range downwards fortunately you need to add capacitance to the oscillator circuit. On LA1NG's pages I found a suitable description on where and how.The operation frequency for the RX and TX is normally fed from a logic card that we discard. It's simply not possible to reuse this card. It uses a 68HC11, which is a very nice processor and has flash-proms where the software is stored, but since nothing is socketed and reverse-engineering this part would be too time consuming I have choosen to build my own logic instead. Since I only needed the frequency setting it was very simple.
I used a PIC16F676 which has a sole purpose. Upon reset up it outputs the serial configuration data that is needed for each PLL.
The only trimming (except for the PLL fix above) that you need to do on the RX is to trim the preselector. It consists of a three-pole helix filter. In order to trim this you connect a voltage meter to the RSSI output circuit and a signal generator to the antenna input. You will likely need to start with a high input signal and then trim the filter to the maximum RSSI output voltage. Each time you have peaked out the RSSI voltage you decrease the input voltage on the antenna input and do another run over the filters.
If you dont have access to a signal generator you can use a handheld rig instead. The problem here is that the output from the rig is so large you will have a hard time getting it small enough on the input. In order to get it small enough you need to connect both the input of the RS-4000 card and the output of the handheld to two dummy-loads, and then you ask a friend to take the rig for a walk away. Doing this I still had too much signal when my father was 50m away, but it got me close enough.
The RS4000 receiver circuit does not contain a squelch. The may be something on the original logic board but we discarded it. I took the schematic diagram for a Johnson receiver and built a similar circuit. Noice-gate that is. </>
As with the RX there is a three-pole helix filter on the input that needs to be trimmed. Furthermore there is a trimmer between the last two stages. So far I have only done any trimming here to peak output power (actually peak power consumption). I have been told be the real expertise that THIS IS NOT ENOUGH. You really should do a sweep-measurement before doing any trimming to find the peak and then carefully watch what happens as you trim each circuit and then trim the output of the final circuit by adding capacitors manually.
I don't have easy access to the equipment needed here. So waiting for a suitable spot of time where I would have the time to visit a ham who has this equiepment the project has gone into hold.