Last Updated: Wednesday, 19 April 2017 15:48
I've been given a ICOM IC-R71E receiver that did not work and had been stored for a long time.
Symptom : no reception, no VFO or keypad action, no display.
Symptom : no reception, no VFO or keypad action.
The Lithium battery on the RAM board which is plugged on the LOGIC board has been removed by the previous owner. The RAM board uses a static CMOS RAM uPD444C. When the backup battery is removed or flat, the datas stored in the RAM are lost.
The R71 has several parameters stored in this RAM, and when they are lost, the radio doesn't work anymore ! Some pretend that the processor is stored in this RAM. That's NOT THE CASE, it's stored in the processor ROM.
Only memory frequencies and mode shift are stored.
The cure is :
- send the radio back to Icom for repair (not sure if they still can do that after more than 30 years !)
- buy a third party RAM board, there are several on Internet. Look for IK2RND Roberto's offer.
- reprogram the board yourself !
Guess what ? I tried the last solution !
On Internet, i've found several infos about this problem and the easiest solution is the one offered by N2CBU. He has written a small software that can reprogram the RAM with the help of a simple programmer and file.
I quickly built this interface on a perf board. Durty but operational !
The main problem i had was to find a PC that was old enough to work under DOS at a relatively low speed and with a parallel port.
I had to try 2 different PCs before it worked. In the BIOS settings, i had to change the LPT mode to EPP. Once i did this, it worked at first try !
After this repair, i could store, clear the memories and the display worked normaly. The keypad worked again, but i still had other problems. The 2 digits on screen displaying the channel number had some strange behaviour and the receiver was still mute...
DC-DC converter DP-2
Symptom : The receiver did not give any sign of life in CW/SSB/RTTY mode. I had some sound in FM only.
The S-meter was stucked at middle scale.
It appeared that there was no voltage on -10V line coming out from the MATRIX board.
The small metal case marked DP-2 was very hot and lately i had the L2 coil that made a lot of smoke and the PSU made some mechanical hum... In short, i had a short circuit !!
I unsoldered this DP-2 module and opened it. I made some reverse ingeneering and tested all components.
Meanwhile, i found another ham making the same work and who published the DP2 internal diagram. I measured the small transformer and the secondary is 2mH and the primary is a center taped 300 uH. Another ham had to rewind the secondary that burned and counted 125 turns and 2 times 26 turns for the primary.
Finaly "only" the 2SD648C transistor was burned.
I didn't have any substitution for this later, and i tried several transistors i had before finding one that worked and did not heat too much or did not burn after a while. I used a BD139 for which i had to file the legs until they fitted in the board. The transistor case is higher than the original one, and the module cover does not completely close the case... But that's not a big deal.
After that i noticed that C13 (100uF/10V) on the -10V output on the MATRIX board was short ! This was probably the reason of all this mess ! After replacing it and the L2 coil, the radio came back to life !
Finaly, i had a rather clean -9.5V on the -10V line. I had noise on all modes, and the S-meter was at S0 level !
But still the channel number display problem...
The PSU is known to be very badly designed. It heats a LOT and has a lot of problems. It is strange that ICOM engineers who have designed such a good receiver could design such a bad power supply !
Mine had 17.4V output , no regulation and getting very hot, heating all the cabinet !
At least for all my tests i decided to supply the receiver with an external 13.5V power supply with current limiter. The receiver consumes 1.3 A @ 13.5 V
I finally decided to have a look at the PSU. I checked all components, printed board failures and wiring, but found nothing wrong or broken. I double checked again and replaced the transistors, just to be sure. Nope !! Still no regulation ...
Just to be sure, i simulated the circuit with help of LTSPICE, and guess what ? The predicted output was 17V !!
Using LTSPICE, i changed the R2 resistor from 56 to 560 Ohm (what i had) and BINGO ! i got 14V at the regulated output.
I confirmed this by changing this resistor on the PSU board, and i got a clean variable 13.8V !
Is this a design error or do i have a particular situation with my board ??
For those interested, here is the LTSPICE file.