IC-R71A Negative Voltage Supply

[WB4LNM]

My Icom IC-R71A was having some 'strange' problems, and I've traced one problem as the negative ten volt supply on the 'Matrix Board' being inoperative. Failure of this PS is noted in the Icom troubleshooting guide as affecting the audio output and S-meter.

So, I have disconnected the inoperative PS from the matrix card and connected the -10 volt line to an external supply. However, this just manifested the full symptom as mentioned in the troubleshooting guide!

Turning the voltage down to -1.2 volts or so (lowest the supply can go) allowed the receiver to seem to operate properly, with 50 microvolts showing up as S-9 on the meter. Signals on 75 meter LSB sound very good.

The receiver is still open on the bench while I continue to think about this. Not sure why it seems to be working fine with the -10 volt bus at -1.2 volts and essentially inoperative if it is at -10 VDC, where is should be. Could be another problem somewhere, but sounds like everything works well at the reduced bias.

The receiver main PS (a known weak point) has been completely rebuilt with an LDO regulator that is running at only about half the current the regulator is capable of. It runs much cooler than the original discrete supply. That voltage is solid.

Just puzzled why the receiver doesn't seem to 'want' the negative bias to be -10 VDC. The original 'CP-2' negative power supply module is unobtainable, so I'll need to build one from scratch. This would not be difficult, but I'm just wondering if that is necessary (Icom engineers thought so!).

Anyone had a similar situation and a good solution. 73 WB4LNM

 

[k9rzz]

First, WELCOME ABOARD!

Dunno how much help you're going to find with your issues here, so also check out the Yahoo group for the R-71a. https://groups.yahoo.com/neo/groups/ICOMR71A/info

Those guys seem to know that radio inside and out! (802 members)

 

[WB4LNM]

IC-R71A Bias problem resolved

No need to respond to my earlier message on the - 10 volt bias abnormalities. The problem has been resolved.

I also just built a small 'linear' power supply to generate the -10 VDC and attached it to the dividing bulkhead between the power supply and the memory card. For others benefit, this only draws about 6-7 mA at all settings.

Good sensitivity all the way through 30 MHz. Meets or exceeds the specifications. The first mixer JFETs were replaced with J310's back in the mid-1990's when I'd found the original owner had blown them out somehow.

73 WB4LNM

 

[NS2F2018]

Problemwith AGC

No need to respond to my earlier message on the - 10 volt bias abnormalities. The problem has been resolved.

I also just built a small 'linear' power supply to generate the -10 VDC and attached it to the dividing bulkhead between the power supply and the memory card. For others benefit, this only draws about 6-7 mA at all settings.

Good sensitivity all the way through 30 MHz. Meets or exceeds the specifications. The first mixer JFETs were replaced with J310's back in the mid-1990's when I'd found the original owner had blown them out somehow.

73 WB4LNM

Hi, I think I've a same problem too with -10V rail. Mine got a malfunction AGC which is always on OFF position no matter what the AGC switch position. My suspect is a -10v supply from matrix board.

Could you please share with me how to repair the circuit that provide -10v? Or share small 'linier' power supply replacement experience with me?

Your help is greatly appreciated.

Regards

Fitra
YC0NGA / NS2F

 

[WB4LNM]

It has been a while. I just used a small linear supply I had and put it beneath the chassis. Disconnected the switching supply. Seemed to work. No luck with getting parts from Icom. Its too old I guess......