[TMC] Update: GPR-90RXD Receiver, SSB & Code reception

[Mark Johnson]

Team;

About 3 weeks ago I posted a question to the TMC reflector about my GPR-90RXD and its poor ability to copy code and SSB.

Not satisfied, I pulled the receiver out and started scoping the BFO injection. The actual injection voltage to the 6AL5 was less than  5 volts Peak to Peak. Not nearly enough. I've now got it around 18 volts Peak to Peak and I can copy SSB and CW very nicely. 

To accomplish this here is what I did. Scoping the actual BFO output, I found the signal quite strong going into  coupling cap C163, (390 pf). Output was very low. To insure the 6AL5 was not dragging that down, I replaced the tube. No change. I replaced C163. Slight improvement. I then started subbing in new values for C163 and found a value of 330pf really drove up the signal. That change made a huge difference. The actual value was quite "tight" in that small changes, 360pf, 340pf, etc. made for quite a change. 

Next I started playing with the coupling capacitor, C59. This cap passes the signal to the 6AL5. The spec is 3 pf. Not enough injection. By pure trial and error, I found 39pf did the trick. I can now copy SSB and CW signals very nicely. 

I pulled out an old Radio Handbook and the author warns of too much injection signal from a BFO. Too strong a BFO signal, and you'll get hiss that can mask weak signals. I can't say I notice that but will use the receiver for a period and see if there is a problem. 

Copying SSB and CW is a pleasure now.

Mark V Johnson VE3DJU/VE3DDI

-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://mailman.qth.net/pipermail/tmc/attachments/20081229/15ec6bcf/attachment.htm