Icom 751A Issue

[WX7P]

I recently got an Icom IC-751A through Craigslist dumpster diving.

The receive works great. Even with a BS antenna, I can hear qso's on 20 and all wwv and Hawai'i condx permitting. All of the other functions work, IF shift, the filters, everything. AM and USB are FB. All the memories are there also.

Here's the problem:

There is absolutely NO receive on LSB or CW no matter what band. Completely dead.

I did see one item on an Icom sheet referring to to a potential bad PBT Oscillator. In their scenario, the only receive function that works is FM. I have AM and USB working, so I'm not sure their fix is applicable.



Symptom:http://www.qsl.net/icom/oldicomfaq/pixel_shim.gifNo RX audio in any mode except FM. No TX. Unit is not unlocked.http://www.qsl.net/icom/oldicomfaq/pixel_shim.gif

Probable Cause:PBT oscillator is not running.http://www.qsl.net/icom/oldicomfaq/pixel_shim.gif

Cure:Verify absence of 9.4665 signal at W86 on main unit in 751A, or W134 in the IC-751. Replace faulty X7 on main unit in IC-751A, or X5 in the IC-751.http://www.qsl.net/icom/oldicomfaq/pixel_shim.gifhttp://www.qsl.net/icom/oldicomfaq/pixel_shim.gifhttp://www.qsl.net/icom/oldicomfaq/pixel_shim.gif



The scope is coming out of storage this week. I posted this to see if anyone had experienced this issue before.

[N8YX]

Check with the service manual - particularly, examine the PLL schematics.

There is a set of switching logic which determines the mode(s). One of the lines from the controller isn't setting the state properly. While I haven't experienced this with either of my '751As, I did run across a couple of threads about it on the various radio forums and user groups. Let me dig into my schematics later today and I'll tell you what to look for - and where.

[N8YX]

Look here:

http://www.download.n7tgb.net/Icom_Manuals/IC-751A_Schematic1.pdf

What voltages do you have on J1 of the "Main" board? Look at each with a scope as you toggle through the various modes.