This is a project I have been looking forward to for some time now. I have two ICOM IC-745s one in reasonable condition and one in very used condition. The one in very used condition I have owned for over 25 years. I bought it back when I was going to get my license the Third Time, it's relegated to being the hanger queen.
The above radio arrived in non working condition, which was the expected condition. The first order of business was to obtain a PIEXX memory module.
With the module installed troubleshooting could begin.
As one might guess the main PLL was unlocked, the sub loop functioned properly and since I had placed an earlier order with Mouser for my 820 I went ahead and ordered some trimmer caps for the four VCOs on both PLL boards. (this unit and the hanger queen.)
Both boards were powered up and some basic tests were conducted before installing the board back in the radio. All the VCOs could be adjusted and steered over the required frequency ranges. Each VCO covers 8MHZ of the total range of the PLL.
Upon troubleshooting the hanger queen's PLL board in the unit it was discovered that the PLL chip was bad.
All the correct serial data and control signals from the logic board were present, as well as the proper input signals from the main loop prescalier and reference divider. I ordered a PLL chip and prescalier chis from ICOM (OUCH) I should have bought the PLL chip from RF Parts.
The parts arrived and I proceeded to repair both boards. One board (the original board in the above radio) worked after replacing the trimmers and some adjustment of L8 (which is not documented in the service manual). L8 is the output transformer for the sub loop filter amplifier. The hanger queen PLL didn't work after repair and it was discovered that I had unsoldered a jumper wire that supplies power to the fourth VCO and the first buffer stage. The bottom shield was removed and the joint repaired. This board has yet to be tested. I may go ahead and test it after I deal with what is left to repair.
Bringing us up to date;
I have a numb and weak (outpur RF) 745 along with the usual dirty pots, switches, relays, and connectors.
As frequency increases both power and sensitivity decrease. This is acting like possible first mixer or first LO buffer (located on the RF board)
Receiver sensitivity is 20DB down.
Transmitter power (RTTY mode) starts out at 100W+ on 160M and ends up around 10W on 10M.
I can hear the low pass filters switching when bands are changed. There are a few other checks that need to be made before jumping to conclusions though.
I also have to investigate why the metering circuits are not functioning properly, but first the above issue needs rectification.
More to follow, I will have some time today to work on sensitivity / power problem.
Reply With Quote
Originally Posted by N8YX
