A display problem with my IC-756 Pro11

About the first of February 2016 the display on my fancy Icom IC-756 Pro11 suddenly went blank. This is my main radio for HF and 6M. It is quite old now (2004 I think) but it has very advanced noise reduction and signal processing. I bought it second hand. The original owner brought it to me to fix and it seemed to be "dead". I soon worked out it was working, the S meter was waggling round and when I pressed the PTT it was transmitting. There was no audio output.

My first plan of attack was make some sense of the circuit diagrams and then trace the audio path from the detector to the speaker. I soon discovered the audio system was fine but nothing was getting to the speaker or the external speaker socket. I was using a signal tracer I bought from Tandy when they first opened stores DownUnder many years ago. I did have to modify it with an external RF probe and to correct the bias on at least one of the germanium transistors.

I eventually got brave enough to remove the main board after unplugging many leads and there was a burnt track, the earth track for the speaker and external speaker socket. Some TCW and some cambric sleeving fixed the missing track and I put it back together. That night conditions on the 3.62MHz BoatAnchor net was particularly bad so I used the repaired IC-756 Pro11 and was amazed at just how well the various noise reduction/signal processing cirsuit worked. I handed it back to Peter with the comment, "when you wish to dispose of it I would like first refusal please". About a year later it was mine.

Now how did that fault occur ?? I "interrogated" Peter as to how it was connected in his shack and found it had been wired to a two input speaker box along with a 2 Metre rig. I soon came to the conclusion the earth's on the 2M rig had become disconnected and the only path to earth and the negative of the 12 Volt supply was the thin earth track inside the IC-756 Pro11.

Six Metre DX was going well during January and February 2016 and I was having a good time logging DX contacts during a Sporadic E opening and after four or five interstate contacts as far away as Western Australia, Queensland and Tasmania, I looked at the radio to shift to another frequency and there was no display !!!

After much rummaging through the 105 pages of the service manual (it is a PDF file so not as bad as it sounds) I found the actual circuit diagram of the inverter for the screen back light on a sheet with a lot of other circuit bits. The PDF file is amazing quality, I was able to enlarge portions of these BIG pages up to 300% and so I could print just the portion I needed in good detail. First I found the board layout for the area of interest, that was easy and after a lot of searching I eventually found the actual circuit diagram of the inverter. I suspected that because the display had been going brighter and darker at random intervals, sometimes it almost disappeared, for about six months.

First place I looked was the little SMD transformer that provides the high voltage for the CCFL back light tube in the display. The resistance of the secondary (output) winding was 450 KOhm and jumping round when I heated it with the iron and it then varied down to about 100K.

This transformer generates 1140 Volt from a 9 volt rail and at that turns ratio I would expect it to read less than 1000 Ohm. The Icom circuit diagram even had the Toko part number. I did not know if that would help because Toko had stopped making quite a few items but i figured there may be a load of NOS stuff lying round the world. I put out a cry for HELP on the forum, someone in the USA may still stock these things and I also sent an Email to Icom Australia.

A search on the Toko part number found several sources of a data sheet but there were also worrying signs, no longer in production or no longer available. I was just about to send an email to Icom USA, or look at how I could make a replacement, and I received a forum reply from a fellow in VK4 land telling me Icom Australia are very good with spare parts even for these old rigs, was worth a try.

On a forum post I found a good description of how to get at the offending section of the rig and although it was for earlier versions of the rig the mechanical layout and construction was the same. It was then time to get very brave and open up the BIG beast.

While I was rummaging round the web and pestering fellow forum members, I also looked at getting a spare display and a fellow in India on the forum told me where I could get a display from China. Getting one of these was not such a simple process as "Katrina's" understanding of English was not the same as mine but after several emails each way I eventually got a Proforma invoice and I now have a spare display. The display has a CCFL back light and I thought it was well worth the expense (A$245.00) to have one of these, it would be my luck to have the display fail due to the CCFL lamp soon after I get the rig back on the air.

I sent my email to Icom Australia on a Friday evening and what great service, I got a reply the following Monday morning while I was exploring the other possibilities. Icom Australia had the high voltage transformer in stock.

They responded quickly to my request and I phoned them and they sent me a Proforma invoice so I could pay by credit card. Pity they did not have two, would have bought another as a spare.

The transformer arrived from Icom Australia in a few days but it was a week before I could fit it onto the display board. I had overseas visitors staying with me and was showing them round South Australia. I fitted the transformer and put the rig back together on the 28th February 2016 and to my great relief, the display worked and I was back to full power on the 6M AM net the next day.

As the IC-756 Pro11 is my main 6M AM rig (the other older 6M rig Yaesu FT-625R decided it no longer did AM several months before) I had to use my Yaesu VX8 tiny three band hand held for the Sundays AM net with it putting out a HUGH 1 Watt on 53.1 MHZ into my 6M J-pole antenna and most of the usual 6M crowd can hear me. 5 and X where w was well above their local noise.

Have not had time to look into the problem with the FT-625R yet so if anyone out there knows what may be wrong, me and one of the other regulars on the 6M AM net will be very pleased to find a solution. It works fine on SSB and FM but no AM output.

Here is a description of the ICOM IC-756 Pro11 rig:

The Icom IC-756 Pro II is the worthy successor of the famous Icom 756, Icom 756 Pro, and covers all nine HF ham bands plus 6 meters (receive from 30 kHz - 60 MHz). The high resolution 4.9 inch TFT color dot matrix LCD display provides more operating information than you've ever had before, including a real-time spectrum scope ( 12.5, 25, 50 or 100 kHz). You can even select from seven different display fonts and eight different screen backgrounds. The ProII features a 32-bit floating point DSP Digital IF filter with 51 selectable bandwidths. Soft and sharp filter shapes can be selected in both SSB and CW modes independently. You also get: microphone equalizer, manual notch function, twin peak audio filter, digital twin pass band tuning (PBT), all mode squelch, multi-function analog meter, including PSK31, and much more. No additional filters or high stability crystal oscillators are available or needed. The filters are the sharpest - more selective than any crystal or mechanical filters. The selectivity lets you pull out weak signals like never before.

The transmit audio is fully configurable with the sophisticated 121 position microphone equalizer. A powerful RF speech compressor is great for breaking through noise to grab that DX contact. This radio includes an HM-36 hand mic. Make the IC-ProII the heart of your complete Icom station! Please see page 70 of the February 2002 QST for a complete review of this amazing radio.

http://www.ab4oj.com/icom/pro_notes.html

If you are unfortunate to need a new display here are the details of the supplier - Information courtesy Chetz, VU3DMP:

The TFD50W40-MMA TFT display module is a suitable replacement part for the IC-756Pro2 display. It should also work in the IC-756Pro3. It is available from the following source:

Some photos of the insides of the IC-756 Pro11 including the backlight power supply circuit

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Written by and Copyright, Phil. Storr © Last updated 7th March 2016