Perusing QTH one day a few weeks ago, an advert touted a bespoke IC-756 PRO hf rig at a tantalising price. Sad to say it was spoken for, listed as sale pending, I pm'd the proprietor and asked to be put on "the list" if the deal fell through. Needless to say the deal fell through and a very clean almost new looking 756 Pro made its way to new ownership.
Total cost with shipping was $360 USD. Even if the tx section was gone forever the Pro series make for outstanding rx rigs, plus they have that neat fish finder! So $360 for a rig with a fish finder and the same dsp chip as the illustrious JRC NRD 545 was, to put it bluntly, a steal.
It seems this rig as delivered indeed had tx issues, and I immediately recalled similar issues with other Icom rigs from past experience, particularly the 746 series. Sad to say the same issue also affects the 756 Pro series. To wit, static or rf can impinge upon the rx grounding silicon innards (activated when in tx mode) and eat them. They're in line when the rig is off, and when an antenna is connected anything coming down that antenna can pop the doodads, due to the Icom's rx/tx switching scheme. After testing (apply rx max attenuation via the front panel button and if the tx is good with the rx attenuator enabled, well, you have the disease) this rig def had "the disease".
This is a link to a fix of the malady;
https://www.youtube.com/watch?v=EPnglEOLChESo the rf section in question was investigated and bad diodes and a transistor were noted and replaced. In this condition the receiver section is active during tx and puts all sorts of garbage into the tx signal, it also attenuates the rx signal. This repair fixed the tx howling issue. Note the component placement and road map of the Pro differs slightly from the Pro 3 but you can figure it out as they do the same job. Enough about that, done and done, sorted.
But wait!
The original Pro flavour (subsequently remedied in the Pro2 and later rigs) was designed with another malady that often went unnoticed! Some early adopters of the OG (Original Gansta) Pro noted a weak audio trace of band activity present below the much stronger audio as delivered by the filter setting. In my case the rig would have several kc of the band tuned to present in the audio, this is best heard via headphone but it was present and hard to ignore once you knew what it was. There is a well founded fix for this;
"Correcting the IC756Pro Noise Blanker Drive-Signal Leakage Problem"
https://www.ab4oj.com/icom/756pro_nb/main.htmlHA HA! Listen to the rumble! HA HA!
This is a simple fix consisting of a single lytic cap, and the dif is night and day, seriously. Once modded the rig has none, nada, zilch, zero, null, of the garbage present in the audio all the unmodded rigs have. Enough about that, done and done, sorted.
Then on to some mods to enhance longevity of Project 756 PRO.
The inputs of most modern hf rigs have some form of impulse suppression, a gas discharge thingy or two, most commonly the input to the rx section, and the PRO has such, but what about the dual UHF socket inputs? No protection offered to the tx section at those menacing sockets, so some gdts (gas discharge thingy) good for a bit over 100w were placed across the UHF sockets; center terminal to nearby ground.
https://www.littelfuse.com/products/gas-discharge-tubes/low-to-medium-surge-gdt.aspxEnough about that, done and done, sorted.
Ok but what about the 12v dc input?
What about that?
What if the psu goes wild and delivers 20v or more?!?
Well, for that you employ a mov, a metal oxide varistor, a resistor that changes resistance when confronted by a voltage over a certain set value;
https://en.wikipedia.org/wiki/VaristorI selected a 20v mov for that duty and now the PRO is about as well protected from external threats as can be cheaply afforded. Enough about that, done and done, sorted.
So in summary here we are with a running Icom IC-756 Pro on the cheap, and as you might have suspected, more mods are forthcoming!
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