EB5AGV's Workbench: Icom IC-706 MkIIG final amplifier repair
A new life for old-type power amps
This page is devoted to the repair of an ICOM IC-706 MkIIG final amplifier.
There are two types of final amplifiers for these rigs. First units, used Motorola SRF-J7044 MOSFETs. When they become obsolete, at about serial number 15xxx, Icom modified the final amplifier to use new devices, the (now) common RD70HHF1 (HF/50) and the RD70HVF1 (VHF/UHF). These transistors have a diferent case and can't be fitted to the old PCB without (at least) some physical mods. SRFJ7044 are obsolete and almost impossible to locate, so only solution so far was to buy a new PA BOARD from Icom... not cheap!
I document this repair because I was unable to find a complete final amplifier repair using new transistors in an old PCB. Most of the information I could find told that it was not possible to go this way, due to electronic differences among the new and old transistors.
Well, in this case, I had got a damaged unit to repair, along a new set of transistors (provided by owner; all of them were of the same type, RD70HVF1, but they seem to work fine at HF/50 also). Someone had already fitted new transistors in the old PCB but without any modification, so they were pressed by the PCB and, judging by the condition of the unit (see pictures below), it was not a good idea. So I decided to do the hard work... and this is the story.
NOTE ABOUT THE TRANSISTORS USED
I installed what owner had provided (three RD70HVF1) but if you buy them, get a pair of RD70HHF1 and one RD70HVF1, as Icom originally mounts on new PAs
Unit as received
This is what I found in this poor abused rig:
And once I got the final amplifier PCB out... what a messy job!!!
There were some overheated components:
This is the PCB once I removed the charred finals:
And this is the metal case, once I cleaned the goop. At least, it looked better!:
Repair work
So, after removing the finals, I decided to go the hardware way, modifying the PCB so the larger transistors could properly fit.
I used a damaged unit as a template:
WARNING: before you start to emulate this work, there are some things which should be considered. For the V/UHF transistor, there is a track which you will remove and need to be redone. Here is the original condition of the hole, looking from the bottom side:
You will need also to relocate one ceramic capacitor in the top side, from the original position:
To this one, rotating it:
There is no need to do anything else except be careful with the surrounding components, as in the HF transistors area, there is another track which will be filed, but it has no connection, so there is no need to reado it. Here you have it boxed:
This is another item you need to care when filing the HF transistors holes, one of the biasing pots:
So, once you know what to care for, this is what I did.
I carefully filed one hole (V/U transistor):
Then another one (HF):
And then the last one (second HF):
As I commented, you need to redo one track in the V/U transistor area:
I resoldered all the suspicious joints, once I had checked the components measuring them and finding them to be fine:
It was the moment to install the new transistors. The new units have longer legs so you need to cut them. I marked them with a pencil, cut them and, after aplying thermal compound, fitted them one by one:
In the V/U transistor, I moved a bit the ceramic capacitors in the output side:
This is the final result, once all transistors were installed:
And this is a general view of the final amp, once refurbished:
Adjustment
I followed the Service Manual to set bias for the power amps. Once I did it, I checked and got these results:
- HF: 120W
- 50MHz: 100W
- VHF: 40W
- UHF: 12W
I have not yet checked spectral purity of this mod. On-air tests have been good so far.
Please, feel free to comment with me this mod and your experience, if you attempt it.
Good luck!