TS-790 PA failure repair

Long time no post !

I've been really busy with work and family and so whilst I've been doing work in the shack, carrying out repairs, restorations etc for other amateurs across the country, I've not had any time to write it up in the blog, I usually take some pictures of the work or restorations and so I will try to write them up as some may be useful or interesting to others, or perhaps the techniques used will be useful to someone !

A few things I've completed have been some repairs for other amateurs, including an interesting repair to a TS-790 which was diagnosed as a power block failure in the 144Mhz PA section (M67727). It was indeed a failure, however upon further investigation and very careful disassembly of the hybrid PA 'black block' which involved carefully removing the 'glue' to enable the top cover off - I discovered that the 2nd stage (Base bias ) connector leg was badly discoloured and had a very high resistance to the hybrid circuits substrate. After measuring it the leg actually fell off from the hybrid circuit substrate ! 

Unfortunately I can't find the picture of the failure that I took :( 

I then cleaned the area on the hybrid and the pin itself and successfully  re-soldered the leg to the hybrid circuit and after re assembly surprise surprise the power output returned to normal.

I've no idea why it failed in this manner although I have some thoughts.

It would seem that none of the PA blocks have any heat-sink compound between the hybrid units and the chassis (!) - no idea why but all that I've had through my hands to repair have been the same. So my theory is that the hybrids have hot spots and one of these is around the 2nd voltage feed into the hybrid to 144Mhz and this through heat cycling causes that area to fail either cracking the hybrid substrate or causing the pin to fail its connection.

So if you have problems with power drop off or failure - it might well be worth checking this and if your lucky and haven't - then I'd check of there is any heat sink compound on the PA blocks and if not - add some !!

Certainly with any hi duty cycle use or contest use I would not wish to use it without this update.

So another TS-790 returned to its owner aligned and working very well - result ! 

I wonder how many PA blocks have been changed as faulty (which technically they are!) when a relatively simply fix could have extended their life substantially - also the original blocks are fairly difficult to get hold and substitutes are getting more expensive - so certainly worth checking !

I hope this might be useful to other 790 owners

73s
Chris