Two Transistors Break Down Frequently

This sort of fault is always difficult to diagnose, as there may be some obscure transistor parameter which is marginal, rather than just the obvious current and voltage ratings, so simply doubling a power rating may make matters worse! A higher power device is often slower, and if it is switching it may then have overheating problems. I once solved a problem like that by going the 'wrong way' and using lower power devices which were much faster!
Adding some heatsink or improved cooling may help.
It is often impossible to tell if the fault is voltage current or temperature related.
Del

Very difficult to comment without complete schematic.

1. check whether you are replacing them with matched gain (hfe) power transistors D401 and B546;

2. Is there a suppression diode across the relay coil?

It sounds like these two transistors could be part of an H bridge, are there another pair? If so change all four. The servo motor will most likely be driven directly by these transistors so there will be no relay, but there will still need to be flyback diodes across the collector/emitter junctions, seek and test those.

Please see the taached picture for power circuit control diagram of Servo motor and suggest me where should i install the fly back diod.

Sorry Signode, been on holiday for a week and just read your question. If you haven't already sorted it have a look at my edit of your photo, bit bodgey I realise but shows orientation and position of flyback diodes. Hope this is useful.

Thanks Lemmy70.

I will connect and report you back after few days.