Scorched pcb on Fostex D160

[HiString]

It's been a long time since I posted here due to real world influences (ie: work) but I'm hoping the group wisdom may have some thoughts and/or advice.

Cutting a long story short........I powered up my Fostex D160 recorder for the first time in a couple of years and nothing happened, no lights, no display......nuttin. First thought was the power supply so I opened it up and found four diodes (bridge rectifier) and the pcb under them scorched as per photo. A quick check seemed to indicate that the diodes are all functioning OK and I still have to check for output voltages, etc., but I'd like peoples thoughts on the scorching issue. Diodes are 1A 200V.

 

[pvision]

They are stood off the board, unlike D19 and D21, so I imagine the manufacturers were taking precautions as they knew heat would be generated

Nick Froome

 

[JohnRoberts]

May be normal (not good, just normal for that design)...

Diodes fail as a short circuit from over heating (silicon melting).

JR

 

[HiString]

Sorry for the blurry photo.

D3, 4, 15 and 17 all had shrinkage cracks in the solder joints.

D16 and 18 had incomplete solder joints to the point the one end of D16 was actually loose.

Could this be the cause of the overheating/scorching?

 

[JohnRoberts]

Sorry for the blurry photo.

D3, 4, 15 and 17 all had shrinkage cracks in the solder joints.

D16 and 18 had incomplete solder joints to the point the one end of D16 was actually loose.

Could this be the cause of the overheating/scorching?

If one diode was open circuit the other might have to do more work to pick up the slack, but generally the bad solder connections are caused by elevated temperature over time.

An overheated solder joint will look dull and be mechanically weak. Refresh with a hot iron and fresh solder to supply some flux to reflow the connection.

JR

 

[HiString]

John,

Thanks for your comments.

I understand the possibility of failure over time due to high temps but I don't think it accounts for the lack of solder on three of the four joints on D16 and 18, it looked as though the solder had only partially taken to the diode "legs".

I neglected to add that I replaced D15 - 18 so I'm just about to connect the pcb to the transformer and check voltages before proceeding.

EDIT:

The attempt to check voltages was futile........no current out of the trannie, then found no current into the trannie. Track back through the 240v supply and found a disconnected wire in the mains plug.......LMFAO.

Fixed that problem and then found a couple of readings on the output of the trannie that need further checking.

Cheers,

ChrisO