Need advice on Rebis rack power supply

[musika]

Hello,

I decided to recap this Rebis power supply.  When I unscrewed the pcb and turned it over, I noticed a couple of component traces that looked like they have been running hot.  See photo. I desoldered one of the diodes to take a look at the model number.  They are BZV85c15 zener diodes.  They look to me as if the solder has been melted at the pads during operation.  I will replace them, but I am wondering what the real problem is here.  Are they supposed to be acting as a crowbar for this supply?  Does anyone know.  Should I be replacing the power transistor?  TIP131. ?  Any help with this is restore would be appreciated. 

 

[musika]

photo2.

 

[musika]

photo3.

 

[musika]

photo4.

 

[musika]

photo 5.

 

[jensenmann]

Hard to tell without measuring anything. Does the PSU work?
It looks as if these zeners are used as a voltage reference (2x 14,7V) for the TO92 transistor next to them.
If heat is a problem in regular use then give the new zeners more distance to the PCB. The longer legs will help cooling them and more distance allows more air to flow. But I don´t believe that heat is the problem because underneath the zeners the PCB is not browned. Did you measure them after desoldering if they still work?

 

[musika]

They both measured OK.  Forward voltage bias drop is .682vdc on one and .690vdc  on the other.  I have read that they should measure between .5v and .7v.  Reverse voltage drop is OL on both.  And yes the supply has been OK and voltages checked prior to recap.  Should have noted that I am doing this as a last step since I have recapped all my modules. 6 gates, 2 EQ's, 3 Comps, and 1 de-essser.

 

[analogguru]

Replace each zener-diode with 2 zener-diodes 8,2 V in series and use 1,3W power rating for them.  This will help to reduce the heat.

BTW, originally there is a 15V and a 18V zener-diode in series to achieve the desired output voltage of 40V (with 1,2A).

 

[musika]

"originally there is a 15V and a 18V zener-diode in series"

OK thanks for all this info.  Now I understand how they are used as voltage references.  I could only read the label on one zener diode and so assumed they were both the same.  Wish I had the schematic for this, but have not found one yet.

 

[analogguru]

Wish I had the schematic for this, but have not found one yet.

The principle used is like this: