Rotek 3970 AC/DC calibrator - power supply problem

[velik_kazakov]

Hi all and first, sorry for my bad English.
In my free time I repair some old lab equipment and now have a Rotek 3970 on my desk. Because I need AC/Dc calibrator in my lab I bought this item on ebay. I think that all work well, but have 2 zener diodes in the power supply that is very hot. First I connect the same in parallel. The temperature is drop, but it is still ~60oC. Think that this is no critical for this zeners, but get a decision to ask here (because of my bad English, I read the forrum from lots of years , but never write here before  ). If someone have the same calibrator, please, just to check about the temperature of this components. Other way to solve this is to start and to exchange all caps in the power distribution. I will try to attache some pictures for info and will add the part of service manual - the power supply.
The calibrator have some repairs before to come to me, but check it and all looks to work fine. Maybe will recalibrate it in the future, but want to solve this problem first.
Try to add a video of the calibrator in youtube, but maybe the quality is bad. Sorry for that.

Thanks to all in advance!

[The Soulman]

Hi all and first, sorry for my bad English.

No worries, could be worse. 

I think that all work well, but have 2 zener diodes in the power supply that is very hot. First I connect the same in parallel. The temperature is drop, but it is still ~60oC.

Paralleling zener's doesn't work, they don't share the current very well, maybe not at all and is only one still doing all the work.
Have you measured the voltage across c1 and c2? If it is to high, check the mains voltage and compare to what is selected on the device, maybe you need to change from 220V to 240V setting.
Do AR1 and AR2 get warm? If not and voltage's are ok, than don't worry. 

[2N3055]

Those two zeners are there to drop voltage going to linear regulators. They purpose is to dissipate power so linear regulators doesn't have, so they stay cooler and have better regulation and such... Measure temp on those zeners.. 75°C will burn your finger but will be safe for the diode.... If you want, you can replace them with zeners with same voltage but higher power rating.. That will keep them cooler...

[velik_kazakov]

Paralleling zener's doesn't work, they don't share the current very well, maybe not at all and is only one still doing all the work.
Have you measured the voltage across c1 and c2? If it is to high, check the mains voltage and compare to what is selected on the device, maybe you need to change from 220V to 240V setting.
Do AR1 and AR2 get warm? If not and voltage's are ok, than don't worry. 

Zener in parallel is the first that I can make to decrease the temperature with components that I have on my lab. Will think to do something else, 10w zeners is bigger and will be not easy to install them. The Voltage on c1 and c2 lookig to be fine - it is +-34V. Main voltage is set to 240V (right for Bulgaria). All voltage look to be good after linear regulators and the power transistors is warm, but not very hot. I think that they is OK.

Those two zeners are there to drop voltage going to linear regulators. They purpose is to dissipate power so linear regulators doesn't have, so they stay cooler and have better regulation and such... Measure temp on those zeners.. 75°C will burn your finger but will be safe for the diode.... If you want, you can replace them with zeners with same voltage but higher power rating.. That will keep them cooler...

The temp on the zeners was ~77-79°C, now is ~60°C. The problem is not the zeners, I know that they can work up to 100°C w/o problems, but the PCB is start to destroy from the temp and I looking to save the PCB.

Maybe will try to get all the filtering caps in the calibrator and will exchange all of it. If have some that is not in good condition, then the current from the power supply is bigger and the zeners dissipate more power now.

[Gyro]

It's normally bad practice to mount high disipation components flush to a PCB. What you're seeing is more discoloration than damage (it looks worse than it is) but it would be a good idea to mount some fresh zeners off the board on ceramic standoff beads.

I can't see any large areas of copper on the board providing heatsinking for the zeners, so standing them off can only improve their cooling.

[2N3055]

You can always do something like this if you have space on board.. Old trick...
You can crimp or solder them... It will stand off diode from PCB and serve as heatsink..

I also took a look at the photos.. It seems to me that charring is not from now, but that it was repaired before, and that previous (replaced) diodes or even not very good repair work might have been responsible for board damage...

[velik_kazakov]

You can always do something like this if you have space on board.. Old trick...
You can crimp or solder them... It will stand off diode from PCB and serve as heatsink..

I also took a look at the photos.. It seems to me that charring is not from now, but that it was repaired before, and that previous (replaced) diodes or even not very good repair work might have been responsible for board damage...

Yes, there have some repairs before to get the calibrator. Will try to make something in the weekend to reduce the heating of the PS PCB. There have enough space for this.

I make some mods, now the temperature on zeners rise to 55-57°C, but to the pcb have no more heat and I think that will keep power supply pcb.
Maybe look stranger this mod, but it work for me.