The power supply is dead,primary volts seem ok (have no cct diagram).there are no secondary volts at all,hence no st/by 5v , nothing.have checked all electrolytics, replaced a couple, still no o/p.

Why do you think the boiler transformer is only 16 volts?

This is mostly power pack problem . check for bulge capacitors in the secondary side of your power , if non check vividly the reservoir capacitor of the primary side very well or even consider it change because it seems the power is not having sufficient capacity to power the load or output ,if ok then.Disconnect the output plugs of the power pack & read the output with your multi meter to check for 24v ,5v & 5.5v . if not ok check for defect component or you change the power pack

Check the 13.5V regulator and associated parts. Refer the power supply schematic diagram to get details. If you wish to get some details, http://electronicshelponline.blogspot.com/ Pull up older posts.

If you have an open primary wire, this means the transformer, or one of the Hot wires coming into the unit may be tripped. Check your breakers to see. If it is the transformer at the furnace, the primary side is the 120 volt side of the transformer. The 24 volt side will be dead due to this, as it is the secondary side. If you find you have power going to the indoor unit, but nothing is working, you will need to replace the control transformer. It is 120 Volts primary side X 24 volts on the secondary side. They only cost about $20.00 and are easy to change out. Primary side you will have a blk and wht wire. You may have several wires on the other side, but it will tell you in the directions which 2 wires to use to get 24 volts, very east to do. Look on the diagram and it will show you where the transformer is at the start of the run on the furnace. Again, it may be a blown fuse if you have a fuse box, or a tripped breaker. It will be the incoming side, that is the primary. Please rate me on my help to you.
Shastalaker7
A/C, & Heating Contractor.
PS, If you need my help contact me.

check primary of thransformer AC mains volts, then
check secondary of transformer AC low volts into each rectifier then check DC volts at capacitors then check DC volts at regulators inputs then check Dc volts at outputs
as you go draw cct diag and layout so you know where everything is and how it is connected.

if its the psu you will find the fault, only ever replace items with original parts or direct equivalents.


Hope this helps
Thanks for using FixYa

There shoud be a big elcetrolytic cap connecto the bridge rectifier output, ti should have about 150~160 vdc on that cap, if you do not have the dc on the secondary side, ti may be because it sees short circuits (you will hear fainted ticking sound), the power supply should have 5v and 12v at all time. Try heating up the primary circuits with hair dryer to see if will kick on. Post some picture will help, see example here: http://s807.photobucket.com/home/budm/allalbums

There should be a small valued cap in the primary side (22ufd?) at 250V. Check that. Also, look for other small caps int he range of 2.2ufd with 200V ratings in the same general area.
Dan

sounds like a short in the cpu of the unit

The power supply is the first place to start.

Is there a transformer? Check to see that there is AC on the primary & secondary of the transformer.

If no AC on the secondary, then the transformer is bad. If AC ok on the secondary, then look for a bad rectifier diode/module and/or regulator module. There might also be fuses on the DC side of the power supply.

If the unit is completely dead, it's most likely a power supply issue.

Please let me know if I can be of further assistance to you.

Good luck!

- Jim

this could be caused by noumerous things: 1. there may be all kinds of definitions that got messes up, such as a lock up of any kind. check manual, there should be a list of common faq's and solutions. 2. check for any blown fuses. check resistors. 3. regulator may have gone bad (not sure about that one). 4. there may be an incompatible level of voultage. if its too low it will not power on the system, if it is too high, the high-voultage mechanizm will cause it to shut down. measure the voultage in the system, if you dont know how to do it exactly: Remove both of the power transistors from the primary, then power up the circuit with AC, then provide 5V (variable) to the secondary side, send it back in the way it normally comes out! Now measure accross the transisor side of the opto while varying the 5V above and below 5V (note: the transistor side of the opto is on the primary side of the power supply since it is feedback) If the feedback circuits are working the opto should turn on and off as you vary the voltage above and below 5V. there could be many more options im afraid, but I hope that will do. good luck.