AC problem- blank stat screen (furnace, heat, outside, replacement)

[d4g4m]

1998 unit- Goodman CPKJ30/Janitrol A32-05 newer stat Robertshaw 9725i2
Screen went blank. Removed stat face. No volts at C and R terminals. Both terminals on transformer show about 4 volts but not at C and R ?. But three leads at defrost PCB on condenser are about 10 volts each. [which includes C, Y, O]
Bad transformer or PCB?

[MrRational]

Originally Posted by d4g4m

No volts at C and R terminals.

Check the transformer down at the furnace and see what the R & C show.
And of course the 120V input too.

Bad transformer...?

It happens

[d4g4m]

^^^^
System is split heat pump [compressor/air handler] not a furnace. And only one transformer in air handler
Input to transformer is 240V [reads 252V]

[MrRational]

Originally Posted by d4g4m

Input to transformer is 240V [reads 252V]

And the output from the X-former?

[d4g4m]

^^^^
output on both [24V] terminals is around 8V and it is not 8V at 'R' term on stat. It's zero.

[Nonesuch]

Disconnect transformer from load, if the output still reads less than 24VAC, the transformer is bad.

[d4g4m]

^^^^^
Removed transformer. Did an OHM test. 24v side shows .9 ohm. 240v side shows open [nothing] Transformer is bad and will be replaced. Research shows that .9 is normal and should be same on both sides.
But what I find odd is that even though the 240v side doesn't show a resistance, I can get low volt reading [4 volts] from wires throughout the system. I think there is some volts going into and through the transformer but not enough to get an ohm reading.

[Nonesuch]

Four volts is low enough to just be a phantom.

Look for a transformer that has an accessible fuse (e.g. a automotive blade-type) on the secondary, or at least a self-resetting (PTC) fuse. You can also put a fuse or breaker on the primary, but that isn't normally necessary.

[btuhack]

An open primary? That's not common at all and the control voltage would be -0- to grounded reference anywhere you check if the primary is open.

The primary for a 240/24 step down transformer should be 10 X the secondary resistance.

Are you sure you don't have a blown control voltage fuse somewhere that you've overlooked?

[d4g4m]

^^^^^
There are no fuses in the air handler or on the outside condenser where 24V connects to defrost PCB or 240v connects to contactor/compressor/fan. There is a 240v fuse box attached to the house wall for compressor shut off service work. [besides the compressor and air handler breaker fuses in the garage panel] In any case, I found an exact transformer replacement from an ebay seller.