Going from good reception to 'no' reception suggests one or more defective components. I'd caution against the urge to say, 'It can't be ____ , it was just working fine.'
Test the original antenna and amplifier with the new parts completely disconnected. Then test just the new parts.
Looking for the possibility of open, loose or shorted cables or connectors.
Has the preamp failed? Or lost power?
Successful combining is going to depend on the two antennas being identical. The 4228 design has been altered over the last few years... still, complete loss of reception suggests a short or open circuit... The two cables used between the antennas and combiner need to be identical also.
In the end, the net gain is not going to be much, 3 dB minus the combiner losses... and that assumes both antennas are 'illuminated' by the same amount and phase signal (an optimistic assumption at best).
Please post a link to your reception report, it would be helpful to see what signal conditions you're contending with.