I think the problem is that the new HL tanks are wired differently - specifically the hull.
I encountered the same problem - ordered two new RX boards (gold plate) from MATO. When I hooked them up to an older HL Tiger, I had the same problem you described. I have a new T-90 that I swapped out the RX's and both new RX's worked perfect. I was looking at the upper hull connections on the new T-90 and it is a different configuration for the electronics.
What I think needs to be done is to rewire/solder the hull bridge on older HL upper hulls so that it coincides with the inputs from the new Transmitter/Rx
Try rewiring the upper hull - pretty sure thats the problem
Hope this helps
AL