In any piece of gear of this vintage, the first thing to do is to just go ahead and replace all the electrolytics and paper dielectric caps. Looks like a total of 5 caps in this thing, so not a big deal in terms of time or cost. Materials science has come a LONG way since this thing was made, and those old caps just don't age very well.
Check all the resistors and replace any that have drifted outside their marked tolerance. Vintage carbon composition resistors have a tendency to absorb moisture from the air and shift value, sometimes dramatically.
A check of tube heaters with an ohmmeter is a good start, but if you can find somebody with a tube tester, so much the better. None of the types in this unit are still in production, but they were all pretty common types, and a complete new set could be put together on eBay for a few bucks. Highly recommended if you intend to keep an oldie like this in service, as they are only going to get harder to find in the future. Luckily, none of them are really attractive to the tube audio crowd, which keeps the prices down.
If a recapping and resistor/ tube checking doesn't restore proper operation, then you will want to go through the assembly portion of the manual and trace each connection that is called out. Being a kit, there is always the possibility that the thing NEVER actually worked properly due to improper assembly by the original builder.