AW 23 hum problem

[Kent King]

Before you say "it's a filter cap", please read through the work so far:

I acquired an AW23 early tuner only and restored it. However, it has an awful 60-cycle hum. Now, I have 4 operating AW23 sets here, so I tried this tuner with a couple other amps, they all hum. Using the same amp/spkr pairs with a late tuner chassis - they work perfectly. Last week, I got another (rougher) early tuner chassis. In spite of the condition, I restored it - and it also hums with any of the amp/spkr combos I have operating. Again - these amp/spkr sets work perfectly with a late tuner. 

The two early chassis work very well, sharp tuning, etc., they just have an underlying hum that is quite bad. I did not believe there was any difference between the early and late power supplies, connections are exactly the same: filament, 250v B+, -20v bias, ground and the two 6C6 plate returns...

Any ideas appreciated at this point. And if there is a difference in the early/late power supplies - please enlighten me!

Kent

[Kent King]

I've been communicating with Norman most of today and he graciously led me to the source of the problem. The outer housing of the dial lamp was just touching the SW coil cover, introducing AC onto the entire tuner chassis. So watch out for that one folks...I will not forget in the future!

Kent

[Alex R. Whitaker]

I was going to ask if any shielded cable was grounded on both ends.  That will cause a ground loop hum.  I never would have thought of the dial lamp introducing hum via a coil cover.  That's a very helpful hint to store up for the future.