Modulation hum fix and FM alignment made easy

[maxhifi]

Recently had a cheapo early 60s am/fm 7 tube AC/DCradio on the bench. FM alignment was out, I had done it visually a few years ago with my B&K 415 but sound still had some distortion. I redid it last night using the signal generator method, using unmodulated 10.7MHz at the antennas terminals. I suspect the marker crystal in my B&K isn't accurate.

Instead of peaking the IF transformers for maximum response (maximum DC voltage at ratio detector capacitor), like with an AM radio I put a piece of tape on my alignment screwdriver like a flag, and rotated the each adjustment screw through its range, noting the -3dB points, and the position of the "flag". Then I moved the screw so the "flag" was in the exact middle of the two -3dB points.

I did this for all the adjustments except the ratio detector. That I did by feeding an AM modulated signal though the radio, and adjusted the ratio detector for minimum audio output. This didn't exactly correspond with centered D.C. Voltage as I expected, though it was close.

All this was done and I thought it worked ok but after running a while had a noticeable modulation hum, on AM and FM. A new "across the line" capacitor cured it, I used 0.1 instead of the factory 0.05.

In the end the ratio detector still didn't sound completely perfect so I adjusted it about 2 degrees by ear, and now the radio sounds really good on the college and public stations I like. On the strong super modulated pop music station there's a touch of distortion, and although I wonder why I will never listen to those stations anyway.

Interestingly, while I was testing the tubes to check for heater to cathode leakage trying to figure out the source of the modulation hum I noticed another weird thing. The 17EW8 tubes both tested bad on my EICO 667, whose manual puts only 12.6V on their filaments. On my TV-7, they both tested good. I believe the TV-7 because the radio is a low hours set, and the radio works well and is sensitive. The EICO works well with every other tube I have used it with, and even when I adjusted the heater voltage to 19V, they still tested bad.