#1
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Modulation hum fix and FM alignment made easy
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. Last edited by maxhifi; 03-29-2017 at 12:02 PM. |
#2
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Occasionally a tester hates a tube type....I've got a cheap Heathkit emissions tester for the OLD fat pin tubes (which my better tester can't handle), and that thing HATES type 78 all I've tried including good new ones from vendors that test their stock get failed by the heathkit....I've started testing those tubes by radio only.
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Tom C. Zenith: The quality stays in EVEN after the name falls off! What I want. --> http://www.videokarma.org/showpost.p...62&postcount=4 |
#3
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I think the data is just wrong for the EICO, it's by all accounts a good tube tester. The 17EW8 is a Japanese tube, maybe was a late addition to the test data or something.
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