#46
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After all this discussion, I decided to pull the 7" Motorola off the shelf. I never restored the set, but I did the preliminary checking of the condition of the set. Of course, the set works very poorly and I don't remember what I did to check the basics. IIRC, I bridged the 'lytics and installed silicon diodes to replace the seleniums. I didn't do anything to it but plug it in. It hummed and it has a dim display on the screen. BTW, this set was dated Feb, 1949 and uses 1/2 of a 12SN7 for the 1st audio. More much later! |
#47
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It's just a typical Bakelite socket.
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#48
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#49
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This set is one of the portable styles and was complete with the antenna and base and in reasonable condition. The CRT seems to be good, but the true test is when the set is restored. |
#50
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I'm not sure this applies but the date is close.
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Audiokarma |
#51
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Well this helps if it were max volume but mine is minimum volume hum.
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#52
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1. Does the hum actually get louder as you turn the volume control down? 2. Do you observe any change of hum as you change the volume control when the grid of the 25L6 is disconnected? 3. Are you absolutely sure that the cap across the 330 ohm resistor in the cathode circuit of the 25L6 is connected correctly? Properly grounded? 4. Do you have a few more 25L6s to try? jr |
#53
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#54
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I bridged the power supply caps made no difference but the 2 300 v caps rated at 100uf and 120 uf I had put in 2- 450 v 470 uf caps in place of the original and I thought maybe this was a problem but it shouldn't as it's just Alittle harder on the diodes when turned on.
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#55
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1. Does the hum actually get louder as you turn the volume control down?
2. Can you measure the voltage of the hum at the speaker? Quote:
jr |
Audiokarma |
#56
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#57
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.012 ac voltage at speaker no hz, grid on, grid off voltage is 1.662 ac at 60hz . No DC voltage grid on or off.
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#58
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Do you measure the same voltage with different 25L6s? jr |
#59
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There was nothing on the grid pin when I measured at the speaker leads and got 1.662 volts ac.But I did use a clip wire to measure the grid with the wire on the grid. 25l6s the same. I've been going over this beast and can't find a thing out of place.
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#60
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Well still cannot find the source of the hum but is it at all possible to get a hum like this from hv, the 1b3 gets its heater voltage from the hv coil so from there I don't know. The Motorola Manuel states that there is a 22 ohm resistor between the crt and the audio amp heaters to keep audio hum down but that resistor is there.
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Audiokarma |
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