Hoffman Amplifiers Tube Amplifier Forum
Other Stuff => Electronic Projects => Topic started by: wsscott on November 16, 2023, 12:55:34 pm
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I'm trying to get an old Channel Master transistor radio working after lying dormant for around 60 years, and have a question about these components.
In this radio, CM uses these "Component Combinations" in biasing the base and emitters on some of the transistors. The specs show most of them as containing a 1000 ohm resistor and a 0.04uf capacitor. The schematic shows these 2 components connected in parallel. This one component contains both the R and the C. They are square in shape, very thin, and green in color. One wire comes out on one end and the other wire comes out of the component in about the middle of it.
I guess they used them in the 1960's to save the limited space on the circuit board.
Anyway, I've never seen them before, and if they need replacing I don't know if they're available, or where to get them, or what they're called, how to test them, and alternatives.
Any help would be appreciated. Thanks.
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Not a high failure item and you will not find a replacement. If you find one that's bad just replace with individual components.
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Thanks Sluckey. When I opened the radio up, it obviously needed batteries. So I replaced them and no sound. I then replaced the electrolytics since their plastic shells were cracked, and I'm still not able to get any signal to pass through the circuit. I've replaced the Oscillator transistor, the Mixer, and the AVC transistors. The audio section seems to be working since I can hear a hiss on the speaker, but that's it. I've got 3 transistors (the Mixer, the 1st IF Amp, and the AVC) that each have low voltage readings on their Base and Emitters, and I thought maybe these components might be affecting the bias and causing the low readings. The remaining 7 transistors read good voltages.
Oh well, I'll have to focus on some other possibility.
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Sluckey-I've noticed in this transistor radio (6V, Pos. ground) that when it is turned off, and I connect my voltmeter with the negative probe to chassis, and the positive probe to any ground point on a component, like the Negative pin on an electrolytic, or its outer case, or an IF transformer case, etc., that I get a DC Voltage reading of -6 volts, or a hair less. I also get this same reading when I check the voltage across the terminals of the speaker.
Then if I turn the power on, all of the voltages go dramatically down and into the mV range.
Does this indicate a short somewhere in the circuit? If so, how is the best way to try to find the short in this type of radio?
Thanks.
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how is the best way to try to find the short
a short typically generates heat, probe you finger around anything burn? (ONLY recommended in low volt, low current stuff:)
one way to split stuff up, remove power to specific sections, Output stage, input stage,.......
another thing I do, isolate the PS from the system, load the PS with an approximate "system load" with a resistor, does the PS still put out normal volts?
if NOT;
fix PS, but be aware the PS "fail" might be caused by a sub-system fail
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I don't think that is an issue at all. Just ignore voltage readings when the power switch is turned OFF.
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if I turn the power on, all of the voltages go dramatically down and into the mV range.
missed this;
I connect my voltmeter with the negative probe to chassis, and the positive probe to any ground point on a component
on the plus side, the tips for shorts are still valid :laugh:
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Shocking!