While I cannot claim to be familiar with that unit, it is likely that the real problems will be along the lines of a few common problems from radios of that era. This assumes that the unts were/have been installed correctly.
1)Chances are very good if it uses a rotary switch that has a plastic center- that switch will be the source of most of your problems. That plastic center gets brittle and breaks.
If it is a toggle switch, it may still be a source of the problem, flip it back and forth a number of times to see if performance improves, if it does, or if excessive noise is noted in the switch operation, I would consider replacing the switch just as a matter of routine maintenance.
2) It is possible that one or more capacitors have failed- electrolytics essentially. These will reveal themselves either as a persistent 60 cycle hum, or if the faulty unit is circuit traced with an audio generator- Driver stage, or final stage may produce an adequate signal prior to an electrolytic, but not after that electrolytic.
Often the Electrolytics are nominally rated for the application to keep costs down during production. Only failing after a number of years, or months.
3) It is also possible that there is one or more transistors that are showing excessive leakage. This results in almost normal operation, save for weak volume, distorted signal, or some times no sound at all.
4) Voltage divider in the power supply has at least one resistor out of rated value range, or one has burned out completely.
5) If one or both units are simply "dead", the problem may be located in the diode rectifiers of the power supply. These fairly often have their juntions fail, and they just "open".
Troubleshooting a unit that has passed the above inspections, and still fails to operate, can be checked by powering up the unit, and touching each transistor lead with a screw driver blade. This will inject a certain amount of noise into the circuit. Start from the output stages and work back.
: I'm trying to repair a friend's 1969 AM/FM intercom made by FASCO (model 8407). Any experience with it? A schematic would be fabulous.
The other is another can only smaller with a white plastic covered rectangular appendage on the back that has five solder on connections. It's mounted to the front of the chassis wall.
Any guidance here would be appreciated.
: Hi,
: Sams does not list that model. They start with the 8408.
: While I cannot claim to be familiar with that unit, it is likely that the real problems will be along the lines of a few common problems from radios of that era. This assumes that the unts were/have been installed correctly.
: 1)Chances are very good if it uses a rotary switch that has a plastic center- that switch will be the source of most of your problems. That plastic center gets brittle and breaks.
: If it is a toggle switch, it may still be a source of the problem, flip it back and forth a number of times to see if performance improves, if it does, or if excessive noise is noted in the switch operation, I would consider replacing the switch just as a matter of routine maintenance.
: 2) It is possible that one or more capacitors have failed- electrolytics essentially. These will reveal themselves either as a persistent 60 cycle hum, or if the faulty unit is circuit traced with an audio generator- Driver stage, or final stage may produce an adequate signal prior to an electrolytic, but not after that electrolytic.
: Often the Electrolytics are nominally rated for the application to keep costs down during production. Only failing after a number of years, or months.
: 3) It is also possible that there is one or more transistors that are showing excessive leakage. This results in almost normal operation, save for weak volume, distorted signal, or some times no sound at all.
: 4) Voltage divider in the power supply has at least one resistor out of rated value range, or one has burned out completely.
: 5) If one or both units are simply "dead", the problem may be located in the diode rectifiers of the power supply. These fairly often have their juntions fail, and they just "open".
: Troubleshooting a unit that has passed the above inspections, and still fails to operate, can be checked by powering up the unit, and touching each transistor lead with a screw driver blade. This will inject a certain amount of noise into the circuit. Start from the output stages and work back.
:
:
:
: : I'm trying to repair a friend's 1969 AM/FM intercom made by FASCO (model 8407). Any experience with it? A schematic would be fabulous.
Does the "AM" section work? If so, you may just need to realign the units. (I am making an assumption that this was one that was "wireless", therefore it operated as a "walkie-talkie" of that same vintage, which was essentially operated on CB channel 16.). If it is a "hardwire connection" between units, those are probably duplexing coils. Similar to the ones you find in telephones. In either instance, I would start from the audio output, and work back through the RF stages, and the duplexing coils. If it is a hardwired unit, the coils should still be good, they are only seeing low voltage levels, and these would be connecting into the circuit at the point of either a line level pre-amp, or just before the driver stage transistors which drive the output transistors..
If the "AM" section does not work, address that first. It may simply be that a number of transistors failed. If you can remove one of the driver transistors with ease, take it out of the circuit completely, and test it with which ever meter you are most confident in it's performance, VTVM's and DMM's have a hig resistance setting usually, and this will come in handy if you think you have an blown transistor junction. Check for "diode effect" between all of the terminals. If you do not see a substantial difference in resistance when switching the lead polarity, or if any of the pairs show "open" in both directions (infinite resistance, or nearly so), you have the located the problem- it is right there. (I am assuming you do not even have one of the simple transistor testers.)
If you use a capacitor on both the input and ground reference of your signal generator output, you can safely inject a signal into the circuit. Just start at a low signal level initially. Just take care not to short out anything.
: Thanks for the incredibly fast response. I've checked out the power supply and the rotary switch. The fact is that I can't seem to get any signal to show up at the transistors at all. I work with mostly vintage AM radios, so I have no familiarity with the type of mirophone used in the main unit.
: There are actually two components that I can't identify in the intercom portion of the unit, so I'm a little reluctant to capacitor couple an audio
: signal in without being sure. I'd do it anyway if it were mine, but this one belongs to a friend. One of the components looks like a small stack of laminated metal with probably a coil on top. It's packaged in a metal can about 3/4 inch in diameter and an inch tall. It's mounted above the AM radio RF section on a metal chassis shelf. It has five small guage wires coming out of it.
: The other is another can only smaller with a white plastic covered rectangular appendage on the back that has five solder on connections. It's mounted to the front of the chassis wall.
: Any guidance here would be appreciated.
Steve
: Hi,
: It sounds as if you are describing other inductors.
: Does the "AM" section work? If so, you may just need to realign the units. (I am making an assumption that this was one that was "wireless", therefore it operated as a "walkie-talkie" of that same vintage, which was essentially operated on CB channel 16.). If it is a "hardwire connection" between units, those are probably duplexing coils. Similar to the ones you find in telephones. In either instance, I would start from the audio output, and work back through the RF stages, and the duplexing coils. If it is a hardwired unit, the coils should still be good, they are only seeing low voltage levels, and these would be connecting into the circuit at the point of either a line level pre-amp, or just before the driver stage transistors which drive the output transistors..
: If the "AM" section does not work, address that first. It may simply be that a number of transistors failed. If you can remove one of the driver transistors with ease, take it out of the circuit completely, and test it with which ever meter you are most confident in it's performance, VTVM's and DMM's have a hig resistance setting usually, and this will come in handy if you think you have an blown transistor junction. Check for "diode effect" between all of the terminals. If you do not see a substantial difference in resistance when switching the lead polarity, or if any of the pairs show "open" in both directions (infinite resistance, or nearly so), you have the located the problem- it is right there. (I am assuming you do not even have one of the simple transistor testers.)
: If you use a capacitor on both the input and ground reference of your signal generator output, you can safely inject a signal into the circuit. Just start at a low signal level initially. Just take care not to short out anything.
: : Thanks for the incredibly fast response. I've checked out the power supply and the rotary switch. The fact is that I can't seem to get any signal to show up at the transistors at all. I work with mostly vintage AM radios, so I have no familiarity with the type of mirophone used in the main unit.
: : There are actually two components that I can't identify in the intercom portion of the unit, so I'm a little reluctant to capacitor couple an audio
: : signal in without being sure. I'd do it anyway if it were mine, but this one belongs to a friend. One of the components looks like a small stack of laminated metal with probably a coil on top. It's packaged in a metal can about 3/4 inch in diameter and an inch tall. It's mounted above the AM radio RF section on a metal chassis shelf. It has five small guage wires coming out of it.
: : The other is another can only smaller with a white plastic covered rectangular appendage on the back that has five solder on connections. It's mounted to the front of the chassis wall.
: : Any guidance here would be appreciated.