First, there is a steady 60 Hz hum, even with the volume down. All the paper caps were replaced by a previous owner and check out fine on my capacitor tester. I read that the hum-bucking coil on the speaker might be reversed, but I couldn't figure out where that was! The second symptom is motorboating at higher volumes. I checked the grid load resistors in the audio output stage, and they were 10x the value on the schematic. After a tedious replacement effort, there was no improvement.
Any ideas? I'm out of gas! Thanks!
: Hi! I'm restoring an Emerson 250, the AM-only version of the peak top mantel set. Someone had repaced the heater element with a power resistor that was simply too hot for the small cabinet, so I decided to try replacing it with a 1N4005 diode, as I've seen referenced in ARC and OTB. I've replaced the power supply caps and swapped out the weak tubes, and now the radio plays strong, despite the slightly reduced voltage. There are 2 problems that I can't seem to get rid of, and that's where I need your help.
: First, there is a steady 60 Hz hum, even with the volume down. All the paper caps were replaced by a previous owner and check out fine on my capacitor tester. I read that the hum-bucking coil on the speaker might be reversed, but I couldn't figure out where that was! The second symptom is motorboating at higher volumes. I checked the grid load resistors in the audio output stage, and they were 10x the value on the schematic. After a tedious replacement effort, there was no improvement.
: Any ideas? I'm out of gas! Thanks!
I'm still unsure about how to get rid of the hum/motorboating. Any other ideas? I'll let you know the results when I add the power resistor this evening.
Regards, David
: Hi David, Firstly, using a diode to drop the heater voltage in place of a ballast only reduces it to .707 of the mains voltage i.e. 85 volts, not half. So you need some extra resistance to reduce the voltage to the correct level. If your set uses three 6.3 volt tubes (two 78s & 77) and two 25 volt tubes (25Z5 & 43) they add up to approx. 69 volts, still requiring 21 volts drop in a resistor of 70 ohms, 10 watt. 68 or 75 ohms is probably the closest readily available. There will be a lot less heat than from the original resistor. You mentioned replacing all the paper capacitors. The symptoms you're decribing is usually due to failed electrolytics. It's very important to replace these as apart from causing hum and motorboating, they can short and damage the rectifier, etc. Use 10 µf in place of the 4 & 8 µf and 22 µf in place of the 16 µf, all 160 volt. Leave the original can electrolytics in place for cosmetic appearance but disconnect them and place new flying lead types under the chassis, making sure of the correct polarity.
: Don Black.
: : Hi! I'm restoring an Emerson 250, the AM-only version of the peak top mantel set. Someone had repaced the heater element with a power resistor that was simply too hot for the small cabinet, so I decided to try replacing it with a 1N4005 diode, as I've seen referenced in ARC and OTB. I've replaced the power supply caps and swapped out the weak tubes, and now the radio plays strong, despite the slightly reduced voltage. There are 2 problems that I can't seem to get rid of, and that's where I need your help.
: : First, there is a steady 60 Hz hum, even with the volume down. All the paper caps were replaced by a previous owner and check out fine on my capacitor tester. I read that the hum-bucking coil on the speaker might be reversed, but I couldn't figure out where that was! The second symptom is motorboating at higher volumes. I checked the grid load resistors in the audio output stage, and they were 10x the value on the schematic. After a tedious replacement effort, there was no improvement.
: : Any ideas? I'm out of gas! Thanks!
: Thanks for the ideas, Don. I had earlier replaced all of the electrolytics as you recommended, but hadn't added the extra resistor with the diode. I'll do that this evening. I've actually replaced the three power electrolytics twice, the second time to make sure I wasn't using a faulty replacement, so I don't thin that's the problem.
: I'm still unsure about how to get rid of the hum/motorboating. Any other ideas? I'll let you know the results when I add the power resistor this evening.
: Regards, David
: : Hi David, Firstly, using a diode to drop the heater voltage in place of a ballast only reduces it to .707 of the mains voltage i.e. 85 volts, not half. So you need some extra resistance to reduce the voltage to the correct level. If your set uses three 6.3 volt tubes (two 78s & 77) and two 25 volt tubes (25Z5 & 43) they add up to approx. 69 volts, still requiring 21 volts drop in a resistor of 70 ohms, 10 watt. 68 or 75 ohms is probably the closest readily available. There will be a lot less heat than from the original resistor. You mentioned replacing all the paper capacitors. The symptoms you're decribing is usually due to failed electrolytics. It's very important to replace these as apart from causing hum and motorboating, they can short and damage the rectifier, etc. Use 10 ?f in place of the 4 & 8 ?f and 22 ?f in place of the 16 ?f, all 160 volt. Leave the original can electrolytics in place for cosmetic appearance but disconnect them and place new flying lead types under the chassis, making sure of the correct polarity.
: : Don Black.
: : : Hi! I'm restoring an Emerson 250, the AM-only version of the peak top mantel set. Someone had repaced the heater element with a power resistor that was simply too hot for the small cabinet, so I decided to try replacing it with a 1N4005 diode, as I've seen referenced in ARC and OTB. I've replaced the power supply caps and swapped out the weak tubes, and now the radio plays strong, despite the slightly reduced voltage. There are 2 problems that I can't seem to get rid of, and that's where I need your help.
: : : First, there is a steady 60 Hz hum, even with the volume down. All the paper caps were replaced by a previous owner and check out fine on my capacitor tester. I read that the hum-bucking coil on the speaker might be reversed, but I couldn't figure out where that was! The second symptom is motorboating at higher volumes. I checked the grid load resistors in the audio output stage, and they were 10x the value on the schematic. After a tedious replacement effort, there was no improvement.
: : : Any ideas? I'm out of gas! Thanks!
: : Thanks for the ideas, Don. I had earlier replaced all of the electrolytics as you recommended, but hadn't added the extra resistor with the diode. I'll do that this evening. I've actually replaced the three power electrolytics twice, the second time to make sure I wasn't using a faulty replacement, so I don't thin that's the problem.
: : I'm still unsure about how to get rid of the hum/motorboating. Any other ideas? I'll let you know the results when I add the power resistor this evening.
: : Regards, David
: : : Hi David, Firstly, using a diode to drop the heater voltage in place of a ballast only reduces it to .707 of the mains voltage i.e. 85 volts, not half. So you need some extra resistance to reduce the voltage to the correct level. If your set uses three 6.3 volt tubes (two 78s & 77) and two 25 volt tubes (25Z5 & 43) they add up to approx. 69 volts, still requiring 21 volts drop in a resistor of 70 ohms, 10 watt. 68 or 75 ohms is probably the closest readily available. There will be a lot less heat than from the original resistor. You mentioned replacing all the paper capacitors. The symptoms you're decribing is usually due to failed electrolytics. It's very important to replace these as apart from causing hum and motorboating, they can short and damage the rectifier, etc. Use 10 ?f in place of the 4 & 8 ?f and 22 ?f in place of the 16 ?f, all 160 volt. Leave the original can electrolytics in place for cosmetic appearance but disconnect them and place new flying lead types under the chassis, making sure of the correct polarity.
: : : Don Black.
: : : : Hi! I'm restoring an Emerson 250, the AM-only version of the peak top mantel set. Someone had repaced the heater element with a power resistor that was simply too hot for the small cabinet, so I decided to try replacing it with a 1N4005 diode, as I've seen referenced in ARC and OTB. I've replaced the power supply caps and swapped out the weak tubes, and now the radio plays strong, despite the slightly reduced voltage. There are 2 problems that I can't seem to get rid of, and that's where I need your help.
: : : : First, there is a steady 60 Hz hum, even with the volume down. All the paper caps were replaced by a previous owner and check out fine on my capacitor tester. I read that the hum-bucking coil on the speaker might be reversed, but I couldn't figure out where that was! The second symptom is motorboating at higher volumes. I checked the grid load resistors in the audio output stage, and they were 10x the value on the schematic. After a tedious replacement effort, there was no improvement.
: : : : Any ideas? I'm out of gas! Thanks!
: You said you replaced some resistors. Did you check/replace ALL of them?
: These old buggers like to change values, yet they look great. Denis: Installed the resistor in series with the diode. Some volume reduction as a result, but nothing major. I still get a loud hum at zero volume, which I think I've isolated to the audio stage, but component replacement hasn't eliminated it. Past 3/4 volume, the set squeals and then motorboats. Any suggestions?
: : : Thanks for the ideas, Don. I had earlier replaced all of the electrolytics as you recommended, but hadn't added the extra resistor with the diode. I'll do that this evening. I've actually replaced the three power electrolytics twice, the second time to make sure I wasn't using a faulty replacement, so I don't thin that's the problem.
: : : I'm still unsure about how to get rid of the hum/motorboating. Any other ideas? I'll let you know the results when I add the power resistor this evening.
: : : Regards, David
: : : : Hi David, Firstly, using a diode to drop the heater voltage in place of a ballast only reduces it to .707 of the mains voltage i.e. 85 volts, not half. So you need some extra resistance to reduce the voltage to the correct level. If your set uses three 6.3 volt tubes (two 78s & 77) and two 25 volt tubes (25Z5 & 43) they add up to approx. 69 volts, still requiring 21 volts drop in a resistor of 70 ohms, 10 watt. 68 or 75 ohms is probably the closest readily available. There will be a lot less heat than from the original resistor. You mentioned replacing all the paper capacitors. The symptoms you're decribing is usually due to failed electrolytics. It's very important to replace these as apart from causing hum and motorboating, they can short and damage the rectifier, etc. Use 10 ?f in place of the 4 & 8 ?f and 22 ?f in place of the 16 ?f, all 160 volt. Leave the original can electrolytics in place for cosmetic appearance but disconnect them and place new flying lead types under the chassis, making sure of the correct polarity.
: : : : Don Black.
: : : : : Hi! I'm restoring an Emerson 250, the AM-only version of the peak top mantel set. Someone had repaced the heater element with a power resistor that was simply too hot for the small cabinet, so I decided to try replacing it with a 1N4005 diode, as I've seen referenced in ARC and OTB. I've replaced the power supply caps and swapped out the weak tubes, and now the radio plays strong, despite the slightly reduced voltage. There are 2 problems that I can't seem to get rid of, and that's where I need your help.
: : : : : First, there is a steady 60 Hz hum, even with the volume down. All the paper caps were replaced by a previous owner and check out fine on my capacitor tester. I read that the hum-bucking coil on the speaker might be reversed, but I couldn't figure out where that was! The second symptom is motorboating at higher volumes. I checked the grid load resistors in the audio output stage, and they were 10x the value on the schematic. After a tedious replacement effort, there was no improvement.
: : : : : Any ideas? I'm out of gas! Thanks!
Also double check to see that you did not leave a cold solder joint, or short a lead, or connected a lead to the wrong terminal. Any of these could potentially cause the remaining troubles.
If you want to see if it might be dirt before you proceed with anything major, use some "Rail-Zip" on the tube pins, and maybe try to get a pipe cleaner saturated with the liquid in on the tube contacts. Rail-Zip can be found at most hobby stores that handle model railroad supplies, or try www.walthers.com.
: Thanks for the help. I replaced the caps, but didn't work through all of the resistors. I'll give that a try and report back.
: : You said you replaced some resistors. Did you check/replace ALL of them?
: : These old buggers like to change values, yet they look great. Denis: Installed the resistor in series with the diode. Some volume reduction as a result, but nothing major. I still get a loud hum at zero volume, which I think I've isolated to the audio stage, but component replacement hasn't eliminated it. Past 3/4 volume, the set squeals and then motorboats. Any suggestions?
: : : : Thanks for the ideas, Don. I had earlier replaced all of the electrolytics as you recommended, but hadn't added the extra resistor with the diode. I'll do that this evening. I've actually replaced the three power electrolytics twice, the second time to make sure I wasn't using a faulty replacement, so I don't thin that's the problem.
: : : : I'm still unsure about how to get rid of the hum/motorboating. Any other ideas? I'll let you know the results when I add the power resistor this evening.
: : : : Regards, David
: : : : : Hi David, Firstly, using a diode to drop the heater voltage in place of a ballast only reduces it to .707 of the mains voltage i.e. 85 volts, not half. So you need some extra resistance to reduce the voltage to the correct level. If your set uses three 6.3 volt tubes (two 78s & 77) and two 25 volt tubes (25Z5 & 43) they add up to approx. 69 volts, still requiring 21 volts drop in a resistor of 70 ohms, 10 watt. 68 or 75 ohms is probably the closest readily available. There will be a lot less heat than from the original resistor. You mentioned replacing all the paper capacitors. The symptoms you're decribing is usually due to failed electrolytics. It's very important to replace these as apart from causing hum and motorboating, they can short and damage the rectifier, etc. Use 10 ?f in place of the 4 & 8 ?f and 22 ?f in place of the 16 ?f, all 160 volt. Leave the original can electrolytics in place for cosmetic appearance but disconnect them and place new flying lead types under the chassis, making sure of the correct polarity.
: : : : : Don Black.
: : : : : : Hi! I'm restoring an Emerson 250, the AM-only version of the peak top mantel set. Someone had repaced the heater element with a power resistor that was simply too hot for the small cabinet, so I decided to try replacing it with a 1N4005 diode, as I've seen referenced in ARC and OTB. I've replaced the power supply caps and swapped out the weak tubes, and now the radio plays strong, despite the slightly reduced voltage. There are 2 problems that I can't seem to get rid of, and that's where I need your help.
: : : : : : First, there is a steady 60 Hz hum, even with the volume down. All the paper caps were replaced by a previous owner and check out fine on my capacitor tester. I read that the hum-bucking coil on the speaker might be reversed, but I couldn't figure out where that was! The second symptom is motorboating at higher volumes. I checked the grid load resistors in the audio output stage, and they were 10x the value on the schematic. After a tedious replacement effort, there was no improvement.
: : : : : : Any ideas? I'm out of gas! Thanks!
: Hi,
: If there is still no improvement after all of that, check to see if your 2nd IF transformer has an open secondary, it might still be capacitively coupled if enough windings are intact, and also check your output transformer primary. If you have not tested the tubes, there might also be some dirt, or even a loose element in the second detector/first AF stage, and/or the final AF stage.
: Also double check to see that you did not leave a cold solder joint, or short a lead, or connected a lead to the wrong terminal. Any of these could potentially cause the remaining troubles.
: If you want to see if it might be dirt before you proceed with anything major, use some "Rail-Zip" on the tube pins, and maybe try to get a pipe cleaner saturated with the liquid in on the tube contacts. Rail-Zip can be found at most hobby stores that handle model railroad supplies, or try www.walthers.com.
:
: : Thanks for the help. I replaced the caps, but didn't work through all of the resistors. I'll give that a try and report back.
:
: : : You said you replaced some resistors. Did you check/replace ALL of them?
: : : These old buggers like to change values, yet they look great. Denis: Installed the resistor in series with the diode. Some volume reduction as a result, but nothing major. I still get a loud hum at zero volume, which I think I've isolated to the audio stage, but component replacement hasn't eliminated it. Past 3/4 volume, the set squeals and then motorboats. Any suggestions?
:
: : : : : Thanks for the ideas, Don. I had earlier replaced all of the electrolytics as you recommended, but hadn't added the extra resistor with the diode. I'll do that this evening. I've actually replaced the three power electrolytics twice, the second time to make sure I wasn't using a faulty replacement, so I don't thin that's the problem.
:
: : : : : I'm still unsure about how to get rid of the hum/motorboating. Any other ideas? I'll let you know the results when I add the power resistor this evening.
:
: : : : : Regards, David
:
: : : : : : Hi David, Firstly, using a diode to drop the heater voltage in place of a ballast only reduces it to .707 of the mains voltage i.e. 85 volts, not half. So you need some extra resistance to reduce the voltage to the correct level. If your set uses three 6.3 volt tubes (two 78s & 77) and two 25 volt tubes (25Z5 & 43) they add up to approx. 69 volts, still requiring 21 volts drop in a resistor of 70 ohms, 10 watt. 68 or 75 ohms is probably the closest readily available. There will be a lot less heat than from the original resistor. You mentioned replacing all the paper capacitors. The symptoms you're decribing is usually due to failed electrolytics. It's very important to replace these as apart from causing hum and motorboating, they can short and damage the rectifier, etc. Use 10 ?f in place of the 4 & 8 ?f and 22 ?f in place of the 16 ?f, all 160 volt. Leave the original can electrolytics in place for cosmetic appearance but disconnect them and place new flying lead types under the chassis, making sure of the correct polarity.
: : : : : : Don Black.
:
: : : : : : : Hi! I'm restoring an Emerson 250, the AM-only version of the peak top mantel set. Someone had repaced the heater element with a power resistor that was simply too hot for the small cabinet, so I decided to try replacing it with a 1N4005 diode, as I've seen referenced in ARC and OTB. I've replaced the power supply caps and swapped out the weak tubes, and now the radio plays strong, despite the slightly reduced voltage. There are 2 problems that I can't seem to get rid of, and that's where I need your help.
:
: : : : : : : First, there is a steady 60 Hz hum, even with the volume down. All the paper caps were replaced by a previous owner and check out fine on my capacitor tester. I read that the hum-bucking coil on the speaker might be reversed, but I couldn't figure out where that was! The second symptom is motorboating at higher volumes. I checked the grid load resistors in the audio output stage, and they were 10x the value on the schematic. After a tedious replacement effort, there was no improvement.
:
: : : : : : : Any ideas? I'm out of gas! Thanks!