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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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Old Audio Receiver Hum?
Recently inherited an old Scott AM/FM Receiver model 319RA (no tubes)
that had been stored for a very long time. Checked all the fuses and none were blown so turned it on and heard loud power supply hum using headphones over the entire Volume Control range (even when set to minimum). Appreciate any suggestions and tips from someone who has had experience restoring old audio amps and receivers. Thanks Gene |
#2
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wrote in message oups.com... Recently inherited an old Scott AM/FM Receiver model 319RA (no tubes) that had been stored for a very long time. Checked all the fuses and none were blown so turned it on and heard loud power supply hum using headphones over the entire Volume Control range (even when set to minimum). Appreciate any suggestions and tips from someone who has had experience restoring old audio amps and receivers. Thanks Gene The power supply smoothing capacitor will have dried out. Replace it with a similar value and all will be well. You hear the hum all the time as it is the signal input to the power amp that is varied by the volume control, not the output of the amp itself. The other point is that you will hear a lot of noise and hum if the headphone connection is directly across the loudspeaker (in effect) with no padding. Put a resistor of 220R-330R in series with the headphones (each channel if stereo) and the noise will disappear as if by magic. -- Woody harrogate2 at ntlworld dot com |
#3
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Sounds like the main filter cap has dried up, look at the
power supply you should see a large electrolytic filter cap. Change that with a new one that should fix you up. Failing that check all the filter caps. kip |
#4
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What test tools do you have?? DVM, soldering iron, scope, etc?
Safety training? there are usually unsafe 120vAC points in older equipment. The household power is dangerous because at 60 cycles/second if you get a shock it will try to make your heart go at 60/second instead of about 60 per minute. The individual heart muscles will fibrilate in different directions, out of sync, many times unable to recover. Every house should own a defibrillator device, they are cheap these days wrote in message oups.com... Recently inherited an old Scott AM/FM Receiver model 319RA (no tubes) that had been stored for a very long time. Checked all the fuses and none were blown so turned it on and heard loud power supply hum using headphones over the entire Volume Control range (even when set to minimum). Appreciate any suggestions and tips from someone who has had experience restoring old audio amps and receivers. Thanks Gene |
#5
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"Bradley1234" wrote in message news:68YCd.16730$Y57.13606@trnddc08... What test tools do you have?? DVM, soldering iron, scope, etc? Safety training? there are usually unsafe 120vAC points in older equipment. The household power is dangerous because at 60 cycles/second if you get a shock it will try to make your heart go at 60/second instead of about 60 per minute. The individual heart muscles will fibrilate in different directions, out of sync, many times unable to recover. Every house should own a defibrillator device, they are cheap these days What a load of crap! The danger from electricity is NOTHING to do with supply frequency - it is all about current. A current of 30mA - that is 0.03A - passing through your body across your heart will usually be fatal. 10mA may cause problems, but in a healthy person will usually not be fatal. 120V (to earth) as used in the US is specifically chosen since it is rarely high enough to cause 30mA current flow through the body: by the same token here in the UK and across most of Europe, 110V (actually 55-0-55) is used professionally for power tools etc. However our normal mains is 220V in Europe, 240V in the UK (that is to earth) and can be fatal. I understand that in the US 240V is available across both supply lines but it is actually 120-0-120 and thus much less dangerous. -- Woody harrogate2 at ntlworld dot com |
#6
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"harrogate2" wrote in message ... What a load of crap! The danger from electricity is NOTHING to do with supply frequency - it is all about current. A current of 30mA - that is 0.03A - passing through your body across your heart will usually be fatal. 10mA may cause problems, but in a healthy person will usually not be fatal. 120V (to earth) as used in the US is specifically chosen since it is rarely high enough to cause 30mA current flow through the body: by the same token here in the UK and across most of Europe, 110V (actually 55-0-55) is used professionally for power tools etc. The number of people who believe that normal household current is not lethal or that powerlines are insulated and do not pose a hazard is alarming. Electrocutions may result from contact with an object as seemingly innocuous as a broken light bulb or as lethal as an overhead powerline, and have affected workers since the first electrical fatality was recorded in France in 1879 when a stage carpenter was killed by an alternating current of 250 volts. 2 However our normal mains is 220V in Europe, 240V in the UK (that is to earth) and can be fatal. I understand that in the US 240V is available across both supply lines but it is actually 120-0-120 and thus much less dangerous. -- Woody harrogate2 at ntlworld dot com My my, you seem to be very educated in this subject if what I said was a load of (incorrect information.) Can you explain what happens to the heart to make it stop? Why doesnt a defibrillator, which probably jolts more than 30mA, cause a person to die? It would be proper for me to quote sources, rather than assume; When current greater than the 16 mA "let go current" passes through the forearm, it stimulates involuntary contraction of both flexor and extensor muscles. When the stronger flexors dominate, victims may be unable to release the energized object they have grasped as long as the current flows. If current exceeding 20 mA continues to pass through the chest for an extended time, death could occur from respiratory paralysis. Currents of 100 mA or more, up to 2 Amps, may cause ventricular fibrillation, probably the most common cause of death from electric shock. 11 Ventricular fibrillation is the uneven pumping of the heart due to the uncoordinated, asynchronous contraction of the ventricular muscle fibers of the heart that leads quickly to death from lack of oxygen to the brain. Ventricular fibrillation is terminated by the use of a defibrillator, which provides a pulse shock to the chest to restore the heart rhythm. 11. Dalziel CF, Lee WR [1968]. Re-evaluation of Lethal Electric Currents. IEEE Trans. Ind. Gen. Appl. IGA-4:467-476. http://www.cdc.gov/niosh/pdfs/98-131.pdf |
#7
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"Bradley1234" wrote in message news:6p_Cd.15961$hc7.10745@trnddc06... | The number of people who believe that normal household current is not | lethal or that powerlines are insulated and do not pose a hazard is | alarming. Electrocutions may result from contact with an object as seemingly | innocuous as a broken light bulb or as lethal as an overhead powerline, and | have affected workers since the first electrical fatality was recorded in | France in 1879 when a stage carpenter was killed by an alternating current | of 250 volts. I heard of 80 VDC from an aircraft starter killing one guy under normal (non-hospital) conditions. N |
#8
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Capacitors. Replace 'em.
Recently inherited an old Scott AM/FM Receiver model 319RA (no tubes) that had been stored for a very long time. Checked all the fuses and none were blown so turned it on and heard loud power supply hum using headphones over the entire Volume Control range (even when set to minimum). Appreciate any suggestions and tips from someone who has had experience restoring old audio amps and receivers. Thanks Gene webpa |
#9
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With old equipment....go for straight for the Caps.
BobbyB Tucson, AZ |
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