I recently recapped a Zenith TransOceanic R-600, which has four electrolytic filter caps in a can. The radio worked fine, with no noticable hum, before replacing the filter caps. The negatives of the four filter caps are tied to the can (B-), which is insulated from the chassis.

I decided not to restuff the can and to hang replacements under the chassis, leaving the can in place for ornamentation. So, I cut the negative B- terminal from the can, and haywired the replacement caps from the respective positive terminals. I didn't cut the four positive terminals from the can, figuring that with the old negative terminals cut, the old caps would do no harm. I've done this in the past with no problem.

With the replacement filter caps, the set had a bad hum, which resisted troubleshooting. I decided to try cutting the positive terminals to the old can, and the hum disappeared.

Problem solved, but I can't figure out why leaving the positive terminals energized caused the hum. Any thoughts?

Maybe a short from cap to cap of the old can, and not cap to ground usualy found with old condensers. Just my 2 cents worth.

Thanks, JK, that's a possibility. But if there were such a short, wouldn't it have caused a problem before I replaced the filter caps?

:Thanks, JK, that's a possibility. But if there were such a short, wouldn't it have caused a problem before I replaced the filter caps?

I think that it may have just acted like it tied the 2 sections togather. Putting new caps in paralel with the old ones, i think to leak from cap tp cap, was bypassing the new caps, it was the path of least rsistance. It was easier to bypass the new caps, and tale the high resistance path to the ground. I hope that may make some sense.

Doug,
I suggest that the old caps (B-) remained connected to the can internally, and created a leakage path back into the other circuits. The caps look like an open circuit (high impedance) to the their respective power supply connection point(s)with B- disconnected, and don't neglect the connecting wires to the cap can inducing energy into other wiring. If you removed the wiring at the old cap can to fix the problem, my theory leaks, but if they were removed at the power supply connection point my theory might hold water. Sounds like you might have created 4 antenna and a and a source of radiated energy.

marv

:Thanks, JK, that's a possibility. But if there were such a short, wouldn't it have caused a problem before I replaced the filter caps?

Marv, I disconnected right at the cap can's terminals.

I think JK nailed it. With four different e-caps in the can, all the negative leads were tied together by the can itself. Each positive terminal was at a different voltage (e.g., one at 90V B+ and one at 10V A voltage). So there were all kinds of cross-ties and backfeeds between the positive terminals thru the old caps.

I'm now wondering how I got by with this in the past. The thing that makes a Zenith TransOceanic a little different is the filament circuit. Unlike a normal AC/DC set, the tube's filaments are not connected across the AC line. Instead, the filament feed (about 9V) is derived from the B+ through a voltage-drop resistor. The voltage differential across the e-cap on the filament circuit and the other e-caps is very large.

:I recently recapped a Zenith TransOceanic R-600, which has four electrolytic filter caps in a can. The radio worked fine, with no noticable hum, before replacing the filter caps. The negatives of the four filter caps are tied to the can (B-), which is insulated from the chassis.
:
:I decided not to restuff the can and to hang replacements under the chassis, leaving the can in place for ornamentation. So, I cut the negative B- terminal from the can, and haywired the replacement caps from the respective positive terminals. I didn't cut the four positive terminals from the can, figuring that with the old negative terminals cut, the old caps would do no harm. I've done this in the past with no problem.
:
:With the replacement filter caps, the set had a bad hum, which resisted troubleshooting. I decided to try cutting the positive terminals to the old can, and the hum disappeared.
:
:Problem solved, but I can't figure out why leaving the positive terminals energized caused the hum. Any thoughts?
Hi Doug
Heres a thought, open ground on the original capicitors, which would radiate and enormous amount of AC, which more than likely was picked up by the Audio stages.
Mark

Thanks, Mark. I now realize that just cutting the can's negative terminal is very bad practice. I'm pretty sure that I have seen radios with this repair, performed by old-time repairmen.

I think just cutting the positive terminals is OK, but I realize that it would be frowned upon by those who would restuff the old can.

Nice try everyone, but that´s not it. Here´s the simple reason. Original caps were bad and probably had leakage. They´re all tied together by the can. Any filtering provided by the various resistors within the set is reduced since current is allowed to leak past the resistors through these leaky electrolytics. Current will leak through one leaky condenser, through the can, and then through another leaky condenser, into another circuit, placing incorrect voltages all over the place, and voiding any filtering created by various filtering resistors. These cans often have the filament wiring and cathode wiring and B wiring all being filtered from one can. Leakage can throw incorrect voltages all over the place, especially into the filament wiring. Also, with leaks going from B+ to the cathode and filament wiring (which is at the low end of the B chain), there will be excessive loading of the power supply, which will cause hum. Even if serious loading isn´t taking place, any shortcut around filtering to the filaments will cause serious hum due to the nature of these filaments.

No radiation is taking place. There´s no AC voltage to radiate.

Thomas

Thomas,
Never heard of DC hum, but maybe you're right!!!

:Nice try everyone, but that´s not it. Here´s the simple reason. Original caps were bad and probably had leakage. They´re all tied together by the can. Any filtering provided by the various resistors within the set is reduced since current is allowed to leak past the resistors through these leaky electrolytics. Current will leak through one leaky condenser, through the can, and then through another leaky condenser, into another circuit, placing incorrect voltages all over the place, and voiding any filtering created by various filtering resistors. These cans often have the filament wiring and cathode wiring and B wiring all being filtered from one can. Leakage can throw incorrect voltages all over the place, especially into the filament wiring. Also, with leaks going from B+ to the cathode and filament wiring (which is at the low end of the B chain), there will be excessive loading of the power supply, which will cause hum. Even if serious loading isn´t taking place, any shortcut around filtering to the filaments will cause serious hum due to the nature of these filaments.
:
:No radiation is taking place. There´s no AC voltage to radiate.
:
:Thomas

Well, there is a such thing as direct current hum when current flowing in one direction is pulsating. Does sound kind of odd for DC to have hum to it, though, doesn´t it? At any rate, there´s a certain amount of hum that´s permissable at the output section of the radio and/or right at the rectifier, that isn´t permissable anywhere else in the radio. If this hum leaks its way into the primary amplifier tubes, whether RF or AF, it´ll be greatly amplified. Also, hum to any degree that gets into any of the filaments will cause severe hum, since they are extremely sensitive to slight fluctuations in current. At any rate, the radio is designed with specific filter resistors and circuitry. Various parts tap their current at appropriate places. If you have a bunch of leaky condensers that tie to all of these points and also tie to eachother, proper distribution and filtering won´t be possible. Furthermore, excessive loading of the power supply may take place, which would cause hum since an excessive load would not leave enough excess voltage for the new electrolytics to charge up on properly. They would then fail to fill in the lows in the DC ripple.

Thomas

Also, one more thing that came to mind......even if the electrolytics aren´t leaky, if they are at all still functional (capable of retaining a charge, even if not that originally specified), they will effectively couple across resistors, which will reduce the filtering action of the resistors. To demonstrate this to yourself, take a radio with a field coil or filter resistor in the power supply. Connect across this a condenser of 1 MFD or larger, preferrably larger. If you use an electrolytic, be sure that the positive side faces the positive side of the resistor or field coil. This side would be the side that is closest electrically to the rectifier cathode, regardless of where the coil or resistor is placed (say in a negatively filtered radio where the field coil is on the B- side). The condenser will effectively bridge across the ripple that the field coil or resistor normally eliminated. As we all know, resistors filter out hum, too, which is evident in those radios lacking chokes(such as the transoceanic that started this thread). They have a sort of sponge affect on the circuit even though they do not have an inductive-choking affect. Bridging across a filter resistor or field coil with a condenser will effectively bridge across the ripple that normally gets absorbed by the resistor or coil.

Some of you may have already experienced this when rebuilding the early "tuned" filters in which very low value condensers were used, and a very low value (say .1 MFD) condenser was bridged across the field coil or choke. Originally this condenser worked in resonance with the choke or coil to buffer out hum. Replacing other condensers with much higher values threw off tuning of the supply, and replacing this condenser with a higher value certainly threw off tuning of the coil, creating more hum than if the condenser wasn´t there at all.

Thomas

:Also, one more thing that came to mind......even if the electrolytics aren´t leaky, if they are at all still functional (capable of retaining a charge, even if not that originally specified), they will effectively couple across resistors, which will reduce the filtering action of the resistors. To demonstrate this to yourself, take a radio with a field coil or filter resistor in the power supply. Connect across this a condenser of 1 MFD or larger, preferrably larger. If you use an electrolytic, be sure that the positive side faces the positive side of the resistor or field coil. This side would be the side that is closest electrically to the rectifier cathode, regardless of where the coil or resistor is placed (say in a negatively filtered radio where the field coil is on the B- side). The condenser will effectively bridge across the ripple that the field coil or resistor normally eliminated. As we all know, resistors filter out hum, too, which is evident in those radios lacking chokes(such as the transoceanic that started this thread). They have a sort of sponge affect on the circuit even though they do not have an inductive-choking affect. Bridging across a filter resistor or field coil with a condenser will effectively bridge across the ripple that normally gets absorbed by the resistor or coil.
:
:Some of you may have already experienced this when rebuilding the early "tuned" filters in which very low value condensers were used, and a very low value (say .1 MFD) condenser was bridged across the field coil or choke. Originally this condenser worked in resonance with the choke or coil to buffer out hum. Replacing other condensers with much higher values threw off tuning of the supply, and replacing this condenser with a higher value certainly threw off tuning of the coil, creating more hum than if the condenser wasn´t there at all.
:
:Thomas
HI Thomas
Well all I know is I was always taught that filter capacitors filter out the AC riding on the DC too produce a more pure DC, if there are problems with the filter capacitors then AC will be riding on the DC producing AC Hum.
Mark

Well, to clarify some of the eroneous things which you have been told, alternating current flows in two directions. Direct current flows only in one direction. Hence the name. It may pulsate. It may remain at one specific rate of flow. It never reverses direction, though. The only way you can have AC riding on the DC of the power supply is if your rectifier is faulty, or if radio and/or audio frequencies are induced and/or capacitively coupled into the wiring. The rectifier itself can only pass current in one direction (it can pass alternating current radio frequency interference due to capacitive coupling through the metal pieces of the tube...this capacitance is not enough to pass 60 cycle ALTERNATING current). It will pass the current in your home in ONE DIRECTION, 60 times a second. For half of each of those 60 times, current will flow, and for the other half, current will not flow since the current in your home will be flowing opposite to that allowed through the rectifier for those times. To clarify what I have said, I do not mean that current only flows 30 times a second. I mean that for each cycle of 60 cycle alternating current, current first flows one way and then another. For each half of each of those 60 cycles in which the current flows in the direction favored by the rectifier, current will be allowed through the rectifier. The current will flow in ONE direction, and will pulsate at 60 times a second. With full wave rectification, each rectifier performs as stated above. The two of them (there are usually two in a full wave rectification circuit) are wired out of phase with eachother so that one is always conducting when the other is not. The net result is direct current which pulsates at 120 times a second... for half of the duration of each of these times current flows, and for the other half of the duration of each of these times current does not flow.

The electrolytics in a radio set are charged up every time current flows through the rectifier. Every time current doesn´t flow through the rectifier, current is taken from the electrolytics. The electrolytics fill in the gaps. In most filter circuits, there will be an electrolytic across from the + to - of the circuit. Resistors or chokes will be placed in the circuit to aid in filtering. To simplify the explanation, let´s say that a resistor is placed between the rectifier and the rest of the radio--on the positive side of the circuit. Another electrolytic will be placed after this resistor from the resistor to the negative side of the circuit. The resistor will slow the current draw from the first electrolytic, and the second electrolytic will smooth out any ripple that the first one didn´t. The power supply is set up so that the voltage at the first electrolytic is going to be more than the radio needs. Placing a resistor in series with this point and the rest of the radio reduces the current to the appropriate level for the radio, and, at the same time, allows the electrolytic to charge up a bit more. If a choke were used, it would have one more asset. Magnetic fields that build up and break down within the choke due to voltage ripple, create an effect similar to moving magnets on a generator. These build ups and break downs of magnetic fields generate or induce additional currents within the coil which give the current inertia, and make it want to keep flowing whenever it is at a low, and make it slow down just a bit when it wants to speed up...this further aids in smoothing out ripples in the current. Still, none of the ripples are due to alternating current--current that flows back and forth. They are only due to current that is building up and breaking down in intensity--never changing direction.

Regarding the original radio problem for this thread, the original electrolytics were connected to various points within the B+ circuit, with all of their negatives tied together. This common negative was then connected to the negative side of the B circuit. To simplify things, let´s stick to the one resistor circuit described above. We have an electrolytic prior to this resistor from + to -, and we have an electrolytic after the resistor, on the radio side of the resistor, going from + to -. Both condensers common at the same - point. Now, let´s say that we chop their negative leads from the negative power supply wire, but leave the leads connected together. (We of course put in new electrolytics wired as the old ones were initially wired). The old electrolytics, tied together at their commons, effectively form a condenser across the filter resistor. Condensers are said to pass alternating currents. They can also pass pulsating direct currents provided that they have a way of discharging between pulses. In amplifiers this discharge route is provided by the grid biasing resistor, which leaks off voltage through the B circuit. In the case of our circuit above, the filter resistor forms the leak for the condenser which is connected across it (two condensers wired in series, joined at their commons). Normally the filter resistor would dampen the voltage drawn off of the first electrolytic and off of the rectifier. However, there is a condenser connected across this resistor, which will easily pass a pulse of current. Now, you´d think that since this is direct current, after the pulse went through the condenser once, it´d be charged up, and the condenser wouldn´t pass anymore pulses. However, it is in parallel with a resistor. The resistor will discharge the condenser in between pulses, making it ready for each next pulse. In this way the condenser will pass the full jolt of each DC ripple from the rectifier.

Thomas

:Well, to clarify some of the eroneous things which you have been told, alternating current flows in two directions. Direct current flows only in one direction. Hence the name. It may pulsate. It may remain at one specific rate of flow. It never reverses direction, though. The only way you can have AC riding on the DC of the power supply is if your rectifier is faulty, or if radio and/or audio frequencies are induced and/or capacitively coupled into the wiring. The rectifier itself can only pass current in one direction (it can pass alternating current radio frequency interference due to capacitive coupling through the metal pieces of the tube...this capacitance is not enough to pass 60 cycle ALTERNATING current). It will pass the current in your home in ONE DIRECTION, 60 times a second. For half of each of those 60 times, current will flow, and for the other half, current will not flow since the current in your home will be flowing opposite to that allowed through the rectifier for those times. To clarify what I have said, I do not mean that current only flows 30 times a second. I mean that for each cycle of 60 cycle alternating current, current first flows one way and then another. For each half of each of those 60 cycles in which the current flows in the direction favored by the rectifier, current will be allowed through the rectifier. The current will flow in ONE direction, and will pulsate at 60 times a second. With full wave rectification, each rectifier performs as stated above. The two of them (there are usually two in a full wave rectification circuit) are wired out of phase with eachother so that one is always conducting when the other is not. The net result is direct current which pulsates at 120 times a second... for half of the duration of each of these times current flows, and for the other half of the duration of each of these times current does not flow.
:
:The electrolytics in a radio set are charged up every time current flows through the rectifier. Every time current doesn´t flow through the rectifier, current is taken from the electrolytics. The electrolytics fill in the gaps. In most filter circuits, there will be an electrolytic across from the + to - of the circuit. Resistors or chokes will be placed in the circuit to aid in filtering. To simplify the explanation, let´s say that a resistor is placed between the rectifier and the rest of the radio--on the positive side of the circuit. Another electrolytic will be placed after this resistor from the resistor to the negative side of the circuit. The resistor will slow the current draw from the first electrolytic, and the second electrolytic will smooth out any ripple that the first one didn´t. The power supply is set up so that the voltage at the first electrolytic is going to be more than the radio needs. Placing a resistor in series with this point and the rest of the radio reduces the current to the appropriate level for the radio, and, at the same time, allows the electrolytic to charge up a bit more. If a choke were used, it would have one more asset. Magnetic fields that build up and break down within the choke due to voltage ripple, create an effect similar to moving magnets on a generator. These build ups and break downs of magnetic fields generate or induce additional currents within the coil which give the current inertia, and make it want to keep flowing whenever it is at a low, and make it slow down just a bit when it wants to speed up...this further aids in smoothing out ripples in the current. Still, none of the ripples are due to alternating current--current that flows back and forth. They are only due to current that is building up and breaking down in intensity--never changing direction.
:
:Regarding the original radio problem for this thread, the original electrolytics were connected to various points within the B+ circuit, with all of their negatives tied together. This common negative was then connected to the negative side of the B circuit. To simplify things, let´s stick to the one resistor circuit described above. We have an electrolytic prior to this resistor from + to -, and we have an electrolytic after the resistor, on the radio side of the resistor, going from + to -. Both condensers common at the same - point. Now, let´s say that we chop their negative leads from the negative power supply wire, but leave the leads connected together. (We of course put in new electrolytics wired as the old ones were initially wired). The old electrolytics, tied together at their commons, effectively form a condenser across the filter resistor. Condensers are said to pass alternating currents. They can also pass pulsating direct currents provided that they have a way of discharging between pulses. In amplifiers this discharge route is provided by the grid biasing resistor, which leaks off voltage through the B circuit. In the case of our circuit above, the filter resistor forms the leak for the condenser which is connected across it (two condensers wired in series, joined at their commons). Normally the filter resistor would dampen the voltage drawn off of the first electrolytic and off of the rectifier. However, there is a condenser connected across this resistor, which will easily pass a pulse of current. Now, you´d think that since this is direct current, after the pulse went through the condenser once, it´d be charged up, and the condenser wouldn´t pass anymore pulses. However, it is in parallel with a resistor. The resistor will discharge the condenser in between pulses, making it ready for each next pulse. In this way the condenser will pass the full jolt of each DC ripple from the rectifier.
:
:Thomas
Hi Thomas
In Looking at the definition of an electrolytic capacitor in the elctronics dictionary, heres what it says, they pass AC and block DC, they further improve the DC From factor, in other words they remove any AC ripple in the DC and filter it too ground.

Read any other book and you'll read what I said. The electrolytics don't filter out anything. They retain charges from every peak of the rippling DC. The retained charge fills in each low. There's no AC on a rectifier. Rectifiers only pass current one way.

to,
too,
two

:Read any other book and you'll read what I said. The electrolytics don't filter out anything. They retain charges from every peak of the rippling DC. The retained charge fills in each low. There's no AC on a rectifier. Rectifiers only pass current one way.
:
:to,
:too,
:two
:Hi Thomas
Well then perhaps you can enlighten me as too what the Hum is in a tube radio when the Power Supply Filter capacitors are bad? I've never heard of such a thing as DC Hum.
Mark

:

:Read any other book and you'll read what I said. The electrolytics don't filter out anything. They retain charges from every peak of the rippling DC. The retained charge fills in each low. There's no AC on a rectifier. Rectifiers only pass current one way.
:
:to,
:too,
:two
:Hi Thomas
Sooo sorry! you will have too excuse an old fool who has not delt with theory for awhile, dug back in my info and discovered you are right, the hum is produced by the ripple voltage from the rectifier when the filter capacitors are defective, guess thats what happens when you don't use electronic theory for a long long time:)
Mark
:

Hi Mark:
TO add a little note here the TWO (2) of us, Thomas and I, are getting TOO confused with your use of the words (to,two,too)
very confusing.

I get all mixed up if you use too when you mean to.

Please help me... ...lol

just kidding but sincere...lol

Congratulations.