Crosley D25 Hum

12/21/2005 1:16:01 PMStan L

I have recapped this radio and tested / replaced tubes and resistors as necessary. It hums on power up. The hum is affected by the volume control. The IF cans seem to be ok as there is no positive voltage and no static. The socket voltages are in tolerance. The loop antenna measures 2.7 Ohms. I did notice that when checking voltages, when touching pin 2 of the 12SQ7, the volume of the hum greatly increased. I'm sure it is something simple (it always is when you find it out). Any suggestions??

12/21/2005 2:07:16 PMrghines1

Hi Stan,
Getting a loud hum when you touch pin 2 of the 12SQ7 is normal. The 12SQ7 tube is a possible candidate for hum via coupling through heater filament to the control grid or cathode. Give another tube a try. Think this is a metal tube, therefore pin 1 needs a good condition to the chassis to shield properly.

Richard

12/21/2005 2:12:18 PMThomas Dermody

Does the hum go away completely when the volume control is turned down? Try shorting pin 2 of the 12SQ7 to pin 3. Does the hum go away? If so, then you have a couple of possible problems. If you disconnect the lead of the 2nd IF transformer that goes to the high side of the volume control and the hum goes away, then you have trouble in the RF section. If it doesn't go away, you may want to look for bad parts associated with pin 2 of the 12SQ7 tube and the volume control. Disconnect all leads from the volume control. Check for leakage between the terminals and the case. If there is leakage, you can disassemble the control and clean it in soap and water. Then go to your automotive store and purchase di-electric grease. Put a very thin film of this onto the carbon element. Reassemble control.

Be sure that pin 1 of the 12SQ7 is connected to the chassis. Be sure that no power wires run near pin 2 or its associated condenser and resistor. Be sure that the resistor is connected either to pin 3 or to some other B- source. If you find that the condenser is picking up interference, you can try wrapping it with wire. Connect this wire to pin 3. Also make sure that the tube is well shielded.

I noticed in your schematic a few rather odly connected components. These would be C11, R4, and R5. Notice how the condenser goes from B+ to a divider resistor network which connects to the grid of the 12SQ7 tube. Normally this couple of resistors would be just one resistor which is used for biasing the grid. I am assuming that this is a sort of hum bucking assembly, as it is only used without the electrodynamic speaker. An electrodynamic speaker usually has its own hum bucking coil built into the speaker which nulifies hum by inserting hum into the voice coil which is out of phase with the hum coming from the output tube. Check the condenser (C11) for shorts and leakage. Also check the resistors for opens and drifting. You may even want to try disconnecting C11. See if the hum goes away. With this circuit in proper working order, though, it should not cause trouble. If it is causing trouble, one of the components is likely at fault.

Be sure that the chassis is connected to B- through the condenser shown in the schematic.

If you have trouble in the RF section, the most likely cause of the hum is a poorly connected antenna. You say that you replaced all filter condensers, so this is not likely your trouble. There could be stray interference getting into one of the grids. Adequate shielding will prevent this. This is most likely not your problem, though, due to the low impedance coils used in the RF section.

Thomas

12/21/2005 5:53:48 PMStan L

:Yes to first two questions. I should have mentioned at the start that the hum is ALL I get...no static or anything. I did try the old Norm Leal suggestion of trying to back into alignment by starting with the oscilator and backing through the the IF stages. I managed the faintest of signals around 1600 but lost it. Obviously, I'm not able to get a diggen signal through either. Maybe I should just set it aside for awhile...I was so excited about fixing the clock I may have overlooked something in the radio.
:
:Does the hum go away completely when the volume control is turned down? Try shorting pin 2 of the 12SQ7 to pin 3. Does the hum go away? If so, then you have a couple of possible problems. If you disconnect the lead of the 2nd IF transformer that goes to the high side of the volume control and the hum goes away, then you have trouble in the RF section. If it doesn't go away, you may want to look for bad parts associated with pin 2 of the 12SQ7 tube and the volume control. Disconnect all leads from the volume control. Check for leakage between the terminals and the case. If there is leakage, you can disassemble the control and clean it in soap and water. Then go to your automotive store and purchase di-electric grease. Put a very thin film of this onto the carbon element. Reassemble control.
:
:Be sure that pin 1 of the 12SQ7 is connected to the chassis. Be sure that no power wires run near pin 2 or its associated condenser and resistor. Be sure that the resistor is connected either to pin 3 or to some other B- source. If you find that the condenser is picking up interference, you can try wrapping it with wire. Connect this wire to pin 3. Also make sure that the tube is well shielded.
:
:I noticed in your schematic a few rather odly connected components. These would be C11, R4, and R5. Notice how the condenser goes from B+ to a divider resistor network which connects to the grid of the 12SQ7 tube. Normally this couple of resistors would be just one resistor which is used for biasing the grid. I am assuming that this is a sort of hum bucking assembly, as it is only used without the electrodynamic speaker. An electrodynamic speaker usually has its own hum bucking coil built into the speaker which nulifies hum by inserting hum into the voice coil which is out of phase with the hum coming from the output tube. Check the condenser (C11) for shorts and leakage. Also check the resistors for opens and drifting. You may even want to try disconnecting C11. See if the hum goes away. With this circuit in proper working order, though, it should not cause trouble. If it is causing trouble, one of the components is likely at fault.
:
:Be sure that the chassis is connected to B- through the condenser shown in the schematic.
:
:If you have trouble in the RF section, the most likely cause of the hum is a poorly connected antenna. You say that you replaced all filter condensers, so this is not likely your trouble. There could be stray interference getting into one of the grids. Adequate shielding will prevent this. This is most likely not your problem, though, due to the low impedance coils used in the RF section.
:
:Thomas

12/22/2005 2:27:40 PMDon

Stan,

Perhaps a silly question but, are you certain that your radio-phono switch is set properly? Sometimes its the obvious stuff that messes things up.

Don

12/22/2005 6:09:48 PMStan Laramore

Hi Don
your post prompted me to recheck the switch an it was wired wrong. When I first took the back off, the switch wires more of less just fell off their connections. I grabbed a pencil and market the redstripe wire on the left side and the yellow wire on the right side. I did nor realize thet the yellow wire actually cane from the top switch terminal. Great catch Don. PS ever tried to align a phonograph?? ;>) Thanks to all...it plays GREAT!
:Stan,
:
:Perhaps a silly question but, are you certain that your radio-phono switch is set properly? Sometimes its the obvious stuff that messes things up.
:
:Don

12/22/2005 8:46:33 PMDon

I can't count the number of times thatI've wasted hours of time trying to remedy problems that were caused by my own ham hands.. Glad to hear this was a simple one!

Merry Christmas
Don

12/21/2005 2:48:38 PMPoston Drake

Hi Stan,
Some excellent suggestions by Thomas. Take his advice and check the areas he has mentioned.

One other thing, I almost hate to mention it, but are you absolutely certain that the hum is originating from the radio, and not from some external source? Be sure and rule out this possibility before undertaking extensive troubleshooting.

By the way, one way to narrow down the offending stage in a case like this is to remove all the tubes except rectifier and output tubes. Power up radio and see if hum remains. If so, then it has to be coming from the AF output stage or the power supply. If hum goes away, insert 1st AF tube and try again. Continue replacing tubes one at a time, backward through the stages, until the hum is first detected. This will identify the stage where the trouble is located, you can then go through it thoroughly using some of the tips Thomas has mentioned.

Poston

:I have recapped this radio and tested / replaced tubes and resistors as necessary. It hums on power up. The hum is affected by the volume control. The IF cans seem to be ok as there is no positive voltage and no static. The socket voltages are in tolerance. The loop antenna measures 2.7 Ohms. I did notice that when checking voltages, when touching pin 2 of the 12SQ7, the volume of the hum greatly increased. I'm sure it is something simple (it always is when you find it out). Any suggestions??

12/21/2005 5:16:34 PMThomas Dermody

Keep in mind that if you pull any of the tubes from and AC-DC set, the remaining ones won't light. With that in mind, if you wish to cut out the RF section, you must actually go under the chassis and disconnect the B+ wire from this section.

Thomas

12/22/2005 1:28:20 AMPoston Drake

Stan,
Sorry, when I made my earlier post I had overlooked the fact that your D25 was an AC/DC radio. But here is how you handle it in that case:

Instead of removing any tubes, short the grid of the output tube to common negative. If you still hear hum, this indicates problems in either the AF output stage or the power supply. If not, remove the jumper and short the grid of the 1st AF tube, then likewise backward through the stages.

Poston

:Keep in mind that if you pull any of the tubes from and AC-DC set, the remaining ones won't light. With that in mind, if you wish to cut out the RF section, you must actually go under the chassis and disconnect the B+ wire from this section.
:
:Thomas