Firestone-Rider 18-8 Replaced All capacitors. The selenium rectifier did not work, using a 1n007 diode in series along with a resistor 10 ohm/1-watt, radio plays well but has what repair book calls "motor boating sound" not a humm along with stations I listen to. 19t8 tube grid pin has neg reading, all tubes,transformer, chokes & resistors checked ok, any ideas???

:Firestone-Rider 18-8 Replaced All capacitors. The selenium rectifier did not work, using a 1n007 diode in series along with a resistor 10 ohm/1-watt, radio plays well but has what repair book calls "motor boating sound" not a humm along with stations I listen to. 19t8 tube grid pin has neg reading, all tubes,transformer, chokes & resistors checked ok, any ideas???
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Almost all motorboating is due to faulty filter capacitors in an otherwist stable set. Have you changed them all?

::Firestone-Rider 18-8 Replaced All capacitors. The selenium rectifier did not work, using a 1n007 diode in series along with a resistor 10 ohm/1-watt, radio plays well but has what repair book calls "motor boating sound" not a humm along with stations I listen to. 19t8 tube grid pin has neg reading, all tubes,transformer, chokes & resistors checked ok, any ideas???
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:Almost all motorboating is due to faulty filter capacitors in an otherwist stable set. Have you changed them all?
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YES

This one has a 4 section electrolytic. Make sure the values are correct and connected correctly. The 40 MDF goes before the field coil, the 20 MFD after the coil.

Antenna connected properly? Antenna or grid wires near any plate or speaker wires? Are the original electrolytics removed from the circuit? Usually motorboating caused by electrolytics is due to developed internal resistance, which would be bypassed if new ones were paralleled on the old ones, but it is still a good idea to remove the old ones from the circuit.

In any case, keep grid and plate wires away from eachother, and keep speaker wires away from antenna wires and all other signal wires within the set. If you want you can short out the control grid of each stage to see where the motorboating lies, or if it is throughout the set. Shorting out a particular grid will eliminate stages prior to and including the stage containing the shorted grid, but may also include stages after that stage if feedback is taking place between stages prior to and after that stage.

T.

:This one has a 4 section electrolytic. Make sure the values are correct and connected correctly. The 40 MDF goes before the field coil, the 20 MFD after the coil.
:Warren; Hi there are 5 electrolytics C-28 I'm reading as one & placed neg (think I'm correct) to chassis.

Agree with all. One of the old timers (before I became an old timer myself) taught me to work backwards on the radio. The speaker (s**t) came out the back, and the food came from the antenna. You are the doctor.

Now, then, assuming that this set functioned well for many decades, there is something going on in at least one stage. You cannot rule out that one or more replacement capacitors might bad. Other than the first one, you can bridge another one in parallel and see if it clears up. The voltage dividers usually go up in value, not down, so probably it's not them. Assume you replaced all the paper caps too. Finally, beware of the B- an chassis references for measuring and connecting. They are not always the same.

::This one has a 4 section electrolytic. Make sure the values are correct and connected correctly. The 40 MDF goes before the field coil, the 20 MFD after the coil.
::Warren; Hi there are 5 electrolytics C-28 I'm reading as one & placed neg (think I'm correct) to chassis.
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Are you sure that the filter cap negative goes to chassis?
Usually AC/DC radios run a B- bus (on/off switch lug common?) for the return side of the power supply and keep the chassis isolated from the power supply for safety.

:Agree with all. One of the old timers (before I became an old timer myself) taught me to work backwards on the radio. The speaker (s**t) came out the back, and the food came from the antenna. You are the doctor.
:
:Now, then, assuming that this set functioned well for many decades, there is something going on in at least one stage. You cannot rule out that one or more replacement capacitors might bad. Other than the first one, you can bridge another one in parallel and see if it clears up. The voltage dividers usually go up in value, not down, so probably it's not them. Assume you replaced all the paper caps too. Finally, beware of the B- an chassis references for measuring and connecting. They are not always the same.
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:::This one has a 4 section electrolytic. Make sure the values are correct and connected correctly. The 40 MDF goes before the field coil, the 20 MFD after the coil.
:::Warren; Hi there are 5 electrolytics C-28 I'm reading as one & placed neg (think I'm correct) to chassis.
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: Are you sure that the filter cap negative goes to chassis?
: Usually AC/DC radios run a B- bus (on/off switch lug common?) for the return side of the power supply and keep the chassis isolated from the power supply for safety.
:
::Agree with all. One of the old timers (before I became an old timer myself) taught me to work backwards on the radio. The speaker (s**t) came out the back, and the food came from the antenna. You are the doctor.
::
::Now, then, assuming that this set functioned well for many decades, there is something going on in at least one stage. You cannot rule out that one or more replacement capacitors might bad. Other than the first one, you can bridge another one in parallel and see if it clears up. The voltage dividers usually go up in value, not down, so probably it's not them. Assume you replaced all the paper caps too. Finally, beware of the B- an chassis references for measuring and connecting. They are not always the same.
::
::::This one has a 4 section electrolytic. Make sure the values are correct and connected correctly. The 40 MDF goes before the field coil, the 20 MFD after the coil.
::::Warren; Hi there are 5 electrolytics C-28 I'm reading as one & placed neg (think I'm correct) to chassis.
:In reply to Walters answer I have all the electrolytics neg to ground, but the thought had crossed my mind that it might be pos to ground. The radio is a Firestone #4A12 in rider 18-7to10, 2 sets of electrolytics C-33 & C-28.Can some take a look??::
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C-33 All negative leads go to chassis ground.

:C-33 All negative leads go to chassis ground.
:There is C-28 also,neg to ground?

Yes, that's right. Same negative chassis ground
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