Hi,

I am repairing a Silvertone Model 18 AM/FM radio. I replaced the filter caps and it performs great on AM. No hum, plenty of volume, great sensitivity. On the FM band it only receivs one strong local station with an external FM antenna. I have not done anything else to the radio because it was running so good on AM. Is there a certian part of the circuit that I should be looking at in terms of poor performance/senitivity on FM?

Thanks,

Jeff

Hi Jeff,
There are a few ideas that come to mind:

1. If your antenna is Y-shaped (similar to what is used on a Hi-Fi receiver), you may want to try positioning the antenna so one section of the "Y" is mounted up, while the other section is hanging down.

(I tried this on my Stewart-Warner AM-FM, and it really improved the FM reception.)

2. Check your B+ voltage and make sure it's within specs. A low B+ reading will cause poor FM reception. Replacing the filter caps is a good start; however, to increase the B+, you may want to replace all of the capacitors especially the wax-paper types. (If there are any that are deteriorating, they may cause the B+ to decrease.)

3. I'm not sure if your set is AC/DC. If it is, it may be either the rectifier tube or the selenium rectifier. If it has a selenium, you can substitute a modern-day silicon diode (such as a 1N4004) with a 47 Ohm 2-Watt resistor in series.

It's surprising how replacing a selenium rectifier will increase the B+.

Good luck,
Mark

:Hi,
:
:I am repairing a Silvertone Model 18 AM/FM radio. I replaced the filter caps and it performs great on AM. No hum, plenty of volume, great sensitivity. On the FM band it only receivs one strong local station with an external FM antenna. I have not done anything else to the radio because it was running so good on AM. Is there a certian part of the circuit that I should be looking at in terms of poor performance/senitivity on FM?
:
:Thanks,
:
:Jeff

Hi,
One other thing to check that Mark didn't, is that detector diodes might be aging. Not common, but aggravating when it does. They tend to be unpredictable in how they fail, but often they lose reverse resistance and fail to act as rectifiers.

If everything Mark has suggested yields no significant improvement, and there seems to be a "high noise floor" (hiss), then check the diodes in the FM detector.

:Hi Jeff,
:There are a few ideas that come to mind:
:
:1. If your antenna is Y-shaped (similar to what is used on a Hi-Fi receiver), you may want to try positioning the antenna so one section of the "Y" is mounted up, while the other section is hanging down.
:
:(I tried this on my Stewart-Warner AM-FM, and it really improved the FM reception.)
:
:2. Check your B+ voltage and make sure it's within specs. A low B+ reading will cause poor FM reception. Replacing the filter caps is a good start; however, to increase the B+, you may want to replace all of the capacitors especially the wax-paper types. (If there are any that are deteriorating, they may cause the B+ to decrease.)
:
:3. I'm not sure if your set is AC/DC. If it is, it may be either the rectifier tube or the selenium rectifier. If it has a selenium, you can substitute a modern-day silicon diode (such as a 1N4004) with a 47 Ohm 2-Watt resistor in series.
:
:It's surprising how replacing a selenium rectifier will increase the B+.
:
:Good luck,
:Mark
:
:
::Hi,
::
::I am repairing a Silvertone Model 18 AM/FM radio. I replaced the filter caps and it performs great on AM. No hum, plenty of volume, great sensitivity. On the FM band it only receivs one strong local station with an external FM antenna. I have not done anything else to the radio because it was running so good on AM. Is there a certian part of the circuit that I should be looking at in terms of poor performance/senitivity on FM?
::
::Thanks,
::
::Jeff

:Hi,
:One other thing to check that Mark didn't, is that detector diodes might be aging. Not common, but aggravating when it does. They tend to be unpredictable in how they fail, but often they lose reverse resistance and fail to act as rectifiers.
:
:If everything Mark has suggested yields no significant improvement, and there seems to be a "high noise floor" (hiss), then check the diodes in the FM detector.

Detector diodes in this case are part of the 6T8 tube.
-Bill M

Hi

Just something to add about 6T8. This tube has three sections. I have seen quite a few with one section of filament burned out. Be sure all three sections light.

Norm

::Hi,
::One other thing to check that Mark didn't, is that detector diodes might be aging. Not common, but aggravating when it does. They tend to be unpredictable in how they fail, but often they lose reverse resistance and fail to act as rectifiers.
::
::If everything Mark has suggested yields no significant improvement, and there seems to be a "high noise floor" (hiss), then check the diodes in the FM detector.
:
:Detector diodes in this case are part of the 6T8 tube.
:-Bill M
: