I'll admit that I'm not as versed in FM circuits as AM. Okay, not at all. Still, I'd like to fix this one. I don't suppose there's an "FM for Dummies"?
Anyway, any suggestions would be appreciated.
Terry
:I'll admit that I'm not as versed in FM circuits as AM. Okay, not at all. Still, I'd like to fix this one. I don't suppose there's an "FM for Dummies"?
:Anyway, any suggestions would be appreciated.
:Terry
:
Johnnysan-
::I'll admit that I'm not as versed in FM circuits as AM. Okay, not at all. Still, I'd like to fix this one. I don't suppose there's an "FM for Dummies"?
::Anyway, any suggestions would be appreciated.
::Terry
::
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:Troubleshooting should be pretty much the same for FM; the signal has to be stopping somewhere. Sounds like a bad connection/open coil. Do you have an FM generator and/or oscilloscope?
:Johnnysan-
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My fear is that I've accidentally moved a coil or other component while subbing the caps.
T.
Get the set operating: |
:::I picked up a nice Silvertone AM/FM model 18 on eBay. When I got it the FM was weak. After replacing a couple of caps, (no where near the tuner section) I lost the FM entirely. There was a discussion a few weeks ago where the book "FM simplified" was recommended. I bought it from Amazon. If this is FM simplified, I'd hate see FM complicated. This is a college text book full of highly technical stuff. I know because there was test left in it from El Camino College. The guy got a 95. ::Troubleshooting should be pretty much the same for FM; the signal has to be stopping somewhere. Sounds like a bad connection/open coil. Do you have an FM generator and/or oscilloscope? ::
:::I'll admit that I'm not as versed in FM circuits as AM. Okay, not at all. Still, I'd like to fix this one. I don't suppose there's an "FM for Dummies"?
:::Anyway, any suggestions would be appreciated.
:::Terry
:::
::
::
::
::Johnnysan-
::
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::
:
:Scope, yes- I also have a Heathkit IG-107 generator that goes to 107mhz.
:My fear is that I've accidentally moved a coil or other component while subbing the caps.
:T.
:
:
: : : :  : : : : : : : : :Sir Johnny . . . . . : : :Ready to go for an FM sleigh ride around the amusement park? : : :Initially I am assuning that AM is fine and that should clear most of intra mode band switching action, along with your set having a quasi-operative power supply. : : :Replace the Ray-Show Detectivator summing capacitor . . C-29 . . if not having done 'dat already. : : :If you have another 6T8 to sub in, would be nice . . . that toobie is very prone to spot welds popping open and loosing an elements inter connection. : : :An evaluative work around is to additionally solder tack on two 1n914---1n4148 *** silicon diodes across the FM diode sections of the 6T8 at the socket pins already soldered connections. : : :Diode #1 . . . cathode (banded end) to 6T8 pin 3 . . .anode goes to pin 2. : : :Diode #1 . . . cathode (banded end) to 6T8 pin 7 . . .anode goes to pin 1. : : :*** ( Common as dirt in SS electronics design / construction since the late '60's.) : : :Then: : : :Confirm ohmmic continuity of FM . .T5 transformers primary and its separate 3 secondary connections . : : :(Your earlier stages T3 tranfo must be intact or else the AM could not be getting passed through.) : : : :Get the set operating: : : :Now lets divide and conquer, to see where the FM aspect is being lost. : : :Initially take DC metering and see what voltage level is present at the 1st grid of the limiter stage. : : :That would be pin #1 of the 6BA6 stage JUST in front of your 6T8. : : :Being a h i i i i i gh Z test point use either a VTVM or a 20-50 k per volt sensitivity . . . VOM for metering. : : :Expect to be finding a negative voltage, which will increase in level as you take the tuning condenser, and tune into, and then peak upon a FM station. : : :Then it will diminish in level as you start tuning off from it. : : :The HIGHER the negative voltage level, will be indicative of the STRONGER the received signal. : : :If none of the above are occurring , we needs to stops and pulls out our Crayolas and go back to the drawing boards, as the FM signal is not being converted or making it to this stage. : : :You can additionally try the same testing at the 1st grid of the 6BA6 1st FM IF, but the potential voltage swing will not be nearly as pronounced. : : :Standing by for feedback . . . to see if the signal is even making it down to those tested stages. : : : : : :73's de Edd : : :  : |
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::::I picked up a nice Silvertone AM/FM model 18 on eBay. When I got it the FM was weak. After replacing a couple of caps, (no where near the tuner section) I lost the FM entirely. There was a discussion a few weeks ago where the book "FM simplified" was recommended. I bought it from Amazon. If this is FM simplified, I'd hate see FM complicated. This is a college text book full of highly technical stuff. I know because there was test left in it from El Camino College. The guy got a 95.
::::I'll admit that I'm not as versed in FM circuits as AM. Okay, not at all. Still, I'd like to fix this one. I don't suppose there's an "FM for Dummies"?
::::Anyway, any suggestions would be appreciated.
::::Terry
::::
:::
:::
:::
:
:::Troubleshooting should be pretty much the same for FM; the signal has to be stopping somewhere. Sounds like a bad connection/open coil. Do you have an FM generator and/or oscilloscope?
:::Johnnysan-
:::
:::
:
:::
:::
::
::Scope, yes- I also have a Heathkit IG-107 generator that goes to 107mhz.
::My fear is that I've accidentally moved a coil or other component while subbing the caps.
::T.
::
:
Johnnysan-
::
:: :: :: :: :: :: :: :: :: :: :: :: ::Sir Johnny . . . . . :: :: ::Ready to go for an FM sleigh ride around the amusement park? :: :: ::Initially I am assuning that AM is fine and that should clear most of intra mode band switching action, along with your set having a quasi-operative power supply. :: :: ::Replace the Ray-Show Detectivator summing capacitor . . C-29 . . if not having done 'dat already. :: :: ::If you have another 6T8 to sub in, would be nice . . . that toobie is very prone to spot welds popping open and loosing an elements inter connection. :: :: ::An evaluative work around is to additionally solder tack on two 1n914---1n4148 *** silicon diodes across the FM diode sections of the 6T8 at the socket pins already soldered connections. :: :: ::Diode #1 . . . cathode (banded end) to 6T8 pin 3 . . .anode goes to pin 2. :: :: ::Diode #1 . . . cathode (banded end) to 6T8 pin 7 . . .anode goes to pin 1. :: :: ::*** ( Common as dirt in SS electronics design / construction since the late '60's.) :: :: ::Then: :: :: ::Confirm ohmmic continuity of FM . .T5 transformers primary and its separate 3 secondary connections . :: :: ::(Your earlier stages T3 tranfo must be intact or else the AM could not be getting passed through.) :: :: :: ::Get the set operating: :: :: ::Now lets divide and conquer, to see where the FM aspect is being lost. :: :: ::Initially take DC metering and see what voltage level is present at the 1st grid of the limiter stage. :: :: ::That would be pin #1 of the 6BA6 stage JUST in front of your 6T8. :: :: ::Being a h i i i i i gh Z test point use either a VTVM or a 20-50 k per volt sensitivity . . . VOM for metering. :: :: ::Expect to be finding a negative voltage, which will increase in level as you take the tuning condenser, and tune into, and then peak upon a FM station. :: :: ::Then it will diminish in level as you start tuning off from it. :: :: ::The HIGHER the negative voltage level, will be indicative of the STRONGER the received signal. :: :: ::If none of the above are occurring , we needs to stops and pulls out our Crayolas and go back to the drawing boards, as the FM signal is not being converted or making it to this stage. :: :: ::You can additionally try the same testing at the 1st grid of the 6BA6 1st FM IF, but the potential voltage swing will not be nearly as pronounced. :: :: ::Standing by for feedback . . . to see if the signal is even making it down to those tested stages. :: :: :: :: :: ::73's de Edd :: :: :: :: |
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:::::I picked up a nice Silvertone AM/FM model 18 on eBay. When I got it the FM was weak. After replacing a couple of caps, (no where near the tuner section) I lost the FM entirely. There was a discussion a few weeks ago where the book "FM simplified" was recommended. I bought it from Amazon. If this is FM simplified, I'd hate see FM complicated. This is a college text book full of highly technical stuff. I know because there was test left in it from El Camino College. The guy got a 95.
:::::I'll admit that I'm not as versed in FM circuits as AM. Okay, not at all. Still, I'd like to fix this one. I don't suppose there's an "FM for Dummies"?
:::::Anyway, any suggestions would be appreciated.
:::::Terry
:::::
::::
::::
::::
::
::::Troubleshooting should be pretty much the same for FM; the signal has to be stopping somewhere. Sounds like a bad connection/open coil. Do you have an FM generator and/or oscilloscope?
::::Johnnysan-
::::
::::
::
::::
::::
:::
:::Scope, yes- I also have a Heathkit IG-107 generator that goes to 107mhz.
:::My fear is that I've accidentally moved a coil or other component while subbing the caps.
:::T.
:::
::
:
:
:
:Terry, if you need the Sams I can email it to you.
:Johnnysan-
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Also, thanks Edd. I will try your suggestions. However, like I said, when I got the set, the FM worked, it was just very weak. It only picked up 3 stations. I was going to recap and then check alignment. I have all of the alignment info. All I did was replace 3 caps in the audio section and then the FM stopped. I must have disturbed something. I know enough about FM to know that parts placement is EXTREMELY critical. I'm expecting some parts today so I'll get back on the bench and keep you posted as to my progress. I'm also working on an RCA BP-10 I picked up at a thrift store. It is very cool.
Anyway-
More later-
Thanks again-
Terry