Stromberg Carlson FM-443-A Eye tube, motionless

3/27/2011 10:48:29 PMDave Froehlich

Hello All,
I'm working on a Stromberg Carlson FM-443-A. This is different from an FM-443. Anyway, the eye tube doesn't move with signal strength. Something's not working. Somehow, unless someone has the schematic, I'll have to take a picture of the one inside the cabinet.

Thanks,

Dave

3/27/2011 11:40:13 PMWarren

Just about any radio that uses the eye tube works on the same principal. Can look at any radio schematic with this feature. Works off the AVC line. Needs B+ and there is a 1 meg resistor that’s normally in the socket itself. See this link.

http://www.radioremembered.org/tuneye.htm

3/28/2011 3:42:30 PMBill G.

Hi Dave,
I think Warren left out the puch line. That 1 Meg. resistor is commonly open on these and needs replacement.
When someone posts here with this problem this is the most likely cause. Many don't know that the resistor is even there.
It commonly opens because it is rated at 1/4 watt and it is required to dissipate less than 1/4 watt. What the engineer forgot to take in account was that the resistor's rating was for free space, air circulating. In that socket, air does not circulate so the resistor overheats and opens.

All the Best,

Bill Grimm

3/28/2011 5:43:35 PMLewis L

:Hi Dave,
: I think Warren left out the puch line. That 1 Meg. resistor is commonly open on these and needs replacement.
: When someone posts here with this problem this is the most likely cause. Many don't know that the resistor is even there.
: It commonly opens because it is rated at 1/4 watt and it is required to dissipate less than 1/4 watt. What the engineer forgot to take in account was that the resistor's rating was for free space, air circulating. In that socket, air does not circulate so the resistor overheats and opens.
:
:All the Best,
:
:Bill Grimm

I have to disagree with you there, Bill, as a one meg will not get very warm in that position in the radio. Measure the AVC Voltage going into the tube. It should get more negative as stations are tuned in, the stronger the station, the more negative the AVC Voltage. I think they go open just for the smell of it as they get older. I know I am getting more cantankerous as I get older. {;>)
Lewis
P. S. And I don't care if I did maybe spell it wrong.
LL
:
:

3/28/2011 8:32:51 PMThomas Dermody

The 1 meg resistor won't get warm, and doesn't get warm, because the triode section of the eye tube consumes about as much current as a 6Q7, which often has something similar at its plate. However, those old carbon composition resistors often drift in value. It is not uncommon to find this resistor much higher in value or completely open. In most of my radios the resistor did not open, but changed value enough to warrant replacing with a new resistor. They do sometimes open, though.

Also consider that if you did not change the AVC capacitor, you should probably do so. The AVC circuit is a very high impedance circuit, and so even small leakage on the AVC capacitor will pull down the voltage in this circuit. Without AVC voltage the eye on most radios will remain open. The AVC resistor is usually about 3.3 megohms, and so leakage in the millions of ohms will easily affect any voltage being delivered by this resistor.

3/28/2011 11:12:18 PMDave Froehlich

Hello Thomas, All,
There is no 1 Meg ohm resistor here. However there are two 470K ohm resistors in the circuit. There are two connections this tube responds to. One is before the first IF tube and the other is before the last stage. All the resistors seem to be good. There are disc capacitors in there. There are zero paper capacitors. There are some very tiny capacitors. This 443-A is different from the 443 in a few places but it's basically the same circuit. The 443 is on a website about "Made In Rochester, NY" But the resistors are good. I don't have the settings for my Seco 107 tube tester for that Eye tube. I believe it's an EM34. If anyone knows the settings I can test it.
Anyway I traced out all the connections are they're all good. So it could be the tube. Also I don't hear any AVC action taking place when I listen to it. Weak stations have low volume and strong stations are much louder. So the problem might still be somewhere else.
Thanks to Norm, I hooked up the FM audio output tube correctly. Someone mis-wired it. It looks like this was done at the factory. How something like this could pass QC, I have no idea.

Thanks,

Dave
:The 1 meg resistor won't get warm, and doesn't get warm, because the triode section of the eye tube consumes about as much current as a 6Q7, which often has something similar at its plate. However, those old carbon composition resistors often drift in value. It is not uncommon to find this resistor much higher in value or completely open. In most of my radios the resistor did not open, but changed value enough to warrant replacing with a new resistor. They do sometimes open, though.
:
:Also consider that if you did not change the AVC capacitor, you should probably do so. The AVC circuit is a very high impedance circuit, and so even small leakage on the AVC capacitor will pull down the voltage in this circuit. Without AVC voltage the eye on most radios will remain open. The AVC resistor is usually about 3.3 megohms, and so leakage in the millions of ohms will easily affect any voltage being delivered by this resistor.
:
:T.
:

3/29/2011 12:57:38 AMWarren

If the tube turns green it's good. test for the negative voltage on the AVC buss line. Without that voltage the tube don't work.

3/29/2011 2:02:22 AMDave Froehlich

Warren,
EM84s don't light up green. They kind of look like the color of a Black and white tv scan.

http://www.mbzponton.org/n2awa/radio_Grundig_2066PX_EM87_tubes_not_EM84.jpg

http://www.jogis-roehrenbude.de/Roehren-Geschichtliches/Mag_Augen/Baender/EM84.htm

It looks a little blue to me. Yes it lights up blue but the shadow is very wide. It wont narrow. So I'll check voltages. I'll figure it out.

I might have said 34 in my last message. Sorry about that. I meant to say EM84.

Dave

Dave
:If the tube turns green it's good. test for the negative voltage on the AVC buss line. Without that voltage the tube don't work.
:

3/29/2011 2:51:34 AMWarren

Okay, I know what you mean about that tube. It's not really like the so called eye tube. I have the same indicator tube in one of my German made sets.

I made the photo copys for your Mopar radio. Will be sending it off to you. Your E-Mail rejected my last message. Thinks it's spam.

3/29/2011 7:39:33 PMEdd

Sir Dave. . . . .

" Dat deres FM receivers toonin' eye toobie . . . . it don't do be wukkin' with no AVC voltages-es !

1 . . . Le Tuning eye tube:

This being an FM receiver, is dependent on its working with a SAMPLING of the normally developed negative voltage taken from the 1st grid circuitry of the FM limiter tube. . . . which is a product of amplified signal strength.

Go directly to that sets V4 tubes limiter stage . . . . . but, do not pass GO and do NOT collect $200 . . . just yet .

Now there, you measure its first grid voltage with either a low loading impedance VTVM or digital voltmeter and then tune onto and off from the strongest received FM station .

As you tune right onto the station a strong negavive voltage should peak up and then decline on your tuning either direction off from that station.

If you no findee voltage, move on back to V3's 1st grid, the 2nd IF amp stage, where it should also produce a like genearation of negative voltage on signal, but not quite so pronounced, with accumulated stages gain not being quite as high, that early on in the receiver.

Look at my referencing and note its GREEN highlighted trail from the low side of that Limiter stage input transformer and it then passing down thru a set of isolation / filtering resistors and capacitors and ending up a the control grid of the EM84 eye tube.

And seems like their scale illumination is a blue-green colorization of its normally white phosporized rectangular scale . . . a la . . . . :

If you were getting the voltage swing back up at the 1st grid of the Limiter, it should equally be passed on down to that EM84, pending no abnormatlites in that coupling network.

If you want to test your eye tube proper, merely get a 9V battery and a battery clip with its pigtail leads and drop them across a 10-500K pots extreme terminal ends and tack a 1k resistor onto the central wiper terminal.

Two more added on leads connect up, with one going to the to the battery/pot positive terminal, and another lead going to the free end of the just installed 1k range limiting/isolation resistor.

The battery positive terminal lead goes to the Stromberger chassis ground and the 1K resistor lead gets connected to the 1st grid of the eye tube.

That circuitry then lets you apply a variable negative voltage to the eye tube and test its functioning / closure capabilities.

If it doesn't close then, check its 470K plate load resistor, its B+ supply level, or possibly a connection problem at the socket, or if the string of R13-C26-R12-and C39 are STILL connected on, temporarily cut them loose at the first grid, as they could be bleeding the voltage downwards or off.

1 . . . Le Audio problem

From your given circuit descriptions, I just might have thought that you were confronting a cathode follower audio stage tied onto the end of that sets FM audio output, but not so.

Examining the sets circuitry we can see detected FM audio and its initial derivation from #6 of the ratio detector transformer and its passage down the BLUE circuit path where it is initially offered with ALL of its potential 19 and 38Khz Stereo signal elements intact, passing over to the right side MULTIPLEX Output jack.

After the audio signal passes thru the R17-C33 high pass filter network, it is offered, then less its ultrasonic and upper AF elements as a detected audio output, feeding over to the DETECTOR output.

The final audio path is then on down thru C35 coupler and into the 1st grid of V5 Audio preamp stage. Nothing abnormal with that stage with its output being taken off at the 6C4 plate and routed over to the right via the BLUE/RED TRACE path and ending up at the C16 and AUDIO Output jack.

I said nothing abnormal, excluding the altering of that stages gain via the YELLOW boxed in circuitry.

Instead of using frontal RF stage gain alteration, they are altering the gain of this audio stage by having it at normal level by using the DISTANT / LOCAL switch to ground out the bottom of the R26--- 4.7 K cathode resistor, when put in the DISTANT position.

If switched to the LOCAL position, that cathode is then series lifted an additonal 47K above ground with some serious gain decline . . . plus at no extra charge . . a bit of negative feedback is now also being introduced.

Sooooo . . . . . if you have eventually ended up with the audio being taken off from the plate circuitry and its voltage swing being developed across the 100K plate load resistor and then coupled on out via the C16 coupling capacitor. . . all must be well, circuitry wise.

Just remember that LOCAL --- DISTANT switch setting does tie in with the sets audio ciruitry . . . . and not our normally expected RF controlling aspect.

Thassit . . . . .

Now . . . see why's dat eye toobies no makee blinkie-blinkie.

73's de Edd

STROMBERG TUNER UNIT SCHEMATIC:

:Okay, I know what you mean about that tube. It's not really like the so called eye tube. I have the same indicator tube in one of my German made sets.
:
:I made the photo copys for your Mopar radio. Will be sending it off to you. Your E-Mail rejected my last message. Thinks it's spam.
:

3/31/2011 12:33:34 AMDave Froehlich

Edd,
This is a bit different from the FM-443-A. In the 443A there's another 470K ohm resistor connected to the first IF tube grid too. Just a second I'll look at the schematic to see what pin it is. I can disconnect that connection and just go back to how 443 was.
The voltage on pin 1 on the EM84 stays -1.1 volts no matter how it's tuned. So something is wrong. But what? I'm sorry I don't see it. But I'll keep looking. I'll follow what you're asking me to do too.
AVC isn't working either. All weak stations have very low low low volume and all strong stations blast. I'll keep checking. I'll find it.

Thanks Very Much,

Dave

:
:

:
:
:
:
:
:
:
:
:
:
:
:Sir Dave. . . . .
:
:
:
:
:" Dat deres FM receivers toonin' eye toobie . . . . it don't do be wukkin' with no AVC voltages-es !
:
:
:
:
:1 . . . Le Tuning eye tube:
:
:
:This being an FM receiver, is dependent on its working with a SAMPLING of the normally developed negative voltage taken from the 1st grid circuitry of the FM limiter tube. . . . which is a product of amplified signal strength.
:
:
:Go directly to that sets V4 tubes limiter stage . . . . . but, do not pass GO and do NOT collect $200 . . . just yet .
:
:
:Now there, you measure its first grid voltage with either a low loading impedance VTVM or digital voltmeter and then tune onto and off from the strongest received FM station .
:
:
:As you tune right onto the station a strong negavive voltage should peak up and then decline on your tuning either direction off from that station.
:
:
:If you no findee voltage, move on back to V3's 1st grid, the 2nd IF amp stage, where it should also produce a like genearation of negative voltage on signal, but not quite so pronounced, with accumulated stages gain not being quite as high, that early on in the receiver.
:
:
:Look at my referencing and note its GREEN highlighted trail from the low side of that Limiter stage input transformer and it then passing down thru a set of isolation / filtering resistors and capacitors and ending up a the control grid of the EM84 eye tube.
:
:
:And seems like their scale illumination is a blue-green colorization of its normally white phosporized rectangular scale . . . a la . . . . :
:
:
:
:
:
:If you were getting the voltage swing back up at the 1st grid of the Limiter, it should equally be passed on down to that EM84, pending no abnormatlites in that coupling network.
:
:
:If you want to test your eye tube proper, merely get a 9V battery and a battery clip with its pigtail leads and drop them across a 10-500K pots extreme terminal ends and tack a 1k resistor onto the central wiper terminal.
:
:
:Two more added on leads connect up, with one going to the to the battery/pot positive terminal, and another lead going to the free end of the just installed 1k range limiting/isolation resistor.
:
:
:The battery positive terminal lead goes to the Stromberger chassis ground and the 1K resistor lead gets connected to the 1st grid of the eye tube.
:
:
:That circuitry then lets you apply a variable negative voltage to the eye tube and test its functioning / closure capabilities.
:
:
:If it doesn't close then, check its 470K plate load resistor, its B+ supply level, or possibly a connection problem at the socket, or if the string of R13-C26-R12-and C39 are STILL connected on, temporarily cut them loose at the first grid, as they could be bleeding the voltage downwards or off.
:
:
:
:
:
:1 . . . Le Audio problem
:
:
:From your given circuit descriptions, I just might have thought that you were confronting a cathode follower audio stage tied onto the end of that sets FM audio output, but not so.
:
:
:Examining the sets circuitry we can see detected FM audio and its initial derivation from #6 of the ratio detector transformer and its passage down the BLUE circuit path where it is initially offered with ALL of its potential 19 and 38Khz Stereo signal elements intact, passing over to the right side MULTIPLEX Output jack.
:
:
: After the audio signal passes thru the R17-C33 high pass filter network, it is offered, then less its ultrasonic and upper AF elements as a detected audio output, feeding over to the DETECTOR output.
:
:
:The final audio path is then on down thru C35 coupler and into the 1st grid of V5 Audio preamp stage. Nothing abnormal with that stage with its output being taken off at the 6C4 plate and routed over to the right via the BLUE/RED TRACE path and ending up at the C16 and AUDIO Output jack.
:
:
:I said nothing abnormal, excluding the altering of that stages gain via the YELLOW boxed in circuitry.
:
:
:Instead of using frontal RF stage gain alteration, they are altering the gain of this audio stage by having it at normal level by using the DISTANT / LOCAL switch to ground out the bottom of the R26--- 4.7 K cathode resistor, when put in the DISTANT position.
:
:
:If switched to the LOCAL position, that cathode is then series lifted an additonal 47K above ground with some serious gain decline . . . plus at no extra charge . . a bit of negative feedback is now also being introduced.
:
:
:Sooooo . . . . . if you have eventually ended up with the audio being taken off from the plate circuitry and its voltage swing being developed across the 100K plate load resistor and then coupled on out via the C16 coupling capacitor. . . all must be well, circuitry wise.
:
:
:Just remember that LOCAL --- DISTANT switch setting does tie in with the sets audio ciruitry . . . . and not our normally expected RF controlling aspect.
:
:
:
:
:
:Thassit . . . . .
:
:
:Now . . . see why's dat eye toobies no makee blinkie-blinkie.:
:
:
:
:
:73's de Edd
:
:
:

:

:
:
:

:
:
:
: STROMBERG TUNER UNIT SCHEMATIC:
:
:
:
:
:
:
:
:
:
:

:
:
:
:
:
::Okay, I know what you mean about that tube. It's not really like the so called eye tube. I have the same indicator tube in one of my German made sets.
::
::I made the photo copys for your Mopar radio. Will be sending it off to you. Your E-Mail rejected my last message. Thinks it's spam.
::
:
:

3/31/2011 12:52:25 AMDave Froehlich

Edd,
This tuner is very different from the 443. There is a different output tube and the schematic says that there's supposed to 45 volts on pin 1 of the limiter tube. -.7 sounds more correct. The local distant switch is connected to the antenna terminals to attenuate the antenna when it's set to local. There is no such network connected to the cathode of the FM Audio amp tube either.
That voltage on Pin 1 of the limiter never changes no matter how its tuned. So what do I look at next?

Thanks,

Dave
:Edd,
: This is a bit different from the FM-443-A. In the 443A there's another 470K ohm resistor connected to the first IF tube grid too. Just a second I'll look at the schematic to see what pin it is. I can disconnect that connection and just go back to how 443 was.
: The voltage on pin 1 on the EM84 stays -1.1 volts no matter how it's tuned. So something is wrong. But what? I'm sorry I don't see it. But I'll keep looking. I'll follow what you're asking me to do too.
: AVC isn't working either. All weak stations have very low low low volume and all strong stations blast. I'll keep checking. I'll find it.
:
:Thanks Very Much,
:
:Dave
:
::
::

::
::
::
::
::
::
::
::
::
::
::
::Sir Dave. . . . .
::
::
::
::
::" Dat deres FM receivers toonin' eye toobie . . . . it don't do be wukkin' with no AVC voltages-es !
::
::
::
::
::1 . . . Le Tuning eye tube:
::
::
::This being an FM receiver, is dependent on its working with a SAMPLING of the normally developed negative voltage taken from the 1st grid circuitry of the FM limiter tube. . . . which is a product of amplified signal strength.
::
::
::Go directly to that sets V4 tubes limiter stage . . . . . but, do not pass GO and do NOT collect $200 . . . just yet .
::
::
::Now there, you measure its first grid voltage with either a low loading impedance VTVM or digital voltmeter and then tune onto and off from the strongest received FM station .
::
::
::As you tune right onto the station a strong negavive voltage should peak up and then decline on your tuning either direction off from that station.
::
::
::If you no findee voltage, move on back to V3's 1st grid, the 2nd IF amp stage, where it should also produce a like genearation of negative voltage on signal, but not quite so pronounced, with accumulated stages gain not being quite as high, that early on in the receiver.
::
::
::Look at my referencing and note its GREEN highlighted trail from the low side of that Limiter stage input transformer and it then passing down thru a set of isolation / filtering resistors and capacitors and ending up a the control grid of the EM84 eye tube.
::
::
::And seems like their scale illumination is a blue-green colorization of its normally white phosporized rectangular scale . . . a la . . . . :
::
::
::
::
::
::If you were getting the voltage swing back up at the 1st grid of the Limiter, it should equally be passed on down to that EM84, pending no abnormatlites in that coupling network.
::
::
::If you want to test your eye tube proper, merely get a 9V battery and a battery clip with its pigtail leads and drop them across a 10-500K pots extreme terminal ends and tack a 1k resistor onto the central wiper terminal.
::
::
::Two more added on leads connect up, with one going to the to the battery/pot positive terminal, and another lead going to the free end of the just installed 1k range limiting/isolation resistor.
::
::
::The battery positive terminal lead goes to the Stromberger chassis ground and the 1K resistor lead gets connected to the 1st grid of the eye tube.
::
::
::That circuitry then lets you apply a variable negative voltage to the eye tube and test its functioning / closure capabilities.
::
::
::If it doesn't close then, check its 470K plate load resistor, its B+ supply level, or possibly a connection problem at the socket, or if the string of R13-C26-R12-and C39 are STILL connected on, temporarily cut them loose at the first grid, as they could be bleeding the voltage downwards or off.
::
::
::
::
::
::1 . . . Le Audio problem
::
::
::From your given circuit descriptions, I just might have thought that you were confronting a cathode follower audio stage tied onto the end of that sets FM audio output, but not so.
::
::
::Examining the sets circuitry we can see detected FM audio and its initial derivation from #6 of the ratio detector transformer and its passage down the BLUE circuit path where it is initially offered with ALL of its potential 19 and 38Khz Stereo signal elements intact, passing over to the right side MULTIPLEX Output jack.
::
::
:: After the audio signal passes thru the R17-C33 high pass filter network, it is offered, then less its ultrasonic and upper AF elements as a detected audio output, feeding over to the DETECTOR output.
::
::
::The final audio path is then on down thru C35 coupler and into the 1st grid of V5 Audio preamp stage. Nothing abnormal with that stage with its output being taken off at the 6C4 plate and routed over to the right via the BLUE/RED TRACE path and ending up at the C16 and AUDIO Output jack.
::
::
::I said nothing abnormal, excluding the altering of that stages gain via the YELLOW boxed in circuitry.
::
::
::Instead of using frontal RF stage gain alteration, they are altering the gain of this audio stage by having it at normal level by using the DISTANT / LOCAL switch to ground out the bottom of the R26--- 4.7 K cathode resistor, when put in the DISTANT position.
::
::
::If switched to the LOCAL position, that cathode is then series lifted an additonal 47K above ground with some serious gain decline . . . plus at no extra charge . . a bit of negative feedback is now also being introduced.
::
::
::Sooooo . . . . . if you have eventually ended up with the audio being taken off from the plate circuitry and its voltage swing being developed across the 100K plate load resistor and then coupled on out via the C16 coupling capacitor. . . all must be well, circuitry wise.
::
::
::Just remember that LOCAL --- DISTANT switch setting does tie in with the sets audio ciruitry . . . . and not our normally expected RF controlling aspect.
::
::
::
::
::
::Thassit . . . . .
::
::
::Now . . . see why's dat eye toobies no makee blinkie-blinkie.::
::
::
::
::
::73's de Edd
::
::
::

::

::
::
::

::
::
::
:: STROMBERG TUNER UNIT SCHEMATIC:
::
::
::
::
::
::
::
::
::
::

::
::
::
::
::
:::Okay, I know what you mean about that tube. It's not really like the so called eye tube. I have the same indicator tube in one of my German made sets.
:::
:::I made the photo copys for your Mopar radio. Will be sending it off to you. Your E-Mail rejected my last message. Thinks it's spam.
:::
::
::
:
:

3/31/2011 1:08:58 AMDave Froehlich

Edd,
The voltage on the first grid of the limiter tube is -.7 volts. I can tune the radio all I want and it stays at -.7 volts. Same with the first grid of the second IF tube. -.7 volts. When I try to measure the grid of the first IF tube the signal goes away. On the grid of the eye tube it's always -1.1 volts. It doesn't change. But how does it start out as -.7 volts and arrive as -1.1 volts? It makes no sense. There's nothing connected to the wire. What do look at next? I'll keep checking.

Thanks,

Dave
:Edd,
: This tuner is very different from the 443. There is a different output tube and the schematic says that there's supposed to 45 volts on pin 1 of the limiter tube. -.7 sounds more correct. The local distant switch is connected to the antenna terminals to attenuate the antenna when it's set to local. There is no such network connected to the cathode of the FM Audio amp tube either.
: That voltage on Pin 1 of the limiter never changes no matter how its tuned. So what do I look at next?
:
:Thanks,
:
:Dave
::Edd,
:: This is a bit different from the FM-443-A. In the 443A there's another 470K ohm resistor connected to the first IF tube grid too. Just a second I'll look at the schematic to see what pin it is. I can disconnect that connection and just go back to how 443 was.
:: The voltage on pin 1 on the EM84 stays -1.1 volts no matter how it's tuned. So something is wrong. But what? I'm sorry I don't see it. But I'll keep looking. I'll follow what you're asking me to do too.
:: AVC isn't working either. All weak stations have very low low low volume and all strong stations blast. I'll keep checking. I'll find it.
::
::Thanks Very Much,
::
::Dave
::
:::
:::

:::
:::
:::
:::
:::
:::
:::
:::
:::
:::
:::
:::Sir Dave. . . . .
:::
:::
:::
:::
:::" Dat deres FM receivers toonin' eye toobie . . . . it don't do be wukkin' with no AVC voltages-es !
:::
:::
:::
:::
:::1 . . . Le Tuning eye tube:
:::
:::
:::This being an FM receiver, is dependent on its working with a SAMPLING of the normally developed negative voltage taken from the 1st grid circuitry of the FM limiter tube. . . . which is a product of amplified signal strength.
:::
:::
:::Go directly to that sets V4 tubes limiter stage . . . . . but, do not pass GO and do NOT collect $200 . . . just yet .
:::
:::
:::Now there, you measure its first grid voltage with either a low loading impedance VTVM or digital voltmeter and then tune onto and off from the strongest received FM station .
:::
:::
:::As you tune right onto the station a strong negavive voltage should peak up and then decline on your tuning either direction off from that station.
:::
:::
:::If you no findee voltage, move on back to V3's 1st grid, the 2nd IF amp stage, where it should also produce a like genearation of negative voltage on signal, but not quite so pronounced, with accumulated stages gain not being quite as high, that early on in the receiver.
:::
:::
:::Look at my referencing and note its GREEN highlighted trail from the low side of that Limiter stage input transformer and it then passing down thru a set of isolation / filtering resistors and capacitors and ending up a the control grid of the EM84 eye tube.
:::
:::
:::And seems like their scale illumination is a blue-green colorization of its normally white phosporized rectangular scale . . . a la . . . . :
:::
:::
:::
:::
:::
:::If you were getting the voltage swing back up at the 1st grid of the Limiter, it should equally be passed on down to that EM84, pending no abnormatlites in that coupling network.
:::
:::
:::If you want to test your eye tube proper, merely get a 9V battery and a battery clip with its pigtail leads and drop them across a 10-500K pots extreme terminal ends and tack a 1k resistor onto the central wiper terminal.
:::
:::
:::Two more added on leads connect up, with one going to the to the battery/pot positive terminal, and another lead going to the free end of the just installed 1k range limiting/isolation resistor.
:::
:::
:::The battery positive terminal lead goes to the Stromberger chassis ground and the 1K resistor lead gets connected to the 1st grid of the eye tube.
:::
:::
:::That circuitry then lets you apply a variable negative voltage to the eye tube and test its functioning / closure capabilities.
:::
:::
:::If it doesn't close then, check its 470K plate load resistor, its B+ supply level, or possibly a connection problem at the socket, or if the string of R13-C26-R12-and C39 are STILL connected on, temporarily cut them loose at the first grid, as they could be bleeding the voltage downwards or off.
:::
:::
:::
:::
:::
:::1 . . . Le Audio problem
:::
:::
:::From your given circuit descriptions, I just might have thought that you were confronting a cathode follower audio stage tied onto the end of that sets FM audio output, but not so.
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:::Examining the sets circuitry we can see detected FM audio and its initial derivation from #6 of the ratio detector transformer and its passage down the BLUE circuit path where it is initially offered with ALL of its potential 19 and 38Khz Stereo signal elements intact, passing over to the right side MULTIPLEX Output jack.
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::: After the audio signal passes thru the R17-C33 high pass filter network, it is offered, then less its ultrasonic and upper AF elements as a detected audio output, feeding over to the DETECTOR output.
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:::The final audio path is then on down thru C35 coupler and into the 1st grid of V5 Audio preamp stage. Nothing abnormal with that stage with its output being taken off at the 6C4 plate and routed over to the right via the BLUE/RED TRACE path and ending up at the C16 and AUDIO Output jack.
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:::I said nothing abnormal, excluding the altering of that stages gain via the YELLOW boxed in circuitry.
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:::Instead of using frontal RF stage gain alteration, they are altering the gain of this audio stage by having it at normal level by using the DISTANT / LOCAL switch to ground out the bottom of the R26--- 4.7 K cathode resistor, when put in the DISTANT position.
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:::If switched to the LOCAL position, that cathode is then series lifted an additonal 47K above ground with some serious gain decline . . . plus at no extra charge . . a bit of negative feedback is now also being introduced.
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:::Sooooo . . . . . if you have eventually ended up with the audio being taken off from the plate circuitry and its voltage swing being developed across the 100K plate load resistor and then coupled on out via the C16 coupling capacitor. . . all must be well, circuitry wise.
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:::Just remember that LOCAL --- DISTANT switch setting does tie in with the sets audio ciruitry . . . . and not our normally expected RF controlling aspect.
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:::Thassit . . . . .
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:::Now . . . see why's dat eye toobies no makee blinkie-blinkie.:::
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:::73's de Edd
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::: STROMBERG TUNER UNIT SCHEMATIC:
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::::Okay, I know what you mean about that tube. It's not really like the so called eye tube. I have the same indicator tube in one of my German made sets.
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::::I made the photo copys for your Mopar radio. Will be sending it off to you. Your E-Mail rejected my last message. Thinks it's spam.
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3/31/2011 1:22:56 AMThomas Dermody

A while back I worked on a Magnavox consol for a friend of mine. It exhibited similar symptoms. First, many of these radios require big antennas (like the FM ribbon antenna you buy at Radio Shack). Second, sometimes the capacitors in the IF transformers oxidize and throw off the tuning of the IF transformer. I managed to save the ones in the Magnavox by injecting dielectric grease into the base. The IF circuitry did require some touching up, though. Don't touch the discriminator transformer, however, until everything else is as good as it can be. Then, if you want, you can check to see if the discriminator is working properly. I believe that on the radio I was working on the discriminator would produce 0 volts at a certain test point when it was set properly and the station was tuned on center.

The eye tube on this radio would not move at all, though, and after not finding any trouble I found that with a minor tune-up and a nice antenna, things came to life.

All stations should be fairly equal in volume, or, if weak, should have lots of amplification hiss and atmosphere noise surrounding them. They should be fairly equal in volume, though, even if distorted. Signal strength should not affect audio strength, as the two are not related in any way. Audio strength is created by frequency deviation. The more deviation, the louder the audio. The limiter clips the carrier so that it is constant in amplitude, so that static is eliminated. The only way it can do this is with a strong signal. The only way negative voltage will develop at its grid is with a strong signal. Make all of the stations blast, and the strong ones will deflect the eye tube. You shouldn't be hearing dead space between stations. There should be lots of noise, or else you are either not using a large enough antenna, or the RF circuits aren't amplifying enough.

Also be sure that there are no leakage paths in the limiter/eye tube circuit other than the 100K resistor.

3/31/2011 1:26:25 AMThomas Dermody

You should not have to turn down the volume everytime you tune in a station. When you tune off of a station the hiss should be similar in volume. If not, then you are not getting enough amplification, either at the antenna or in the radio.