I have been working part time on the 1933 Airline 62-84 AM radio. I have audio but the front end is dead. I resoldered the grid cap on one of the 56 tubes, thanks, Warren. I had the tuning cap assembly out and repaired the tuning shaft. Now that is mechanically good, torn speaker repaired, new filter caps and most of the wax caps replaced and some resistors. Now back to troubleshooting today.

http://www.nostalgiaair.org/PagesByModel/660/M0011660.pdf

I hooked my Heathkit SG-8 signal generator to the cap which come off the 56 2nd detector where the signal goes to the tone control. I had a signal from there before. The lead I'm using is a coax cable with a ground clamp to the radio chassis.

Here is the mystery. With the sig gen turned off, and the lead clamped to the tone control, I am getting AM reception and can tune in stations. Disconnect the test lead anywhere, and the audio fades out. Pull the lead out of the jack at the signal generator, no more audio. Substituting a 10 mfd capacitor for the coax gives a bit of muted sound, louder for a millisec when disconnected.

Can somebody explain this phenomenon? There must be a bad cap here but which one? Or is it something else?

:I have been working part time on the 1933 Airline 62-84 AM radio. I have audio but the front end is dead. I resoldered the grid cap on one of the 56 tubes, thanks, Warren. I had the tuning cap assembly out and repaired the tuning shaft. Now that is mechanically good, torn speaker repaired, new filter caps and most of the wax caps replaced and some resistors. Now back to troubleshooting today.
:
: http://www.nostalgiaair.org/PagesByModel/660/M0011660.pdf
:
:I hooked my Heathkit SG-8 signal generator to the cap which come off the 56 2nd detector where the signal goes to the tone control. I had a signal from there before. The lead I'm using is a coax cable with a ground clamp to the radio chassis.
:
:Here is the mystery. With the sig gen turned off, and the lead clamped to the tone control, I am getting AM reception and can tune in stations. Disconnect the test lead anywhere, and the audio fades out. Pull the lead out of the jack at the signal generator, no more audio. Substituting a 10 mfd capacitor for the coax gives a bit of muted sound, louder for a millisec when disconnected.
:
:Can somebody explain this phenomenon? There must be a bad cap here but which one? Or is it something else?
:
Hi john, have you checked your IF cans for a possible open coil? or ant coil?

:Hi john, have you checked your IF cans for a possible open coil? or ant coil?
:

No open coils before, but I should check again.

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::Hi john, have you checked your IF cans for a possible open coil? or ant coil?
::
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:No open coils before, but I should check again.
:
No open coils found. I will go over the electrolytic cap connections tomorrow. The original rectangular box caps had rolled wax caps bridged across them. I've installed new 8 mfd units, but might have crossed something up. My radio has variations from the schematic, improvements, I imagine.

I had the revelation at sunrise this AM. Measured the R of the coax cable plugged in to the sig gen - 1.2 megohms. Tried a 1.2 meg resistor with alligator clips in place of the test lead, and the radio plays. It also plays with a 850 K resistor. The voltage drop across the resistor is about 100 vdc.
I just need to replace some more parts and get that B+ voltage down. :>)

John, You said you replaced 'most' of your wax caps. I would also consider replacing the remainder while you are working on it. Most caps become unreliable as they age. Only ones I have not replaced in the past are the mica ones.
--Dave

:I had the revelation at sunrise this AM. Measured the R of the coax cable plugged in to the sig gen - 1.2 megohms. Tried a 1.2 meg resistor with alligator clips in place of the test lead, and the radio plays. It also plays with a 850 K resistor. The voltage drop across the resistor is about 100 vdc.
:I just need to replace some more parts and get that B+ voltage down. :>)
:

Agreed. I need to reorder, so am making do with what I've got for now.
I've also added a 1 amp slo-blo fuse on the line cord, so at least it is relatively safe to operate.

:John, You said you replaced 'most' of your wax caps. I would also consider replacing the remainder while you are working on it. Most caps become unreliable as they age. Only ones I have not replaced in the past are the mica ones.
:--Dave
:
::I had the revelation at sunrise this AM. Measured the R of the coax cable plugged in to the sig gen - 1.2 megohms. Tried a 1.2 meg resistor with alligator clips in place of the test lead, and the radio plays. It also plays with a 850 K resistor. The voltage drop across the resistor is about 100 vdc.
::I just need to replace some more parts and get that B+ voltage down. :>)
::
:
:

:Agreed. I need to reorder, so am making do with what I've got for now.
:I've also added a 1 amp slo-blo fuse on the line cord, so at least it is relatively safe to operate.
:
::John, You said you replaced 'most' of your wax caps. I would also consider replacing the remainder while you are working on it. Most caps become unreliable as they age. Only ones I have not replaced in the past are the mica ones.
::--Dave
::
:::I had the revelation at sunrise this AM. Measured the R of the coax cable plugged in to the sig gen - 1.2 megohms. Tried a 1.2 meg resistor with alligator clips in place of the test lead, and the radio plays. It also plays with a 850 K resistor. The voltage drop across the resistor is about 100 vdc.
:::I just need to replace some more parts and get that B+ voltage down. :>)

Here's what it sounds like to me.....all the tube grids in the front of the radio are connected to the AVC bus. If there is not a ground connection to the grids through a resistor, the grids will get electrons stuck to them as the electrons go from the cathode to the plate, and the tube will eventually get cut off. Placing the 1.2 meg resistor on a grid provides a ground for all the front end tubes, and they start conducting again, and the radio starts playing, albeit without AVC action. Ergo, I think you have a problem in the AVC ciruit that is causing the tubes to cut off without the 1.2 meg (I think you said) resistor. The tube grids, as well as the entire AVC bus, should have a very high negative Voltage on them in the cut-off state.
Lewis
Lewis
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:Here's what it sounds like to me.....all the tube grids in the front of the radio are connected to the AVC bus. If there is not a ground connection to the grids through a resistor, the grids will get electrons stuck to them as the electrons go from the cathode to the plate, and the tube will eventually get cut off. Placing the 1.2 meg resistor on a grid provides a ground for all the front end tubes, and they start conducting again, and the radio starts playing, albeit without AVC action. Ergo, I think you have a problem in the AVC circuit that is causing the tubes to cut off without the 1.2 meg (I think you said) resistor. The tube grids, as well as the entire AVC bus, should have a very high negative Voltage on them in the cut-off state.
:Lewis

Thank you, Lewis. I think you may have nailed it. The radio behaves as you describe. With a lower resistor clipped in, you can hear the tinny sound of tubes going into cutoff. High negative voltage measurements as well.

I replaced a couple more dogbone resistors yesterday with no effect. I have not replaced any of the 0.25 mfd caps, as I don't got none. My suspicions are with them, as there are 3 of them all tied to ground in that area.

Placing the 1.2 meg resistor on a grid provides a ground for all the front end tubes, and they start conducting again, and the radio starts playing, albeit without AVC action. Ergo, I think you have a problem in the AVC circuit that is causing the tubes to cut off without the 1.2 meg (I think you said) resistor. The tube grids, as well as the entire AVC bus, should have a very high negative Voltage on them in the cut-off state.
::Lewis

:I replaced a couple more dogbone resistors yesterday with no effect. I have not replaced any of the 0.25 mfd caps, as I don't got none. My suspicions are with them, as there are 3 of them all tied to ground in that area.
:
AVC is certainly not working. The tube can be pulled to no effect. I cleaned ground connections and tried bypassing the .25 caps and got no results. I will now go over all soldered connections in the AVC region.

:Placing the 1.2 meg resistor on a grid provides a ground for all the front end tubes, and they start conducting again, and the radio starts playing, albeit without AVC action. Ergo, I think you have a problem in the AVC circuit that is causing the tubes to cut off without the 1.2 meg (I think you said) resistor. The tube grids, as well as the entire AVC bus, should have a very high negative Voltage on them in the cut-off state.
:::Lewis
:
::I replaced a couple more dogbone resistors yesterday with no effect. I have not replaced any of the 0.25 mfd caps, as I don't got none. My suspicions are with them, as there are 3 of them all tied to ground in that area.
::
:AVC is certainly not working. The tube can be pulled to no effect. I cleaned ground connections and tried bypassing the .25 caps and got no results. I will now go over all soldered connections in the AVC region.

John:
That AVC section is going to be a booger to understand for troubleshooting, what with weird biasing and such, the plate of the tube is connected to the AVC line, with IF coming into the grid and so on. But, there is an R2, if I read right, says 50 M (which is probably 50K) and another resistor (100 M)? anyway, check those resistors or replace with a 47 K and a 100K, and I think your front end problems are solved and you will only have a bunch of little thingies to fix to get a good working radio.
Lewis
:

:That AVC section is going to be a booger to understand for troubleshooting, what with weird biasing and such, the plate of the tube is connected to the AVC line, with IF coming into the grid and so on. But, there is an R2, if I read right, says 50 M (which is probably 50K) and another resistor (100 M)? anyway, check those resistors or replace with a 47 K and a 100K, and I think your front end problems are solved and you will only have a bunch of little thingies to fix to get a good working radio.
:Lewis

Thanks. Sadly, I've already replaced those R's and the R's in the voltage divider section and checked the others and so on. I know what you mean about the AVC. I will just try to put good parts in place of bad and cross my fingers with some of this.
On the bright side, the radio plays, meaning all the major components are good enough to operate, with a few more tweaking sessions left to accomplish the goal.
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:On the bright side, the radio plays, meaning all the major components are good enough to operate, with a few more tweaking sessions left to accomplish the goal.
:::

I am coming to the conclusion that AVC is operating after all. I do get some variations in volume when I touch the AVC grid. The radio simply won't keep playing if I disconnect the 1.2 meg resistor to ground.

Here is my theory now. There are 10 old tubes, no longer operating at their optimum. The resistor compensates for the reduced performance of the tubes. I just feel lucky to have stumbled upon this 'solution'. :>)

Here is an update for anyone following this.
Having tweaked and replaced components as much as I could, I used a variable resistor to find the optimum resistance which allows the best performance. I now have a 30 Kohm resistor between the lead from the tone control to the 56 grid and chassis ground. The resistor between 12 and 24 vdc, depending on frequency.
So far, I see no ill effects from having it there.

:Here is an update for anyone following this.
:Having tweaked and replaced components as much as I could, I used a variable resistor to find the optimum resistance which allows the best performance. I now have a 30 Kohm resistor between the lead from the tone control to the 56 grid and chassis ground. The resistor between 12 and 24 vdc, depending on frequency.
:So far, I see no ill effects from having it there.
:
The voltage across the R varies from 12 to 24 vdc as I tune from the low end of the dial to the middle. Then it begins to decrease as I tune to to the upper end of the dial.

::Here is an update for anyone following this.
::Having tweaked and replaced components as much as I could, I used a variable resistor to find the optimum resistance which allows the best performance. I now have a 30 Kohm resistor between the lead from the tone control to the 56 grid and chassis ground. The resistor between 12 and 24 vdc, depending on frequency.
::So far, I see no ill effects from having it there.
::
:The voltage across the R varies from 12 to 24 vdc as I tune from the low end of the dial to the middle. Then it begins to decrease as I tune to to the upper end of the dial.
:
I have had to increase the R to 100 K, dropping -30 to -70 vdc. I now know that the voltage across the resistor rises when a stronger station is tuned in, so this is all about the AVC not working as it was intended.