Stewart-Warner Radio - Edd, I need your help!

3/26/2012 7:08:31 PMMmakazoo

Edd: Can you help me to understand how the AVC works in this set? I can't understand it. It seems to have a separate circuit for the tuning eye and another for the AVC. You do those nice color coded explanations - it would help me a lot. Chassis R-149, Models 1491 to 1499. Thanks. Mark from Kalamazoo

3/27/2012 9:16:28 AMLewis L.

:Edd: Can you help me to understand how the AVC works in this set? I can't understand it. It seems to have a separate circuit for the tuning eye and another for the AVC. You do those nice color coded explanations - it would help me a lot. Chassis R-149, Models 1491 to 1499. Thanks. Mark from Kalamazoo

Mark:
I think you're right. There seems to be a seperate amplifier and 6H6 for each circuit, AVC and tuning indicator. Maybe the designer of that circuit had a relative in the tube selling business. It would seem to me that you should get negative AVC Voltage from both 6H6s, and it should vary with signal strength. Do you have a problem with AVC or tuning eye?
Lewis

3/27/2012 7:53:56 PMMmakazoo

Lewis: Tuning eye works great. I am having a problem with reception. Only the strongest stations come in. Seems like the AVC may not be working. I have checked the antenna and RF stages. They seem okay, but I'm not sure where the problem is. I have an AM transmitter at home to play music from internet through my radios. That station comes in good - fully closes the tuning eye. AF stage is great - booming sound. Mark

3/28/2012 5:32:21 AMLewis L.

:Lewis: Tuning eye works great. I am having a problem with reception. Only the strongest stations come in. Seems like the AVC may not be working. I have checked the antenna and RF stages. They seem okay, but I'm not sure where the problem is. I have an AM transmitter at home to play music from internet through my radios. That station comes in good - fully closes the tuning eye. AF stage is great - booming sound. Mark

AVC is a negative Voltage placed on the tube grids to lower the gain of the stage, and when it is not working, you have distortion on strong stations and fading in and out on weaker ones. Sounds to me more like a RF amp problem, maybe an antenna one. I'll get back to you when I've had more coffee, and see about getting the radio working on weak stations, too. I'll study the schematic a little more and write later.
Lewis

3/28/2012 12:07:38 PMLewis L.

::Lewis: Tuning eye works great. I am having a problem with reception. Only the strongest stations come in. Seems like the AVC may not be working. I have checked the antenna and RF stages. They seem okay, but I'm not sure where the problem is. I have an AM transmitter at home to play music from internet through my radios. That station comes in good - fully closes the tuning eye. AF stage is great - booming sound. Mark
:
:
:AVC is a negative Voltage placed on the tube grids to lower the gain of the stage, and when it is not working, you have distortion on strong stations and fading in and out on weaker ones. Sounds to me more like a RF amp problem, maybe an antenna one. I'll get back to you when I've had more coffee, and see about getting the radio working on weak stations, too. I'll study the schematic a little more and write later.
:Lewis
:
Hokay, I'm a little more awake now, so let's analyze the schematic. It would appear that this radio has two seperate IF lines, one from the upper 6H6 that only controls the IF tube and the 6G5 tuning eye, and another from the lower 6H6 for the RF amplifier and converter stages. There is a small negative DC Voltage applied to these AVC lines from Resistors 81 and 87. So, to check the AVC, get a high resistance Voltmeter, such as a VTVM or a modern digital meter, as you will never be able to check this grid Voltage with an analog meter. The tuning eye works, so that AVC line is OK. The other line should go negative with received signal strength, about the same as the working one.

The AVC only reduces the amplification of the tube, so unless you have a high negative Voltage on the AVC line, your problem is somewhere else. That radio has a lot of RF gain, so it might pick up a strong station with a stage completely dead. My gut tells me to check the RF stage for trouble first.
Lewis

3/28/2012 5:53:11 PMMmakazoo

Okay, Lewis, I hope to have time this weekend to look into it and will post what I find. Thanks. Mark from Kalamazoo

3/28/2012 6:30:35 PMEdd

Sir Mark. . . . .

Good Grief . . .Charlie Brown ! . . . . ( and you too Mark.)

Sir Lewis . . . .Déjà vu . . .. . . . now those were my same identical thoughts . . . .out of my mind . . . . only less, my having poked them up and submitted them .

In THAT pending interim I was looking around for a better schematic of that SW 149 chassis, upon which to perform a technical explanatory mark up.

Alas . . . not to be . . . . as it was never created by Bietmans drafting and was way too early for being in a Samuels coverage.

Give me some side composing time and I will see what I can do on enhancing a 'mo better copy of it that I have on a Cee Dee containing it.

SOME INITIAL EXPLORATORY TESTS:

Me also thinks that AVC is NOT the problem, as the usual failure mode of AVC circuitry is to DIMINISH its controlling action /effect.

Soooooo if your AVC action is being LESS . . . . its net end effect will be to let the RF portion of the set run with MORE gain, and that would be conducive to more sensitivity to weaker stations reception.

To test that set with it running full out on its frontal RF gain, all you would have to do is ground out any component on the tri juncture of:

Resistor #4 510K and

capacitor #C43--.004 ufd and

the bottom leg of #84 dedicated AVC I.F. transformers secondary

(These are located around the LOWER 6H6 AVC detector stage. )

Try that on a middle of the road strength of received AM BCB station

If it doesn't come up a bit, then the AVC is not the problem.
Next put yourself another "benchmark" receiver set right BESIDE your SW and tune it into a weak station and see how your SW's performance compares on receptions of weaker stations ?

Lastly, remember that your SW is using an internal antenna coil and is somewhat dependeent upon signal getting piped into it from any length of "aerial wire " that you have connected to it.

If the SW is still deficient, lets enact a simple test of the RF front end of that unit. Take loose whatever aerial that you NOW have connected and get about 8 feet of hook up wire coming down from a ceiling corner apex and connect it into the grid cap of the 6A8 and evaluate the set as a conventional receiver, tune in on a middle of the road signal strength BCB station again and take advantage of your tuning eye's evaluation. A quarter to mid closure referencing would be nice to work with.

Then . . . . AND THEN . . . . . you unclip and move the "test aerial wire" over to the firstus -a- gridamus of that 6K7 RF amp stage and see what it is doing for you . . . gain wise .

Thats probably enough to chew on for now . . . . ZUJ'ing

73's de Edd

The chance that you'll forget something is directly proportional to.....to....ah.....ah..........

:::Lewis: Tuning eye works great. I am having a problem with reception. Only the strongest stations come in. Seems like the AVC may not be working. I have checked the antenna and RF stages. They seem okay, but I'm not sure where the problem is. I have an AM transmitter at home to play music from internet through my radios. That station comes in good - fully closes the tuning eye. AF stage is great - booming sound. Mark
::
::
::AVC is a negative Voltage placed on the tube grids to lower the gain of the stage, and when it is not working, you have distortion on strong stations and fading in and out on weaker ones. Sounds to me more like a RF amp problem, maybe an antenna one. I'll get back to you when I've had more coffee, and see about getting the radio working on weak stations, too. I'll study the schematic a little more and write later.
::Lewis
::
:Hokay, I'm a little more awake now, so let's analyze the schematic. It would appear that this radio has two seperate IF lines, one from the upper 6H6 that only controls the IF tube and the 6G5 tuning eye, and another from the lower 6H6 for the RF amplifier and converter stages. There is a small negative DC Voltage applied to these AVC lines from Resistors 81 and 87. So, to check the AVC, get a high resistance Voltmeter, such as a VTVM or a modern digital meter, as you will never be able to check this grid Voltage with an analog meter. The tuning eye works, so that AVC line is OK. The other line should go negative with received signal strength, about the same as the working one.
:
:The AVC only reduces the amplification of the tube, so unless you have a high negative Voltage on the AVC line, your problem is somewhere else. That radio has a lot of RF gain, so it might pick up a strong station with a stage completely dead. My gut tells me to check the RF stage for trouble first.
:Lewis
:

3/29/2012 2:52:25 PMMmakazoo

Edd and Lewis: Thank you for some direction in pursuing my problem. And Edd, I do hope you can find a better copy of the schematic so that you can do your color mark up. I am just having a hard time understanding the AVC set up on this radio. Looks like a waste of extra tubes compared to other radios. I didn't think that the AVC was the problem, but couldn't comfortably rule it out because of the complexity. Mark from Kalamazoo

3/30/2012 4:45:58 AMLewis L.

:Edd and Lewis: Thank you for some direction in pursuing my problem. And Edd, I do hope you can find a better copy of the schematic so that you can do your color mark up. I am just having a hard time understanding the AVC set up on this radio. Looks like a waste of extra tubes compared to other radios. I didn't think that the AVC was the problem, but couldn't comfortably rule it out because of the complexity. Mark from Kalamazoo
:

Mark:
I can't think of any reason that they would do it that way either, except as I said, someone must have had a relative in the tube business. Sure seems a complicaated way to do it to me. How 'bout you, Edd?
What'cha think?
Lewis
P. S. Clock still working fine after what, three years?

4/17/2012 2:42:52 PMMmakazoo

Edd: I clipped an aerial to the grid of the 6A8 and tried the radio. Performs about the same, a little less gain compared to the antenna connection. MOved it to grid of 6K7 and about the same, maybe a little more gain compared to the 6A8. The only station that seems to come in with any strength is around 1480 (but doesn't move the tuning eye). Middle of the dial is quiet. 590 and 1360 (both local stations) come in with volume cranked nearly all the way up. My SSTran comes in loud at 1430 and closes the tuning eye - the two halves actually overlap. Mark from Kalamazoo

4/21/2012 10:43:36 AMEdd

Sir Mark . . . . .

Stil l l l l l cleaning up that units schematic and doing some enhancement is still a work in progress.

Your poor sensitivity on that set is still suggestive of a RF or mixer stage proplem, and not AGC.

However . . . . agreed . . . . that the units performance from your SSTRAN is so swamped with its nearby (feeble 100 MW) signal that it thinks that a 50KW clear channel signal is coming in.

Was any wiring replacement involved, such that a mis wiring might have occured in rewiring?

As for the RF and Mixer coils, do they ohm out in the low ohms category, with no chance of a copper sulfate encrusted wiring junction, having eaten the wire open, and just the conductive chemical bridge ?

Are all of the RF and mixer plate and screen voltages up to level . . . .drifted supply resistors ?

Was hoping that your antenna inputting between the RF input or Mixer input could have been evaluating any signal differentiation by the use of the tuning eye tube as an ersatz "field strength meter" . . . alas . . . . its performance being too weak to be able to realize such.

Guess that you have already inter swapped your RF and IF 6K7 tubes positions?

Standing by . . . .and further wukkin' on da Schematique II

73's de Edd

Bare feet magnetize sharp metal objects so they always point upward from the floor.

:Edd: I clipped an aerial to the grid of the 6A8 and tried the radio. Performs about the same, a little less gain compared to the antenna connection. MOved it to grid of 6K7 and about the same, maybe a little more gain compared to the 6A8. The only station that seems to come in with any strength is around 1480 (but doesn't move the tuning eye). Middle of the dial is quiet. 590 and 1360 (both local stations) come in with volume cranked nearly all the way up. My SSTran comes in loud at 1430 and closes the tuning eye - the two halves actually overlap. Mark from Kalamazoo
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