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 Post subject: Re: 1st and 2nd IF coils - AA5 radio
PostPosted: Feb Sat 16, 2019 4:56 pm 
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Location: S. Dartmouth MA 02748-1225 USA
The frame/rotor of the tuning condenser is at AVC potential. If the frame has been shorted because of failing grommets or remounted with different hardware that fault may kill the oscillator and or cause the radio to have excessive gain to the point of overload.

I do not know if the images of the chassis are correct, however, the screws that mount the tuning condenser pass through the PC board, those screws should be in contact with the foil on the board that is the AVC circuit. If the screws are corroded, back them off and cleanup. Sometimes the screw head is soldered to the trace on the board. Look for breaks in the trace. Broken traces can be solder spliced with a bit of bare copper wire...

GL

Chas

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 Post subject: Re: 1st and 2nd IF coils - AA5 radio
PostPosted: Feb Sat 16, 2019 5:01 pm 
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Joined: Nov Fri 04, 2011 11:14 pm
Posts: 39
Ok......

The green wire, which looks like it was replaced by the PO, was not hooked up to the right location. Must have broken off, and they had no schematic and took a guess, maybe....

I have now hooked the green wire from the antenna loop, over to the green wire that goes to V1 pin 7. We now have oscillation! Picking up the squeal from an adjacent working AM radio.

Also noted that the adjustable screws for tuning A5 and A6, are socked down tight. Guessing that is not correct. Took the radio outside to try to pickup some strong local stations...nothing but noise. Undoubtedly, alignment has been tinkered with.

++++++++++++++++++

At this point, I traced and pin-out L2, just so that I could put some numbers on my schematic such that things make sense. Everything traces out correctly.

Should I retest voltages and resistance per the table again?

-Brett


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 Post subject: Re: 1st and 2nd IF coils - AA5 radio
PostPosted: Feb Sat 16, 2019 5:25 pm 
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Sounds like the radio is sort of operational. It's not unheard of that an owner would think alignment adjustments needed to be "tightened down."

Turn on your signal generator and hook up a short wire as sort of transmitting antenna; don't make a direct connection to the radio yet. See if the radio receives anything. I think the next step is to do the full alignment procedure.


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 Post subject: Re: 1st and 2nd IF coils - AA5 radio
PostPosted: Feb Sat 16, 2019 8:49 pm 
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Here is my set up:

NOTE: I have a Heathkit IG-102 that I have not used before. Pictured is the way I'm currently using it. I have it set at RF out, 1.000 MHz. The scope seems to support that output. This is a pretty minty unit with very low time. (said the previous owner) I have not recapped this unit. It is as original. I know at minimum, I have to change out the dual 20MFD paper cap.

With about 1 foot of wire connected to the center terminal of the RF output, I get no tone on the Westinghouse. BUT, I also don't hear a tone on two good working AM transistor radios.

Thoughts?


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 Post subject: Re: 1st and 2nd IF coils - AA5 radio
PostPosted: Feb Sat 16, 2019 9:13 pm 
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I would suggest using a digital readout radio set to 1000 Kilocycles, with the generators hot lead next to the top of the radios cabinet, and see if you get a tone. I believe you have to set it to internal modulation. Also, not sure of the fine and coarse attenuation, but set those for the strongest output signal to begin with (which would seem should be set to low).

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Last edited by fifties on Feb Sat 16, 2019 9:15 pm, edited 1 time in total.

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 Post subject: Re: 1st and 2nd IF coils - AA5 radio
PostPosted: Feb Sat 16, 2019 9:15 pm 
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In your picture, the modulation is off---it has to be on to get a tone.
OOPS--don't mind me---i'm just the echo....;)

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 Post subject: Re: 1st and 2nd IF coils - AA5 radio
PostPosted: Feb Sat 16, 2019 9:48 pm 
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Joined: Nov Fri 04, 2011 11:14 pm
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Oops. Yes, modulation is now on, and yes....I get a tone, (at ~ 1000KHz) on the two transistor radios as well as the Westinghouse in question. Sorry, I don't have a radio with digital readout, or a digital frequency counter.

I also replaced the paper cap on the Heathkit. At least we know that piece of test gear is working!

I'm starting to read about Alignment.

-Brett


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 Post subject: Re: 1st and 2nd IF coils - AA5 radio
PostPosted: Feb Sat 16, 2019 11:00 pm 
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Inputs about the alignment. Always do it in this order: IF, osc, antenna. It's important that everytime you adjust the oscillator trimmer, you need to re-align the antenna section.

Since you don't have a calibrated digital readout...

- IF = 455khz. For this radio, it does not need to be absolutely exact. When aligning the two IF filters, adjust the signal generator to 455khz on the dial, then fine tune the signal generator until you hear the tone on the radio. Then perform IF alignment to peak the loudness of the tone. I suspect IF alignment is not far off

- Oscillator adjustment. This is to align the oscillator to the dial. I think the schematic says to use 1625khz. Actually any frequency near top end of the dial. Calibrate the signal generator with another radio on a known station. Example: tune to a station on 1600khz, then adjust the signal generator so the tone is on top of the station. In fact, if at this point the Westinghouse radio can pick a real station, you can use that instead of the signal generator.

- Antenna adjustment. Schematic says to use 1400khz. Don't even worry about the exact frequency. You can use any signal near the top of the dial. Again, you can just turn the radio dial to a weak station on top end of the dial, then adjust the antenna trimmer until the signal strength is peaked.


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 Post subject: Re: 1st and 2nd IF coils - AA5 radio
PostPosted: Feb Sat 16, 2019 11:40 pm 
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What can you tell me about having to use a dummy antenna?


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 Post subject: Re: 1st and 2nd IF coils - AA5 radio
PostPosted: Feb Sat 16, 2019 11:49 pm 
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Caperodder wrote:
What can you tell me about having to use a dummy antenna?

That would refer to connecting a small value cap, .01, .05, etc., to the hot lead of the generator, and then directly to the radios antenna.

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 Post subject: Re: 1st and 2nd IF coils - AA5 radio
PostPosted: Feb Sun 17, 2019 12:14 am 
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...yes. I think the term "dummy antenna" might be misleading. It's just a small capacitor to avoid direct contact between the signal generator output and the radio.


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 Post subject: Re: 1st and 2nd IF coils - AA5 radio
PostPosted: Feb Sun 17, 2019 12:17 am 
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Thanks!


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 Post subject: Re: 1st and 2nd IF coils - AA5 radio
PostPosted: Feb Sun 17, 2019 12:20 am 
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From "Elements of Radio Servicing":


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 Post subject: Re: 1st and 2nd IF coils - AA5 radio
PostPosted: Feb Sun 17, 2019 1:18 am 
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Ok. First alignment I've ever done. Seems to have worked per the plan.

For the first time...since I've owned this radio, I can actually pick up a couple of extremely weak stations. None of the local stations are coming through. Also, a lot of static across the entire dial.

Clues?


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 Post subject: Re: 1st and 2nd IF coils - AA5 radio
PostPosted: Feb Sun 17, 2019 1:57 am 
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Congrats. This is much better than 48 hours ago :)

Maybe alignment is still off.

- Perform the oscillator test. Tune the radio to a known station on lower end of dial; use an adjacent radio to verify if oscillator is aligned to the high side. Example: Tune Westinghouse radio to a known station at 650khz; the adjacent radio should pick up the oscillator at (650 + 455) 1105khz.

(explanation for above: it's possible the oscillator is aligned to the wrong side...455khz below received frequency instead of above)

- Repeat IF alignment at 455khz. Peak 2nd IF, then 1st IF in that order

- Repeat antenna alignment: tune radio to a known station at top of dial. Adjust antenna trimmer.

You posted resistance checks before; if the above does not result in improvement, check voltages this time


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 Post subject: Re: 1st and 2nd IF coils - AA5 radio
PostPosted: Feb Sun 17, 2019 2:39 am 
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Ok I’ll try that. To be clear.....I’m only receiving 3 or 4 stations, and just barely. I think I’ll go through the test points again, and check the voltages too.


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 Post subject: Re: 1st and 2nd IF coils - AA5 radio
PostPosted: Feb Sun 17, 2019 3:16 am 
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Quote:
...I can actually pick up a couple of extremely weak stations. None of the local stations are coming through..

Are those stations showing up at the correct place on the dial? You said "weak" station. Do you mean you're only getting the distant stations but the local strong stations are missing?

Keep at it. Radio should work fine if all components are ok and there are no wiring mistakes. You can check all those. Only thing left would be alignment. We can give suggestions based on only what info you share with us.


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 Post subject: Re: 1st and 2nd IF coils - AA5 radio
PostPosted: Feb Sun 17, 2019 6:55 am 
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Caperodder wrote:
Ok. First alignment I've ever done. Seems to have worked per the plan.

For the first time...since I've owned this radio, I can actually pick up a couple of extremely weak stations. None of the local stations are coming through. Also, a lot of static across the entire dial.

Clues?

Might be an AGC problem, overly cutting out stronger transmissions, while letting the weaker ones through. IDK how to trace the AGC circuit; maybe AJJ can help on that.

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 Post subject: Re: 1st and 2nd IF coils - AA5 radio
PostPosted: Feb Sun 17, 2019 7:14 am 
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Fortunately, we're already familiar with this circuit from previous troubleshooting.

AGC/AVC (automatic gain/volume control) circuit has two components. That 3.3meg resistor and that 0.01 mfd capacitor.

The function of AGC is to lower the gain of strong stations. Otherwise, strong stations will be too loud as you tune across the dial.

As high level signal enters into volume control, a corresponding negative voltage is generated through that resistor to other parts of the radio circuit, thus reducing signal level.

The resistor-capacitor pair creates a "RC time constant" so that AGC action is gradual, not instantaneous.

https://en.wikipedia.org/wiki/RC_time_constant

This is not to suggest the issue is definitely caused by the AGC circuit, but it's worthwhile to double check this circuit and its components.


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 Post subject: Re: 1st and 2nd IF coils - AA5 radio
PostPosted: Feb Sun 17, 2019 3:33 pm 
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Ok. A few more bits of info that may help.

1. Turns out I do have a digital AM radio as part of my Bose Wave radio. This radio will auto scan the entire AM band without stopping! So sadly, there are no "boomers". Manually tuning, 1030 out of Boston, get a fairly decent quality audio.
2. This morning, daylight, and I'm only finding one station on the Westinghouse - 880. I can receive this on the Westinghouse, (extremely hard to hear), as well as on two transistor radios, (fairly distant but legible), so the dial is ~correct.
3. Amazing how much noise my house generates. I replaced my florescent bench lamp with an LED one. Even iPhone chargers generate noise. I've done what I can to eliminate the noise, but the radio is still very noisy across the spectrum.

I also replaced the bumblebee capacitor with a modern one.

-Brett


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