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 Post subject: Alignment issue?
PostPosted: Feb Sun 02, 2020 9:18 am 
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Joined: Oct Mon 02, 2017 8:53 am
Posts: 399
Location: Panama City, FL 32401
The other day I finished recapping a Zenith Royal 700. On 3 strong stations only it has two peaks receiving the same station, similar to an FM receiver. The second peak is the strongest tuning up the band. I didn't notice whether it was doing it before. Will a simple alignment fix that? The reception otherwise is great across the band. Thanks in advance.

-Mike.


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 Post subject: Re: Alignment issue?
PostPosted: Feb Sun 02, 2020 9:45 am 
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Joined: Feb Wed 07, 2018 6:52 pm
Posts: 670
Location: Stone Mountain, GA
Could be it is overdriving the IF if it is too strong. The peaks will be where it just starts clipping.

If other stations are fine, then I'd say the alignment is fine.

You could verify with a scope but difficult to do without loading the IF. Could be the AGC is not working right.

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 Post subject: Re: Alignment issue?
PostPosted: Feb Sun 02, 2020 10:36 am 
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Joined: Oct Mon 02, 2017 8:53 am
Posts: 399
Location: Panama City, FL 32401
The 3 stations are local, so the radio is definitely dealing with some overload. I have noticed this on some other radios I have but most can handle the strong signals without an issue.


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 Post subject: Re: Alignment issue?
PostPosted: Feb Sun 02, 2020 4:48 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 20963
Location: Utica, NY 13502 (USA)
I agree that it likely is an AGC (AVC) issue. I would first check electrolytic polarities. Positive leads to ground in this radio.

Dave


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 Post subject: Re: Alignment issue?
PostPosted: Feb Mon 03, 2020 1:44 am 
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Joined: Oct Mon 02, 2017 8:53 am
Posts: 399
Location: Panama City, FL 32401
The chassis on this 700 ends in Z3. It has a negative ground as verified by the multimeter. The electrolytics were installed correctly. I noticed when listening late last night the band seemed a little quiet. So today I tweaked the IF's and found the 2nd one was way off,by an entire turn. Massive difference. No more excessive bandwidth from strong stations, and weaker ones heard in between strong ones. Was hearing plenty of weak watery signals around 3PM. Thanks also to Fifties from a previous thread where he mentioned the 2nd IF can often be off. Thanks so much!


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 Post subject: Re: Alignment issue?
PostPosted: Feb Mon 03, 2020 11:09 pm 
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Joined: Jan Thu 16, 2020 12:29 am
Posts: 292
This may be nothing to do with you issue, however I could point out there is no such thing as installing the AGC electrolytic capacitor the correct way around, in many (not all) vintage transistor radios. It astonishes me how across more than half a century of radio design & repairs, nobody else appears to have noticed this.

One reason why the AGC filter electrolytic in vintage radios often fail, is they spend most of their life (when the radio is running) in a reverse polarized condition. If you check the voltage across most of these capacitors with a meter (DVM), you will find off station they have a small DC voltage of the correct polarity applied, due the the bias conditions on the AGC line. However, when a strong station is tuned, the polarity reverses. The solution here is to fit a bipolar electrolytic, which is what I do in a lot of my vintage transistor radios, also with a 0.1uF monolithic ceramic capacitor across that to make sure its RF tight. If RF voltages develop across this capacitor, usually the IF bursts into oscillation.


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 Post subject: Re: Alignment issue?
PostPosted: Feb Tue 04, 2020 2:03 am 
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Joined: Oct Mon 02, 2017 8:53 am
Posts: 399
Location: Panama City, FL 32401
ACORNVALVE wrote:
This may be nothing to do with you issue, however I could point out there is no such thing as installing the AGC electrolytic capacitor the correct way around, in many (not all) vintage transistor radios. It astonishes me how across more than half a century of radio design & repairs, nobody else appears to have noticed this.

One reason why the AGC filter electrolytic in vintage radios often fail, is they spend most of their life (when the radio is running) in a reverse polarized condition. If you check the voltage across most of these capacitors with a meter (DVM), you will find off station they have a small DC voltage of the correct polarity applied, due the the bias conditions on the AGC line. However, when a strong station is tuned, the polarity reverses. The solution here is to fit a bipolar electrolytic, which is what I do in a lot of my vintage transistor radios, also with a 0.1uF monolithic ceramic capacitor across that to make sure its RF tight. If RF voltages develop across this capacitor, usually the IF bursts into oscillation.


Oh now you tell me, after I've recapped all my radios with polarized caps! :mrgreen:
Thanks for the info!


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 Post subject: Re: Alignment issue?
PostPosted: Feb Tue 04, 2020 4:41 am 
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Joined: Jan Thu 16, 2020 12:29 am
Posts: 292
Zenith Royal wrote:
ACORNVALVE wrote:
This may be nothing to do with you issue, however I could point out there is no such thing as installing the AGC electrolytic capacitor the correct way around, in many (not all) vintage transistor radios. It astonishes me how across more than half a century of radio design & repairs, nobody else appears to have noticed this.

One reason why the AGC filter electrolytic in vintage radios often fail, is they spend most of their life (when the radio is running) in a reverse polarized condition. If you check the voltage across most of these capacitors with a meter (DVM), you will find off station they have a small DC voltage of the correct polarity applied, due the the bias conditions on the AGC line. However, when a strong station is tuned, the polarity reverses. The solution here is to fit a bipolar electrolytic, which is what I do in a lot of my vintage transistor radios, also with a 0.1uF monolithic ceramic capacitor across that to make sure its RF tight. If RF voltages develop across this capacitor, usually the IF bursts into oscillation.


Oh now you tell me, after I've recapped all my radios with polarized caps! :mrgreen:
Thanks for the info!


its only the agc cap that is sometimes the issue, the rest are fine.


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 Post subject: Re: Alignment issue?
PostPosted: Feb Tue 04, 2020 6:41 am 
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Joined: Aug Wed 24, 2011 4:35 am
Posts: 4795
Location: Sunnyvale CA
Zenith Royal wrote:
Oh now you tell me, after I've recapped all my radios with polarized caps! :mrgreen:
Thanks for the info!


You did it correctly, which you can easily verify by putting a voltmeter across the cap in question. With no signal, its maybe .3V +positive on the positive end, with strong signal, it drops to nearly zero and, it's effectively a DC signal.

I just checked a Royal 750 on various strength station. The voltmeter is attached across the AGC cap with the positive lead attached to the positive end of the AGC cap, and the negative lead attached to the negative end of the AGC cap. The voltage remains positive, and properly polarized, at all times. I am sure you will find the same situation in your radio, should you check.

Actually, some of the other caps actually do swing positive and negative, by very small values, but not enough to cause an issue - as the people designing it at the time fully understood.


No signal:

Attachment:
royal_750_agc_nosignal.jpg
royal_750_agc_nosignal.jpg [ 246.67 KiB | Viewed 449 times ]


weak station:

Attachment:
royal_750_agc_weak.jpg
royal_750_agc_weak.jpg [ 251.9 KiB | Viewed 449 times ]


Strong signal:

Attachment:
royal_750_agc_strong.jpg
royal_750_agc_strong.jpg [ 253.38 KiB | Viewed 449 times ]


Brett


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 Post subject: Re: Alignment issue?
PostPosted: Feb Tue 04, 2020 8:29 am 
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Joined: Oct Mon 02, 2017 8:53 am
Posts: 399
Location: Panama City, FL 32401
Great pics Brett. According to one source Zenith used 7 KHZ bandwidth filtering, (in this series radios) which accounts for the excellent sounding audio. The 500H used 10 KHZ skirts.


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