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 Post subject: AIR-O-EAR 710-WN (Nova-Tech) Aircraft Receiver
PostPosted: May Sat 10, 2014 9:53 pm 
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Location: Bethpage, NY
Hi! has anyone come across this receiver's schematic? It looks to be based on a Granco 610U FM receiver, but with an added squelch control. I probably can replace the electrolytics and Black Beauty (bumble bee) capacitors - they are prone to failure), but one thing gets me is a 50mfd 150v electrolytic from the base of the squelch control to ground; the ground has the positive of the electrolytic to ground! Is this part mounted incorrectly? (it seems to be a relatively new part, not original). Anyone have a schematic for this receiver, I would gladly appreciate it. I bet the 610U schematic is a good start, but this squelch electrolytic has me stumped! :?


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 Post subject: Re: AIR-O-EAR 710-WN (Nova-Tech) Aircraft Receiver
PostPosted: May Mon 12, 2014 5:13 pm 
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2nd request, if no schematic is available, could another owner of one of these radios open theirs to see what REAL value capacitor is supposed to come from the squelch control to ground (speaker shell)? I suspect someone put this one in instead of say, a .47mfd paper cap read wrong. I have never seen an electrolytic used this way, and won't plug it in until I get some confirmation. Thanks anyone!


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 Post subject: Re: AIR-O-EAR 710-WN (Nova-Tech) Aircraft Receiver
PostPosted: Jun Fri 13, 2014 11:36 pm 
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Location: Walden, NY USA
I sent you three pages of the model 711 via a PM. I hope the files came through. If not, send me your regular email and I will attach them there

Hope this helps

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 Post subject: Re: AIR-O-EAR 710-WN (Nova-Tech) Aircraft Receiver
PostPosted: Jun Thu 19, 2014 4:45 pm 
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Did you get the schematics?

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 Post subject: Re: AIR-O-EAR 710-WN (Nova-Tech) Aircraft Receiver
PostPosted: Jun Wed 25, 2014 2:16 pm 
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Yes, but it was for the 711, not 710. I have a 711 and it works great after replacing the filter caps. I guess I'll just wing this one based on the existing parts. at least I found out that there is a selinium rectifier off the squelch control, and not a capacitor. I removed the backward electrolytic on mine and the radio howled when touching the squelch control; I will replace the two bumblebee caps that always fail.


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 Post subject: Re: AIR-O-EAR 710-WN (Nova-Tech) Aircraft Receiver
PostPosted: Jul Tue 01, 2014 1:15 pm 
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Location: Warner Robins, GA
Measure the voltage on the squelch control where the capacitor connected. Most likely the voltage there will be negative which will explain why that capacitor has its positive lead grounded.


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 Post subject: Re: AIR-O-EAR 710-WN (Nova-Tech) Aircraft Receiver
PostPosted: Jul Wed 03, 2019 8:55 pm 
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Joined: Dec Thu 13, 2007 7:15 pm
Posts: 12
Location: Apex, NC
Hi, both the 710 an 711 radios use a similar but very primitive squelch circuit that uses both the B+ from the SS rectifier and -165VDC (B-) supplied from a separate SS diode with a electrolytic filter capacitor + side at ground. The squelch pot applies a negative voltage to the grid of the 1st audio amp, the more negative, the more it reduces the tube amplifying the background noise until a strong signal (pilot or control tower talking) overrides this negative (bias) on the grid. Once the transmission stops, the negative voltage on the grid again (squelches) the background noise.
This radio is a re-purposed Granco 610U FM receiver converted to receive VHF aviation band. The aviation band uses amplitude modulation, so the FM ratio detector is converted to a simple AM envelop detector. I recently purchased a 710-WN and after several hours of tracing out all the circuitry, updated a Granco 610U schematic to include all the Nova Tech modifications including the addition of the squelch circuitry. Also added the radio to RadioMuseum web page.


Attachments:
File comment: Modified Granco 610U schematic for AIR-O-EAR 710-WN (Nova-Tech)
Nova_Tech_AirOEar_710WN_Schematic.jpg
Nova_Tech_AirOEar_710WN_Schematic.jpg [ 203.25 KiB | Viewed 420 times ]
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 Post subject: Re: AIR-O-EAR 710-WN (Nova-Tech) Aircraft Receiver
PostPosted: Jul Wed 03, 2019 9:14 pm 
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Joined: Dec Thu 13, 2007 7:15 pm
Posts: 12
Location: Apex, NC
Hi again, thought I'd add a list of modifications Nova Tech made as seen in the schematic:
1.) Convert FM ratio detector to AM envelop detector.
2.) Added AVC circuitry to RF amp, 1st and 2nd IF amp stages. Granco 610U did not contain any AVC circuitry.
3.) Reduced B+ voltage feeding 2nd IF amp to approx. +48 VDC. Reason is that the 2nd IF stage also acted as a limiter which would cause distortion for AM reception. Voltage reduction would reduce gain so to eliminate clipping of AM signal.
4.) Added 25uf electrolytic cap to 50C5 cathode for fixed bias.
5.) Added 470 ohm resistor across 19T8 filament to reduce cathode emission. Not sure why this was done.
6.) Added headphone and auxiliary speaker jacks to front panel.
7.) And of course, added squelch circuitry, (resistor network, B minus (-DCV supply and squelch pot).


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 Post subject: Re: AIR-O-EAR 710-WN (Nova-Tech) Aircraft Receiver
PostPosted: Jul Thu 04, 2019 2:48 am 
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Joined: Jan Thu 01, 1970 1:00 am
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#5 was probably done to either reduce noise or reduce gain.


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