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 Post subject: FM sensitivity problem? Realistic STA-75 Fixed!
PostPosted: Jun Wed 05, 2019 4:46 pm 
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Joined: Mar Tue 03, 2009 11:12 pm
Posts: 1534
Location: Hutchinson KS
Another fm receiver problem! This thing seems to work ok except it only receives strong signals even with an external antenna. There is no loud hash between stations like the usual fm tuner does. Acts like the fm mute switch is on. I assumed the FM RF amp fet is bad, but replacing it doesn't change anything. I varied the agc line with external power supply, both - and + a couple of volts and that just makes it worse. Maybe I'm barking up the wrong tree and should be looking at the IF stages? Not much service lit on this, just the tiny schematic in the owners manual.

Added edit; Don't know whether to feel sheepish or what, but replacing the electrolytic caps in the 20 some volt regulator circuit fixed this darn thing! Accidentally scoped the supply B+ going to the IF, FM detector, tuning meter, etc. circuit and there was a large amount of ripple. DC wise voltages ok but apparently the hash upsets things! I broke one of the first rules of servicing; scope the B+ lines!


Last edited by SteveT on Aug Thu 15, 2019 7:17 pm, edited 1 time in total.

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 Post subject: Re: FM sensitivity problem? Realistic STA-75
PostPosted: Jun Sat 22, 2019 12:45 am 
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Joined: Sep Thu 23, 2010 6:37 am
Posts: 11707
Location: Powell River BC Canada
At 2uV IHF sensitivity it should be hotter than a firecracker. Back in the day with
Sansui we had endless trouble with bad ICs in the FM IF robbing the gain.

Check out the IC in the snip from the online .pdf

Attachment:
Realistic STA 75 FM IF  IC MAY BE BAD.JPG
Realistic STA 75 FM IF IC MAY BE BAD.JPG [ 23.24 KiB | Viewed 483 times ]

_________________
de
VE7ASO VE7ZSO
Amateur Radio Literacy Club. May we help you read better.
Steve Dow
ve7aso@rac.ca


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 Post subject: Re: FM sensitivity problem? Realistic STA-75
PostPosted: Jun Tue 25, 2019 4:05 pm 
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Joined: Mar Tue 03, 2009 11:12 pm
Posts: 1534
Location: Hutchinson KS
There must be a different versions of this model, the one I have does not have IC's in the IF circuit, just transistors. It's on the back burner for now!


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 Post subject: Re: FM sensitivity problem? Realistic STA-75
PostPosted: Jul Thu 11, 2019 12:16 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 20280
Location: Warner Robins, GA
It could be a simple alignment issue.

If this is something you just bought, does it look like someone has been in there with the golden screwdriver?

If it is an alignment issue, I suggest trying to find the service info before doing the alignment unless you have a lot of experience aligning FM tuners.


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