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 Post subject: Realistic DX-302 Receiver
PostPosted: Nov Thu 16, 2017 7:42 pm 
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Joined: Aug Thu 25, 2016 10:03 pm
Posts: 27
Location: Houston, Texas 77072
Does anyone have the service manual for the Realistic DX-302 receiver? Preferably in a pdf?

I found a previous post (2014!) that gave a link to one but that link is dead now.
I googled it and found another one, but seems a bit shady, wants to go through a download
service and asking or CC info and such. No Way!

Or if someone has a readable schematic would even help. I have one I downloaded but it's very poor quality
and almost unusable for troubleshooting.

It has been sitting for at least 15 years, so when I went to fire it up I discovered the infamous nylon gear for the MHZ
dial was split. Then I find that somehow the tuning is way off, MHZ starts at 5, goes through about 27 or so and then starts at 0 !!!
I did not have any of the linkage disconnected at all. Only pulled the gear out the front.

And... has plenty of static, does change when you connect or disconnect the antenna (long wire) but Zero signal regardless of where you tune to,
same static no change.
Almost like only the display is changing but perhaps it's not really changing the tuning... but the variable caps are opening and closing.

I bought this one new in about 1982 so hate to let it go to waste. Now that I'm retired, kind of wanted to get back into SWL some.

Thanks!

Ron Taylor
Houston, Texas


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 Post subject: Re: Realistic DX-302 Receiver
PostPosted: Nov Thu 16, 2017 8:57 pm 
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Joined: Jun Sun 19, 2011 2:31 pm
Posts: 4777
Ron,

Check your PM, I uploaded my copy to Dropbox (about 6 meg PDF) and sent you the link via PM.

Rodger WQ9E


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 Post subject: Re: Realistic DX-302 Receiver
PostPosted: Nov Thu 16, 2017 9:35 pm 
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Joined: Sep Sat 06, 2008 3:04 am
Posts: 184
Location: Benton City, WA
Ron_Taylor wrote:
Does anyone have the service manual for the Realistic DX-302 receiver? Preferably in a pdf?

Ron, I know it took me a bit to figure out how to tune in a station with mine. Also they have a very common problem of the nylon gear, usually the mhz gear, cracking so it does not change the preselector tuning when you rotate it. I have a service manual in pdf and added a larger size schematic to it so I can read it or get a printed copy if I need.

I bought the FRG7700 receiver also as several some recommended it but I found the DX302 had much better sounding audio to my ears and my DX302 seemed a little more sensitive and selective than the frog.

Good luck.
Jim


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 Post subject: Re: Realistic DX-302 Receiver
PostPosted: Nov Sat 18, 2017 1:24 pm 
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Joined: Apr Thu 20, 2017 2:35 pm
Posts: 36
The DX-302 was the first digital communications receiver that I ever owned. Much disappointment with its performance. Terrible bassy audio with no highs. Synthesizer noise. IF bandwidths both too narrow. Very poor dynamic range.

I went back to the DX-160a receiver and added a frequency display. Much superior to the 302. The FRG-7, FRG-7000 or the 7700 blew the DX-302 out of water. Not even close. The R-390a later purchased has been a staple in my shack since.

I wish you luck in restoring yours.

Al


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 Post subject: Re: Realistic DX-302 Receiver
PostPosted: Nov Sat 18, 2017 4:41 pm 
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Joined: Jun Sun 19, 2011 2:31 pm
Posts: 4777
Al,

I agree with all of your criticisms of the DX-302 except I bet that most of the noise you heard was from the digital counter since the Wadley Loop doesn't generate high levels of synthesizer noise like the PLL/VCO synthesis circuits that were used during that time period. The oscillators in the DX-302 do have some phase noise, as does every oscillator including even crystal oscillators, but it is minuscule compared to the tremendous noise sidebands created when a phase locked loop system tries to rapidly both step and control the stability of a voltage controlled oscillator.

It is the rapid shift in VCO frequency that creates the tremendous phase noise sidebands and this is why the "hybrid" synthesis receivers like the National HRO-500 and Galaxy/Hy Gain R-530/1530 were fairly quiet since the synthesis section only involved choosing the very coarse frequency steps which allowed the designer to use a fairly relaxed lock up time resulting in greatly reduced noise sidebands. The Drake DSR/MSR receivers followed this same basic approach of using a PLL/VCO for the coarse range and a regular analog VFO for the fine tuning and Drake continued this practice with their 7 line gear. Ten Tec also experimented with this hybrid approach in an attempt to reduce the impact of phase noise.

Under a lot of conditions phase noise isn't a huge deal but it comes into play when trying to listen to weaker signals, especially in a crowded band. The easiest way to visualize the impact of significant phase noise is you have the receiver tuned to a specific station with a fairly narrow IF bandwidth selected and although the signal is weak there is no interference directly on that frequency. This would normally result in very good reception. But if the receiver has high phase noise what is happening is the "noise" output from the synthesizer is actually a spectrum around the desired synthesizer output frequency and these additional output frequencies then mix with other (undesired) signals in the band and also convert them to the receiver IF frequency. The phase noise output drops off as it is removed further from the desired output frequency but when it mixes with strong signals the resulting IF product may still be stronger than the desired signal and even if not this spectrum of intermixed signals is greatly increasing the effective noise floor.

The first brand new transceiver I bought was a Yaesu FT-980 and it had a very noisy synthesizer. The Collins KWM/HF 380 transceivers were similarly bad as was the 651S-1 receiver and pretty much all of the synthesized ham gear of that era. Once DDS (direct digital synthesis) became cost effective there was a huge leap forward in performance and reduction of phase noise. The DX-302 has a number of shortcomings because it was intended to hit a fairly low price point. However during this time period if you had given a design team the requirement of producing a PLL/VCO synthesized receiver and Wadley Loop receiver both to hit a $400 price point you would have ended up with two pretty decent receivers but I am confident the Wadley design would have had the better overall performance.

Rodger WQ9E


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