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 Post subject: Radio Shack DX394 receiver repair
PostPosted: Mar Mon 18, 2019 7:35 am 
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Joined: Sep Sat 06, 2008 3:04 am
Posts: 211
Location: Benton City, WA
I recently acquired a Radio Shack DX394 on ebay. Unfortunately, it does not work. Google has not been my friend. I got a gazillion hits but all seemed to be pointing to the user manual or the service manual and little or no other information.

The radio appears to work. The frequency display works and changes with dial movement. The audio amp works (if you call white noise "working."). When I tune to what should be a strong local AM broadcast station the S meter shows nothing and no audio is heard (other than the white noise) which tells me that the RF Amp is not working at all.

Does anyone else have any experience with this receiver? Do you know of any decent web sites that highlight DX394 info or mods or repairs?

Thank you.


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 Post subject: Re: Radio Shack DX394 receiver repair
PostPosted: Mar Mon 18, 2019 8:34 am 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Albuquerque, NM 87123
ManualsLib has the service manual.


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 Post subject: Re: Radio Shack DX394 receiver repair
PostPosted: Mar Mon 18, 2019 12:07 pm 
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Do you have a signal generator? Try injecting a high level signal into the antenna input and see if there is any response. Also try using the whip antenna input to bypass the input attenuator switches.

It could be a lot of things including a damaged RF amp. You can either use your signal generator or alternatively an antenna coupled, via a .01 DC blocking capacitor, directly to the primary of T6 which will bypass the front end stage and couples the signal directly to the input of the mixer.

I believe this receiver is new enough that it uses SMD components so be prepared to work with them.

Rodger WQ9E


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 Post subject: Re: Radio Shack DX394 receiver repair
PostPosted: Mar Mon 18, 2019 5:00 pm 
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Joined: Sep Sat 06, 2008 3:04 am
Posts: 211
Location: Benton City, WA
rsingl wrote:
Also try using the whip antenna input to bypass the input attenuator switches.

There is no whip antenna! That was also a surprised but no whip antenna was included. I do want to find one though.
rsingl wrote:

It could be a lot of things including a damaged RF amp. You can either use your signal generator or alternatively an antenna coupled, via a .01 DC blocking capacitor, directly to the primary of T6 which will bypass the front end stage and couples the signal directly to the input of the mixer.

The RF amp fet was my initial suspicion. From past experience with 40673's in vhf transceivers of the 70's that thing blew if you even looked at it wrong! This uses a 2SK209 but same principle.

rsingl wrote:

I believe this receiver is new enough that it uses SMD components so be prepared to work with them.

You are right...and you are wrong! The top board with the rf amp is through hole. The bottom logic board is smt.

I do not have a working signal generator. I bought an S9 but turns out it does not work either and there is not a lot that can go wrong with those (a transistor, or a crystal).

I was rather surprised that no signals are getting through the front inincluding very strong signals.


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 Post subject: Re: Radio Shack DX394 receiver repair
PostPosted: Mar Mon 18, 2019 5:03 pm 
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Joined: Sep Sat 06, 2008 3:04 am
Posts: 211
Location: Benton City, WA
Johnnysan wrote:
ManualsLib has the service manual.

I have the service manual, albeit hard to read the schematics and pictorials but that was my point. Every one of the gazillion links Google gave me were for the user manual and/or the service manual. I was looking for the front end. The service manual said it is either the DC-DC converter or it is the rf amp and said to look at Q24, Q25, and Q26. finding them on the service manuals was a challenge since the resolution was so bad. As I said to Rodger, I suspect the rf amp (2SK209) from past experience but also from past experience even with a bad rf amp fet some signal would get through and none is getting through.


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 Post subject: Re: Radio Shack DX394 receiver repair
PostPosted: Mar Mon 18, 2019 8:52 pm 
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Southerner,

I have PDF copies of the service manual along with a separate PDF of the schematic which are very readable, they are 14 meg and 10 meg files respectively to compare with what you have to see if these are different. I can't remember where I got them but if you need them I can upload them to google drive later and send you a link. My daughter has her first high school soccer game starting soon and I have to get there ahead of time to get set up for game photography so I can't upload them until I get home later.

The sensitivity is already somewhat reduced on the medium wave range so a strong over the air signal may not make it through in the MW range without an intact RF amp.

Well that is good that the front end is regular through hole as that will make any needed repair easier. I have a 394 but I can't remember ever having it out of its case. A signal generator will definitely help because it would be nice to signal trace forwards from the 455 Khz. detector and see where the signal loss is occurring. Good luck!

Rodger WQ9E (also a former southerner from the gulf coast of Mississippi :) )


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 Post subject: Re: Radio Shack DX394 receiver repair
PostPosted: Mar Tue 19, 2019 9:09 am 
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Joined: May Sat 12, 2012 1:33 pm
Posts: 1689
Location: Rochester, NY.
What G, S and D voltages are you seeing on the 2SK209?

https://www.google.com/url?sa=t&rct=j&q ... jXC319cMIi


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 Post subject: Re: Radio Shack DX394 receiver repair
PostPosted: Mar Wed 20, 2019 4:18 am 
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Location: Virginia USA
Does the receiver have an internal antenna for AM reception or did you connect an external antenna of some kind?


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 Post subject: Re: Radio Shack DX394 receiver repair
PostPosted: Mar Wed 20, 2019 4:27 am 
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Joined: Sep Sat 06, 2008 3:04 am
Posts: 211
Location: Benton City, WA
bfo wrote:
Does the receiver have an internal antenna for AM reception or did you connect an external antenna of some kind?

The only internal antenna is the AM/BCB ferrite loop which should have worked since I was trying to hear the local 610kc/s AM broadcast station. I did also hook up a 10ft piece of wire and there was never any signal heard and no signal indicated on the S meter.


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 Post subject: Re: Radio Shack DX394 receiver repair
PostPosted: Mar Wed 27, 2019 10:18 am 
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I would try to detect the local oscillator, or the oscillating parts of the converter/mixer(s). A dead oscillator turns the unit into a single fixed channel TRF radio (at the if).

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 Post subject: Re: Radio Shack DX394 receiver repair
PostPosted: May Fri 17, 2019 9:03 am 
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Joined: Sep Sat 06, 2008 3:04 am
Posts: 211
Location: Benton City, WA
rsingl wrote:
Do you have a signal generator? Try injecting a high level signal into the antenna input and see if there is any response. Also try using the whip antenna input to bypass the input attenuator switches.

I do not have a working signal generator

rsingl wrote:
It could be a lot of things including a damaged RF amp. You can either use your signal generator or alternatively an antenna coupled, via a .01 DC blocking capacitor, directly to the primary of T6 which will bypass the front end stage and couples the signal directly to the input of the mixer.

I measure 0 volts across D29 and D30 which are 2 static suppressor 7.5v zener diodes.
I tried hooking a short (2ft) wire to C29 which is the capacitor just ahead of the 3SK195 dual gate mosfet rf amp but no increase in signal so no signal at all even on the local AM powerhouse.
D33 and D34 are also listed as ZSML-7.5Y zeners and I measure 6.15 volts on each side of those zeners.
rsingl wrote:
I believe this receiver is new enough that it uses SMD components so be prepared to work with them.

It is mostly smt including the 3SK195 dual gate mosfets.

It is hard to follow the service manual because the pictorial boad layouts are in gray scale and light. Also the first page of the schematic cut off part of the left side so I do not even see D29 and D30 or C29.

Thank you.
Jim


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 Post subject: Re: Radio Shack DX394 receiver repair
PostPosted: May Fri 17, 2019 7:23 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: SoCal, 91387
An easy way to check if the osc is working is to do the second radio test.


Attachments:
Second Radio Test.Jpeg
Second Radio Test.Jpeg [ 71.1 KiB | Viewed 334 times ]

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 Post subject: Re: Radio Shack DX394 receiver repair
PostPosted: May Fri 17, 2019 8:16 pm 
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Joined: Sep Sat 06, 2008 3:04 am
Posts: 211
Location: Benton City, WA
fifties wrote:
An easy way to check if the osc is working is to do the second radio test.

Nope, no DX394 oscillators heard when sweeping across 550-1710khz slowly. The second receiver was in CW and then in AM but nothing heard from the DX394 when the target receiver was set to 1010 and also at 920.

It does seem strange that the DX394 obviously hears something as the beacon band (150 to 550khz) returns S meter readings on computer generated noise coming from this computer . That is the only time I have seen the DX394 LCD s meter indicate anything on any signal!

Thank you.


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 Post subject: Re: Radio Shack DX394 receiver repair
PostPosted: May Fri 17, 2019 8:49 pm 
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Joined: Sep Sat 06, 2008 3:04 am
Posts: 211
Location: Benton City, WA
One other thing that seemed like unusual behavior is that I would get very loud computer generated hetrodynes in the VLF range from 150 up to 500 but at exactly 500khz the noise would stop and no S meter indication or any other indication of signals.

Again, I can not hear any of the local VLF beacons, just the computer generated RFI noise.

I assume that the 2 static suppressor zener diodes D29 and D30 are shot but not sure about D33 and D34 although it is strange to have 6.15v on both sides of those 2 zeners.

Thank you.

Jim


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 Post subject: Re: Radio Shack DX394 receiver repair
PostPosted: May Fri 17, 2019 9:09 pm 
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Southerner wrote:
fifties wrote:
An easy way to check if the osc is working is to do the second radio test.

Nope, no DX394 oscillators heard when sweeping across 550-1710khz slowly. The second receiver was in CW and then in AM but nothing heard from the DX394 when the target receiver was set to 1010 and also at 920.

This indicates that the osc is dead, and might be your starting point for repair, beginning with the Transistor.

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