dx-394 not picking up

[blainenbecky]

I came across a Radio shack dx-394 (A) at a yard sale last week and I thought it was a good deal at 15 bucks
but when I got it home and plugged her up very thing seemed to be working fine but it will not pick up any signals .
I think it must have been wracked by a surge from the antenna.
I heard on some old net pages

http://forums.qrz.com/showthread.php?368805-HELP!!!-Radio-Shack-DX-394-Receiver-problem

that there is some kind of first stage RF FET transistor that Pops out and need to be changed
have any of you guys had to deal with this or know of a post on line that takes you step by step on how to do it
I really like like this radio and would like to get it going if I can
Thanks for any help you may have

 

[mikewazowski]

Thread moved to HF Equipment forum.

 

[Boombox]

Does it pick up local AM stations at all?

Does it make any difference if you just use the whip antenna?

Have you exercised or checked the attenuator switch on the back of the radio? If it's set to -20 db, you won't get many signals. It works mostly with the Lo-Z coax antenna input. The attenuator switch is in between the Lo-Z coax antenna input and the RF circuitry. So if you're using a coax input antenna, exercising the attenuator switch might clear it up because switches can get oxidised if not used often.

The DX-394's High-Z (whip + High-Z ) inputs are both protected by diodes. There also is a bit of circuitry for a static charge to go through before it hits the RF amp FET.

If you exercised the attenuator switch, and you tried the whip antenna, or tried 15-20 ft. of wire on the Hi-Z input -- AND you you still aren't getting any SW stations -- AND you aren't getting any MW stations off the loop, either -- you may have a blown RF FET.

Also, remember -- some SW conditions have been crappy lately. Last time I tuned the SW bands at night 31 meters was completely dead and there were maybe 5 stations on 49 meters. Some days there are a lot of signals, others not so much. So before you think your radio's dead make sure it wasn't just propagation.

I'm not aware of the RF FET being able to 'pop out'. It would be something you would solder. It's Q1 on the schematic, and looks like a dual gate FET. There are schematics and other information available on the Yahoo DX-394 eGroup.

The plus with the DX-394 is that most of the circuitry is on one board, and compared to a lot of other radios, it's fairly easy to work on. The negative is that a lot of the circuitry is surface mount (I don't think the transistors are, though) so you'd have to be extra careful when soldering.

PS -- if you get it working, and it only tunes one way, shoot some tuner cleaner down the side of the tuner shaft, and work the tuner. You may have to do it several times. Sometimes the metal contacts inside the DX-394's tuner can get oxidised if it isn't used much. I had this happen to mine a couple times. There is enough play down the side of the tuner shaft to let a bit of cleaner through to where it can hit the contacts.

 

[blainenbecky]

(((((Does it pick up local AM stations at all? )))))
I got 2 very small weak signals but could not make out the words .
(((((Does it make any difference if you just use the whip antenna?)))))
no,I tried them both,no difference at all
(((((Have you exercised or checked the attenuator switch on the back of the radio? )))))
yep,tried both positions. but I did not work it over ,I will try that in the morning
and thanks for all the Info

http://www.monitor.co.uk/radio-mods/dx-394/dx-394-manuals/dx-394-service-manual.pdf

 

[Boombox]

Sounds like it could be a blown RF amp FET.... :-(

Service manual should help.

When the DX-394's working, its a good radio. Doesn't need much wire for an antenna. Fairly sensitive radio.

 

[blainenbecky]

pic test
this is the bottom side of the board ,looking at Q-1 does this look like a FET or should I be looking on the TOP side of the board ?

 

[blainenbecky]

here is the Top side of the board



and the Bottom side

 

[blainenbecky]

I think this might be one on ebay but its hard to tell what it looks like

3SK195 Encapsulation SOT 143 N Channel Dual Gate mos Type TV Tuner | eBay

 

[majoco]

If it's a dual gate FET it has 4 legs - halfway between your pencil tip and the circle.

http://pdf.datasheetcatalog.com/datasheet/toshiba/1433.pdf

You can see in your pic that it has one fat leg at the bottom right corner.

 

[blainenbecky]

OK thanks .now that I am looking at it I see 2 more of them lower with what I think the same marks
I wonder if they are related to the problem and if I should change them all 3 out ?
Man them things are small.I am going to have to put at least 2 pair of eye glasses on just to see them puppy's,lol
Thanks again

 

[Boombox]

I think Q1is the one on top of your photo there, being that it looks like a dual gate FET and it's closest to the marking that says "Q1".

If you look on page 21 of the PDF you linked, you'll see the outline of it on the PCB (lower left side of the page).

The other two FETs down below it in the photo are probably Q3 & Q4. You'll see there are nearby markings saying "Q3" and "Q4" also. They match the illustration on page 21 of the PDF.

Be really careful with surface mount components. I almost wrecked the headphone jack of my Sangean PR-D4 trying to bypass the dropping resistors (I actually did wreck the left headphone channel). As I learned to my chagrin, surface mount traces and components just don't take much heat. But then, you probably already know that.

If I were in your shoes, I'd just change out Q1, and see what happens. Maybe buy the other two (if they're cheap enough) just in case. The fact you heard *something* in your speaker when you tuned to the AM band tells me the first mixer FET's (Q3 & Q4) may probably be working O.K.

I've heard of other radios' RF amp FET's getting zapped, and usually all the guys do is change out the RF amp FET and they're back in business. Hopefully, that's the case with your 394.

Good luck. Looks like you're on your way to fixing the 394.

 

[blainenbecky]

thank you so much for the help

(((((Be really careful with surface mount components. I almost wrecked the headphone jack of my Sangean PR-D4 trying to bypass the dropping resistors (I actually did wreck the left headphone channel). As I learned to my chagrin, surface mount traces and components just don't take much heat. But then, you probably already know that. )))))

actually the last time I tried to change a surface mount component it was a disaster but I have since acquired a better soldering iron with adjustable heat and a finer tip so hopefully it goes a little better this time .
the only
3SK195 DUAL GATE I can find is on eBay and the guy is on vacation so looks like it will take 3 to 4 weeks for the part to come in .
thanks again for the help and I will post how it all comes out if I every get that tiny thing mounted back on the board

 

[blainenbecky]

my part came in today
man I can't believe how small these things are

 

[KC8ESL]

Do you have a heat gun by chance? Heat up the board and pluck the smd component with tweezers.

https://www.sparkfun.com/tutorials/100

 

[blainenbecky]

I have a large heat gun about the size of a hair dryer is that the kind you are referring to ?
and would the neighboring components turn lose as well ?
I do have one of the plunger sucker type solder remover things ,I have used it before but not on any thing this dang small .
thanks

 

[KC8ESL]

Yes, the large style... I've used them in a pinch on my bench when my reflow station was down. It worked but you need to be real careful and not overheat the board. Look at the spark fun link I shared.

 

[blainenbecky]

well,its a little crooked but its on and working fine .
picking up stations now really good
Thanks for all your helps guys
and down below there is a link to a you-tube showing the radio working
https://www.youtube.com/watch?v=_ol4Zr3QrpU&feature=youtu.be



http://youtu.be/_ol4Zr3QrpU

https://www.youtube.com/watch?v=_ol4Zr3QrpU&feature=youtu.be

 

[blainenbecky]

now that my radio is working I am doing a little research on youtube and I see they made a A model as well as a B model
I am trying to figure out which one I have but I have no sticker indicating A or B
here is a pic of a B model from a screen shot off of youtube


and here is a pic of my radio



dose anyone know how to tell which one I have by other means other then the sticker ?

 

[Blackswan73]

If there is no sticker, it is probably an A model

 

[KC8ESL]

Welcome to the horribly nerve racking club of SMT! For those of us who are wondering (myself included) what iron/temp/wattage did you use to do the job?

Not burning a trace or lifting a pad is an accomplishment in the world of SMT and direct heat soldering irons.