Radio Shack DX 394 is off freq

hanlonmi06

Member
Premium Subscriber
Joined
Jul 16, 2009
Messages
143
Location
Pittsfield Twp, Michigan
I've dug out my old DX-394 for kicks recently and noticed that the receive is off by about 5khz in AM mode and it is very difficult if not impossible to tune anything in either LSB or USB. For just puttering around on some large shortwave broadcasters I can get by, but I happened across some traffic on 11.175 mhz and struggled to make anything out, so now I am motivated to take a run at a repair. I have a copy of the service manual, but its been a while since getting into this kind of troubleshooting. Any tips on where focus my efforts? Admittedly, I'm a noob at something like this, but I'd like to learn and try.
 

Kfred

Member
Joined
Sep 16, 2007
Messages
83
Location
Early, TX
not an electronics tech, but i would start with the power supply. capacitors age and change value over time. some resistor values also drift. When that is working properly look at VFO , same problem. Schematic should have voltage amounts listed at test point. Dont know this particular radio. some radio's had instability built in at the factory. kfred
 

hanlonmi06

Member
Premium Subscriber
Joined
Jul 16, 2009
Messages
143
Location
Pittsfield Twp, Michigan
"yes"....

I have some older equipment and I've had trouble with the signal generator that I have. I don't have the quartz sensor, but according to the manual that's called out for a clock adjustment which I wont be performing. I've got a 100mhz sillyscope, various meters, freq counter...all hobby grade type stuff (though the oscilloscope is a good name brand, I forget the maker, but it is an older digital scope).

I don't expect this to be a professional job, and this radios is more sentimental than anything else but I'd like to take a crack at seeing what I can do. I have done something like this before with the gear I have, but its just not something I do on a regular basis. I read the the manual a couple times, and as stated there, I will be starting with the power supply's and seeing where that leads. Everything 'works', so maybe I get lucky and its a leaky cap that's throwing things off a tad. I do have a background in industrial electrical equipment, electrical controls engineering in manufacturing, so I'm "wicked smaht" with a Fluke in that fashion...so, anyways, Its a fun little project that I'd like to take a run at.
 

fineshot1

Member
Joined
Sep 17, 2004
Messages
2,529
Location
NJ USA (Republic of NJ)
Apparently there are 3 different (A, B and C versions) versions of this receiver. Anyone know how to tell the difference
between them? Perhaps an actual A, B or C in the model number? Just wondering in case I see a used on for sale
somewhere.
 

hanlonmi06

Member
Premium Subscriber
Joined
Jul 16, 2009
Messages
143
Location
Pittsfield Twp, Michigan
I've read that as well. I will be getting started on this project this weekend. I will try to find out how to tell the differences since I am curious as well.
 

merlin

Active Member
Joined
Jul 3, 2003
Messages
882
Location
South East Idaho
Sounds like the VCO reference has slipped a bit. that could be a voltage regulator issue, or the radio took a thump. You need an accurate receiver or equipment to properly adjust the reference then never touch it.
A redneck way is tuning WWV and do the adjustment, everything else should be calibrated.
I had one of these years back, not a bad receiver for bargain basement.
It helps if you understand PLL synthesizers and their workings.
 
Last edited:

hanlonmi06

Member
Premium Subscriber
Joined
Jul 16, 2009
Messages
143
Location
Pittsfield Twp, Michigan
So on a quick first pass it looks like the serial number starts with a letter followed by numbers. I have a C001786 so I assume I have a "C" version.

I got the radio benched, covers off, volt meter out and started at the top with the "reception check" flow chart in the service manual, which the first step once determining you have an issue is checking the DC-DC converter and second step checking the voltage regulator. The DC-DC should have an output of 5v, mine comes in at 5.034. Moving on to the voltage regulator Q30, I have high voltage readings on all three legs of the PNP transistor.

I am 0.5v high on the base, its called out as 13.0v, I read 13.5. The print shows about 0.1v drop from collector to emitter, 13.8 to 13.7. My voltages are 14.25 and 14.25, so essentially its high, and doesn't really reflect the voltage drop I should see. So, do I have a leaky capacitor issue, or actually a "mostly bad" voltage regulator? At any rate, it looks like as others have suggested, we have some sort of power supply issue. This I think I can handle, determining a bad component or two and doing a parts swap. Hopefully someone can maybe pull up a copy of the schematic and offer some guidance?

C144 looks like the big ole 'lytic cap for filtering connected right with Q30 voltage regulator. Physically there's no obvious signs of problems. If it was leaky or failing, would it cause the voltage to be high or low at Q30, or does it just depend?
 

majoco

Member
Joined
Dec 25, 2008
Messages
3,872
Location
New Zealand
I wouldn't be overly concerned about the voltages around Q30 - almost every place that it feeds goes through another voltage regulator. AM and FM can be off by a few kHz and still be intelligible but SSB? No! I see there are two crystals by the big chip, one is 32.768kHz which is just to run the clock, but the 4.5MHz seems to do all the timing and frequency control via the microprocessor - but it doesn't have a trimmer to adjust the frequency! However the first couple of sections of the alignment procedure detail how to adjust the VCO and the 2nd oscillator - BUT - and it's a big BUT - you have to adjust the frequency to better than 5Hz! So theoretically your frequency counter should be at least 4 times better, say to within 1Hz! So first of all, get your counter tested! There is a very good method of doing that against WWV using an o'scope and an audio analyser program but it's quite tricky. Anyway, read through the alignment instruction first. DON'T touch the IF alignment - it rarely goes out far enough to make much difference.
 

hanlonmi06

Member
Premium Subscriber
Joined
Jul 16, 2009
Messages
143
Location
Pittsfield Twp, Michigan
merlin - I had a pretty good background in electronics in college. 20 years ago......had a real good semester course in communications electronics where we went in to real detail on modulation types, pll circuits, did lots of lab work. a long time ago. I've more or less forgotten all the details but suffice to say I can muddle my way through at the hobby level.

majaco - thanks for the guidance and suggestions.

when i get back down to the shop, i will try to at least complete the voltage checks, stick to the service manual troubleshooting process...i appreciate the suggestions, its like a fun learning process. second opinions n all. i will tinker with the freq counter, make sure it still works. its as much an excuse to just get down there and putter, as it is to have a goal to try to make something work better. ill do my checks, and maybe get set up with the adjustment process and post my findings.

ive basically owned this radio since i bought it new. it was never anything remarkable, but i have a distinct memory of this thing being my attainable unicorn since I figured i could never afford a Drake, or JRC NRD-xxx out of the the old WRTH ad sections.
 

hanlonmi06

Member
Premium Subscriber
Joined
Jul 16, 2009
Messages
143
Location
Pittsfield Twp, Michigan
I started the VCO alignment procedure. Got everything hooked up:

step 1: set freq to 29.999, measure for 21v +/- 0.5v and I get 21.32v
step 2: set freq to 150khz and measure for 21v +/- 0.5v and I get 1.944, with a freq of 45.1541

I've stopped at this point, haven't made any adjustments, as the procedure calls out the tolerance of 21v +/- 0.5v. Being nearly 20 volts off seems way off to me, so I didn't want to start turning things right out of the gate...
 

hanlonmi06

Member
Premium Subscriber
Joined
Jul 16, 2009
Messages
143
Location
Pittsfield Twp, Michigan
...which doesn't make sense because i can scroll from 29.999mhz on down the bands and the voltage continuously goes lower as the freq goes lower. So at 10.000 mhz, the voltage is about 5v, which is correct according to print. Could there be a typo in the service manual and it should be like 2.1v at 150khz?
 

majoco

Member
Joined
Dec 25, 2008
Messages
3,872
Location
New Zealand
Hmmm - that doesn't seem to be right according to my intuition! I would expect those voltages to control a phase-locked loop oscillator and the voltages you are getting is what I would expect - high voltage produces low capacitance in the varactor and therefore high frequency, at the low frequency end you want high capacitance from low voltage which is what you are getting - BUT the instructions say you should get 21v at both steps. I'm thinking a misprint in the manual! Especially as you are getting the 45.15Mhz requirement at TP6. The block diagram right at the start of the manual shows that the first oscillator injection frequencies are 45.15MHz which you have already measured (and correct) and right up the high frequency end should be 74.999MHz - can you check that please? If you do get 75MHz (or near enough!) then the PLL is working correctly and the manual is a misprint!

OOPS - your post came as I was typing mine! The 10v at TP5 is a DC reading which is what controls the PLL.
 

hanlonmi06

Member
Premium Subscriber
Joined
Jul 16, 2009
Messages
143
Location
Pittsfield Twp, Michigan
At 29.999 I'm reading 75.0075mhz on the meter.

What I was referring to with TP5 being at 5 volts at 10.000mhz was the notation at the bottom of the prints that the voltages called out in the ovals are all with volume down, radio in AM mode and at 10.000mhz. So when I realized that, I scrolled the tuning knob down from 29.999 and watched the voltage steadily drop as I tuned. So, from what I can glean from the readings called out in the prints and my limited understanding at this point, the readings all match and the service manual has a typo probably, but brings me no closer to understanding why the radio is off by 5k.

I can tune WWV at 10 and 15 mhz pretty well and I definitely have be 4-6khz down in freq to get it tuned in.
 

majoco

Member
Joined
Dec 25, 2008
Messages
3,872
Location
New Zealand
Good - I suggest what you do now is go on to re-align the 2nd VFO as per the alignment instructions - this could be out by 5kHz too.

One thing has me a bit baffled - I see from the owners handbook that there is a 'fine tune' control. Now I've had problems with these in the past - does it tell you anywhere to set this to a reference point when you're not using it? If you tune to the nominal WWV - can you bring it back up to the correct frequency with the fine tune control? - or does it only have an effect on LSB or USB?
 
Last edited:

hanlonmi06

Member
Premium Subscriber
Joined
Jul 16, 2009
Messages
143
Location
Pittsfield Twp, Michigan
So far as I can tell the fine tune adjusts the frequency in all modes. It allows the final digit to be adjusted, so 28.500.x, or 1600.x or 150.x can all be adjusted.

As for the 2nd local oscillator adjustments, the mystery deepens...

Step 1. I read 44.5459, adjusting VR2 has no affect at all
Step 2. I read 44.5459, I can adjust L6 and get it only as low as 44.5426 and no lower.
Step 3. going back and forth, Vr2 has no affect, L6 adjusts the reading in either of the two channels selected in steps 1 and 2, and always shows the same frequency on the meter.
Step 4. I read 44.5426 after completing the adjustment and "loop" of step 3.

According to the procedure for each channel selection it calls out a different reading. I don't see that on my readings, and VR2 has no affect on anything. Based on the probable type discovered\ earlier, I'm a little leery on how well to trust the procedures specifics.
 

Boombox

Member
Joined
Sep 2, 2012
Messages
987
Not an electronics expert here. Either way, hope you can get the DX-394 fixed. They are fun radios.
 

majoco

Member
Joined
Dec 25, 2008
Messages
3,872
Location
New Zealand
Well, both of your frequencies are about 150Hz out of spec. As the adjustment of VR2 does nothing and both frequencies are out I would suggest that VR2 is suspect OR you haven't got the right adjustment - it's easily done of some of the cramped boards, but I see that VR2 and L6 are right next door to each other so it would be hard to get the wrong one. Have you got a can of switch cleaner? - give VR2 a squirt and wind it from one end to the other a few times and see what happens.

Is the fine tune control just like the main tuning - is it an endless control that keeps on moving the 100Hz frequency - can you turn it up or down so that the frequency indicated is correct?
 

hanlonmi06

Member
Premium Subscriber
Joined
Jul 16, 2009
Messages
143
Location
Pittsfield Twp, Michigan
The fine tune is just like the main tuning. you can scroll it endlessly. Its not like a shift knob where you can "slide". Its more like a course/fine set of knobs.

I shot VR2 with some cleaner and swept it full range. No change in what I described before. Good point on making sure I'm on the correct component, but as you saw its pretty easy to see all the labeling.

I started probing around, pushing gently on the board, and in the area of these components to see if I could detect a bad solder joint. In that process, I started to visually look more closely at both sides of the board and did happen to notice what looks to be physical damage to D1. Chasing that down it looks like it feeds Q9 which according to the block diagram is part of the AGC circuit, but also goes to the main CPU.

I'll try to get a close up view of it with a zoomed in image of some sort, my eye sight isn't exactly 20-20, but it sure does look like D1 took a hit at some point. Whether it has anything to do with the frequency being off I have no idea, but this is feeling like its turning into a basket case, which actually makes me want to resurrect it even more. Through various info gathering searches I've also managed to see what the going rate is for one of these unremarkable radios.....unreal...
 

majoco

Member
Joined
Dec 25, 2008
Messages
3,872
Location
New Zealand
It's a good idea to look closely for damaged components and dodgy solder joints. Plugs and sockets are another weak point so you might like to go around and disconnect/reconnect every one you can find one at a time. Quite often a bad solder joint will work for years then suddenly pack a sad. I'm running out of ideas and just working from the manual without actually seeing the radio is not the easiest way of doing things. I have three bench top HF receivers here, only one I bought new back in the early 80's, a Kenwood R2000 which occasionally goes funny and needs a bit of TLC, the other two I bought on our local auction, a JRC NRD-515 and a Debeg 7313 which both needed a lot of work to get them up to speed. The Debeg is a Siemens designed marine receiver which is almost devoid of bell's'whistles but has amazing audio on AM and the JRC is brilliant for winkling out the weak stations.

Those cheap microscopes that plug into a USB port on EPay are good for enlarging things - you can take them off the stand and hold them - they have inbuilt lighting too. Easy to read the writing on an SMD chip!

The fine and main tuning is done by a copper wheel with slots in it and a pair of contacts which although they look opposite each other are actually a quarter-slot offset. When you turn the wheel one contact will make or break before the other - with a couple of bits of logic you can tell which way it was turned and by how much. You might like to give those contacts a squirt of the switch cleaner too. Just a thought - what is the minimum tuning step on the main tuner? Could you find how much the tuning is really off frequency by exactly 5kHz by turning the mode to CW and listening to a SW broadcast band to find two adjacent stations which will be 5kHz apart - tune one to zero beat then tune to the adjacent one and see if it indicates that they are indeed 5kHz apart. You could try this on the AM BC band too if you can hear two stations 10kHz apart. Just another thought, can you find out of the the offset is there in all modes, AM, USB, LSB and CW - it may not be a tuning fault rather than a filter shifted in frequency.

You'll fix it in the end, it sound like just a single fault as the rest of the receiver seems to be working - good luck! Push it to the back of the bench for a week then come back to it - sometimes you can't see the wood for trees and a holiday is good for clarifying the mind!
 
Top