Radio Shack DX 394 is off freq

[merlin]

...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?

Your VCO is working fine, you just need to get it to proper frequency.
Set your tuning to to WWV you know you can receive with all other setting mid point. (BFO, Fine tune etc)
Now adjust the VCO trimmer until you center WWV. That is it, you are done.
Now check low frequencies for accuracy//high frequencies for accuracy. If your reference oscillator is properly set, there will be no skewing across the range of the radio.
A note here is the VCO should hold 'lock' at 29.9999 and 150.0000 Khz.
The way it works is the VCO takes small voltage at highest frequency, then the increasing voltage 'pulls' the frequency down with varicaps.

 

[majoco]

What Merlin says - did you go back and do the VCO setup procedure where we found the typo? You need to get the frequencies right rather than the voltages. Way back in post #13 I said

The block diagram right at the start of the manual shows that the first oscillator injection frequencies are 45.150MHz which you have already measured 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!

You must get those frequencies spot on +/- 10Hz, you said

At 29.999 I'm reading 75.0075mhz on the meter.

which is not close enough, that's 7500Hz out!

 

[merlin]

What Merlin says - did you go back and do the VCO setup procedure where we found the typo? You need to get the frequencies right rather than the voltages. Way back in post #13 I said
You must get those frequencies spot on +/- 10Hz, you said which is not close enough, that's 7500Hz out!

Puzzled,,so your meter is accurate to within 10 Hz,,any reason you can't adjust the VCO?
If the VCO will not get any closer than the 7500 Hz you note, then you need to check the critical capacitors in the tank circuit including the varicaps. You also need to check the frequency of the reference oscillator.
That can fudge the frequency and still get a lock on the PLL. Let me look a few things up.

 

[merlin]

OK, I see the only adjustable element for the PLL is L11 near pin one and 2 of the PLL IC.
Test point 5 follows the loop filter that goes to the VCO. the buffered output at test point 6 should be the VCO frequency to monitor with a counter, the first mixer injection.
You didn't state if this is high or low, because depending on modes, this frequency will be different.
Can you get WWV on USB/CW, AND LSB ?

 

[hanlonmi06]

Got real busy these last two days. I'll be in the shop tonight, so updates forthcoming.

Going back through this, I think on the VCO alignment when I stopped at the step 2 part where the manual says 21 volts and I read 2 something, I think I panic-skipped the actual adjustments. When I get back down to the shop I can step through it all again. And also, realized my freq counter doesn't have down to the 10's of Hz resolution, so there's another strike. Everything being said here is making sense though, pretty stoked to get back down there and go through it all.

 

[merlin]

Well, I don't see an adjustable VCO here, and the VCO voltage should be like 2.1 volts for 29.999 Mhz and 21.x volts for 160 Khz,(TP5) that shows the PLL is working. The reference is cristal X4 at 33 Mhz with an adustable L11 to resonate the cristal. This can vary that by a small amount but out of oscilation, you loose all VCO and no receive. The voltage source for this is the DC-DC converter and has to have that 21 volts.
IC5 pin 6 shows 32 volts through series resistors R145 and R132. Check R145 and R132.
The load side is test point 5 and the VCO control voltage.
If your scope can see the 45 Mhz IF, probe TP1 AM mode tune a strong BC station and peak the 45 Mhz IF chain. Next, do the 455 Khz IF alignment. I probe the input to the AM detector and peak the 455 Khz.
At this point, frequency is not the problem. You need to go through the BFO alignment so they match the sideband filters, This will need a good signal generator with an accurate frequency counter.
I use a Tectronics CFG208, more than adequate.
If sidebands still don't clarify, you have a problem in sideband filters or BFO oscilators.(451.6 and/or451.8 Khz).
In general, this receive is strikingly similar to the receive in Kenwoods TS-440.
(They got the VCO voltage/frequency backwards in your manual.)

 

[hanlonmi06]

So I went back through and figured out where the misstep happened. On the VCO alignment, we discovered the typo about the voltages, but what I failed to mention, which is completely on me for not mentioning, is that step 3 of the VCO alignment where it states to adjust L11 for 45.150mhz, I cannot get any change at all on the 45.1543 frequency that I read when probing TP6 at the 150khz frequency. When I flip over to 29.999mhz, the freq meter shows the 75.0075, adjusting L11 has some affect, but its "sluggish". With a 'generous' turn on L11, the 75.0075 doesn't change a whole lot. That same "generous" turn at 45.1543 has no affect at all.

So, to everyone's point, I think we are still stuck on the VCO not behaving? Need to try to figure out what it is preventing the adjustment to the VCO, because everything essentially follows after that?

Merlin, I have not located IC 5. I took voltage readings of IC1, a bunch of the transistors q24, 25, 26, voltage readings all seemed reasonable, however IC5 I suspect is hiding inside another soldered metal case that I'd have to open up to get at, which I will look at how to open up tonight.

And also, I don't trust my chincy hobby grade little freq meter. Its worked for what I needed it for previously. It does show numbers that make sense, but I'm willing to make a small investment here, so I went to that auction site and did a quick search for frequency counters and, well, there's plenty to choose from. Any recommendations for a decent model, say around $200 price point?

 

[hanlonmi06]

Update:

I finally found the problem. In the alignment procedure for the 2nd local OSC, adjusting VR2 is supposed to cause a change on the frequency counter reading when probing TP1. In my situation there was no change when adjusting VR2. I finally had the wherewithal to probe for DC on VR2 and had absolutely nothing. The source for VR2 comes through CN3, pin 5, which is connected to pin 9 of IC206. I pulled apart the face of the radio to gain access to IC206. Probing pin 9 right on the IC gave me the 3.4 volts called out on the print. I had a broken wire that was virtually impossible to see with the naked eye as there was brown glue physically holding the wire in place on the the circuit board, but the conductor had actually broken right at the solder joint.

So, repeating the alignment procedure all over again, the numbers didn't all exactly match, but everything actually responded to my adjustments. I tried to get everything as close as possible in each step, and moved forward. Right now, I can tune WWV right in on the 10 and 15mhz freqs, and currently have a ham contest on 40m tuned in on LSB reasonably well where as before SSB just wouldn't tune at all. This is definitely a usable radio for my requirements.

It never ceases to amaze me what symptoms wind up showing up with various faults in a system. I sincerely appreciate everyone's replies and assistance in trying to figure out what was going on. I guess I chalk it up to inexperience, but I cant figure a way in hindsight that I could have found the VR2 situation sooner, before tweaking so many other adjustments. Things were immediately not making sense from the first adjustment, so I guess its just a learning experience.

Pic of broken wire. IC206 is on the opposite side of the board:

 

[merlin]

Your persistence has paid off. One tiny part of the circuit at a time til you find what doesn't work right.
I once had similar circuits acting up. A flakey(but checked good) tantalum capacitor. Not thermal or mechanical, just flakey. took a month to find.
Glad you got your receiver back to A1.

 

[air-scan]

Update:

I finally found the problem. In the alignment procedure for the 2nd local OSC, adjusting VR2 is supposed to cause a change on the frequency counter reading when probing TP1. In my situation there was no change when adjusting VR2. I finally had the wherewithal to probe for DC on VR2 and had absolutely nothing. The source for VR2 comes through CN3, pin 5, which is connected to pin 9 of IC206. I pulled apart the face of the radio to gain access to IC206. Probing pin 9 right on the IC gave me the 3.4 volts called out on the print. I had a broken wire that was virtually impossible to see with the naked eye as there was brown glue physically holding the wire in place on the the circuit board, but the conductor had actually broken right at the solder joint.

So, repeating the alignment procedure all over again, the numbers didn't all exactly match, but everything actually responded to my adjustments. I tried to get everything as close as possible in each step, and moved forward. Right now, I can tune WWV right in on the 10 and 15mhz freqs, and currently have a ham contest on 40m tuned in on LSB reasonably well where as before SSB just wouldn't tune at all. This is definitely a usable radio for my requirements.

It never ceases to amaze me what symptoms wind up showing up with various faults in a system. I sincerely appreciate everyone's replies and assistance in trying to figure out what was going on. I guess I chalk it up to inexperience, but I cant figure a way in hindsight that I could have found the VR2 situation sooner, before tweaking so many other adjustments. Things were immediately not making sense from the first adjustment, so I guess its just a learning experience.

Pic of broken wire. IC206 is on the opposite side of the board:

Just FYI that compound is brown which isn't supposed to be. I don't remember the color of it new but it becomes conductive which can also make alignment a pit. If that brown stuff isn't scraped off more problems will come again. Probably very soon. I can't count how many times Youtuber's like Mr Carlson's Lab, The Radio Shop, TRX Bench, 12VoltVids has pointed out the exact same thing we see in your image as being a potential problem with that browned up conductive compound.

Which is also corrosive, probably why that wire snapped off to begin with. Just look at it. You can still see a tiny piece of the wire on the pad with the caked on brown stuff!

I certainly would find it worth the effort to scrape it all off very carefully to prevent shorts. All of it.

 

[hanlonmi06]

I'm simply dumbfounded to learn of that. I still have the radio on the bench, so I will certainly do as you described. It looks like I need to spend more time on Mr. Carlson's lab channel! Merlin and majoco, I've been steadily trying to digest everything you described on the circuit theory.

 

[air-scan]

Its a good idea to de-solder those wires and clip off the caked on ends and strip them to good wires. I bet those other 4 are about to go. You should have enough slack in that wire to allow this. Also go through the entire radio make sure that browned up circuit eater stuff is cleared. It'll save that radio.

 

[majoco]

Well done - we knew you would find the problem in the the end! That pair of white/black wires in the black insulation looks awfully short so perhaps it was doomed to fail right from the start! Try to give it a bit of slack when you repair it - where did it break - from that "#5" vacant hole next to the other wires or from that larger piece of PC track that appears to have a single strand of wire soldered on to it?

 

[hanlonmi06]

It was the #5, next to the other wires. The white is actually the surrounded by a shield which would be the "black" wire. The "black" wire is actually heat shrink, I played dentist last night and carefully got after most all of the brown goo. The rest of the connections seem to be in good shape. I think I am going to try the alignment procedure again after replacing one of the metal shield covers and now that this brown stuff could have been affecting the adjustments. As mentioned, once fixing the wire, the adjustments actually started working, but I couldn't get the numbers bang on. We'll see how it goes, then I will get my screwdrivers dipped in gold, so that I can say I've got the golden touch with the golden screwdriver! In all seriousness, I'm excited. I spun the dial on the bench again last night and it was nice hearing a few SSB hams in the clear, and some big SW broadcasters coming out of this little rig.

Edit: on second thought, I may just wrap it up, pop the case back together and use it for a bit first before getting hasty with the adjustments...

 

[rescue161]

Outstanding job! Always good to see someone repair instead of throwing it in the trash. Mr. Carlson's Lab is a really good channel. I really enjoy watching and learning new things.

 

[superman2112ms]

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.

I have one and the serial number starts with a C. That could be it. IDK. How ever I bought it used and never used it. Are you Interested in it?

 

[RichardKramer]

I have 2 DX-394's; one is off by 1KHz USB the other 2KHz USB - I just put a marked label on each one to tune off by that amount when programming the freq.

 

[Boombox]

I have 2 DX-394's; one is off by 1KHz USB the other 2KHz USB - I just put a marked label on each one to tune off by that amount when programming the freq.

I think a certain amount of that was standard with DX-394s. The old yahoo DX-394 groups used to discuss that.