Repair of CC Radio SW (shortwave)

[Tycho_Brahe]

I bought this 'parts only' radio from ebay as a project.  Thought it might be fun to try to fix it, though I don't know what I'm doing.

Doesn't power up at all.  No display, no sound.   I tried using batteries and the adapter.   I have probed the circuit boards at different spots and I do see 9 volts and 3.4 volts at various places.   So, I think the boards are getting power.   

Nothing obvious, like leaky or bulging electrolytic caps or obvious broken wires or solder joints.  I've disconnected and reconnected most of the harnesses.   Here's a link to schematic to a roughly similar radio: https://www.manualslib.com/manual/762278/Roadstar-Tra-2350.html

On a whim, I started pressing random components on one of the boards and it displayed things in a cycle (almost like the display was scanning top to bottom).   Today, I tried it again and managed to get it to display a mostly normal image (the clock, the battery symbol, etc. all at the same time).    What I did is press on the two spots where I drew the arrow below.    That's pretty much the only place that will cause the display to appear.   But still, I can't get it to make any sound yet.

So, my question is, what should be my next step?  The first image below shows the arrows where, when I am pressing down it will display a normal image.   

The second image is showing you that there are tabs holding a big metal shielding box on the other side.   So I'd have to take that off in order to see if there are any weird solder joints on the other side (haven't done that yet).

What's the most likely culprit for these symptoms?   A cold solder joint, or can the board itself even go bad (like, the layers and traces inside, or whatever)?  Thanks!

[Tycho_Brahe]

I think I might have noticed the problem spot.   I walked past the mess of components spread out on my workbench and see that the main circuit board looks like it has a small crease in it.   That's the best way I can describe it.   It's near the volume pot and I suspect that at one time the radio was dropped on its front/face - forcing the pot into the board (which it's soldered and anchored to).

So it looks like the board may be fractured somewhat.  Nearly impossible to see without the right light and angle.

Anyway, if I flex the board this way and that, I can make the radio do new things (still not functioning perfectly).

Question - do boards like this nearly always just have only one or two sides?  I mean, no traces in the middle (like a computer has) right?   If so, I'm thinking maybe I can isolate an area and examine top and bottom and locate and fix broken traces and components.

[bob91343]

You are on the right track.  The board is probably cracked and you may be able to repair it.  You might need a microscope to see the fracture.  As for your last question, there is no standard answer.

I have a similar radio.  I paid $12 for it, new, and it has FM, AM, and shortwave and a clock.  Not too useful, as it has no BFO and the tuning steps are 5 kHz on shortwave and you can't run an external antenna.  I got it to track down some interference on the 40m band but shortly after I got it, the interference has largely gone away.

The internal speaker sucks and I can't use the headphones as I have hearing aids.

Take a pen or pencil and gently press around the board to find a sensitive spot.

[BrokenYugo]

This is certainly a single sided board. If it is cracked all you have to do is follow the crack, scrape back the solder mask of each cracked trace on either side and solder jumpers across the gap.

[Tycho_Brahe]

Thanks for all the help.   Took me a while to get back to this.  Had to purchase a microscope so I could solder those tiny traces.   

Okay, so there were over 15 broken traces (it's been a couple weeks so I forgot exactly - I think it was 16 or 17) and I managed to bridge the solder for each.   Now, it works mostly.   The display works completely.   F.M. band works perfectly.

A.M. and Shortwave bands do not.   No matter which band and which frequency, the sound is the same.  Light background noise really.  Not even normal household interference.  For the A.M. band it should be picking up that, at least.

Next, I turned my signal generator to 540khz and set it to do A.M. modulation at 1,000hz through a small coil and resistor just sitting near my radios hovering over the workbench.   I got a known-good radio and tuned it into that frequency and heard it loud and clear.    I tuned off that frequency and it quickly went away and was replaced by static.   So, this is what I would expect.

I grab the broken radio and do the same.   I can hear the modulated tone through the broken one on 540khz.   However, I then tune anywhere on the A.M. band and can still hear the modulated tone.   Even if I tune up to 1600khz.   About the same volume there and no static.    I then switch to shortwave.   I can hear it everywhere.   

Not sure what this means.   Obviously, it's picking up frequencies through the loopstick (A.M.) and/or antenna (shortwave) because that's the only way the signal generator is being picked up.  But not a single radio station is ever picked up.

Any ideas why?

[SoundTech-LG]

You may be dealing with COB technology. Circuit on Board. I don't see any active components on the top side of the board. You do not show the back side. when looking at the back side do you see any black blobs with multiple traces leading into the blobs? If so, it's COB. A very cheap way for manufacturers to produce  a circuit board. Unfortunately there is little you can do to repair these if it is a COB problem.
See photo.

 

[BrokenYugo]

Sounds like maybe the test signal is blowing through the malfunctioning AM front end, 540 being close enough to the probably 455kHz IF (if it's a superhet) that it gets through and you hear it all over. Like the oscillator isn't running for some reason or something, or perhaps the antenna is disconnected.

We really need to see the board under the display board. That appears to be where the actual radio parts are.

[Tycho_Brahe]

As requested, some photos of the RF board.  Set 1.

[Tycho_Brahe]

Set 2 of the RF board photos.

[Tycho_Brahe]

And lastly, set 3 of the RF board photos.  I will add that the LCD display does have a signal meter.   And at no time, does it move past 1/10 for the A.M. / Shortwave bands.   Yet I am able to hear the local signal generator tone.  I suppose the weaker over the air stations are just getting through too weakly yet my local signal generator might be a lot more powerful compared?   And AGC takes over and messes with my mind.   So I have been wondering all along if somehow the signal just isn't getting through, past a certain point (or getting through very weakly).

After seeing these, any idea on where to start probing first and what to look for?