Help me diagnose a failure in a modern AM/FM/SWB digital radio - Page 1
Forum section: electronics / repair

author: gooseEL34
author_rank: Regular Contributor
timestamp: December 30, 2015, 08:29:55 pm
content: Goodwill purchase. $2.99. Realistic model DX-440 "Voice of the World" portable0) No power on with 9v wall adaptor1) Initial inspection revealed green crustiness on the center pin of the 9v DC in jack.2) slight corrosion on the springs/negative contacts of D battery holder (6x1.5 "D" plus 2x1.5 "AA" for memory. I hit them with3) unit powers on with D-cell batteries and I can clearly tune in 90.3 (local NPR FM station) but no sound out of main speaker (dead silence). The led signal strength meter (1-5 LEDs) goes to 5 so I know I am getting signal.4) AKG headphones (known to be good) into 1/8" headphone jack give some signal but with these issues5) Antenna is loose at the pivot bolt, but I believe it is working as I can tune in stations.a) constant pop-pop-pop at regular intervals and when popping, the audio is not heardb) popping continues for a bit after unit is powered down (power on off is a momentary switch, not a make break mechanical)c) no effect with the volume slider (unit stays at constant volume regardless of slider positionI am thinking filtering in the power supply, but I am no EE and I usually tinker only with vacuum tube guitar amps. The closest thing I could guess, was that it was something with a bad electrolytic filer cap in the power supply.Will post some shots, ASAP when I get back cover off.

author: voltz
author_rank: Frequent Contributor
timestamp: December 30, 2015, 08:52:24 pm
content: Starting point, the manual:http://www.dream-machine.myzen.co.uk/Radio%20Shack%20-%20Realistic/DX-440%20%28Service%20Manual%29.pdfVery last page has the schematic.Instant guess would be audio chip IC301 (bottom right hand corner). Any audio going in to pin 2 or 13? pin 10 is power. Any audio coming out? Power supply all ok?

author: DimitriP
author_rank: Super Contributor
timestamp: December 30, 2015, 09:08:39 pm
content: 0) Center pin is negative, outer barrel is positive.Inspect the jack for a broken solder connection.On the same side as the power jack , above the headphone jack, there is an unpopulated 5-pin DIN connection. It's a LINE OUT output before the volume control etc .Check for audio there .

author: Zad
author_rank: Super Contributor
timestamp: December 30, 2015, 09:35:25 pm
content: Inspect the headphone etc sockets and PCB for oxidisation between switching contacts, and for dry joints on the PCB. Pop-pop-pop sounds are often insufficient power getting through to the audio stage, so it charges the capacitor, the amp stage starts up, runs for a fraction of a second and drops out, repeat ad-infinitum.

author: cncjerry
author_rank: Unknown
timestamp: December 31, 2015, 01:03:58 am
content: A number of the old Radioshack DX radios had capacitor problems in the audio section. Weak or no audio was the symptom. I fixed my radio, can't remember the model, in about 30 minutes by replacing two electrolytic caps. The hardest part of the repair was getting it open.

author: cvanc
author_rank: Frequent Contributor
timestamp: December 31, 2015, 01:43:10 am
content: I know nothing about this radio, but on the vary last page of your manual, lower right corner where the 9 volt battery is shown, there is a second 3 volt battery. What's up with that? Could it be some button cell soldered on the board and long dead?

author: Towger
author_rank: Super Contributor
timestamp: December 31, 2015, 07:41:41 am
content: The 3V would be the two AA memory backup. I also vote for the capacitors, the Sony SW radios of the same vintage also suffer from them. Down to the poping on the audio when the audio IC caps go bad.