Radio with no audio - Page 1
Forum section: electronics / repair

author: carl0s
author_rank: Unknown
timestamp: April 29, 2021, 10:39:42 am
content: Hi everyone.Just before I throw this in the bin.We have a Revo Supersignal radio. One of these modern things with the Frontier Silicon chip inside for connecting with internet radio and spotify etc, as well as DAB.It has no audio output. It became intermittent, and now has given up altogether.The OLED display works, but no sound either through loudspeaker or headphone.I thought it would be an ideal project for learning how to fix things.I have failed though, and am ready to put it in the bin.I have tested most but not all of the electrolytic capacitors. Mostly out of circuit, but the PCB quality is very poor and tracks lift easily.I have found that the L7805 5v regulator is showing something like 0.8v on its input pin and 1.1v on its output pin, while in circuit. but it tests fine out of circuit - I put 12v on it and get 5v out no problem.I removed the SCR which was a Littlefuse S2006D. It was damaged in the removal and I was unable to buy a replacement - I could have got the 400v version, S4006D, but the handling charge from Farnell meant it was to be too expensive, so I bought a different SCR (BT151S-800R) and I don't think it's compatible - now the unit does not power on at all unless I temporarily short the SCR's gate and anode, but still then, there is no audio.The unit is modular in that it has a separate amplifier board, and I have disconnected this and still observed the 5v rail showing 1.1v etc, so I think I have eliminated the amp board as being faulty. Also the receiver board (Frontier Silicon chip on it) is separate, and I believe that is working fine, from a 3.3v rail, since it shows a received signal and the DAB radio station text etc.Any ideas, before I chuck it?I do have a scope, but it's not yet unpacked from the house move. I put a small speaker driver on the Left / Right audio feed that goes into the amplifier circuit and didn't hear any sound. I know I should have probably unpacked the scope for that really...In the photos, all the caps with the purple mark are ones that I have removed and tested out of circuit. I have tested some others in-circuit, which I know is not the right way, but if the 100uF cap came back as about 100uF while tested in circuit, and it's all on its own, I've sort of figured that might be a good enough test, given that component removal on this cheap thin PCB is harmful to the PCB.The three diodes around the L7805 seem to check out fine as well.Also, this was bought as a 'refurbished' model, and straightaway I could see some soldering-iron burns to sides of caps, and sloppy looking replacement of some of the 220uF SMD electrolytics, so that was a previous failure point it would seem, but those particular ones test OK right now.I haven't checked any of the tiny SMD caps (0402 size etc). Should I perhaps be looked for a shorted one of those?

author: carl0s
author_rank: Unknown
timestamp: April 29, 2021, 11:52:15 am
content: I have tested more caps now and they are OK too.It seems like the input to the 5V regulator comes via these two small tranistors and a fuse. I tested all fuses by the way and they're all OK.the 18v and 5v to the amplifier board are all less than 1 volt as well. Only the 3.3v side seems to be alive. 18v is the voltage from the wall-wart AC adapter. So although it's powering the 3.3v LDO wherever that is, it seems that it is still getting lost somewhere as it doesn't make it over to the amplifier board where there is a pin on the header labelled '18v'.

author: carl0s
author_rank: Unknown
timestamp: April 29, 2021, 10:02:15 pm
content: I have just tested even more caps, and they're all fine.There was another metal shield/cover that I hadn't looked inside. I've removed this cover and can see that this contains an 18v regulator - SC4524D, and a broken-looking inductor. I don't think the damage to this inductor should affect its operation though, should it? Testing it in-circuit with my AT825 LCR meter shows 5 micro-henries, and it looks to have been labelled 4R7 before becoming broken.

author: bob91343
author_rank: Super Contributor
timestamp: April 29, 2021, 11:11:19 pm
content: Measure the input to the regulator with an Ohmmeter (power off). If you see low resistance you have a short. Make sure the Ohmmeter positive lead is on the voltage line.

author: carl0s
author_rank: Unknown
timestamp: April 30, 2021, 10:37:00 am
content: Quote from: bob91343 on April 29, 2021, 11:11:19 pmThanks Bob.I see a continuously rising reading, from 50K but rising up by about 1K per second. It was reading 80K just when I stopped taking the reading, but still rising.Did you mean have my positive test lead on the barrel jack socket centre pin (voltage line in to the board), and negative test lead on the +Vin of the 18v regulator?

author: bob91343
author_rank: Super Contributor
timestamp: April 30, 2021, 11:10:03 pm
content: No. The 18V regulator has an input pin and a ground pin. The Ohmmeter negative should be on the ground pin. Positive on the input pin. Make the same test on the output pin to ground.Incidentally, not all Ohmmeters have the polarity you might think, from the color of the leads. Check with another meter to make sure the negative goes to ground.