Roberts DAB Radio repairs - Page 1
Forum section: electronics / repair

author: Richard_L
author_rank: Contributor
timestamp: September 15, 2015, 02:21:57 pm
content: I recently acquired a couple of broken Roberts Vintage DAB radios. All the electronics inside are modern - the only 'vintage' thing about them is their retro look.http://www.amazon.co.uk/Roberts-Vintage-Portable-Radio-Battery/dp/B005LUZ5YK/ref=sr_1_1?ie=UTF8&qid=1442322364&sr=8-1&keywords=roberts+vintage+dab+radioAnyway, I've got around to trying to fix the first of them and its fault is that no sound comes from the speaker. Everything else works perfectly. Having extracted the main PCB from the case I've narrowed the fault down to within the metal can in the top right of the first photo.Audio signals go into that can from the volume control pot below it but they never make it out to the speaker wire connector located at the top right of the PCB.(see attached photo) The speaker wires both hold a steady 5V when the speaker is in use.The can, which I soon discovered was half full of glue, was a right pain to remove. It's removal revealed a lot of stretchy yellow goop under which were hidden 2 thru-hole inductors, 3 thru-hole electrolytic caps, one SMD IC and a few SMD passives. (see attached photo for PCB under the can with glue and thru-hole parts removed)The IC that the can was hiding looks like it's a TI TPA2005D1 class D amplifier in an MSOP package with a power-pad under it.www.ti.com/lit/ds/symlink/tpa2005d1.pdfI haven't had time to resolder the can's thru-hole components yet to check if I can detect an audio signal right at the IC's output pins. I had to remove them all to finish cleaning off the glue and to shine a torch at the right angle to be able to read the tiny markings on top of the amplifier IC.I've also been trying to identify the capacitors I've removed in order to check if they're still in spec but so far I've not been able to positively ID them. The text 'GD' (within an oval outline) and 'SH7(HT)' is written on them but googling that hasn't helped me much. (see attached pic of capacitors)The 2x 2.2uF, 50V have both been checked for capacitance and ESR. Their properties are both very similar. Results for one of them are below:@100Hz: 2.1uF, 26R@1kHz: 1.9uF, 11.2R@10kHz: 1.8uF, 6.9R@100kHz: 1.2uF, 6.1RThe 1x 47uF, 6.3V capacitor from inside the can has also been checked:@100Hz: 45uF, 3.6R@1kHz: 39uF, 1.4R@10kHz: 30uF, 1.2R@100kHz: 10uF, 0.9RAre these supposed to be low ESR caps given their role? (see attached photo of my incompetent attempt to work out a schematic for the contents of the can). Even if it's the amplifier IC that's broken, I think it would be a good idea to replace the capacitors too because of the mechanical stress they were subject to during the removal of the glue-filled can.So, in summary:1. Can anyone identify and find a datasheet for these capacitors?2. Any tips for replacing the amplifier IC given it's got a 'power pad' underneath it?EDIT: Just got round to searching Farnell for replacement capacitors. No low-ESR series caps turned up in the search results, so I've ordered Rubycon ML caps along with a few amplifier ICs. I'll update this post again when I've attempted the repair with the new parts.thanks,Richard

author: sherlock
author_rank: Contributor
timestamp: October 12, 2015, 06:52:26 pm
content: I'm guessing that your IC may be blown. You can get a data sheet for it athttp://www.ti.com/lit/ds/symlink/tpa2005d1.pdf.There are some user application stuff there too. Low ESR caps are used a lot in switching power supplies but it probably wouldn't hurt if you used them in your radio either. I think the only one specifically mentioned in the data sheet is the power supply bypass cap. That IC might be a bear to remove. You're probably going to have to use a hot air gun for that. you'll have to heat the IC and the adjacent area until the solder melts and you can remove it with some tweezers. A good idea would be to put a piece of aluminum foil over the board with a hole cut into it where the IC is. The foil will protect the rest of the board somewhat... Good luck with your project.sherlock

author: steve207a
author_rank: Contributor
timestamp: October 31, 2015, 07:26:21 pm
content: Hi probably the ic has failed but do check the shutdown voltage on pin 1 is not activated as this turns off the amp chip see data sheet

author: tec5c
author_rank: Frequent Contributor
timestamp: November 01, 2015, 01:07:39 am
content: Those caps are from a one hung low company called "The Huakai Capacitance".Their website is quite poor as it offers no documentation/datasheets (need to use google translate too....at least I did).Website here:http://www.huakai-cap.com/It seems that their low ESR range are marked "LEH..." on the can. From the photo you attached of the caps, I can only see one marking on all of the caps which reads, SH7(HT). This appears to be a general purpose high temp (105deg) rated cap.Just inspect the others to ensure see what markings they have on them in order to see if you need to be worrying about low ESR replacements.