Bose Wave Radio (Model AWR131) - Page 1
Forum section: electronics / repair

author: twjonckheere
author_rank: Newbie
timestamp: July 29, 2015, 01:38:35 am
content: Looking for some schematics to repair this radio, but I have not found any..So far I've noticed that a display driver is only receiving 4.2 volts when the IC (National Semiconductor MM58342) datasheet says it needs 4.5 to 5.5 volts. When it turns on, all of the display lights turn on at about medium brightness and stay this way. The radio works other than that. Would a minor shortage of voltage cause this? I've also ordered a new MM58342 to see if replacing the display driver IC will fix things.Any other ideas?Thanks.

author: SL4P
author_rank: Super Contributor
timestamp: July 29, 2015, 03:17:07 am
content: Without seeing the actual unit, I doubt the display 'lights' are driven by the chip.That MM58342 (VFD driver) drives the display segments of the VFD display tube.The low 5V rail voltage will most likely be due to something loading down the supply, (expect to find something warmer than usual in the PSU circuit), or the PSU itself is being 'told' to deliver less power than is specc'ed - hence the dim illumination.The electronics and display probably have a reasonable tolerance to voltage droop - so they operate normally - at least outwardly. If you can, see if you can 'disconnect' the 5V supply to individual parts of the circuit - to see if any single one is pulling the rail down.Or equally possible - the input side to the 5V regulator is sagging - so the regulator can't actually get up to the nominal 5V potential - that dirty-side voltage should probably be something like 9-15V or more if other rails are also derived from it.Of course... if the 'lights' you refer to are the actual VFD character segments - then we are chasing a completely different problem (possibly the VFD tube itself, or the HV supply), but the low 5V rail is certianly worth chasing down.

author: twjonckheere
author_rank: Newbie
timestamp: July 29, 2015, 11:27:24 am
content: Thank you for your help.As a beginner I'm aware I probably did not state things right with the lights. By lights I did mean the segments of the VFD display tube. They all come on, but are at about medium brightness.I will use my Flir camera to look for hot spots in the PSU area as suggested.I also did unplug the ribbon cable that goes off to the display and then test the voltage at the board connector. This way if the VFD driver was pulling the voltage down (or the whole display board it connects to), this test would show the true voltage before it could drop down from the VFD's current usage. The voltage was still 4.2 volts. However, something else could of course pull this voltage down before it got there. I'm still tracing that...I couldn't find any regulators other than the one 10 volt regulator (sorry I don't have the # available right now). It had about 19 volts coming in and 9.8 coming out. It's rated to be 2% accurate. Perhaps that slight dip could cause this? It also wasn't hot (no heat sink either), so it's not pushing much load.Thanks again for your help!

author: SL4P
author_rank: Super Contributor
timestamp: July 29, 2015, 11:31:28 am
content: You sound like you're exactly on the right track.I'm not experienced with those VFD displays, but someone else may have a thought on what voltages you're likely to see - with awareness that only some segments (I.e. pins) are illuminated at a given time!The 10V reg putting out 9V8 sounds perfectly reasonable.

author: twjonckheere
author_rank: Newbie
timestamp: July 30, 2015, 03:30:35 am
content: Updates.I received and replaced the display driver and the display now works! However, it's quite dim. Which is probably due to the low voltage. So there are probably two issues.Question. What other things should I look for in regards to low volts? I looked at the circuit under the Flir and nothing was over 45 degrees C. Caps ESRs were good. The one regulator was good. Could it be some bad resistors that don't divide the voltages correctly? Any other things to look for?Thanks. At least so far the radio could still pass as a good nighttime radio.