Bose Wave Radio 1994 ac hum

[jasonbay13]

i've got an old bose awr1-1w from 1994 that i re-capped. almost all of them were bad. most tested with esr of over 30 some over 100.

it works now, but when first plugging it in and turning it on the speakers will play whatever sound i am giving the unit but also an ac hum that gets louder until it clips the sound levels and then it automatically mutes itself (possibly self-protection?)
if i mute and un-mute a few times the hum gets softer and softer until its pretty much gone and can only be heard less than inches away and also plays the audio clean after this. what could be going on here? maybe one of the caps has a bad solder joint or could i have used a wrong value somewhere? weird thing is the hum doesnt seem to get softer while its on, only in the transition from and to mute.

should also be noted it looks like the speakers have a slight dc offset, just a tad pulled in when turned on.

[stj]

Re: Bose Wave Radio 1994 ac hum

could be a ground problem.
i would re-check the soldering.

[rhomanski]

Re: Bose Wave Radio 1994 ac hum

One of the capacitors reversed?

[keeney123]

Re: Bose Wave Radio 1994 ac hum

i've got an old bose awr1-1w from 1994 that i re-capped. almost all of them were bad. most tested with esr of over 30 some over 100.

it works now, but when first plugging it in and turning it on the speakers will play whatever sound i am giving the unit but also an ac hum that gets louder until it clips the sound levels and then it automatically mutes itself (possibly self-protection?)
if i mute and un-mute a few times the hum gets softer and softer until its pretty much gone and can only be heard less than inches away and also plays the audio clean after this. what could be going on here? maybe one of the caps has a bad solder joint or could i have used a wrong value somewhere? weird thing is the hum doesnt seem to get softer while its on, only in the transition from and to mute.



should also be noted it looks like the speakers have a slight dc offset, just a tad pulled in when turned on.

I would think they would have some DC offset pots somewhere on the board. Do you have a schematic to it?

[R_J]

Re: Bose Wave Radio 1994 ac hum

Is the main filter cap. C401 ? whats the voltage reading on it? (+13.5) and did you replace it? if yes, what value did you replace it with?

[jasonbay13]

Re: Bose Wave Radio 1994 ac hum

i have had the thing plugged in but "off" all day and when i turn it on i do not get that hum, i have not checked to see if just waiting a while before trying to turn it on helps.
i may check all the solder connection and reversed caps.
dc offset pots? i do have a schematic, but not a service manual. looks like i actually have the 1993 version, attached the schematic.
the main filter cap is the only one i did not replace as it looked to be in shape still based solely off of esr and dmm capacitance test. leakage unknown. only electrolytics were replaced.
i shall get a voltage off it when i pull it back apart to check connections/values/polarities.

[Longbow]

Re: Bose Wave Radio 1994 ac hum

i've got an old bose awr1-1w from 1994 that i re-capped. almost all of them were bad. most tested with esr of over 30 some over 100.

Dr. Huckstable, you have failed to describe the villainy. Before replacing all the capacitors was the unit completely dead? There are a lot of electrolytic capacitors in this unit. Hum in both speakers? Present in the line outputs? Power supply voltages correct?

[juliadream]

Re: Bose Wave Radio 1994 ac hum

I've seen one diode in a bridge rectifier play these kind of heat related tricks.. and also voltage regulator chips.. they leak at only specific temps.. above and below they are just about acceptable.. which might explain why it's a very low hum again once the machine has been allowed to get up to operating temps after peaking during the warmup cycle..

Shouldn't be that hard to hunt down with a meter on ac range.. the culprit should be pretty obvious.. I'm going to point nice long accusing fingers at U401 and 2 .. they are responsible for the +12 and +5v supplies which feed all the low signal level circuitry..