Bose 802 Mk2 help

[joe27979]

hi all, I'm new to the forum and was recommended by a friend.

I have 6 Bose 802 II speakers that I'm repairing for my dad. I downloaded the correct manual and have required two of the cabinets. all of the drivers popped ok with a 1.5v battery and both the 20uf and 5uf capacitors charged up ok when testing with my multimeter, however when I test them through the amp it seems like only drivers 5,1,4 seem to work! any help appreciated..

as all the speakers are in series I'm wondering if the capacitors are shorting out at running voltage as its higher than my multimeter. both capacitors have a rating of 100v

 

many thanks guys

[mi-ul]

WHat impedance is there across each speaker?

 

As they're in series, if one fails then the speaker will stop doing anything. If only some are working then it may be the foam surrounds that have given up but that should be visibly obvious with the front off.

 

Mi-ul

[themadhippy]

as all the speakers are in series I'm wondering if the capacitors are shorting out at running voltage

doutfull,if that was the case youd hear a diffrence in the freuency responce,along with measuring the individual drivers impedance id test each driver with a signal.

As they're in series, if one fails then the speaker will stop doing anything

only if they fail open circuit,if the windings have got a bit hot and melted its possible theve shorted out

[joe27979]

thanks for the reply guys, I'm measuring between 0.9 and 1.3ohms across them all, none are open circuit. some of the cone foams are a little dented.

 

whatsoever you mean or how do I test each driver with a signal? sine wave 15khz 15v? I don't have a silly scope ;(

thanks again

[Brian]

All 8 drivers are in series with the capacitors connected across some of them to tweak the frequency response.

 

Start by disconnecting each capacitor at one end. This will leave just the 8 drivers in series across the terminals. Apply a signal but not 15k, use something more sensible like 1k. If you don't get noise from every drive unit then there are two possibilities...

 

1) The quiet driver(s) is borked or

2) You've messed up the wiring

 

Until you get to this stage you're wasting your time.

[joe27979]

thanks Brian,

I'll try the capacitor test and a 1k test frequency.

the wiring is exactly as per Bose service manual so although the impedance looks good one or more drivers may be borked :0)

 

can I download a 1khz test mp3 file to test or should I really be using an oscilloscope?

cheers

[david.elsbury]

A 'scope is a tool to display and measure waveforms, it doesn't generate them. I imagine downloading a tone would be fine, or Audacity will generate then for you.

 

David

[joe27979]

cheers David, Bose quote " connect a sine wave oscillator" I instantly thought scope but you are indeed correct. no signal generator (apart from wife's constant 15.6Mhz) so I'll download some test tones

[joe27979]

one cab repaired all working fine.

next cab is a little strange, all drivers test 1.4ohm impedance, disconnected one leg of both caps and yet only drivers 1,4,5 are working. they are all in series as per wiring diagram. 1khz tone is a bit tricky to test as they are all so close to each other in cabinet it's hard to distinguish between them. guess I'll swap drivers 1 and 2 to see if problem moves?

[timsabre]

if you push gently on the cones do they all move freely in and out?

Have had some 802's with cooked drivers where they were locked solid due to distortion of the voice coil. (the foam surrounds were also completely trashed)

[joe27979]

thanks Tim, yes they all look and feel perfect, they even produce a small amount of milli volts across the terminals as I depress the cones and the coil passes by the magnet.

 

if most of the drivers are reading 1.3/1.4 ohms then I should get 11.4ish or less total circuit impedance..isn't that a little high for an 8ohm cab?

[paulears]

Not really - because the resistance and impedance are quite different at this end of the range.

[joe27979]

thanks Paul, after reading a bit more my DMM will only read DC resistance, a rudimentary way of converting is multiplying this figure by 1.3 to get the impedance value.

 

the quest continues, I'll try swapping a couple of drivers round today and then report back.

thanks for everyone's input so far-I'm enjoying the learning curve of this topic!

[timtheenchanteruk]

bink test CD

 

better than MP3s, .Wav and all the test tones you will ever need!!

[joe27979]

I found 500hz the best frequency to test at. although they tested ok 5of the drivers failed to generate any sound!! that's why only three worked! next thing is to find genuine drivers in the uk!