1997 Bose 301 Series IV no more tweeters diagnose help

[Phil916]

Hello,

i'm totaly novice here and trying to diagnose these which have no tweeter sound and woofer seems very low sound to me
when i connect the tweeters directly, rear oriented one (3") give good cristal clear sound but the front oriented one (2") nothing at all.
servicing manual inserted

thanks

 

[rayma]

Check the RT-1 lamp, it is probably open circuit. Also check C1 and C2 for value.
But from your note, one tweeter may be bad, so measure the tweeter voice coil resistance.

 

[Phil916]

thanks, spot on few Megaohms open circuit and i discovered this lamp is a fuse green dot on it
can i replace it with 1,5ohms 25W resistor ?
speakers resistance
2" reads 3.26ohms
3" reads 3.46ohms
8" reads 5.2ohms

 

[rayma]

Then the drivers may be ok. At least for testing, you can use that low value resistor in place of the lamp.
Later replace the resistor with a 1A or 2A fast blow fuse if you like to play the speakers loudly.

Do you have means to test the two capacitors?

 

[Phil916]

thanks, will solder a fuse here no problem

here are capacitors tests results, seems very low ESR and leakage value to me though i don't know original specs ...

 

[rayma]

Those seem fine. The lamp probably protected the capacitors.

 

[Phil916]

👍 so my resistor idea was very bad,
just a 1A fuse and it will sing again keeping protection
thanks

 

[rayma]

You could install a fuse holder on the back so access is easy.
A 1A fuse may blow more easily that you like, but will give good protection.

 

[Phil916]

i will use fuse holder of course 😉
i found no access from outside other than dismounting woofer, where do you see his better location ?

is their some upgrade that can be made to these 301 series IV ?

 

[rayma]

The fuse could be somewhere on the back, near the speaker wire terminals.

About all you could do is replace the capacitors with better quality ones, especially C1.
Maybe add some stuffing in the cabinet.

 

[Phil916]

ok so i will have to dismount woofer anyway so it can stay on same place of actual fuse lamp.

i can upgrade C1 to 4,7uf 50V or more polypropylene capacitor
is this Dayton 4,7uf ok ? and their 20uF version for 22uF replacement may be okish without to much filter frequency movement ?
https://www.soundimports.eu/en/dayton-audio-dmpc-4-7.html

 

[rayma]

Yes, all of those are fine. The film capacitor may be much physically larger, though.
I would also try stuffing the cabinet, if it's empty.

 

[Phil916]

thanks

 

[Phil916]

about stuffing there is some sort of white whool inside

 

[mtidge]

What is the device you are using to measure the caps? Where can I get one?

 

[Phil916]

It's a cheap components tester, similar than
TC-1 TC-7 ones
though this one has also basic dvm, frequency generator and measurement function. eBay Aliexpress will give you plenty sellers.
Search for m328 component tester
Very good value for his price

 

[Galu]

20uF version for 22uF replacement may be okish without to much filter frequency movement ?

Sound Imports also stock 22 uF film capacitors. For example: https://www.soundimports.eu/en/audyn-q2-22-0-250.html

 

[Galu]

While you're looking at Sound Imports, you may wish to consider their tweeter protection lamp: https://www.soundimports.eu/en/260-231.html

 

[Galu]

so my resistor idea was very bad

Not really, as a green dot protection lamp offers a series resistance of 1.5 ohm when cold.

 

[Phil916]

thanks Galu👍
i put a Fast 1,6A fuse i had on hand to repair defective one and they are singing again,
good enough for my hears, will see for new caps at next repair 😉

for component tester here is a direct link to finished version of my tester (it exist 5$ cheaper but as > soldering kit < so if you are not learning soldering and value your time, not worth solving 5bucks imho )
https://fr.aliexpress.com/item/32673908843.html