I've got a pair of Ohm Acoustics speakers from the 80's that have two fuse bulbs in the crossovers. One bulb is blown in each crossover. Ohm is notoriously tight lipped about giving out info on diy repairs of their speakers. They want you to send them to them for big repair bills. Not interested. Anyway, I need to find a replacement or simply a modern alternative to the blown bulbs. I've only taken one crossover apart. The impedance of the good bulb is 2 ohms. I've found several other fuse protection blubs online that look identical to mine, but with varying ratings, and none list impedance. The only markings on mine are "C 9" stamped in the glass end, and an "S" stamped into the other side of the same end. I would assume that a speaker engineer would factor in the impedance of the bulb when designing the crossover, yes? Although it is my understanding that the impedance of these bulbs does increase as the wattage they receives increases. The bulbs are in the fabric insulation.
Maybe get a picture of the bulb. As a guess I would go for a 12V halogen bulb of the same size roughly.
BTW, That crossover build reminds me of my speaker build quality when I was in junior high school.
BTW, That crossover build reminds me of my speaker build quality when I was in junior high school.
It looks like one of these...
https://www.simplyspeakers.com/ev-speaker-protection-bulb-f01u109322.html
Mine...
It "looks" like one of these...https://www.simplyspeakers.com/ev-speaker-protection-bulb-f01u109322.html..., but they come in different ratings.
And, there is a polyswitch in parallel with one of them (looks like a small ceramic capacitor).
I would expect the linked EV bulb is right. Was likely a common automotive interior light for years.
Test the polyswitch about the same as the bulbs. Should be less than an ohm cold. Funny thing is sort of behaves nearly the same as the bulbs.
Both the bulbs and the polyswitch and the bulbs will have low resistance when cold and transition to higher resistance with enough constant current flow.
Both the bulbs and the polyswitch and the bulbs will have low resistance when cold and transition to higher resistance with enough constant current flow.
I would put 12 volts into a good one and measure the current draw. That way you can get the exact replacement. Assuming the good bulb is the same type as the bad one.
Thx. The poly switches must be bad too. One measures 6 ohms and the other measures 21 ohms. That's out of circuit.
I'd venture to say most home speakers don't have fuses anymore.
Would it be the worst idea in the world to just do away with the fuses and poly switches?
Any chance there are part numbers on the poly fuses? The resistance numbers you measured could be right. Might be able to look up the expected resistances.
Considering they appear to be in parallel with one of the bulbs, they may be some finer tuning of the resistance values for the bulbs.
That wiring looks lousy...yes, it would be a good idea to rearrange everything on a separate board, remove that primitive and inefficient protection and buy one of these.
https://www.diyaudio.com/community/...tion-and-delay-gb.340694/page-47#post-7271913
If you have blown out light bulbs protecting your drivers, don’t you think you need the protection?
simon7000, I bought these used. I'm guessing some prior owner abused them. As far as I know, most home speakers these days don't have "protection". At least none I've had (I've owned a LOT of speakers).
PS...can someone explain why the "quote" button on the latest response is missing? It only appears after the next "latest" response is posted.
PS...can someone explain why the "quote" button on the latest response is missing? It only appears after the next "latest" response is posted.
Thanks, but I'm looking at two options...remove the protection circuit entirely or replace all the bulbs and the polyswitch. Problem is, I don't have the part numbers or specs, and Ohm doesn't give them out.
One measures 6 ohms. The other measures 20 ohms. Everything I read about them is they should be totally open... no real resistance, unless triggered.
The writing is "mexico" on one side and something that looks like a "xD" over "-050" over "843G" on the other side. The writing is old and tiny.
Among the latest modifications that were made here in this forum, this possibility was eliminated so as not to repeat appointments unnecessarily. It is assumed that if you are going to answer the last post, you do not need to quote it. It makes sense, from my point of view.
As far as I understand polyswitch, it should have some low resistance and will have higher resistance in a fault condition. Any chance you can draw a schematic from what is there? There is a good chance the polyswitches are good.
You may just need to get a replacement bulb.
Removing the bulb and polyswitch protection may alter to overall balance of the crossover. You may be able to do it, just may require some additional crossover tweaking in that case.
Also there is a good chance you can secure a close enough replacement bulb at your local auto parts store. Mostly look for 12V 2A.
You may just need to get a replacement bulb.
Removing the bulb and polyswitch protection may alter to overall balance of the crossover. You may be able to do it, just may require some additional crossover tweaking in that case.
Also there is a good chance you can secure a close enough replacement bulb at your local auto parts store. Mostly look for 12V 2A.
Can you give us the exact model of your Ohm speaker? I would like to explore the web about it. Maybe I'll find the xover diagram and that might help get all the protection part out.
And yes, you are right, a well used speaker cabinet would not need protection, usually when they go bad (mainly the tweeter, the weakest part) it is because the amplifier failed and delivered DC.
And yes, you are right, a well used speaker cabinet would not need protection, usually when they go bad (mainly the tweeter, the weakest part) it is because the amplifier failed and delivered DC.