Jumping a bad resistor for testing to see if it's bad.

Tony M

I have to fix the crossovers in my RTi12's.

The tweeters don't work but I know they are good. I direct wired them with very low power and they are fine.

So the crossovers are at fault as is the known issues with this line.

Can I take a small piece of wire and at very low volume, jump across a ceramic resistor's leads to see if sound comes out of the tweeters?

I've never heard anybody mention this testing method so maybe it's a no-no.

I have an ohm/multi meter that's nice. Any advice to checking if they're ok when there is no visible damage signs to those ceramic resistors?

Thanks.

F1nut

I don't see why not. Of course, you can just remove the resistor and test with your meter.

Tony M

Thanks for the confirmation.

I'm new to replacing crossover components. I've replaced a couple of bad components in older speakers in the past but I knew what was bad by visually seeing the bad things.

I just received a soldering station I ordered from Parts Express and can't wait to fix a couple pairs of speakers. I can confidently replace rotten foam surrounds now. Now onto crossover fixing and refreshing. Maybe fixing electronics is in my future too.

pitdogg2

You're in a slippery slope Tony
Lol

Tony M

I hope to put one more hobby / trade under my belt before the end comes. This is one skill I've been dreaming about for years. I'm almost done with my catch-up to do list and then can concentrate on selling, clearing space and getting good at fixing broken electronics and speakers.

My life will have become complete then.

delkal

Just test the resistors in place with your multimeter. Set it to the lowest ohm reading you have and place the probes on each side of the resistor leads. They should read the correct value.

Then do the same for the inductors. They should read close to zero ohms. Then start at the speaker terminals and trace that you get current to the capacitor (common multitestors do not work across capacitors)

mhardy6647

So -- don't forget Ohm's Law when one is testing "passive" components (resistors, capacitors and inductors/transformers).

The "value" of a component in situ in a circuit will reflect the circuit that it is in. It is - potentially - possible to test, e.g., a resistor in situ by putting the probes right on either side of the resistor... but still an accurate test would have to be "out of circuit".

To the original question: jumpering (shunting) a questionable resistor with another resistor will (should!) work IF the resistor in question is open (infinite resistance). If it is shorted or simply "out of tolerance", the result of shunting the questionable resistor with a known, good resistor will be a net resistance that is less than that of either individual component. It's that Ohm's Law thing: 1/Rt = 1/R1 + 1/R2 (etc.).

The other thing that may be worth mentioning -- testing a resistor with a meter will give an accurate result as long as the meter is a high impedance device and if the incrementally added resistance of the test leads is low. The latter's not usually a problem and "modern" (last couple-three decades ) DMMs are high impedance devices -- but an old-time analog multitester has such a low input impedance (sometimes as low as 1000 ohms/Volt) that it will "load" the resistor under test and misreport the value of the resistor (that Ohm's Law thing again).

Accurate and meaningful quantitative testing get even trickier with capacitors -- we can talk about that if anyone wants

mhardy6647

Tony M wrote: »

I hope to put one more hobby / trade under my belt before the end comes. This is one skill I've been dreaming about for years. I'm almost done with my catch-up to do list and then can concentrate on selling, clearing space and getting good at fixing broken electronics and speakers.

My life will have become complete then.

Best route to success (IMO, and not that you asked!) -- apprentice (so to speak) yourself to someone who knows what they're doing.

Next best thing: start here (older is better, but the newer versions are easier to find)
http://electriciantraining.tpub.com/
http://www.fcctests.com/neets/Neets.htm

Here's another really great resource:
http://www.tubebooks.org/

I make no apologies for being (heh-heh) biased towards vacuum-tube era references -- the function of vacuum tubes and the design and troubleshooting
of vacuum tube circuits (particularly audio circuits!!!) is way more straightforward to understand - IMO - than solid state devices or circuit design.

YMMV.

mhardy6647

Oh -- derp -- Given the original issue (tweeters not receiving signal from a crossover) -- empirically at least in simple crossovers, the component that's most
often at fault (strictly speaking IME, and predominantly with vintage loudspeakers) are the capacitors in the crossover.

As implied above, testing capacitors isn't quite trivial for the average Jacques.
There are two parameters (actually more than that) which are important. The capacitance (which is what one'd expect, of course!) and the equivalent series resistance (ESR). The other important factor is dissipation factor -- the bottom line is that capacitor testing is a bit subtle and arcane and typically substitution is the 'standard of care'

The fact that capacitances in parallel are simply additive: Ct = C1 + C2 means that is IS easy to 'test' for a bad capacitor by shunting (jumpering) in a known good one of a reasonable value.

In summary: substitution is the "best" (easiest and most straightforward) way to test, capacitors IMO (and FWIW).

EDIT: Oh, and don't forget the most obvious -- check for continuity of the crossover circuit supplying the tweeters! More to the point, make sure all of the wires are hooked where they're supposed to be

DaveHo

Tony just solder in new one and see if it works. Probably faster than wrapping ones head around the last 3 posts. J/k

Great explanation doc!

pitdogg2

Doc is correct of course and I agree Dave resistors are cheap put it in if it works move on if it don't then dig deeper. Resistors are the first to cook and may not readily appear so in my observations other time you DEFINITELY know it has been scrambled

Tony M

Thanks Jessie, Doc, Dave and Ivan.

Years ago, a guy told me to just replace everything except the inductors on a crossover that was bad. He used Parts Express capacitors and resistors to fix what he did. I didn't have that kind of money or any experience at the time. He invited to mentor me and I should've taken him up on his offer but he made me mad. I weighed the madness against the opportunity and chose to not be tutored by him.

I have some play money now. Enough to do simple crossovers, but not my SRS2's yet.

Thanks for all the explaining and the advisements to keep it simple and just replace the suspected parts and see if it comes alive from the input posts.

jdjohn

Do those tweeters have fuse protection? Assuming you already checked that?

This post got my attention since I fried some resistors the other night on a vintage receiver I was 'trouble-shooting'. My troubles have now increased significantly with it.

Tony M

I don't think they have a fuse in-line.

This LSi line is know for blowing those long white/tan ceramic resistors made in Mexico. I think the rating level was too low. I've read somewhere about raising some rated for 5 watts to 10 watts.

mhardy6647

Tony M wrote: »

I don't think they have a fuse in-line.

This LSi line is know for blowing those long white/tan ceramic resistors made in Mexico. I think the rating level was too low. I've read somewhere about raising some rated for 5 ohms to 10 ohms.

Is the issue the resistance (ohms) or the power ratting (watts)?
I'd guess the latter -- but I dunno.

Changing the resistance value would be expected to have a sonic effect.

F1nut

Yep, change the 5 watt to a 10 watt and you'll be ok down the road.

mhardy6647

F1nut wrote: »

Yep, change the 5 watt to a 10 watt and you'll be ok down the road.

That sounds better to me.

Tony M

F1nut wrote: »

Yep, change the 5 watt to a 10 watt and you'll be ok down the road.

mhardy6647 wrote: »

F1nut wrote: »

Yep, change the 5 watt to a 10 watt and you'll be ok down the road.

That sounds better to me.

I just had my first lesson / post graded. Thanks guys. I had time to change it too.

mhardy6647

Tony M wrote: »

F1nut wrote: »

Yep, change the 5 watt to a 10 watt and you'll be ok down the road.

mhardy6647 wrote: »

F1nut wrote: »

Yep, change the 5 watt to a 10 watt and you'll be ok down the road.

That sounds better to me.

I just had my first lesson / post graded. Thanks guys. I had time to change it too.

That's good (i.e., that you could fix it). It's like it never happened.

I note that @F1nut was much kinder and more affirming than was I in his manner of posting... he is such a gentle and uplifting soul.

Tony M