Xover problems need help

Hi everyone. This is my 1st time ever being part of a forum so please bare with me i will post what i can to help others as well. Im stuck and A bit confused trying to troubleshoot a 3 way crossover. This xover is for the peavey SP6. In both cabinets the midrange doesn't work. The driver is not at fault. Approximately 1/2 the components register resistance while the other 1/2 do not ! Some of which are brand new silver series jantzen caps. Even the new caps that have never been used dont show any resistance. Im confused bc other caps do show resistance.. also when i plug a 1/4" cord into the input and measure from the mid And the tweet pos & neg to the exposed 1/4" plug of the cord i get resistance to the tip on both +\- and nothing on the sleeve. (I do have the xovers disconnected from the drivers & out of the cabs). Both xovers show the exact same resistors, caps & inductors good & bad. They seem to have the identical problem. Do i really have 3 resistors, 4 inductors and 4 caps (of which 1 cap is brand new), that are bad ? Per xover !! I have to be missing something here that i must not understand. I can upload pics and elaborate more if needed. Im sure alot of you are wizards, i really need your input. Thank you

 

A Capacitor will show an infinite resistance to DC as measured on a meter, but an impedance to AC depending on the AC's frequency. Resistors show a resistance to both DC and AC at audio frequencies according to their value. If you're measuring capacitors in circuit, then some will show infinite resistance, some won't, depending on what other components are in the circuit, so you can't tell anything about the state of the components from in-circuit DC testing.

What you've indicated about the capacitors out of circuit is perfectly correct. They won't show any resistance on a meter. If they did, then they're faulty and/or there's something wrong with the measurements, for example, you're holding the meter probes, so what you're actually measuring is your body resistance.

What you need to do is to inject a signal into the input, then trace the signal through the circuit with an oscilloscope to find where the signal fails. Then, you'll be in a position to analyse what's happening.

I assume the crossover you're having problems with is a passive high-level crossover, i.e. one made of resistors, capacitors and inductors, not a low-level active crossover with op-amps. The chances of both crossovers having the same fault is very low, possible, but very unlikely. It's more likely that there's something wrong with the way they're set up or connected.

If you can post a photo of the crossovers, and the way they're connected, we may be able to advise further.

Oh, and by the way, have you checked where the bi-amping link is set? This is accessed by unscrewing the input cap. If it's set to bi-amping, then only the bass unit or mid/HF sections will be active if only one cable is plugged in.



S.

 

Thank you and i will upload some pictures.. yes, the biamp option unplugs from full range and plugs into biamp. It is plugged into full range.

 

I use a 1/4" on the input (on the back of the small panel on the left), and the 4 output leads are on the bottom of that panel. 2 goto 15" woofs and the 3rd set is the 6.5 mid and the 4th is the tweet

 

Also can you send explain how i go about tracing the signal with an oscilloscope ? And what specs should i look for when purchasing an oscilloscope ? Does it matter what one i buy ? Thanx

 

To trace signals using an oscilloscope, you need a signal generator to create a signal. This can be the sound card in a laptop or tablet, even a mobile phone will do it if it has a headphone output.
1) Connect the signal generator to the amplifier, and connect the output of the amplifier to the crossover.
2) Set the generator to a sine wave at 1kHz, and connect the oscilloscope input to the output of the amplifier.
3) Set the generator's output level and amplifier's volume control to a suitable level such that you get a decent trace on the oscilloscope of, say 2-3V Peak to Peak. This is sufficient level to see clearly what's happening,but won't strain any of the crossover components. Make sure the sine wave is clean and visually undistorted.
4) Move the oscilloscope probe to the mid-output of the crossover, and make sure it's there and clean.
5) Change the frequency to 100Hz and move the oscilloscope probe to the LF output(s) and make sure it's present and clean there.
6) Change the frequency to 10kHz and move the oscilloscope probe to the HF output and make sure the signal is present and clean there.

That should confirm that all outputs are working. Now, if any of the outputs don't work, then you'll have to find where the output stops, by moving the probe through the circuit. It will be hugely easier to do if you have a copy of the crossover's circuit diagram. Do a search on-line and see if you can find one.

As to which oscilloscope to buy, for this purpose it doesn't much matter, but as an oscilloscope is so useful for all audio measurements and fault-finding, you might as well get something that will suit generally.
As a spec, you need ideally two traces, as this will allow you to view both channels simultaneously or view input and output simultaneously.
A bandwidth of at least 10MHz is needed, as although audio only goes to 20kHz, there are other times when you need to see much higher frequencies, like spurious oscillations. I use a 40MHz 'scope, but even 100MHz wouldn't be excessive.
Minimum sensitivity of, say, 10mV per division, although 1mV per division isn't excessive.
Finally, get a couple of decent probes, switchable 1:1 and 10:1 These aren't expensive, and will make signal tracing a lot more convenient than crocodile clips on odd bits of cable.

Hope this helps.

S.

 

This was an extreme help to me ! Thank you very much ! And yes, i all ready possess the schematic.