Carvin LS1202 Crossover, No High Frequencies

[TomCarlos]

Ok... I am stumped. I give you the Carvin LS1202 Crossover. The low frequency output is working fine. In fact, it looks like it is running full frequency? The issue is that I do not get any HF at the horn. Yes, I checked the horn.... it works fine on another amp.

I am getting signal at the junction of D1 and R3. But after that, it is reduced significantly. I have no idea how K1 works. It is a heat or current sensitive device?

Is there a quick and dirty bypass trick I can do to see if the issue is K1?

Thanks!!







LS1202_Crossover.pdf

[The Dude]

Is the switch set appropriately for how you are using the speaker? If full range, pins 1+ and 1- are used, If biamp, pins 1+ and 1- are for the LF driver and pins 2+ and 2- are for the HF driver.

[TomCarlos]

Yes, the switch is set to Full Range.

[The Dude]

Regardless of K1, there should be a good amount of signal through the two 20 ohm 10W resistors. Is one of them open or maybe cold solder on one?

[TomCarlos]

The 20 ohm resistors measure spot on. I pulled them both, cleaned the board, soldered back into place. I can tell you this is a double sided board. So I will probably need to lift K1 slightly and try to make sure solder is flowing on the top side of the board.

From the schematic, it looks like the normal position for the K1 switch is to put a short across R3 and R4? Is that correct?

[The Dude]

Yes, you can try shorting across the resistors to see if it works.

[TomCarlos]

I put a short across R3 and R4. No difference.

I am going to pull S1 and check that carefully. I wonder if there is something open in the switch.

[TomCarlos]

Update.... The double sided PC board is driving me bonkers. I really want to simplify this crossover network if I can by eliminating K1 and components not needed.

How about this... K1 removed. The white wire going to the NEG lead of the horn will be rerouted to the junction of D1 and R3 (Center of S1.C). In a prototype wiring, the horn works.

Am I missing or losing anything by doing this?

Update: The Carvin woofer is 8 ohms, the horn is 16 ohms.

[g1]

The white wire going to the NEG lead of the horn will be rerouted to the junction of D1 and R3 (Center of S1.C). In a prototype wiring, the horn works.

So you have output at horn if you connect horn before R3, but not if you connect after it? How about between R3 and R4?
What resistance do you measure from H1.A to H1.B with no horn connected?

[TomCarlos]

Thanks G1....

I believe K1 became faulty. There was no signal at H1.A. And trying to remove K1 from the PC board was nearly impossible. So I took the horn's negative lead and connected it to the center of S1.C. I removed K1 and C2 from the PC board. I left D1, R5, R3, and R4 in place. But they are no longer part of the circuit. The speaker cab was then able to produce sound at both the woofer and horn.

The speaker was part of a sound system package I had on Craigslist (selling for my brother). So I made a full disclosure as to my mod and I found a willing buyer. My advice was that if there are any issues in the future, just drop in an Eminence or Dayton crossover, connect that to a jack, and be done with it. But I am still puzzled why Carvin uses a 16 ohm compression drrver.

But I do want to understand how the circuitry after S1.C works. So I'll stare at this a bit longer.

[Helmholtz]

K1 should be a relay. I think it's part of a protection circuit.
Normally R3/R4 are shorted by K1 and the horn gets full HF signal.
However when the HF signal gets excessive (e.g. because of acoustic feedback or amplifier clipping), the rectified (by D1) signal charges C2 high enough to power the coil of K1 and the contact opens, thus reducing the power delivered to the horn.

[TomCarlos]

Thanks again gentlemen... I appreciate your replies.