A-7 questions

[Russellc]

I have managed to assemble my A-7s and they do work! However, a couple of problems and questions have arisen. One cabinet/woofer is measurably louder than the other, at least a few db across the range and as many as 5-6 db in some areas. What is acceptable difference? I did move interconnects around to eliminate the source and the preamp as the cause, and traded speaker wires at the amp end to eliminate it as the cause. In the AM I will trade out the crossovers out. By the way, the cabs are 828, woofers are 515-8g, 511B horns with 902 8B drivers. The x-overs areN501-8B. All drivers appear very clean and cosmetically nice. Any pointers on what to look for in the way of non original diaphrams, cones etc? The seller did tell me he would give a 3 year warranty if a repair is needed. Also, what is the preferred replacement for the insulation/foam on the cabinet walls...regular fiberglass from home depot?
Any help or suggestions would be very much appreciated.

Thanks,
Russellc

[Fitts]

A most interesting problem. Start at the amp. Does swaping left for right still leave the same cabinet with problems? If not the amp or preamp or even the sound source (CD, tuner, turntable) is the problem.

If the signal from the amp is OK, you should probably do a through polarity check on the wiring of all the parts. A single driver out of phase can really make for some strange sound patterns. Don't just check wire colors, follow the wires and make sure every wire goes to the same terminal on each cabinet. Once you are sure all the wires are correct, you may want to check the 4 drivers with an AA battery. Make sure that briefly touching + power to the red or #1 terminal and - power to the black or #2 terminal makes the cones all move away from the magnet. Doing this with the horn drivers is more difficult than the woofers but it can be done if you get the driver under the right light. If any unit does not move in the correct direction, then you willl need to reverse wire that one item.

If the problem still persists, I would suggest you disconnect the crossovers and measure all the components inside with an ohm meter. Compare the same parts in each crossover making sure they all read similar to each other. The coils of wire should show a low resistance. The capacitors should start by dipping low and then go high the first time you check them. The bigger the capacitor, the more pronounced the dip. Because capacitors can hold a charge, be sure to discharge them before you do any retests or they will just read high with little or no dip. Any significant variations between like components should be cause for concern and further evaluation.

Last, use the ohm meter on the drivers. The woofers should have similar readings to each other as should the horn drivers. Keep in mind that the DC resistance as measured by an ohm meter will always be less than the drivers rating. Look for similar readings and don't be too concerned that the values are lower than what the driver says.

Best wishes and good luck.

[Russellc]

Thanks for the response. I have already swapped cable connections to eliminate both the source and the preamp, am heating up the amp now to re-check by switching speaker cables, then on to switching out the crossovers, then I will take your suggestion as to measuring the resistance of the woofers. In checking out the crossovers, I understand about measuring resistance of the coils and resistors, I am a little puzzled on the cap measurement you suggest. I assume you mean to discharge caps ( groung out with a resistor to the chassis box of the x-over? can these components be measured as you suggest in circuit, or do I diconnect one end?) Then watch the reistance change the cap causes? Unfortunately, one of my N501-8b x-overs is asembled with rivets, while the other (NOS in box with all original paperwork) is assembled with screws.

Thanks again for the
informative response
Russellc

[Fitts]

Since one crossover is riveted, maybe you should just test the inputs and outputs. I can't think of much that can go wrong with a crossover. I would not open them unless you find big differences when comparing resistance measurements between units. I'd check out everyting else first. If I had to guess, I think you should get very low steady resistance across the HF output and for the inputs and LF outputs you shoud see the ohm meter dip then rise to a fairly high resistance. Mind you this is just a guess, you should be more concerned with consistency between the two crossovers than my guessing.

Let's say you do want to check inside the crossovers. After you disconnect the crossover completely from the amp, woofers and drivers, test the caps in place. If they test good, then you should be all set. If one tests bad in the circut, then unhook one end and retest. You are right to suspect that coils and resistors in the circut can affect the measurements.

Testing caps with an ohm meter is not the best way but can give you a general sense of how the cap is doing. I usually start by putting a short circuit jumper wire across the cap leads for a few seconds to discharge it just before taking any ohm readings.

A large cap (100 mfd and up) will dip very low and slowly climb up. The caps in a crossover (10 - 20 mfd) should dip and then rise fairly quickly. A small cap like a little disk cap will hardly dip at all. All caps should show high resistance if you leave the ohm meter on long enough.

You can get a feel for testing caps by experimenting with a good one. You can get one cheap from Radio Shack if you don't have one laying around. A 10 mfd 10 volt cap will give you a feel for how they should look on an ohm meter.

Good luck.

[Russellc]

well I swapped eveything around and the one channel remains weak. I opened the boxes and realized my worst fears. While both drivers appear the same, with beautiful rear caps proclaiming them to be 515-8G speakers, on close inspection I saw differences. The "quiet" driver frame is only marked with a QC, and underneath these two letters is a 56. The "louder" driver is marked with a small square box with PI over the number 17. next to this is 8421. I looked at the "quieter" drivers cone and read 1803035662, I believe, the last three digits are kind of blurred. The "louder" drivers cone is numbered 1803035602. Now I know I am going to have a problem, as I paid big bucks for these specific drivers. When I measured the resistance I about blew a gasket. The "louder" driver measured 14 ohms (obviously a 16 0hm unit) While the other measured ( quieter driver) 4.4 ohms!
Can any one tell me if either of these drivers are actually 515-8G drivers? Or what cones these are? My dream is becoming a night mare. supposedly this purchase is warranted, will let you know what I find out. Thanking all in advance for help on figuring this mess out.

Cheers,
Russellc