I bought 13 JBL CBT 70j-1 used at auction.
In way of vetting them, I hooked them all to a tripod and used a mic stand to measure all 13 speakers.
4 of the speakers have a smooth response when measured with omnimic. 9 do not. There is an identical sizable dip at 2Khz on the 9.
Here's a chart of what the speakers frequency response look. See how most have the dip at 2Khz?
Disregard the one very random FR plot in the chart. One of the 13 speakers came with the crossover board melted, so I swapped that board out with the seller, and the replacement crossover board had the same dip at 2Khz as 8 of the others. I just haven't gone back and re-measured all 13 again.
This issue at the crossover point, though I'm not sure how/why? In fairness, my Denon X7200WA doesn't tell me there is a polarity issue when I run Audyssey with these speakers and in the past - when I’ve accidentally wired a speaker out of phase the Denon AVR has told me I have a speaker out of phase. The speakers don't sound bad, and I'm not sure I'd even know there was a problem if I hadn't measured them.
Fortunately, the speakers cleanly come apart into 3 pieces.
- Tweeter Array,
- Woofer Array,
I have narrowed the difference down to the cab/crossover I think.
I swapped the tweeter array between a good speaker and a bad speaker.
The problem stayed with the cabinet/crossover.
I swapped the woofer array between the good speaker and the bad speaker.
The problem still stayed with the cabinet/crossover.
When I look at the cabinet/crossover I don't see anything different between the two crossover boards. Furthermore the wiring harnesses all only fit one way, and the speaker wire in the cabinets is cut to exact length with big and small spade terminals which would prohibit anyone from attaching with the polarity backwards - so it doesn't seem possible the wiring is wrong???
Anyone have any pointers? What's my next step in troubleshooting?
Pics follow - more available at request:
Good and bad side by side:
Top part of cabinet/ showing music/speech switch and dispersion switch. All switches in identical positions. This is just to show wiring looks identical
Close up of crossover board connections. See they only attach one way.
Picture of woofer array:
Picture showing the tweeter array only attaches one way. (Disregard the white wire it’s an RGB 5050 LED light I’ve attached to the speaker to mimic AMC Primes red LED look. It doesn’t interfere with anything)
Dip FR on Left.
Flat FR on Right