KLH Model 5 Crossover Restoration - diyAudio
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Old 27th February 2015, 07:29 PM   #1
rkp is offline rkp  United States
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Default KLH Model 5 Crossover Restoration

The KLH 5 Crossover Treatise – Randall Peede, Camarillo, CA
I recently restored a pair of KLH Model 5 loudspeakers for a client. What I found surprised me and so I’m passing it on. I worked on Serial #’s A50515788 and A50515786, close enough to sequential for me.
I rebuilt the crossovers one at a time for testing comparison. Both crossovers had bad caps. I used Solen’s from Parts Express. I replaced the tweeters with the best I could find on eBay and tested them.
The old tweeters were fried, which is not unusual on this model, and I found out why. After disassembly, I put a crossover on my spectrum analyzer with a full-range, pink-noise source. With the left, 3-position crossover knob (marked 2500-700 CPS) set to the full counter-clockwise, “Decrease” position, the tweeter was receiving full bass response in addition to tweeter response. There was also a -8dB saddle in the middle. The center position (“Normal”) and right position (“Increase) were OK. The right-hand switch (“Above 7000 CPS”) had no effect on the tweeter/bass anomaly. Gee, I wonder why they fry tweeters.
Now let’s look at the boring details. First I’ve seen some reference to metal back plates on the crossover. These are plastic. Next there’s the schematic available online which does not match the actual product. The midrange connections on the crossover measured fine in any switch position as did the woofer.
The left rotary switch is a 12 position model with three sliding plates controlling the connections. The right rotary switch is also a 12 position switch of which only seven connections are used with two sliding plates. So we have lots of permutations to trace.
Without a correct schematic, I started drawing one. I started from the tweeter output side and then from the main inputs. First I found a loop that went nowhere. Then it got weird. I stopped. If I was going to do a science project or thesis, I could finish this analysis, but the point here was to restore/fix.
There were three possibilities: 1. Start from scratch and redesign the crossover – no., 2. Design an add-on external circuit – I did it, but while the bass was gone from the tweeters, the effect was incorrect for other settings. 3. Put a sign on the back of the speaker that says Never, ever use this position (with an arrow). It will destroy your tweeters! Like I said, this wasn’t a science project, so I chose #3.
I did a full-range measurement of both assembled speakers using a calibrated microphone and verifying the amplifier’s flatness of response. Both speakers look great on the spectrum analyzer. I did not test the dreaded “Decrease” position. This whole process consumed about 30 hours, including lying in bed at night staring at the crossover and tracing in my mind. An interesting experience.
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Old 27th February 2015, 07:57 PM   #2
rayma is offline rayma  United States
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Quote:
Originally Posted by rkp View Post
Without a correct schematic, I started drawing one. I started from the tweeter output side and then from the main inputs.
KLH 5 Schematic | The Classic Speaker Pages
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Old 27th February 2015, 08:19 PM   #3
rkp is offline rkp  United States
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Default Thank you, but it still doesn't match the actual product

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Originally Posted by rayma View Post
Thank you, I appreciate your response, but neither factory schematic matches the actual product that was in front of me. The photos are of the metal backed ( they look earlier) crossovers. Mine were plastic.

Best Regards
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Old 27th February 2015, 09:11 PM   #4
rayma is offline rayma  United States
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Originally Posted by rkp View Post
neither factory schematic matches the actual product that was in front of me. The photos are of the metal backed ( they look earlier) crossovers. Mine were plastic.
Maybe someone "rebuilt" them.
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Old 27th February 2015, 11:04 PM   #5
Lojzek is offline Lojzek  Croatia
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According to KLH model 5 schematic from 1969, there is nothing
unusual about the decrease knob position. Rotating SW1 2500-7000 Hz
clockwisely, switches two 4 uF in series, parallel with addition of
adding 0,4 mH as second order filter.

SW2 increases spl of only very high frequencies.
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File Type: jpg KLH Model 5.jpg (25.3 KB, 31 views)
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Old 28th February 2015, 01:41 AM   #6
rkp is offline rkp  United States
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Yes. The schematic does not match the product that was on my bench.
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Old 28th February 2015, 07:23 AM   #7
Lojzek is offline Lojzek  Croatia
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Rayma is right then.
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