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Bohlender Graebener RD75
Bohlender Graebener RD75
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Old 9th June 2017, 05:47 AM   #1
Grazinoz is offline Grazinoz  Australia
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Smile Bohlender Graebener RD75

I have a local who has brought me two dead ones of these for repair, apparently there is no support. I would like to help him but have no data for the drive unit. Does anybody have a dc at the terminals resistance reading for a live one? Any who have installed replacement panels? I have looked inside, all materials look familiar but without a basic resistance sorting these could be a time consuming game...

Curious - Graz
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Old 9th June 2017, 12:25 PM   #2
lowmass is offline lowmass  United States
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http://daveayers.com/rd_ribbons.pdf
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Old 10th June 2017, 09:58 AM   #3
Grazinoz is offline Grazinoz  Australia
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Thanks for the link, I had seen the pdf actually, and the spec sheet.

The unit I have here (taken apart) actually arrived with a fractured dead circuit and I made a panel up based on the actual physical measurements of what was in there, layout circuit, foil gage as measured. The trouble is - specs claim 6 ohms, dc resistance I am reading is different to the claimed specs, and without a working one to measure against claimed specs I could be chasing my tail...

The way they have been put together it actually looks like they are sealed for life, with no consideration for service...

I guess my main curiosity is for the dc measurement of one that works?

Curious - Graz
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Old 10th June 2017, 12:06 PM   #4
lowmass is offline lowmass  United States
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hmmm yea I wondered about that. I have a contact that has a pair. I will send him a line and seeif he can take a measurment.
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Old 11th June 2017, 01:31 AM   #5
Grazinoz is offline Grazinoz  Australia
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Thanks, appreciated!
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Old 11th June 2017, 09:58 PM   #6
Old'n'Cranky is offline Old'n'Cranky  Australia
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Graz, North coast Graz ?

Does this help at all ?

Taken from here -> https://www.parts-express.com/bohlen...0_BP_J500-1000


Bohlender Graebener RD75 Planar Transducer

The RD series of loudspeaker drivers offer the systems designer the advantages of a thin-film, line-source transducer with very high performance potential for fixed installation sound reproduction. Unlike most exotic driver types, the RD series models feature unmatched bandwidth, power handling and ruggedness, as well as the ease of an almost entirely resistive load to the amplifier. All models in the RD Series feature linear, push-pull Ceramic 8 motors linearly driving a polyester diaphragm whose moving mass is actually less than the air load. The bandwidth of the RD Series drivers is extraordinarily large, and is usable from as low as 150Hz to beyond audibility. Program power handling of the RD75 driver is 200 watts. The diaphragm is protected within a chassis made of three layers of heavy gauge, self-reinforced steel.

Specifications: Power handling: 100 watts RMS/400 watts max Nominal impedance: 6 ohms Re: 6.0 ohms Frequency range: 150-18,500 Hz SPL: 88 dB 1W/1m Net weight: 35.5 lbs. Dimensions: 75-5/8" L x 4-11/16" W x 1-1/2" D.
Attached Files
File Type: pdf 264-700-bohlender-graebener-rd75-specifications-44088.pdf (968.1 KB, 29 views)
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Old 12th June 2017, 09:10 PM   #7
lowmass is offline lowmass  United States
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Graz,

Using a Fluke model 115 DVM , he gets 5.5 ohm on one unit and 5.6 ohm on the other.

Hope that helps....
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Old 12th June 2017, 09:50 PM   #8
Grazinoz is offline Grazinoz  Australia
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Appreciated, thanks! The construction of these things is like a big "EMIM" panel, but single sided and circuit style is similar to the Carver's of old. Counter-intuitive really

I have to wonder, assuming quite a few of these were sold to diy people, and as part of systems, how many of these were fixed in the field? Were all service requirements back to base? Are there people sitting on ribbon parts or without parts, able to install these to the original formula?

I can make the panels in house of course, but it seems strange such a popular drive unit would have zero support...


Curious - Graz
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Old 13th June 2017, 12:53 AM   #9
lowmass is offline lowmass  United States
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Hmm yea does seem a bit strange. This type of planer is quite robust,maybe there just wasnt much call for repair??

I have no experience with these but do with the old Strathern's (basically same design). They were very durable so long as you didnt drive them too hard at too low a freq.
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Old 13th June 2017, 04:00 AM   #10
Grazinoz is offline Grazinoz  Australia
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The main failure mode seems to be foil fracture, and it turns out the build process is back to base - no longer...

Apparently there is an installation machine for initial fitting that makes everything uniform/repeatable. I spoke to an ex employee this morning in detail, most enlightening. I could build such equipment but realistically it makes no sense to do so for repairs, as the kit would have to be the same as that made for full production, and it would seem there is not much call for it...

Take care - Graz
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