Bohlender Graebener RD75

[Grazinoz]

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

 

[lowmass]

http://daveayers.com/rd_ribbons.pdf

 

[Grazinoz]

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

 

[lowmass]

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.

 

[Grazinoz]

Thanks, appreciated!

 

[Old'n'Cranky]

Graz, North coast Graz ?

Does this help at all ?

Taken from here -> https://www.parts-express.com/bohle...ducer--264-700?utm_group=264-700_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.

 

[lowmass]

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....

 

[Grazinoz]

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

 

[lowmass]

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.

 

[Grazinoz]

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

 

[tyu]

Less then the Apogee Stage Bass foil.....I would say....I have had more than one pr of the Carver AL3 an other of this type... over the years...I sold Carver big ribbon speakers at the Audio shop I worket at.. in the 80s/90 did not have one fail in over 10 years...but I got into the Apogee in the 90-91....never like the sound of these ribbons.....my ears could always hear there type of buzz.....are just what sounded like buzz to me!.....
All just one mans input....good luck

 

[lowmass]

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

Yea I do remember some people talking about the Stratherns foil fracturing down where the foil crossed over the frame edge. These were the first units I believe that used a lighter gauge foil that gave a DCR of about .5 ohm. Latter units used foil that gave about .3 DCR. When I rebuilt mine I put a small strip of 12 micron kapton tape on that spot as a strain relief.

 

[captainjack115]

You can put a little Nickel Print on the fracture, let it dry over night, then use one or two coats of clear nail polish to keep the repaired area rigid to avoid breaking again.