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Help!  Bohlender Graebener RD50 dead
Help!  Bohlender Graebener RD50 dead
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Old 13th June 2018, 02:05 AM   #1
Robert3750 is offline Robert3750  United States
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Join Date: Jun 2018
Default Help! Bohlender Graebener RD50 dead

I have a pair of VMPS speakers that use the B-G RD50 for the midrange. One of the RD50s stopped working a few days ago. I've looked all over the net for a replacement, but I can't find one. This does not seem like the kind of speaker that the typical speaker repair shop can fix. I've emailed BGRADIA to see if they can fix it. As an alternative, I've been looking at the BG Neo 8S. Six of them in a line array would fit in the space where the RD50 is, but I'm worried that it wouldn't be close to "plug and play". For one thing, the impedance of the RD50 is listed as 4 ohms, and the Neo 8S is 8 ohms. If I use a combination of series and parallel wiring, I could achieve an effective resistance of 5.3 ohms. How much of a difference would that make? I could match the 4 ohms by wiring 8 of them. Would that work?

The low end frequency response of the RD50 is given as 150 Hz. As near as I can tell, the high pass point is set at 124 Hz in the VMPS crossover, which is in a separate external box for the mid and tweets. The woofers use a separate amp and separate digital crossover that I can easily set to different crossover values and types (I had it set at 168 Hz with great success for many years). The low end response of the Neo 8S is given as 300 to 400 Hz, but that's for a single speaker. Would it be lower in the line array configuration?

Since the crossover is in a separate box, it could be modified, but I have no idea how to do that. Any and all help would be greatly appreciated.

Last edited by Robert3750; 13th June 2018 at 03:13 AM.
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Old 15th June 2018, 12:35 AM   #2
jcx is offline jcx  United States
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Default BG is dead

PartsExpress discontinues BG RD40, 75?
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Old 15th June 2018, 12:44 AM   #3
dtaylo3 is offline dtaylo3  United States
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Join Date: Nov 2004
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I think a Chinese company may have purchased the BG IP or inventory. Someone is still making the smaller tweeters for Parts Express and Danny Richie of GR Research. I would ask where they are getting their products from and maybe the supplier still has some old stock or repair capabilities.
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Old 15th June 2018, 03:43 PM   #4
WrineX is offline WrineX  Netherlands
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Location: Den Haag
how long is the RD5-50 and can you calculate how thick the foil is, we might just be able to fix it our selfs. it the simplest planar i ever seen so far.
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Old 17th June 2018, 07:40 AM   #5
ds23man is offline ds23man  Netherlands
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It is not that easy, you will need a special rig to take it apart due the very strong opposite magnets.
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Old 17th June 2018, 07:47 AM   #6
Robert3750 is offline Robert3750  United States
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I decided to go with 8 of the Neo8s in a line array. By wiring them in series and parallel, I'll match the 4 ohm impedance of the RD50.
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Old 30th September 2018, 08:37 PM   #7
kftool is offline kftool  United States
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Join Date: Feb 2007
Location: Richmond, Virginia, USA
Default I can repair RD 40, 50 and 75 drivers.

Quote:
Originally Posted by Robert3750 View Post
I have a pair of VMPS speakers that use the B-G RD50 for the midrange. One of the RD50s stopped working a few days ago. I've looked all over the net for a replacement, but I can't find one. This does not seem like the kind of speaker that the typical speaker repair shop can fix. I've emailed BGRADIA to see if they can fix it. As an alternative, I've been looking at the BG Neo 8S. Six of them in a line array would fit in the space where the RD50 is, but I'm worried that it wouldn't be close to "plug and play". For one thing, the impedance of the RD50 is listed as 4 ohms, and the Neo 8S is 8 ohms. If I use a combination of series and parallel wiring, I could achieve an effective resistance of 5.3 ohms. How much of a difference would that make? I could match the 4 ohms by wiring 8 of them. Would that work?

The low end frequency response of the RD50 is given as 150 Hz. As near as I can tell, the high pass point is set at 124 Hz in the VMPS crossover, which is in a separate external box for the mid and tweets. The woofers use a separate amp and separate digital crossover that I can easily set to different crossover values and types (I had it set at 168 Hz with great success for many years). The low end response of the Neo 8S is given as 300 to 400 Hz, but that's for a single speaker. Would it be lower in the line array configuration?

Since the crossover is in a separate box, it could be modified, but I have no idea how to do that. Any and all help would be greatly appreciated.
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Old 30th September 2018, 09:06 PM   #8
kftool is offline kftool  United States
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Join Date: Feb 2007
Location: Richmond, Virginia, USA
Hi all,

I use 12 RD50 drivers in my system from 200HZ to 6,500HZ. I had blown two of them about 8 years ago and found a pair on the net. A few weeks ago I blew another pair. I was lucky to find another pair but it will be difficult to keep finding more if I continue to play DVDs at 110 DB levels.

I kept the pair I blew years ago and just took them out and inspected the traces. I saw what looked like a knife cut across one of the traces.I took a few strands of the copper wire in a 16 gage corc and used it to check for continuity . Sure enough, the apparent cut was a continuity breach. I looked for a high content silver loaded epoxy to breach the gap.I spoke to a customer service tech at Epoxies Etc. in Rhode Island as they seemed to have what I needed. I settled on a two part, of course, epoxy that was somewhat flexible and had an extremely high silver content.

I paid around $150 for a few grams of each part and will have within a few days. I'm sure this will enable me to resurrect the four RD 50 drivers I have to fix.

I will alzo be raising the crossover point to 350 or 400 HZ. I use Krell KBX electronic crossovers at 24 DB per octave. This will lower the power to the drivers quite effectively so that I can reduce the input power to all the drivers.

A few years ago I purchased two CraneSong Mastering grade Compressor/Limiters butnever got around to installing them. I will not use them as compressors but just as peak limiters.

I'm using 24 Scan Speak 7 inch kevlar Revelator drivers in each channel from 60 HZ to 200Hz. They are great up ro 1K Hx and above so crossing them over higher at 350 or 400Hz wilt create a problem.

Furthermore, I have 8 channels of Rives Audio Parc parametric equalizers. They are three band parametric up to 350 Hz which I'll finally connect if I measure or hear any sonic abnormalities below 350 Hz.

I also have the Acoustisoft software and measurement microphone to check the performance in that range. I also have ears that don't do too badly even though they are 78 years young.

Should any other DIY members wish to contact me regarding their BG planar problems do not hesitate to reply to this posting.

Cheers, KenFritz, Richmond, Va.
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