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Thread: Restoring BGW 750C

  1. #16
    Senior Member SEAWOLF97's Avatar
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    Quote Originally Posted by Chevelle View Post
    Also you can add a 100 Ohm resistor in series with the 300 ohm resistor in the fan supply. The fan will run slower at idle.

    Hjalmar
    On my 750D the fan settings are either HIGH or LOW. The LOW did not work. HIGH was too loud.

    I tried some fixes but no joy. Since there was a lot of room behind it, I mounted a 110v computer "muffin fan" to the air outlet port normally used by the factory fan. Then ran that through a rheostat to control speed.

    Now it's dialed in to run at the fastest speed with lowest noise. 750's case doesn't even get warm now.
    “If you think that’s the dumbest thing you’ve ever heard,
    just wait a couple minutes!”

  2. #17
    Member originaltubino's Avatar
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    Just starting restoration of 750B and 750C

    Quote Originally Posted by Chevelle View Post
    Yes, the 750C always have transformer hum. I fixed the transformer only with the four screws on the side with rubber grommets between the transformer and housing. I did NOT use the metal bracket that secures the transformer to the bottom and also put a piece of a rubber mat under the transformer. This all reduces the hum but you should not ship the 750C without the bracket, otherwise the transformer will get loose...
    Also you can add a 100 Ohm resistor in series with the 300 ohm resistor in the fan supply. The fan will run slower at idle.

    Hjalmar
    This thread has been extremely helpful and even inspiring. I recently became owner of a BGW 750C quite accidentally through a friend, and expected a lot of power but the same faults I had heard with PA amps: a bit harsh, veiled, lacking detail. When I tried it with a good tube preamp and a set of high-efficiency speakers and was very pleasantly surprised, I decided to take it seriously and consider replacing caps etc. I bought a second one (actually a 750B, the one with the clipping lights) so I can keep listening to one while I work on the other.

    The tips here are really useful. I'll try the same grommet and rubber trick used by Hjalmar, and may replace the original huge electrolytic caps too. I'm not sure on the small caps on the board... what to change, what to replace them with...

    I ordered a pair of ultra-quiet 120mm x 120mm fans (made for computer cases) that I won't mind running all the time. They will need a 12vdc supply but I figure I can put a small wallwart inside the case.
    deHavilland Aries SE 845; UltraFi Monaco SE 845
    Tannoy GRF-R, DMT15; '52 Jensen Imperials+JBL 2441, LE15B; TAD 2402 clones

  3. #18
    Member originaltubino's Avatar
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    Quote Originally Posted by Chevelle View Post
    Yes, the 750C always have transformer hum. I fixed the transformer only with the four screws on the side with rubber grommets between the transformer and housing.
    I have started seriously on the 750B. I replaced the quarter-inch jack inputs with RCA phono jacks. I wanted sturdy rubber feet on it, and there are four holes already drilled in the right places. But I wanted to use 10-32 screws and had to drill the holes a bit larger first, but now have wide and very sturdy rubber feet in all corners so I can put it on top of something without fear, and it also makes it easier to pick up.

    I followed your lead on the transformer but used a thin sheet of cork betweeen the side and the transformer. I have a bunch of cork floor samples that are sealed on one side, and these tend to be more solid than most cork. I drilled holes through the cork for the bolts to pass through, and this has worked well. I also put a silicon pad under the transformer...

    The computer fans bought are nearly silent. You can’t hear them at all unless quite close. They run off DC from 5 to 12 Vdc, and I am using a wallwart that is nominally 9vdc but actually puts out about 13 without load. I was concerned that the airflow might be inadequate but I can feel the flow exiting the top sides and it seems like it will do the job nicely.

    Getting the umbilical plug back together was a struggle but a tiny amount of Cramolin conductive grease worked well.

    Next I want to order the big caps, and when I put those in I’ll finish wiring and gluing in the fan wallwart. There is plenty of space in the front right corner, and I can wire it to run whenever the amp is on.

    So far so good!
    deHavilland Aries SE 845; UltraFi Monaco SE 845
    Tannoy GRF-R, DMT15; '52 Jensen Imperials+JBL 2441, LE15B; TAD 2402 clones

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