Restoring BGW 750C

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01-21-2017, 11:27 AM
Chevelle

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Restoring BGW 750C

I bought a bunch of BGW 750C and 250C/D amplifiers cheap from a local PA hire. I know these amps from the time I was also owner of a rental company. We had a lot of them together with Crown DC300 and Phase Linear 400 and did the service in house. I wanted to use one of the 750C for the 2235 in my 4343 monitors.
Disassembling is straight forward. After cleaning and repainting the heavy steel housing I restored the boards. I replaced the electrolytic and tantalum caps with Panasonic FC types, changed the old mica insulators of the output transistors, cleaned the output relay and the pots, changed the emitter resistors and put in new 27.000uF power supply caps. As the cooling fan was much too loud for home hifi I bought a 26dB Pabst fan which is very quiet.
The amp is nearly like new and sounds great with the 2235, damping factor is greater than 230.

Hjalmar
01-21-2017, 11:33 AM
Chevelle

4 Attachment(s)

here some more pictures...
01-25-2017, 10:44 PM
jblbgw_man

2 Attachment(s)

These are a great amp, even by today's standards. I replaced my BGW's with 2 x new Bryston 4BSST'2 and I have to be honest ..... there isn't the AU$11,000.00 improvement in sound quality that I expected, more power yes but sound improvement .... not so sure.

Attachment 75621 Attachment 75622
01-26-2017, 07:55 AM
Ed Zeppeli

Beautiful work Chevelle. Look brand new!

JBLgw_man, beauty set-up as well.
01-30-2017, 02:10 AM
mech986

Hjalmar,

Had you considered changing out any bipolar or non-polar electrolytics in the signal path for film caps? Also, IIRC, isn't there an IC front end which might benefit from Op-Amp rolling with a newer and cleaner unit? Also, did you clean the octal umbilical and replace the speaker relay?
01-30-2017, 11:22 AM
Chevelle

There is only one electrolytic cap in the signal path, 10uf. It was a Tantalum cap.....I put in a Nichicon Muse bipolar which I always use in other hifi components. I cleaned the contacts of the output relay and polished them. If I would know where to buy new relays that fit the socket I would be happy.
I think you mean the round 8-pin connectors for the output boards when you say "octal umbilical". Yes, I had a look at them and used a little deoxid.
The input opamp is a LM318 in a round metal case. I could not find any replacement as an upgrade yet. I also think that the opamp is responsible for the typical sound of the BGW which I like together with my 4343.

Hjalmar
01-30-2017, 11:33 AM
Chevelle

5 Attachment(s)

By the way, I have also finished the 250C. I find it better sounding especially with my 4313B. Much detail and smooth with the 066s.

Hjalmar
01-30-2017, 12:15 PM
SEAWOLF97

I really enjoy my BGW 750D (even more power than an A,B or C) . Also have a 250D. Also have a commercial distribution amp (301 ?) that I never got around to playing with.

CL had an ad for a BGW sub-woofer. Massive thing, too big for my home.

Bought the 750 and later a 203 preamp on advice of an LHF member who is rather expert on the brand. If you have tech questions, he is a great resource.
I had been following the ebay prices of BGW 203's and they just aren't reasonable anymore.
won an auction for a 750H , then the seller backed out "Too heavy to ship" , suspect it was because he didn't get his anticipated windfall. :crying:

Fortunately the 203 has dual outputs , but I daisy chain the 250 thru the 750 ...can run either/or ... ie: the 250 will run with the 750 off.

only maintenance that has been needed was replacing the tantalum caps on the 203.
01-30-2017, 03:55 PM
loach71

Lm 318

I wouldn't bother replacing the LM 318 op amp. If it isn't broken don't fix it. You are doing all the right things with your 750s. The only other thing I would do would be to solda-pullt (solder sucker) all the joints on the pcb. Some production runs of the 750s and 250s used off-spec solder which could cause pops, clicks, farts and other audio irregularities. If none of these sonic problems appear after you have finished your mods then the renewal of the solder joints is unnecessary. Enjoy your big BGW iron... American battleships forever!
01-30-2017, 09:18 PM
jblbgw_man

Quote:

Originally Posted by Chevelle

There is only one electrolytic cap in the signal path, 10uf. It was a Tantalum cap.....I put in a Nichicon Muse bipolar which I always use in other hifi components. I cleaned the contacts of the output relay and polished them. If I would know where to buy new relays that fit the socket I would be happy.
I think you mean the round 8-pin connectors for the output boards when you say "octal umbilical". Yes, I had a look at them and used a little deoxid.
The input opamp is a LM318 in a round metal case. I could not find any replacement as an upgrade yet. I also think that the opamp is responsible for the typical sound of the BGW which I like together with my 4343.

Hjalmar

It's been years since I reverse engineered and built my own BGW 250C's and 202 pre amp in the late 70's, I was working for Philips Micro Electronics at the time here in AU, manufacturing semi's for the global market, including custom thick film hybrids for the US military. Anyway I recall regarding substitution of the LM318 at the time that it's specs were very very good even by today's standards with high slew rates and high bandwidth suitable for video, the LM318 was also available in a plastic package and also a ceramic package, however the metal can version had better thermal noise specs and better temp stability than the plastic package versions, a wise choice by BGW at the time I thought, proved to me that they knew what they were doing, the solid Tantalum caps used by BGW were also at that time generally only found in mil spec equipment. I have a lot of respect for BGW even 35 years after the event.
01-30-2017, 10:11 PM
audiomagnate

One of my 620B's (same modules as 750) needs new supply caps. Could you let me know what you used?
01-31-2017, 12:38 AM
Chevelle

Quote:

Originally Posted by audiomagnate

One of my 620B's (same modules as 750) needs new supply caps. Could you let me know what you used?

The original caps in the 750 have a diameter of 3" and are hardly to get. I choose these:

http://www.ebay.de/itm/310409014383?...%3AMEBIDX%3AIT

You have to alter the holder of the caps a little bit because these have a smaller diameter of 2.5"
01-31-2017, 07:53 AM
srm51555

Chevelle - I love rebuilding threads like this, thanks for sharing!

Jblbgw_man - nice setup
02-01-2017, 02:38 AM
Goldjazz

Nice one. I use a 750C on the woofers of my 4343's. I once had two on the woofers in bridged mode and that was pretty awesome bass I must say. The single one is pretty good too. The only issue for me is some transformer hum. I actually managed to smoke the other one I had by being silly trying to pot the transformer to get rid of the hum and not putting it back together properly. Anyway very solid, decent amps. enjoy.
02-01-2017, 09:09 AM
Chevelle

Quote:

Originally Posted by Goldjazz

Nice one. I use a 750C on the woofers of my 4343's. I once had two on the woofers in bridged mode and that was pretty awesome bass I must say. The single one is pretty good too. The only issue for me is some transformer hum. I actually managed to smoke the other one I had by being silly trying to pot the transformer to get rid of the hum and not putting it back together properly. Anyway very solid, decent amps. enjoy.

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
02-01-2017, 09:36 AM
SEAWOLF97

Quote:

Originally Posted by Chevelle

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.
08-14-2018, 02:04 PM
originaltubino

Just starting restoration of 750B and 750C

Quote:

Originally Posted by Chevelle

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.
08-18-2018, 01:17 PM
originaltubino

Quote:

Originally Posted by Chevelle

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!