BGW 750B output modules

I have a problem with several of these modules for my BGW amplifiers. One module has this problem where -when I plug it in it will work fine for 20 or 30 seconds, then it will make a big "pop" noise and the music will come out distorted. Or, when I turn on the amp, it will make a (snap, crackel, pop) noise, and the music will come out distorted, then it will make another distinct pop noise, and the music will come back out clean again for another minute or two... and then it will make that pop noise again, and the music will come out distorted. Djk, do you have any ideas? It sounds like a cap or something on the module, but I don't know where. Also, I have three other of these modules that are toast. When I turn on the amp with these modules, it imediately cuts off and goes into protection. I have been trying to get some replacement transistors for the modules as well. Djk, I read a previous post to a question you answered about another guy who was trying to replace them too. I know that you said to use MJ15024 for earlier, and MJ15024/25 for later, but I cannot find these anywhere. Some of my modules have Motorola (SJ 7394 7852), or (SJ 7394A 7952/ 7939) or (SJ 7407A 7926) or (RCA 6803407, C, 7913). In the service manual it says to use SJ 7394 (part #1854-7394) for the upper rail of the module, and SJ 7407 (part #1853-7407) for the lower. Does anybody know where I can get some replacements comparable to these at a reasonable cost. Also, does anybody have any additional tips, tricks, comments, or suggestions for testing procedure of the transistiors, -or general maintenance of the modules. I am an amateur and NOT an experienced pro, so I would really appreciate any additional input. Thanks
 
snap and pop

I repaired bgw gear under warranty years ago , this sounds familiar.
try replaceing the *#.%*!!! bead tantalum caps, poping sounds
and interrmittant sound are classic symptoms.

If I remember correctly they are 10uf, use a good quality low leakage electrolytic at the same or higher voltage rating.
 
SJ are house part numbers from Motorola (ON Semiconductor).

The early models use only NPNs, those MJ15024s are also second sourced by RCA/Harris.

If you have trouble finding them I usually have a couple of hundred laying around.

In addition to replacing the tantalum caps you should suck the solder off all small signal transistors that have gold leads and then resolder with 4% silver (if possible). The gold migrates into the tin and becomes brittle and then fractures.

Early models with the TO39 cased transistors have a problem with the heatsink clips failing. Do not grind or abrade these, some may be BeO2 (poison!). Just clean the heatsink grease off and epoxy them back together, use fresh grease (as little as possible).
 
"I repaired bgw gear under warranty years ago , this sounds familiar.
try replaceing the *#.%*!!! bead tantalum caps, poping sounds
and interrmittant sound are classic symptoms."

I also worked on BGW stuff but back in the Chicago area right about from BGW's very beginning to the late 80's. Ampman we ought to get together and exchange BGW stories and others sometime........
Aside from the tantalum cap problem there was also a solder/metalurgical problem with early 750 modules and that can only be taken care of by replacing the entire module. It was a difference of some sort between the hardware and the solder itself and on those modules you will see cracking of the solder around the hardware used to mount the outputs and sometimes around the outputs base and emitter pins.
I don't remember any of the BGW stuff ever using Jap output devices but the last efforts may have. Brian G. Wachner was the kind of guy that built things all with american made parts or not at all. Sadly, Brian passed on from a sudden bout with cancer in the late 90's. All the early 750's that I worked on used RCA output devices and had quasi complementart output stages.

Mark
 
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Most of my experience is with the BGW 750C amps. Once these are repaired and set up properly they run great.

I normally strip and wash the modules. I use MJ15024 / MJ15025 's for outputs and I do match them. I also work back and match all the other transistors. Then it's offset adjustment time.

As a normal course, I was replacing the caps. All this takes a while, but the amps sound better and seem to be tougher when done. I feel the attention is worthwhile.

-Chris
 
wow! I didnt know there were so many ex bgw techs around

all sort of things are coming back now
(whew spececially that bad burrito for lunch)
yeah lots of cracked solder joints on the outputs.
and bad connections on the p.c. that mounts on the input and output jacks.
I still have a 750 around and a little 100. that vintage are all pretty much the same circuit . using a lm318 for the vas.
probably getting hard to find to-99 packages are long gone.

hey Mark would be fun swapping repair horror stories.
ever have the displeasure of repairing a sylvania receiver
from the 1970s they smoke like chimmneys!!!

I made a find today- a 1500va plitron torroid dual c.t. secondarys
$25! coooool
 
"Then it's offset adjustment time"

Never seen an off-set adjust on a BGW. Off-set is determined by the LM318.

"using a lm318 for the vas. "

The LM318 was a 70V/µS opamp, still impressive today.

They did have a 2N3440/2N5415 common emitter pair as the VAS following the LM318.
 
Unfortunately no I don't, wish I did though. It had a gazillion output devices all npn and If memory serves me they were all Delco DTS 410 like Phase Linear used but I could be wrong on that..... this was back in 1973! Those Delco semi's were originally developed for NASA and actually went to the moon! Some were used in the first NASA color TV camera as the drive amplifier that ran the spinning disk color filter wheel.

The 1801 modules were screwed to the sinks by a bunch of screws from the finned side through to the modules, not unlike a Threshold. The 1801 had a huge EI tranny and weighed in at 85 lbs unboxed. You killed yourself getting one up onto a service cart to work on! It was heavy and the fins were sharp. It was the largest but not the most powerful power amp back The Phase Linear 700 was still the most powerful till the BGW 1000 appeared. BGW aptly nicknamed the model 1000 "The Arc Welders Special" It used a cooling tunnel arrangement and was a mono block amp.

BTW, an 1801 chassis would be a great foundation for a class A power amp. There have never been such huge flat back heat sinks on any amp that I can remember!

Mark
 
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Hi Mark,
Gee, and I thought the Marantz 500 (80 lbs by itself) was a force to be reckoned with. Vastly underrated in true Marantz fashion. One of these was the cause of a heria for me. 2.2 KW transformer. These sound great when properly repaired.
-Chris
 
"These sound great when properly repaired."

Yea, I'd like to get my hands on one. Another air tunnel amp. We sold a few of those and one came back for service but being only about 10 miles from MArantz headquartes in Itasca, IL. they got o go back to the factory for service. We only did out of warranty repairs on Marantz since they were so close. The Marantz 500 had a balanced output stage that was not refrenced to ground and was quite the beast. I remember a warning in the service manual that stated not to discharge the Caps with a screw driver as it would vaporize the tool. It would be another good foundation for a class A power amp if one could bear to scrap it out.....

The thing about the 500 that was nice was that there were no large finned heat sinks like the 1801 had..... a dangerous amp if you slipped and fel:eek:

Mark
 
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Hi Mark,
Actually, the 500 was not a balanced out. Neg spkr terminals went to the ground buss, and I mean buss.
These would vapourise a screwdriver as one of my techs found out, and the screw top too.
With 80V rails (I think), it would need a different (lower output V) power transformer for class A. It would be a crime to do that to one of these as they can sound so good. Plus it's a classic. I saw one where the idiot figured it needed more supply capacitance. There were installed were the meters used to be. Made me almost cry.
-Chris
 
I've no doubt got models #s mixed up on that... Could have been the Model 1500. They didn't make many of them either but I see em on E-Bay once in a while. That was a very long time ago......
I looked up things and the model 1000 was the first amp they produced.... in Brians garage! The smaller model 500 went on to replace the Crown DC-300 as the industry standard. I always felt that the Mellinneum Series is what killed the company. Whimpy amps at best and strange output stage that had a floating ground.

Mark
 
HEY WHADDA *******


I own 3 BGW 750. They are ****. No current and a harsh
and unpleasant sound. Tried to run Gauss 18" 4843 on them, it sounded like the speakers were still in their cardboard boxes.
I really wonder why somebody find any interest in them, other than pure nostalgia. Same goes with all the RCA clone amps, ie the 70W from ca 1968. Face it, they are ****. All quasi complementary amps sound like ****, no wonder since they have different open loop gain on pos resp neg half period. No way that could sound acceptable.



Andreas Wahlberg.


Make me an offer on the BGW