BGW 750B output modules

[djsynthesize]

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

[diy guy]

Hi

If I can recall,

The output of the BGW is 2sc3858 & 2sa2922.

I will try to dig up the manual.

[amp-guy]

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.

[djk]

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).

[Mark A. Gulbrandsen]

"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

[anatech]

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

[amp-guy]

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

[djk]

"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.

[anatech]

DC offset was adjusted by padding resistors in the predriver stage and watching the DC voltage of pin 6 on the LM318.

-Chris

[Mark A. Gulbrandsen]

For those that are not aware BGW also manufactured the Bose 1801 modules.... but to Boses's specs of course, not a BGW design in any way. I think just about every 1801 module failed at least twice!!


Mark