That thing oscillates like crazy. None of the experts here could help. Spent almost a year troubleshooting it, finally gave up and jumpered the PCB to take MJ15016’s in the bottom half and it worked the first time as if nothing was ever wrong. Built the same basic thing with as many as 4 steps and voltages as high as +/-160, with appropriate transistors.
Hi wg_ski,
I will congratulate you on being able to modify it to a point where you got it working. Still, that path is not recommended. It worked as designed. If you change the transistors to other types, be prepared for an unstable amplifier until you recompensate it.
-Chris
There are times when you absolutely need to have a unit on the bench in front of you to sort it out. That's the case here. You can't expect remote assistance to be able to solve every problem - even if you're paying for the support.
No. Your amplifier was oscillating like crazy. I've not had that problem myself.
I will congratulate you on being able to modify it to a point where you got it working. Still, that path is not recommended. It worked as designed. If you change the transistors to other types, be prepared for an unstable amplifier until you recompensate it.
-Chris
Hi NMOS,
The RCA transistors were a lot faster than most other brands of transistors. In many service manuals there will be clear instructions on how to re-compensate the amplifier to be stable with other transistor types. If you use a different transistor than the correction is for, there is a big possibility that the recommended compensation will not be effective.
A manufacturer cannot predict what devices will be developed in the future, or if a service tech would use an even older transistor type. A technician who doesn't use a recommended type that is available is really on their own.
Whenever you are working on an amplifier, make certain that the heat sink is well grounded if you have released it. This isn't for safety, although it is an excellent step to take for that. Most amplifiers will oscillate like mad if the heat sink is disconnected from the grounded chassis. The actual issue is that an ungrounded heat sink makes an excellent antenna. The outputs will easily radiate noise into the heat sink, and what has been picked up through capacitive coupling will be radiated to other parts of the amplifier using the same mechanism. It is quite common for perfectly good amplifier circuits to oscillate if the heat sink is isolated from the chassis ground.
-Chris
When you switch supply rails suddenly, that step will be coupled through the collector to the emitter and then to the output. It will appear in the feedback signal as a sharp "pip" a,d possibly be amplified as a signal. This is similar to what Carver amplifiers had a problem with.
The RCA transistors were a lot faster than most other brands of transistors. In many service manuals there will be clear instructions on how to re-compensate the amplifier to be stable with other transistor types. If you use a different transistor than the correction is for, there is a big possibility that the recommended compensation will not be effective.
A manufacturer cannot predict what devices will be developed in the future, or if a service tech would use an even older transistor type. A technician who doesn't use a recommended type that is available is really on their own.
Whenever you are working on an amplifier, make certain that the heat sink is well grounded if you have released it. This isn't for safety, although it is an excellent step to take for that. Most amplifiers will oscillate like mad if the heat sink is disconnected from the grounded chassis. The actual issue is that an ungrounded heat sink makes an excellent antenna. The outputs will easily radiate noise into the heat sink, and what has been picked up through capacitive coupling will be radiated to other parts of the amplifier using the same mechanism. It is quite common for perfectly good amplifier circuits to oscillate if the heat sink is isolated from the chassis ground.
-Chris
When I tried to get that BGW-ish amp to work class H I tried every method of compensation in the book. Nothing fixed the root cause, which was the compound output getting mighty upset when Vcc was switched. The emitter followers just don’t care. The original idea was to make use of a bunch of unpaired 2N3773’s and a pile of 30-0-30 toroids. It finally came down to “a couple dozen 15016 just aren’t that expensive - this just isn’t worth it”.
The takeaway here is that if this is tried with the BGW modules it will probably do the same thing - work perfectly on the later full complementary versions and not with the quasi. Things just aren’t that different. If someone out there does find a way to calm the all NPN stage down, I’m all ears.
The takeaway here is that if this is tried with the BGW modules it will probably do the same thing - work perfectly on the later full complementary versions and not with the quasi. Things just aren’t that different. If someone out there does find a way to calm the all NPN stage down, I’m all ears.
Djs playing the PA hard and very loud - NO LIMITER !
Dance / Electro / House / Techno
Bank of BGW750Bs / 250C /100 + DBX 500 Bass Boom Box + RG Dynamik Expander
Richard Long and associates New York Costumized Sound System
What i want say is: THE BGW 750B no have Problem with
Full Power into Clip every weekend up tp Sunday afternoon.
12 years about 5 times 1 channel from BGW750Bs failed and will be fixed
Thats nothings for amps overloaded into clip every weekend and old 20 years +
Todays Amps like this no way to survive 1 month when driving hard like 750B design.
The BGW 750 design comes from Jack Sondermeyer, he worked later as Peavey Chief Engineer
12 years cant remember BGW750Bs really fail, thats the reason
Its time for Redesign BGW750B MKII + add Class H very hot when driving hard into clip
750 design 12 years never had problem in club.
I work in PA sales and marketing unbeliievable how much amplifiers fails...already in warranty time
this kind of amps will not survive 1 weekend in previous club
Dance / Electro / House / Techno
Bank of BGW750Bs / 250C /100 + DBX 500 Bass Boom Box + RG Dynamik Expander
Richard Long and associates New York Costumized Sound System
What i want say is: THE BGW 750B no have Problem with
Full Power into Clip every weekend up tp Sunday afternoon.
12 years about 5 times 1 channel from BGW750Bs failed and will be fixed
Thats nothings for amps overloaded into clip every weekend and old 20 years +
Todays Amps like this no way to survive 1 month when driving hard like 750B design.
The BGW 750 design comes from Jack Sondermeyer, he worked later as Peavey Chief Engineer
12 years cant remember BGW750Bs really fail, thats the reason
Its time for Redesign BGW750B MKII + add Class H very hot when driving hard into clip
750 design 12 years never had problem in club.
I work in PA sales and marketing unbeliievable how much amplifiers fails...already in warranty time
this kind of amps will not survive 1 weekend in previous club
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Ha yes I know I tested something to do with those thermal switches...
Okay, yes I remember I checked them and two on one module mirrored the other two...resistance wise.
However, now having looked at the cct diagram, and testing again, the thermal switches wired to the amp modules are closed circuit no resistance - signal can pass through.
The thermal switches wired to the chassis are open circuit, which I believe they should be, when they get too hot the switches close, increasing the supply to the fan making it go faster, this I think was mentioned previously...
So the thermal switches look as if they are working correctly...
Where is this fault then...???
I have stripped out the transformer plugs, XLR inputs and phono inputs, and replaced all those with just two simple RCA inputs, hopefully removing any old joints, extra wiring etc.
I have also taken the rca inputs straight to the module wiring to cut out the extra wiring that goes to the gain pots, the pots are now not connected at all. More wiring and components removed from the equation as I am unsure how to find the fault, and thinking make it as simple as possible with less joints and wiring and it should be easier to find a fault....
(I have put 20k of resistance between rca hot and earth, each side)
Next, is to dig my way to the bridge rectifier and clean that area up, got to be around 1/8" of fine black dust on the bridge rectifier.
I do wonder, if the amount of dust in there may well cause some sort of issue.
I must say it is quite a nice project as the wiring is okay to follow around and the cct diagram is slowly becoming clearer and clearer as time goes on.
So glad the modules work! they seem so complex compared to the chassis wiring!
Thanks for Info...
750Bs had selected housepart numbers SJ or RCA for output devices
1. Is it important to match todays TO3 or TO3P devices fo reliable redesign ?
The 750 A/B/C no have base resistor at TO3 output devices,
later BGW 750 D/E/F/G models have base resistor
I remeber we have fixed in 1998 BGW 750C
replace SJ with MJ15003 / 15004 not selected
change broken LM318H too and the amp play next 3 years again in club to drive big subs.....never failed Then the Club closing forever
Hi NMOS,
Yes, matching the BGW drivers and outputs does matter. If you never have, you wouldn't know any different. Matching helps the output stage switch more cleanly at low volumes and to share current more evenly a medium currents. At high currents, emitter resistors force current sharing between the outputs as long as you don't have wide beta differences between the transistors. Current sharing will obviously be better if the outputs were matched well. I'm talking per bank here. If you can get the PNP and NPN transistors roughly matches the distortion will be improved.
I wonder how they killed the LM318? I can't say I have ever seen one of those damaged.
-Chris
Yes, matching the BGW drivers and outputs does matter. If you never have, you wouldn't know any different. Matching helps the output stage switch more cleanly at low volumes and to share current more evenly a medium currents. At high currents, emitter resistors force current sharing between the outputs as long as you don't have wide beta differences between the transistors. Current sharing will obviously be better if the outputs were matched well. I'm talking per bank here. If you can get the PNP and NPN transistors roughly matches the distortion will be improved.
I wonder how they killed the LM318? I can't say I have ever seen one of those damaged.
-Chris
All switches, mechanical joints cleaned, 47uF caps across the main ones, re-assembled, switched on, RHS now with some hisses cracks and pops that died out with the clip lights after around 30sec, then hit play 
Yes! I have beautiful music from both channels
Bi-amp'd at mo just 750 just doing woofers on CDM1-NT's
Think the 750 Makes them sing nicely!
However when switching off, is the relay supposed to cut sound to the speakers?
I get wizzing and hissing after powering off just for a short while.
Oh the fan.. its like having an industrial extractor going, maybe one of these would be quieter?
4806Z | ebm-papst 4000Z Series Axial Fan, 119 x 119 x 38mm, 105m3/h, 12W, 115 V ac | RS Components
Been running for almost 8hrs now at reasonable volume - glad the neighbours went away for Christmas! Not had this sort of bass out of the B&W's before
I don't see how anyone can not like the BGW sound!
(not tried on tweeters yet)
Yes! I have beautiful music from both channels
Bi-amp'd at mo just 750 just doing woofers on CDM1-NT's
Think the 750 Makes them sing nicely!
However when switching off, is the relay supposed to cut sound to the speakers?
I get wizzing and hissing after powering off just for a short while.
Oh the fan.. its like having an industrial extractor going, maybe one of these would be quieter?
4806Z | ebm-papst 4000Z Series Axial Fan, 119 x 119 x 38mm, 105m3/h, 12W, 115 V ac | RS Components
Been running for almost 8hrs now at reasonable volume - glad the neighbours went away for Christmas! Not had this sort of bass out of the B&W's before
I don't see how anyone can not like the BGW sound!
(not tried on tweeters yet)
Hi Deaf Cat,
Yes, the relay should drop out quickly. It also sounds like it doesn't have a delay coming on. Compare the capacitor sizes with the schematic, increasing the size could cause silly problems. I haven't got time to review the schematic right now, but maybe later if you are still having trouble.
-Chris
Yes, the relay should drop out quickly. It also sounds like it doesn't have a delay coming on. Compare the capacitor sizes with the schematic, increasing the size could cause silly problems. I haven't got time to review the schematic right now, but maybe later if you are still having trouble.
-Chris
Hey, thanks for the tip, do you mean the caps on the relay board? or, having added the 47uF across the main caps?
The first thing I did in an attempt to get the amp working through both channels was to change the plug in relay (could not get exactly the same, but thought it was close enough) straight after swapping the relay, I noticed the R2D2 sounds once powered off...so that irregularity was going on before the 47's were added, and before I disassembled the chassis wiring...
Looks like there a good number of caps on the relay board...
The first thing I did in an attempt to get the amp working through both channels was to change the plug in relay (could not get exactly the same, but thought it was close enough) straight after swapping the relay, I noticed the R2D2 sounds once powered off...so that irregularity was going on before the 47's were added, and before I disassembled the chassis wiring...
Looks like there a good number of caps on the relay board...
OK I am perplexed, just switched on and had both channels for around 45 seconds and the right channel dropped out!
Switched the 750 off and on again and the right channel is back again...
Looking at the relay, either two channels are on or off, and they are not switched individually.
Inputs are simplified, gain controls are bypassed, which after stripping down and rebuilding simplifying, gave me the right channel back, I don't understand what else could be causing a failure.
(previously the modules have been swapped around a fair few times and its always the RH channel that did not work, meaning the modules are ok! I hope!)
Could an iffy stereo/mono bridge switch cause the rhs to drop out? I did give it a good dose of contact cleaner...
I shall see if I can understand that part of the cct diagram
Switched the 750 off and on again and the right channel is back again...
Looking at the relay, either two channels are on or off, and they are not switched individually.
Inputs are simplified, gain controls are bypassed, which after stripping down and rebuilding simplifying, gave me the right channel back, I don't understand what else could be causing a failure.
(previously the modules have been swapped around a fair few times and its always the RH channel that did not work, meaning the modules are ok! I hope!)
Could an iffy stereo/mono bridge switch cause the rhs to drop out? I did give it a good dose of contact cleaner...
I shall see if I can understand that part of the cct diagram
Hi Deaf Cat,
A mono - stereo switch can certainly cause one of the channels to cut out. Try moving the switch slowly with a low constant tone playing. You want very little noise and a clean break as the switch moves to the other side.
As for the speaker relay. Was the replacement brand new? The test, you should be using a low level tone for this also. Apply the tone with the amp off. Turn the amp on, you shouldn't hear anything for approx 5 seconds, then you should hear the relay click in. Turn the amp off. The sound should stop almost instantly as you hear the speaker relay drop out.
-Chris
A mono - stereo switch can certainly cause one of the channels to cut out. Try moving the switch slowly with a low constant tone playing. You want very little noise and a clean break as the switch moves to the other side.
As for the speaker relay. Was the replacement brand new? The test, you should be using a low level tone for this also. Apply the tone with the amp off. Turn the amp on, you shouldn't hear anything for approx 5 seconds, then you should hear the relay click in. Turn the amp off. The sound should stop almost instantly as you hear the speaker relay drop out.
-Chris
Evening,
Ahh I have not dared try the mono/stereo switch eeek just in case there is a lot of static crackle etc I may brave it when I have cheaper speakers attached ;-)
Ar yes I have tried the low level signal at turn on and switch off, it plays for around 15/20 sec. (I think) after switched off, with the odd crackle and pop while getting quieter and quieter. Yes the relay was brand new! Hmm will have to check pin outs! Unless its welded its self together for some reason...??
Ahh I have not dared try the mono/stereo switch eeek just in case there is a lot of static crackle etc
I may brave it when I have cheaper speakers attached ;-)Ar yes I have tried the low level signal at turn on and switch off, it plays for around 15/20 sec. (I think) after switched off, with the odd crackle and pop while getting quieter and quieter. Yes the relay was brand new! Hmm will have to check pin outs! Unless its welded its self together for some reason...??
Thanks for the thought, yes lets hope it is just stuck...and can be, free moving, with little effort....
There I was, thinking that new is perfect!
Ha! thinking of day job, if something is real important to get done, or I am short on time, I generally pick up two in case one fails or does not work, leaks or is broken... What a way of being a!
What happened to new will work perfectly and be problem free for years!
Oh My
There I was, thinking that new is perfect!
Ha! thinking of day job, if something is real important to get done, or I am short on time, I generally pick up two in case one fails or does not work, leaks or is broken... What a way of being a!
What happened to new will work perfectly and be problem free for years!
Oh My
protection bypassed - the old relay worked correctly, on/off clicks, not that i tested it with sound.. its just that I cleaned the contacts with v fine sand paper (now learnt not good!) to try and get the right channel working. New relay obviously has different ideas as to how it should act! I hear no clicks at power up or power down.
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