Ooops! I did wonder if it was the right thing to do before I sanded away, but, I thought as it was not conducting anyway, what else could I do...?
New Relay!
Okay
Thanks for explaining why!
Is there anything that could blow a new relay, or is this rather unlikely?
Cheers
Ah not quite yet..
When I read:
"Have you confirmed that the supply voltages are the same but opposite polarities?"
My brain goes
However! I have just found a circuit diagram!
Locate the two main power supply filter capacitors, carefully place the negative lead of meter on the copper bus bar that joins the two caps. Place positive lead of meter on one of the remaining posts on one cap (B+) and then the other (B-).
Well that was a lot more simple that I initially thought! cheers
I shall do that tomorrow when I am not quite so tired!
I think I shall also be able to measure the voltage at each amp module input too.
In the mean time this is the closest I can seem to find for the relay replacement:
MJN2C-DC12 Omron Automation and Safety | Mouser United Kingdom
Do you think this will be ok? the only thing I can see different is that the original was rated at 12A this one is only 10A. However I am having trouble locating anything else similar! - may be just me..
I shall do that tomorrow when I am not quite so tired!
I think I shall also be able to measure the voltage at each amp module input too.
In the mean time this is the closest I can seem to find for the relay replacement:
MJN2C-DC12 Omron Automation and Safety | Mouser United Kingdom
Do you think this will be ok? the only thing I can see different is that the original was rated at 12A this one is only 10A. However I am having trouble locating anything else similar! - may be just me..
Okay, just updating observations, not measured things yet!
After cleaning the old relay contacts:
At switch on:
rhs clip light lit bright, then slowly and gently faded to about half power, and just stayed on, dim.
lhs clip light did not come on at all
rhs nothing out of speaker
lhs music played out of speaker
New relay installed, (with old cover with magnets!)
rhs clip light lit bright, then slowly and gently faded to about half power, and just stayed on, dim.
lhs clip light lit bright, then slowly and gently faded out
rhs nothing out of speaker
lhs music played out of speaker
I switched the amp modules over. Both clip lights light bright again to start with, but this time,
rhs clip light lit bright, then slowly and gently faded out
lhs clip light lit bright, then slowly and gently faded to about half power, and just stayed on, dim.
rhs nothing out of speaker
lhs music played out of speaker
So I gather the two amp modules are ok..?
However the clip light staying on seems to follow the module and
rhs speaker still plays nothing
Oh! and in switching the amp off, there are now hisses crackles and pops! over a good number of seconds..
of if source music is left playing and the 750 is switched off the music continues to play for a while and then crackles and pops for a bit getting quieter and quieter until it vanishes...
Like my Adcom which I believe does not have a muting circuit...??
So I shall do some measurements tomorrow time permitting
After cleaning the old relay contacts:
At switch on:
rhs clip light lit bright, then slowly and gently faded to about half power, and just stayed on, dim.
lhs clip light did not come on at all
rhs nothing out of speaker
lhs music played out of speaker
New relay installed, (with old cover with magnets!)
rhs clip light lit bright, then slowly and gently faded to about half power, and just stayed on, dim.
lhs clip light lit bright, then slowly and gently faded out
rhs nothing out of speaker
lhs music played out of speaker
I switched the amp modules over. Both clip lights light bright again to start with, but this time,
rhs clip light lit bright, then slowly and gently faded out
lhs clip light lit bright, then slowly and gently faded to about half power, and just stayed on, dim.
rhs nothing out of speaker
lhs music played out of speaker
So I gather the two amp modules are ok..?
However the clip light staying on seems to follow the module and
rhs speaker still plays nothing
Oh! and in switching the amp off, there are now hisses crackles and pops! over a good number of seconds..
of if source music is left playing and the 750 is switched off the music continues to play for a while and then crackles and pops for a bit getting quieter and quieter until it vanishes...
Like my Adcom which I believe does not have a muting circuit...??
So I shall do some measurements tomorrow time permitting
The pre driver transistors are a bit over extended in their duty if I recall on these amps, have used careful applications of hot/cold air to try and locate the offending device on the board.
Can still find them likely, nos parts on ebay, unless someone knows of a good substitute for those parts.
If you get things working properly, there are some alternative methods of decoupling the lm318 that really do improve the sound. It involves using a smaller 22-47uf electrolytic in place of the tantalum parts or whatever is there, going from each rail to ground, and also using a film/foil .01uf from rail to rail, under the board.
A nichicon muse bp works wonders in the feedback position, and also in the input, bypassed with a wima mkp10, .22uf. Some of the parts used originally just aren’t helping things.
Can still find them likely, nos parts on ebay, unless someone knows of a good substitute for those parts.
If you get things working properly, there are some alternative methods of decoupling the lm318 that really do improve the sound. It involves using a smaller 22-47uf electrolytic in place of the tantalum parts or whatever is there, going from each rail to ground, and also using a film/foil .01uf from rail to rail, under the board.
A nichicon muse bp works wonders in the feedback position, and also in the input, bypassed with a wima mkp10, .22uf. Some of the parts used originally just aren’t helping things.
The 750 B/C was really very good reliable sounding amps....running in german Clubs up to mid of 2000
Service today no problem, all semiconductor devices are available, easy to get.
- I dont understand why BGW later not upgrade to 750B MKII with Class H Stepdriver
The Heat will decrease a lot
- I dont understand why BGW change circuit design in 750 D/E series und using Japan Devices?
750 D/E series Service impossibile, Japan output Transistors and Input ICs obsolete
Service today no problem, all semiconductor devices are available, easy to get.
- I dont understand why BGW later not upgrade to 750B MKII with Class H Stepdriver
The Heat will decrease a lot
- I dont understand why BGW change circuit design in 750 D/E series und using Japan Devices?
750 D/E series Service impossibile, Japan output Transistors and Input ICs obsolete
FYI
“- I dont understand why BGW later not upgrade to 750B MKII with Class H Stepdriver
The Heat will decrease a lot
- I dont understand why BGW change circuit design in 750 D/E series und using Japan Devices?
750 D/E series Service impossibile, Japan output Transistors and Input ICs obsolete”
The Engineering group after I left BGW took my basic designs from 7000 & 8000 families and used them for the updated 750D/E. They did not use my latest class H that was not fully production ready, as the new power supply and other new parts had not been selected. Several of the dual matched input & other parts were discontinued when the 750D/E series was started.
Duke
“- I dont understand why BGW later not upgrade to 750B MKII with Class H Stepdriver
The Heat will decrease a lot
- I dont understand why BGW change circuit design in 750 D/E series und using Japan Devices?
750 D/E series Service impossibile, Japan output Transistors and Input ICs obsolete”
The Engineering group after I left BGW took my basic designs from 7000 & 8000 families and used them for the updated 750D/E. They did not use my latest class H that was not fully production ready, as the new power supply and other new parts had not been selected. Several of the dual matched input & other parts were discontinued when the 750D/E series was started.
Duke
Phase - Thanks for the info! even though at the moment I am afraid I have no idea as to where to find or what a pre-driver transistor is, I may well research more as and when I have the thing actually working, then can plan some upgrades
Thank you for mentioning part specs to, always good to know, and refer back to.
Duke - wonderful !! thank you for the information on the feedback caps. I have printed out the schematic and found on the drawing the caps you mention. I may well take a pic of the actual cct board and circle the caps I think relate to the schematic and post for checking! I hope this will be ok!?
Thank you for mentioning part specs to, always good to know, and refer back to.
Duke - wonderful !! thank you for the information on the feedback caps. I have printed out the schematic and found on the drawing the caps you mention. I may well take a pic of the actual cct board and circle the caps I think relate to the schematic and post for checking! I hope this will be ok!?
I have now managed to take some measurements, apologies it has taken me so long!
I don't know if I measured right!? but, I unplugged both, amp modules and switched on, I measured everything very carefully! using the Plus of the meter cable on the point to be measured, and the Negative of the meter, I connected to the amps chassis.
Please find attached the relay board measurements.
I am not sure if this is of any use? I was not expecting readings on all the terminals simply as the modules were not connected but please bear in mind I am attempting learning all this complex stuff.
At the bottom of the page is the measurements at the smoothing cap terminals, one is +78.8 the other is -76.7 Vdc
Good first step, pretty much the same values...
I don't know if I measured right!? but, I unplugged both, amp modules and switched on, I measured everything very carefully! using the Plus of the meter cable on the point to be measured, and the Negative of the meter, I connected to the amps chassis.
Please find attached the relay board measurements.
I am not sure if this is of any use? I was not expecting readings on all the terminals simply as the modules were not connected but please bear in mind I am attempting learning all this complex stuff.
At the bottom of the page is the measurements at the smoothing cap terminals, one is +78.8 the other is -76.7 Vdc
Good first step, pretty much the same values...
Next I measured all the pins on both the left and right round connectors. Plus from the meter, on the pins, and negative from the meter on the amps chassis.
(no inputs or speakers connected)
The only things I thought may be a little off are the pins 3 and 4 on each connector, 23V seemed a little low, on the right, compared to all the other readings.
I would have expected the bridge input to have 0V on stereo, but that may be just me.
I must say I was a little shocked to read 90+V but hey I know it was mentioned this may not be the best/safest amp to learn on! Still it looks a nice simple well put together amp compared to some of the modern stuff I have seen inside of.
(no inputs or speakers connected)
The only things I thought may be a little off are the pins 3 and 4 on each connector, 23V seemed a little low, on the right, compared to all the other readings.
I would have expected the bridge input to have 0V on stereo, but that may be just me.
I must say I was a little shocked to read 90+V but hey I know it was mentioned this may not be the best/safest amp to learn on! Still it looks a nice simple well put together amp compared to some of the modern stuff I have seen inside of.
Remember, I have the 750C which has the gain controls on the front. I was advised when I bought the amp that they had been bypassed, but the wires seem to be still connected to the pots so I had a little measure of them too. I hope the drawing below makes sense.
OK so yes they are bypassed, sort of, but they measure different?
Should they not both be bypassed, so resistance is zero, as in, maximum power?
OK so yes they are bypassed, sort of, but they measure different?
Should they not both be bypassed, so resistance is zero, as in, maximum power?
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That topology works just fine with a 5532 op amp and MJ2119x outputs. Compensation needs to be adjusted for the op amp and layout. One of the real advantages of an op amp input stage is never having to match diff pairs. The op amp maker has already done that for you. The other reason they like to use them for big PA amps is no high voltages near the input stage, even under fault conditions.