Hello everyone
Have to know resistor code and value of R22
On right channel i got a RN60/78ohm
on the left channel it was no one
In the attached file some one have written down R22 is a RN65 / 75ohm is this true ??
if some one know the original value and code of R22 plz let me know!
Can some one who got one BGW 750C that dont have bean tutched maby take it apart and look ?
Have to know resistor code and value of R22
On right channel i got a RN60/78ohm
on the left channel it was no one
In the attached file some one have written down R22 is a RN65 / 75ohm is this true ??
if some one know the original value and code of R22 plz let me know!
Can some one who got one BGW 750C that dont have bean tutched maby take it apart and look ?
R22 is the fixed bias resistor.
Note 2 in the schematic speaks of adjusting R22 to get 370mV across emitter resistor for Q10 or Q11.
So every amplifier can have a different value for R22 but the value of the other channel can be a good start.
Note 2 in the schematic speaks of adjusting R22 to get 370mV across emitter resistor for Q10 or Q11.
So every amplifier can have a different value for R22 but the value of the other channel can be a good start.
Sometimes we used RNxx however we has a plastic bin with many values 1% values. Our jig was a rotary switch and ezhooks and a current limited power supply to set bias & offsets values.
Duke
Duke
Yes, you can use a pot. That's how I do it with these amps (fixed them for years). Then any normal, good quality 1% metal film resistor. Don't forget that when these were new, most people used carbon composition resistors with all their horrible issues. Carbon film were better, but not great compared to the metal film. So BGW specified you use a decent resistor, that's all. Anyway, find the value and select the closest fixed 1% value to set your bias. Power dissipation is very low there, so a 0.5 to 0.6 watt part works perfectly.
Bias is a variable. It changes with temperature and line voltages to some extent. So don't go nuts getting a specific value, just get it close and watch it from cold to hot. It should remain close.
Don't forget R24, also a variable value. You balance the output stage so the op amp (LM318) doesn't have to correct as much. Don't replace the op amp. The LM318 is blazingly quick, 50 V/uS and in feed forward 70 V/uS (from memory). You don't want to open the door to trouble trying to improve that part.
Bias is a variable. It changes with temperature and line voltages to some extent. So don't go nuts getting a specific value, just get it close and watch it from cold to hot. It should remain close.
Don't forget R24, also a variable value. You balance the output stage so the op amp (LM318) doesn't have to correct as much. Don't replace the op amp. The LM318 is blazingly quick, 50 V/uS and in feed forward 70 V/uS (from memory). You don't want to open the door to trouble trying to improve that part.
All I know is that these are very good amplifiers. I get excellent performance out of them after undoing the poor work roadies and hacks have done over the years. It probably needs a speaker relay by now.
My typical rebuild is a total strip of the heat sinks so they can be washed (and the chassis). New fan possibly while it's apart. Then you test each output after cleaning and grade them. You may need a new set. Match the beta values. Turning your attention to the PCBs, match the complementary pairs (no, they will never match exactly). Replace anything that needs it, then reassemble. After that, adjust your bias currents and DC offsets. You've cleaned the PCBs if they were soiled. Performance will be very good after this.
A lot of work? Sure it is. How old is it? Well worth it no matter what the used prices are. You now have something no one is selling. That's what many folks don't get. Like an old car that sells for "X" on the open market, restored cars sell for much, much more. They are not the same thing. Same for audio equipment that was good, properly restored they are not what is being sold on the open market. That's as long as they were good to start with. Junk will always be junk.
My typical rebuild is a total strip of the heat sinks so they can be washed (and the chassis). New fan possibly while it's apart. Then you test each output after cleaning and grade them. You may need a new set. Match the beta values. Turning your attention to the PCBs, match the complementary pairs (no, they will never match exactly). Replace anything that needs it, then reassemble. After that, adjust your bias currents and DC offsets. You've cleaned the PCBs if they were soiled. Performance will be very good after this.
A lot of work? Sure it is. How old is it? Well worth it no matter what the used prices are. You now have something no one is selling. That's what many folks don't get. Like an old car that sells for "X" on the open market, restored cars sell for much, much more. They are not the same thing. Same for audio equipment that was good, properly restored they are not what is being sold on the open market. That's as long as they were good to start with. Junk will always be junk.