Grandson amp circuit board # 721 input/driver and 722 output circuit board. I have a Grandson schematic but the part numbers don't match up? Anybody have the correct one? I can't get the DC off set below 50mv on 1 channel. I've swapped bias chips and it stays the same & changed all the diodes in this channel. The voltages in the driver stage are off compared to the other channel. So I'd like to replace the driver transistors which are 2SD525 & 2SB595. I've searched the forum and Google with no lucky. .... Yeesh!
Did you match and install the 4 diff input transistors? Did you replace the servo? With everything good in the front end and the servo working the DC offset should be in the neighborhood of + or - 3 mV.
The amp is old and the caps in the signal path should be replaced as well as the larger wattage resistors.
Your not stuck using the 2SB595's and 2SD525's. You can cross these to on semi parts without any problems and the amplifier will run fine and trouble free. I would be looking at
the outdated resistors that seem to change in value as a possible problem before I would look at the drivers.
If you have the schematic now you will notice the PN's don't match and there isn't an updated schematic out there anywhere that will jive with the numbers. The circuit is very
basic and no schematic is actually needed in order to follow the circuit. It is one of the easiest amplifiers out there to fix.
The amp is old and the caps in the signal path should be replaced as well as the larger wattage resistors.
Your not stuck using the 2SB595's and 2SD525's. You can cross these to on semi parts without any problems and the amplifier will run fine and trouble free. I would be looking at
the outdated resistors that seem to change in value as a possible problem before I would look at the drivers.
If you have the schematic now you will notice the PN's don't match and there isn't an updated schematic out there anywhere that will jive with the numbers. The circuit is very
basic and no schematic is actually needed in order to follow the circuit. It is one of the easiest amplifiers out there to fix.
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Find Q101 which is going to be a MPS8099 ( one of the 4 diff input transistors). You will find (2) 100/10volt caps in series making up a 50mfd non-polar input cap. These are going to the base of Q101. If the circuit board says 701 for example then all the parts in that channel will start with 700. If my memory is correct the front end match up needs to be spot on in order to have the correct voltages at the driver stage. If I were to gamble this is where I would start. You have 51 ohm resistors to the emitters of all the diff transistors. Also check the 15 k resistors R110,R111 that feed these as well as the 3.01K R104,R105, that are tied to the collectors of Q101, and Q103. Also check the 62 ohm resistors going to the emitters of the MPS U57, and MPS U07.
I don't have a Grandson in my possession right now but I am thinking that the numbering went 700's for one side and 800's for the other but I could be mistaken.
I don't have a Grandson in my possession right now but I am thinking that the numbering went 700's for one side and 800's for the other but I could be mistaken.
I don't have a Grandson in my possession right now but I am thinking that the numbering went 700's for one side and 800's for the other but I could be mistaken.
On the PCB# 721 the numbers start with 101 (on the left side oft the board, which oddly enough is input CH B's i/p resistor) and go all the way through the last part of CH A and the common parts. NO separation between A, B and supply like in the schematics.
So there is basically NO logic in finding the corresponding parts. Even the numbers that COULD match, meaning CH B that starts with 101, won't.
Only way is to track it all down.
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On the PCB# 721 the numbers start with 101 (on the left side oft the board, which oddly enough is input CH B's i/p resistor) and go all the way through the last part of CH A and the common parts. NO separation between A, B and supply like in the schematics.
So there is basically NO logic in finding the corresponding parts. Even the numbers that COULD match, meaning CH B that starts with 101, won't.
I'm so glad that you straightened this out.
Like I mentioned earlier its a very basic easy schematic. Track the parts down put the correct numbers next to the mislabled numbers on the schematic and your done.
On the PCB# 721 the numbers start with 101 (on the left side oft the board, which oddly enough is input CH B's i/p resistor) and go all the way through the last part of CH A and the common parts. NO separation between A, B and supply like in the schematics.
So there is basically NO logic in finding the corresponding parts. Even the numbers that COULD match, meaning CH B that starts with 101, won't.
I'm so glad that you straightened this out.
Like I mentioned earlier its a very basic easy schematic. Track the parts down put the correct numbers next to the mislabled numbers on the schematic and your done.
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