Hi All,
I have a series 2 DC 300a that has very distorted and low level output on both channels. There is practically no DC offset in either output, <.02v.
B+ and B- come in at 65vdc, measurements taken at the ua739 op amp are
+ 9.68v and - 10.17 at pins 14 and 7 respectively.
These voltages seem close enough however, where 1.35v should be present at op amp outputs A and B pins 1 and 13, only .63v are present when there should be 1.35v.
Am I at least looking in the right place?
I have a series 2 DC 300a that has very distorted and low level output on both channels. There is practically no DC offset in either output, <.02v.
B+ and B- come in at 65vdc, measurements taken at the ua739 op amp are
+ 9.68v and - 10.17 at pins 14 and 7 respectively.
These voltages seem close enough however, where 1.35v should be present at op amp outputs A and B pins 1 and 13, only .63v are present when there should be 1.35v.
Am I at least looking in the right place?
With the uploaded low quality photostat to hi-fi engine its impossible to tell ,you got a link to a good clear schematic ?
Hi Duncan,
I've attached a copy of the manual I have, I don't recall where I sourced it from, hopefully it`s helpful.
I've attached a copy of the manual I have, I don't recall where I sourced it from, hopefully it`s helpful.
Where does it say that the outputs from op amp should be 1.3V ?
The output from both as far as DC is concerned ( offset ) can be nulled out using R106 and R206 .
Have you already sent an audio tone through the input to see if its amplified at the output , if you keep it low say 1KHz then it should register on your AC range of your meter.
The output from both as far as DC is concerned ( offset ) can be nulled out using R106 and R206 .
Have you already sent an audio tone through the input to see if its amplified at the output , if you keep it low say 1KHz then it should register on your AC range of your meter.
I got the 1.35v values from the schematic.
I`ve injected a 1kHZ sinewave to both inputs simultaneously and traced them to the opamp non inverting inputs ok, that is where the sinewave ceases to be.
What comes out of the opamp resembles nothing like the inputs.
I`ve injected a 1kHZ sinewave to both inputs simultaneously and traced them to the opamp non inverting inputs ok, that is where the sinewave ceases to be.
What comes out of the opamp resembles nothing like the inputs.
I have a built in app in my LInux PC I just magnified the circuit and can see what you are saying 1.35 V , that must be the lowest the offset goes to.
If both channels of the audio IC have the same output then either the chip is faulty or the components round about it are faulty but looking at the schematic each channel is fed independently with basically the same circuit so the odds of both having the same fault relating to mirror image components is low .
What is common to both is D5 which looks like a zenner diode that's worth checking .
If both channels of the audio IC have the same output then either the chip is faulty or the components round about it are faulty but looking at the schematic each channel is fed independently with basically the same circuit so the odds of both having the same fault relating to mirror image components is low .
What is common to both is D5 which looks like a zenner diode that's worth checking .
Had an amp exactly like this one once. Had a similar problem, cause were the zener diode in the amp circuit (don’t remember the details). Try that first one one channel and see the result.
This amp has one of the biggest power transformer out there, weight a ton.
SB
This amp has one of the biggest power transformer out there, weight a ton.
SB
I had suspected the 739 might be faulty and it was replaced. That didn't solve the problem.
I will investigate D5.
I will investigate D5.
Progress
I replaced D4,D5, and C4,C5,C6. I had the intention of replacing non polar C108,109,208,209 but couldn't find replacements that fit.
Channel B is now fully operational and channel A is good up until 10vrms into an 8 ohm load when the negative sine wave triangulates and looks weird.
See attachment.
I replaced D4,D5, and C4,C5,C6. I had the intention of replacing non polar C108,109,208,209 but couldn't find replacements that fit.
Channel B is now fully operational and channel A is good up until 10vrms into an 8 ohm load when the negative sine wave triangulates and looks weird.
See attachment.
