-
Categories
-
Platforms
-
Content
Sorry about that, I kept getting interrupted and I got mixed up.
OK, Q601 Reads
44.9 V==>C
46.7 V==>B
47.4 V==>E
D603 Reads 46.8 V & 47.4 V
R611 Reads 1.45 V & 1.35 V
R633 Reads 45.5 V & 47.7 V
R609 Reads 1.45 V & 0.64 V
R601/602 adjust bias currents so compare left right channel for differences from input to output with schematic voltages.
Also check ripple on supply 48V, with no load, it should be very small.
Do we know what triggered the failure? User? power up, random?
Last edited:
O.K.
There is something strange in the readings of D603 cathode (46.8V) vs. B of Q601(46.7V),
as the base should be higher voltage than the diode cathode.I assume it is a measurement error.
Let's try this :
Measure the voltage on the Emitter of Q607 relative to GND (if possible probe constantly) .
What do we get ?
Q607 pin-out
Turn trim-pot VR601 gently with a small screwdriver ,can you lower that voltage?
If you can, to what voltage?
Last edited:
The goal if the trimpots is to get 0V on the inputs to the blown IC701 on pins 2,3** if they are close.
Pin 1 used to offset bias to effectively disable the IC701
THe 1st one biases current for Q603-Collector
The second is critical for ** DC null on IC701
. .
Q619is like a peak integrator that once Q621 saturates , it activates the protection and turns on LED. I suspect there was parasitic oscillation on the speaker output which caused IC701 to fail.
Pin 1 used to offset bias to effectively disable the IC701
THe 1st one biases current for Q603-Collector
The second is critical for ** DC null on IC701
. .
Q619is like a peak integrator that once Q621 saturates , it activates the protection and turns on LED. I suspect there was parasitic oscillation on the speaker output which caused IC701 to fail.
Sunnysky,
IC701 is not blown.
The "second" i.e VR603 purpose is to set the idel current of the final Trs.
Your explanation of Q619 and Q621 is "off bias"
Q619 is not a peak integrator,it is a current sensor of the final Tr Q625 (Emitter current)
i.e the output current .
Q620 does the same for the other channel.
Q619 and Q620 are connected in a "wired or" arrangement.
If any of them conducts (i.e. due to high enough output current) then Q621 will conduct and trip the "over current sense" of IC701(pin 1) putting the AMP in protection mode (i.e. de-energizing the speakers relay and lighting the protection LED ) .
In this specific AMP we do not have the above condition(i.e no over current protection activated)!
IC701 is not blown.
The "second" i.e VR603 purpose is to set the idel current of the final Trs.
Your explanation of Q619 and Q621 is "off bias"
Q619 is not a peak integrator,it is a current sensor of the final Tr Q625 (Emitter current)
i.e the output current .
Q620 does the same for the other channel.
Q619 and Q620 are connected in a "wired or" arrangement.
If any of them conducts (i.e. due to high enough output current) then Q621 will conduct and trip the "over current sense" of IC701(pin 1) putting the AMP in protection mode (i.e. de-energizing the speakers relay and lighting the protection LED ) .
In this specific AMP we do not have the above condition(i.e no over current protection activated)!
Last edited:
thanks for the corrections.Sunnysky,
IC701 is not blown.
The "second" i.e VR603 purpose is to set the idel current of the final Trs.
Your explanation of Q619 and Q621 is "off bias"
Q619 is not a peak integrator,it is a current sensor of the final Tr Q625 (Emitter current)
i.e the output current .
Q620 does the same for the other channel.
Q619 and Q620 are connected in a "wired or" arrangement.
If any of them conducts (i.e. due to high enough output current) then Q621 will conduct and trip the "over current sense" of IC701(pin 1) putting the AMP in protection mode (i.e. de-energizing the speakers relay and lighting the protection LED ) .
In this specific AMP we do not have the above condition(i.e no over current protection activated)!
What I meant to say was ... now that I am not rushing my fingers on the keyboard.
1) Q619 conducts if speaker positive current exceeds 3A ( 660mV ) with gain on the collector current.
2) but Q620 integrates this result in a ~ 300 ms delay resulting in a rise in disarm pin 1 of IC601 speaker relay enable driver. Higher currents are faster, so it has thermal fuse like responses.
a) My suggestion is compare DC offset of both speaker outputs and null this. or trace back to fix fault.
b) check Pin 1 voltage if +ve, the protection is ON with no speaker, there is a fault
If fault goes on with a speaker , it due to unstable oscillation. ( full scale) STOP and report.
c) If no fault with 8 Ohm speaker (or a 12V car headlamp for a dummy load) then so far so good,
etc...
Q607 reads...
15.6 V==>E
45.5 V==>C
16.2 V==>B
I can get E to go down to 12.8 V and B down to 13.3 V
Please put the trim-pot VR601 in the position to produce 15.6V Emitter voltage on Q601(as it was in the first place).
Volume at minimum.
Measure the voltage on the junction of R645/R647 (0.22ohm ).
What do we get?
Disconnect the receiver form mains power.
Locate R627 (820ohm/2W).
Find it's Leg that is connected to the junction of R645/R647 (0.22ohm ).
Disconnect(un-solder and lift) that leg as marked on the pic below.
Connect well that resistor Leg to chassis GND with a wire.
Power -up the receiver.
Measure the voltage on the junction of R645/R647 (0.22ohm ) and Q607.
What do we get?
Volume at minimum.
Measure the voltage on the junction of R645/R647 (0.22ohm ).
What do we get?
Disconnect the receiver form mains power.
Locate R627 (820ohm/2W).
Find it's Leg that is connected to the junction of R645/R647 (0.22ohm ).
Disconnect(un-solder and lift) that leg as marked on the pic below.
Connect well that resistor Leg to chassis GND with a wire.
Power -up the receiver.
Measure the voltage on the junction of R645/R647 (0.22ohm ) and Q607.
What do we get?
Last edited:
Vout rises to 45V?
VR603 which adjusts crossover and output bias current has enough range to turn off output bias current or increase to 30mA depending if phantom offset if fixed.
Recall Q609
11.0 V ==>E
13.1 V ==>C Vce=2.1 which starves output stages of any DC current Vce= needs to be ~2.4
11.6 V ==>B
I wonder if there are any leaky electrolytic caps that affect offset by 7V like C613, it would have to be in the hundred ohm range tho.
Check voltage at (B) -46.6V this can cause offset if really low.(-20)
VR603 which adjusts crossover and output bias current has enough range to turn off output bias current or increase to 30mA depending if phantom offset if fixed.
Recall Q609
11.0 V ==>E
13.1 V ==>C Vce=2.1 which starves output stages of any DC current Vce= needs to be ~2.4
11.6 V ==>B
I wonder if there are any leaky electrolytic caps that affect offset by 7V like C613, it would have to be in the hundred ohm range tho.
Check voltage at (B) -46.6V this can cause offset if really low.(-20)
Last edited:
I can't find where R645 and R647 meet on the circuit board to get a voltage reading on the junction like you said. They look close together on the schematics but far apart on the circuit board. Is there another way to get a reading?
anything connected to that NODE is in yellow. This is the left PA output
There is a pink positive buffer side and a green negative buffer side to this typical bipolar amplifier. They crossover and share current near 0 and that crossover DC current is adjusted by VR603.
Since you had 0.64V? on IC701-3 which is scaled by R ratios 1 / 11 from the above signal in yellow
The Yellow signal should be 11 times IC701-3.
This large positive offset is very unusual because the offset control VR601 is limited.
WHich leads me to believe something is wrong with the NEGATIVE SUPPLY VOLTAGE B in green.
There is a pink positive buffer side and a green negative buffer side to this typical bipolar amplifier. They crossover and share current near 0 and that crossover DC current is adjusted by VR603.
Since you had 0.64V? on IC701-3 which is scaled by R ratios 1 / 11 from the above signal in yellow
The Yellow signal should be 11 times IC701-3.
This large positive offset is very unusual because the offset control VR601 is limited.
WHich leads me to believe something is wrong with the NEGATIVE SUPPLY VOLTAGE B in green.
Last edited:
Rick the physical distance is not important.
Simply verify there is an electrical connection between R645 and R647(ohm test)-that is the "junction".
You can measure on each of the resistors where it is most convenient.
Sunnysky,
I thought so but it isn't the case,look at #14 to #18.
What I'm trying to do now is disconnect the DC loop from the output (junction of R645/647)
to the Emitter of Q603, by connecting R627 bottom side directly to GND.
This would create a situation in which it would be easier to find the fault location.
I thought so but it isn't the case,look at #14 to #18.
What I'm trying to do now is disconnect the DC loop from the output (junction of R645/647)
to the Emitter of Q603, by connecting R627 bottom side directly to GND.
This would create a situation in which it would be easier to find the fault location.
It's R627 not R641.
D605 could be a factor, but we should check the "DC main path" first.
right... the only high impedance part in this design which has near unity DC loop gain and AC gain of ~ 40 is the front end Q603. About 50k leakage across Vce will pull the output up to 7V.
The AC gain permits 1Vp to reach 40Vp if the 1000uF does not have higher ESR yet.
This failure mode is unusual but possible.
@dorke , you are doing a great job analyzing this failure. I've lost a lot of my attention to detail skills, not working with such designs for decades. keep up the good work!
The AC gain permits 1Vp to reach 40Vp if the 1000uF does not have higher ESR yet.
This failure mode is unusual but possible.
@dorke , you are doing a great job analyzing this failure. I've lost a lot of my attention to detail skills, not working with such designs for decades. keep up the good work!
Last edited:
OK, R645/R647 Junction read 11.1 V
Then I Cut & lifted the leg of R627 that was connected to the R645/R647 junction and connected that leg to chassis GND and now the junction reads 39.9 V
And Q607 now reads...
42.4 V ==>E
43.0 V ==>C
43.0 V ==>B
Similar threads
