Hi !
Mine looks just fine once powered up according to the setup procedure,
but enough for today.
Bias trimming tomorrow.
JM
Hi !
On powering ON, everything seems to be OK, leds lights are on,
but I don't get any current across R25 or R26, the digital mV meter
stays steady on 0mV - no jump to 100mV and decrease, nothing at all : 0V
VR1 and VR2 have been adjusted to same value 480 ohm (turning screw clockwise).
VCC+ is +50V VEE- is -50V
Should I proceed and decrease VR1 and VR2, or do I have something wrong ?
I hope I won't be a pain ...............
Thanks in adavnce !
JM
On powering ON, everything seems to be OK, leds lights are on,
but I don't get any current across R25 or R26, the digital mV meter
stays steady on 0mV - no jump to 100mV and decrease, nothing at all : 0V
VR1 and VR2 have been adjusted to same value 480 ohm (turning screw clockwise).
VCC+ is +50V VEE- is -50V
Should I proceed and decrease VR1 and VR2, or do I have something wrong ?
I hope I won't be a pain ...............
Thanks in adavnce !
JM
Good news, it took about 12 turns to get at about 10mV.
Thank you, I hope everything will be fine now.
JM
Next issue:
One of the 1R 2W resistor (on the R26 side) just fried !!
when I was at about 300mV apparently ..........
I must say that I had to turn both trimmers quite a lot to have current rise.
What would happen is that it would rise to some value, well below 400mV
and then go back to 0mV.
I guess I'm should go backwards with the trimmers, and use other resistors.
Is it OK to use other value than 1R ?
JM
One of the 1R 2W resistor (on the R26 side) just fried !!
when I was at about 300mV apparently ..........
I must say that I had to turn both trimmers quite a lot to have current rise.
What would happen is that it would rise to some value, well below 400mV
and then go back to 0mV.
I guess I'm should go backwards with the trimmers, and use other resistors.
Is it OK to use other value than 1R ?
JM
Your amplifier is probably oscillating! Connect a scope at the output; this will tell you whether or not the amp is oscillating.
Check proper heatsink grounding.
That is quite normal. I had the same behavior. Due to gain of input stage transistors but Shaan can explain much better.
You know what ?
One of the 1R was fried, the other one OK.
I checked the other one : showed 2.7 ohm on the multimeter !!!!
They are Mouser parts, new, out of the bag !!
The heatsink is well grounded, to 0V and Earth.
EDIT: enough for today. I just made new 10W 1R (actually 1.33 ohm with what I had in a drawer)
and we'll see what happens tomorrow. Maybe trim it slower and let the heatsink warm and current stabilize.
JM
One of the 1R was fried, the other one OK.
I checked the other one : showed 2.7 ohm on the multimeter !!!!
They are Mouser parts, new, out of the bag !!
The heatsink is well grounded, to 0V and Earth.
EDIT: enough for today. I just made new 10W 1R (actually 1.33 ohm with what I had in a drawer)
and we'll see what happens tomorrow. Maybe trim it slower and let the heatsink warm and current stabilize.
JM
Last edited:
Started the setup from start with my new 1 ohm resistors:
PSU's fuse blew up immediately after diodes lit OK.
Q14 is dead (on the side of yesterday's problem), legs shorted.
I don't know if I'm going to unsolder and replace Q14.
And what else to check ?
JM
The 1R 2w resistor acted as a fuse yesterday, but my today's 1R 10w
was no fuse ! Me stupid ........
Anyway, what should I do (after Q14 replacement) to solve this oscillation issue ? The heatsink is earth ground and 0V already.
JM
Hi Jean.
Check all the BJTs for collector-emitter short. A good way to test is to set the meter to continuity test mode, then (for NPN) place red probe to collector and black probe to emitter. Then use a wire and short base and emitter. The meter should show open circuit. If it shows short circuit or some random value then replace the BJT. Reverse the probes for PNP.
If Q14 is dead then replace it. Instead of trying to melt the pins together and pull the dead mosfet out, you may want to cut all three pins from the mosfets and then get individual pin off the board, will be faster, cleaner and easier.
Also, as a further protection in addition to the internal zener diodes of the MOSFETs, you may install a 9V or 10V zener and 1N4148 diode in series to each MOSFET's gate and source pin. For N mosfets the zener's negative pin goes to gate and the 4148's negative pin goes to source while both their positive pins are soldered together. For P mosfets the zener's positive pin goes to gate and the 4148's positive pin goes to source while both their negative pins are soldered together. The additional 4148 diode with the zeners are required to keep the nonlinear zener capacitance invisible to the VAS stage.
You can use 10ohm resistors instead of 1ohm, in which case the real value will be 1/10th the measured value of current. You may also want to install 1A or 500mA rated fuse to the PSU for the duration of test.
Sorry for late response. Had been a bit busy preparing for the next 20 days lockdown.
I doubt that it's oscillation if the boards are Rev1 or the rev1 mod is installed. Please try the procedures in my last post and update.
Thanks a lot, I will do this.
It may take time as I'm going to miss some parts.
I'm going to start populating my 2nd pcb meanwhile.
Thanks again for your help.
Good luck for lockdown ............. it is really a special experience.
Already 5 days here .......
Cheers
JM
I've found some hints for other mosfets, should be from 0.45 to 0.7 Nm maximum.
Now another question.
I just did the VAS trimming measurement on R26 legs, up to 400mV, no problem,
turning VR1 exactly like VR2.
Once there, I checked the value on R25 legs, and it showed 320mV.
Is this OK ?
At this point VR2 is 103 ohm, and VR1 shows 32 ohm.
Should I leave it like this and proceed now to Offset trimming ?
Thanks again
JM