What caused the problem?
Hopefully, you haven't:
.... because the negative binding post on that amplifier must stay floating at all times... and there's no short circuit protection (nor any type of current limiting) to save the transistors... you know.... the ones that you coincidentally reported as blown.
Hopefully, you haven't:
- Connected the negative binding post to ground
- Used a type of measurement equipment that would make ground connection via mains cable earth pin, if an alligator clip or similar was connected to the grounded measurement equipment case... a non-differential (bog standard) oscilloscope probe comes to mind.
.... because the negative binding post on that amplifier must stay floating at all times... and there's no short circuit protection (nor any type of current limiting) to save the transistors... you know.... the ones that you coincidentally reported as blown.
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Sadly I have no clue of what caused to problem. I got the amp already broke. The last owner told me that it just stopped.
I've not been involved in powered on live measurements so no risk of accidental earth connection while measuring.
Inside, S6 was blown : it is the fuse of the +20VL.
I've traced that the +20VL_AMP1 section got a short : I've unsoldered the resistors of calibration (R119, R120) to +20VL_AMP1 and +20VL_AMP2, removed two jumpers (P7) and then no problem. Then, I've reconnected one at a time. When +20VL_AMP1 was connected, the fuse was blowing.
I've found that Q78 was E-C shorted : the 2SB1184 I am trying to replace becoming short, I guess it just pumps all the voltage of the positive half of the push pull, and so draw a lot of current.
Hey all ! Soooooooo ....
I've been repairing the amp these last two days :
After many fuses blown due to false diagnosis of faulty transistors, of bad biaising due to bad schematics readings, and components mounted upside down, the amp is finally powering on safely and give consistents reading. I hope it will last.
At the end, driver and output transistor pairs have been changed.
Now :
Also :
I was willing to test for high frequency oscillations. Output transistors are 30MHz, but drivers are 250MHz.
Someone said that HF oscillations were unlikely in this design, I didn't quite understood why (I see feedback is done through LM458 which is 1MHz opamp, and through some resistors so no coupling caps, is it enough ?)
My scope is "only" 40MHz; are the HF oscillations supposed to be in this range or much higher ? I guess everything is possible depending on the transistors, so here I would say 30MHz would be a maximum ?
Thanks ! 🙂
I've been repairing the amp these last two days :
After many fuses blown due to false diagnosis of faulty transistors, of bad biaising due to bad schematics readings, and components mounted upside down, the amp is finally powering on safely and give consistents reading. I hope it will last.
At the end, driver and output transistor pairs have been changed.
Now :
- I will biais the amp. The biaising is done thanks to a 0.22 resistor along the power rail that power pre-driver, driver and output transistors.
- We are supposed to measure 30mV across it (so 136mA split in these three pairs).
- When I am moving the trim to rise the voltage from 0mV to 30mV, it works well.
- However, at around 20mV, if I stop to turn the trim, the voltage keep rising on its own
Also :
I was willing to test for high frequency oscillations. Output transistors are 30MHz, but drivers are 250MHz.
Someone said that HF oscillations were unlikely in this design, I didn't quite understood why (I see feedback is done through LM458 which is 1MHz opamp, and through some resistors so no coupling caps, is it enough ?)
My scope is "only" 40MHz; are the HF oscillations supposed to be in this range or much higher ? I guess everything is possible depending on the transistors, so here I would say 30MHz would be a maximum ?
Thanks ! 🙂
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It is normal on many designs and due to current gain of a transistor increasing with temperature along with the intrinsic base/emitter forward voltage decreasing.
Set the bias correctly when the amp is fully warmed/hot. The bias generator transistor I assume is in contact with the heatsink and that is to try and cancel this effect, the bias voltage across this transistor falls with temperature reducing bias current. You may well find the voltage rises and then falls back as the heatsink equalises in temperature.
Set the bias correctly when the amp is fully warmed/hot. The bias generator transistor I assume is in contact with the heatsink and that is to try and cancel this effect, the bias voltage across this transistor falls with temperature reducing bias current. You may well find the voltage rises and then falls back as the heatsink equalises in temperature.
Ohhh I see the trick. Really clever, remind me some tempco oscillator compensations I've realized before.
And about the HF oscillations potential problem, do you have any hint ?
And about the HF oscillations potential problem, do you have any hint ?
Use a scope to look for low level high frequency signal superimposed on the output line and also look at how it behaves to a proper square wave input (but keep output voltage low and in the 5 volt peak region). Look for ringing and other unwanted behaviour. Measure before any series output inductor though as that will add a ringing to the signal (which isn't a problem).
I've been testing the amp, no oscillations as far as I can tell, signal is quite neat. With square, not so far from my source (it's a dac so already a kind of squarish signal). Maybe a really little oscillations, but i think due to my scope (the same is there with nothing connected). In any case, nothing more than a few mV..
I've biaised the beast. Biais is really fluctuating, so I've made everything within 29mV and 31mV steady, I guess it would not impact sound ?
1mV through 0.22 ohm is 4mA, split between the three transistors, look okay.
Sound is nice so I guess everything is tuned !
Thanks again for the help, been quite useful !!!
I've biaised the beast. Biais is really fluctuating, so I've made everything within 29mV and 31mV steady, I guess it would not impact sound ?
1mV through 0.22 ohm is 4mA, split between the three transistors, look okay.
Sound is nice so I guess everything is tuned !
Thanks again for the help, been quite useful !!!
It will not impact the sound. Even the tiniest of bias currents is enough to pretty much eliminate crossover distortion. I'm sure you could set it to say 5mv and you would hear no difference.
Yeah that's what I thought, once conducting I guess transistor is just on, but manual says 30mV that's why I've choosed it. I've noticed channel left (the one I've touched) is slightly less loud than the other, I am going to look at the scope along the path tomorrow to check it out