Exicon bias...
I learned that (without incident) the hard way, I was surprised by the extent to which this is so.
I want to put them in all of my amps...
I learned that (without incident) the hard way, I was surprised by the extent to which this is so.
I want to put them in all of my amps...
Attachments
In the semiconductor business this is a well-known weakness of MOS devices I think.
Gate voltage specs (threshold, plateau) in MOSFETs have a wide manufacturing spread, and can change somewhat over time, basically being sensitive to lattice defects and stray ions within/near the gate oxide layer, creating embedded charges that affect the electric field across the oxide.
Until they figured out to reduce Na+ and K+ ion impurity levels the gate voltages were subject to large, history-sensitive, changes in use due to sodium ions being mobile enough to migrate across SiO2 in response to gate voltage. More info how this is countered I found here: Mobile Ions
Modern devices are better than older ones, but you basically need to allow for this manufacturing spread, and as I discovered with my original Hitachi-style MOSFET amp, biasing will drift somewhat over the decades - I suggest checking the biasing every decade or so (temperature of heatsinks after warming up on idle is a good proxy).
Blimey 8 of them nice.
Yeah I’ll keep that in mind. I can see the interest in this gear now still like me valve gear but this all beats the mass produced solid state gear plus room to make improvements. Quite therapeutic for sure.
This unit I’ll start a new thread is a matrix m400 not able to find any info on it at all. Mind you got it for peanuts and it’s immaculate inside apart from in places 3” inches of dust. Ew.
I’ll get some more work done on this other amp done new heatsink for other board plus now moving things around so bit of careful planning there. Kinda starting all over again. It’ll get done. Tempted to just test that new board out of the unit with the tranny connected up and Variac. There will be no grounding but should be okay!!!
Yeah I’ll keep that in mind. I can see the interest in this gear now still like me valve gear but this all beats the mass produced solid state gear plus room to make improvements. Quite therapeutic for sure.
This unit I’ll start a new thread is a matrix m400 not able to find any info on it at all. Mind you got it for peanuts and it’s immaculate inside apart from in places 3” inches of dust. Ew.
I’ll get some more work done on this other amp done new heatsink for other board plus now moving things around so bit of careful planning there. Kinda starting all over again. It’ll get done. Tempted to just test that new board out of the unit with the tranny connected up and Variac. There will be no grounding but should be okay!!!
Well all installed but the heatsink is not grounded which think they needs to be. Good news is I’m down to mV on output new pcb but could smell burning then noticed those poor little resistors where starting to burn up. So that’s as far as I got. So got some heavy duty 5 watt resistors coming tomorrow so once there in I’ll fire her up again. Cor be glad when all done.
You don't need to "ground" the heatsink.
If the power line is earthed to the chassis and/or power supply ground that's all you need, Some people use a resistor (between power earth and chassis), some people use a Thermistor ( CL60 comes to mind) and some go to the full extend of using a rectifier for protection.
I just attach the ground ( 0V point) from the power supply after the rectifiers and cap bank to the chassis and never had problems.
That's is a bummer, what resistor(s) did heat up (burned) ?
If the power line is earthed to the chassis and/or power supply ground that's all you need, Some people use a resistor (between power earth and chassis), some people use a Thermistor ( CL60 comes to mind) and some go to the full extend of using a rectifier for protection.
I just attach the ground ( 0V point) from the power supply after the rectifiers and cap bank to the chassis and never had problems.
That's is a bummer, what resistor(s) did heat up (burned) ?
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Err .. umm .. It'd probably be a good time to put the fuses back into those clips -- if those are the resistors that are getting toasty -- rather than fitting some big, honkin' resistors.
If you wanna be super-safe, you could start with a lower-rated fuse, maybe 1A or 1-1/2A. Just don't go so low that the price-per-fuse goes up dramatically.
Just a thought ..
Regards
If you wanna be super-safe, you could start with a lower-rated fuse, maybe 1A or 1-1/2A. Just don't go so low that the price-per-fuse goes up dramatically.
Just a thought ..
Regards
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Just fitted the fan relays not hooked up yet. I’ll put some 1 amp in there in but then fire up see what happens. Did notice yesterday the LEDs didn’t light up at all either board so assuming the resistors where not letting in enough voltage. Odd one as they lit up before.
Also noticed those heatsinks on the BDs where getting warm. Did not realise the wattage on them. That was good pointer on your part thumps up.
Any idea why someone would mark lots of caps and resisters with red lines on them? All I can think of is they have been replaced or tracing out part of the circuit.
I picked up a Genexxa MPA-46 for only 6 quid bargain. Plus it’s mint so be using the tone control board out of that. It’s actually an op amp funny enough. Good transistors in there as well.
Not had any replies on this matrix mp400 I’ll have to check I’ve posted it on correct forum.
Anyhow off to fire this up hopefully it won’t go up in puff of smoke ha ha.
Also noticed those heatsinks on the BDs where getting warm. Did not realise the wattage on them. That was good pointer on your part thumps up.
Any idea why someone would mark lots of caps and resisters with red lines on them? All I can think of is they have been replaced or tracing out part of the circuit.
I picked up a Genexxa MPA-46 for only 6 quid bargain. Plus it’s mint so be using the tone control board out of that. It’s actually an op amp funny enough. Good transistors in there as well.
Not had any replies on this matrix mp400 I’ll have to check I’ve posted it on correct forum.
Anyhow off to fire this up hopefully it won’t go up in puff of smoke ha ha.
From post #193
Those resistors shouldn't get too hot when there is no signal applied.
You could still have excessive current and 0v on the output.
Measure the ac volt drop across the 100r resistors.
Sid.
Those resistors shouldn't get too hot when there is no signal applied.
You could still have excessive current and 0v on the output.
Measure the ac volt drop across the 100r resistors.
Sid.
I need to check the mains connection on the original board as I’m only get mV on the mains input! Other side around 37v. The output has dropped to 1.4 v on new board still to high but better than 40 plus volts for sure. Led I’ve put in wrong way round so it’s no lightning up. Otherside not lighting up due to only mV at mains input of tranny. Taken resistors out running 500mV quick blow. There fine I’m sure if there was major problem they would blow straight on switch on at 240v. So just need to check that mains in on original board put in new led and then it’s getting the bias sorted. Hopefully that will resolve the issue with the output. Worst case scenario is a fuse or resistor across the +/- side of output. Can’t really see what else would be causing it to be in voltage range as opposed to mV. Strange one. But at least me board worked out a okay. Was waiting for a bang ew.
Well and it goes on. So fault on old board. The mains neutral on tranny broke on connector so was not making proper contact hence not full power getting to the module. Fixed now still with 500mA quick blow fuses new board still no led lit up(possibly bias too low) other side led lit up but left fuse blew within nano seconds. So 100r power resistors installed not on new board yet but old board. All lit up. So possible short somewhere.
Now changes to take into consideration. Transformers now bolted to chassis the mounting plate taken out. New heatsinks which have thick sil pads attached. So the heatsinks are no longer in direct physical contact with the chassis as they where previously. So assumption not making a ground connection at all like they where when attached to the original mounting. So why did the fuse blow definitely a short and inrush of current. But that would not explain why the otherside not earthed fuses fine.
Now good news is new board the output is now down to 11.4mV but no lit led!
So question begs where do I go from here?
I can measure voltage drop across resistors both sides but assuming that’s just to get an idea of mA? Now even if we know what those are both sides again what purpose or what or how is that going to help and help with what?
The bias obviously needs setting for sure now I know the Maplin gives a good guide to setting bias but the configuration is different on this module. So is the bias set with the resistors in situ? Then where do we get the mA readings from to know where in the ball park figure.
Quite a bit of food for thought there ha.
Now changes to take into consideration. Transformers now bolted to chassis the mounting plate taken out. New heatsinks which have thick sil pads attached. So the heatsinks are no longer in direct physical contact with the chassis as they where previously. So assumption not making a ground connection at all like they where when attached to the original mounting. So why did the fuse blow definitely a short and inrush of current. But that would not explain why the otherside not earthed fuses fine.
Now good news is new board the output is now down to 11.4mV but no lit led!
So question begs where do I go from here?
I can measure voltage drop across resistors both sides but assuming that’s just to get an idea of mA? Now even if we know what those are both sides again what purpose or what or how is that going to help and help with what?
The bias obviously needs setting for sure now I know the Maplin gives a good guide to setting bias but the configuration is different on this module. So is the bias set with the resistors in situ? Then where do we get the mA readings from to know where in the ball park figure.
Quite a bit of food for thought there ha.
Yes the bias will be set with the resisors in place. There is no other easy way to measure the current in this design.
You measure the ac voltage drop (with a true RMS Meter) across the 100r resistor and do the sums.
I=V/R
For 100mA through 100r you would get a 10v drop and 1watt dissipation. (I feel like I'm repeating myself here)
Therefore if you have 5Vac drop across the 100r resistor then you have 50mA.
Once again this is NOT an accurate measurement of bias current but it will tell you if you are in the right ball park.
Sid.
Cheers Sid
What I thought no easy way on this.
Math fine and yes you have already told me that ha.
I’ll put everything back as it was then measure not touching it at present not intending having to buy a new tranny for sure well I wouldn’t. I’ll sort me superhet out total refurbished for a few days had enough of this one and I know won’t get a headache with it like this unit. Ew you’d think they would at least put test points on it. Blimey I’ve got gear here 70 years old with measuring points in circuit you’d think Morden stuff they’d of thought that. This PA system got for six quid got test points all round the board that’s 1990s.
Anyhow that’s the rant over.
What I thought no easy way on this.
Math fine and yes you have already told me that ha.
I’ll put everything back as it was then measure not touching it at present not intending having to buy a new tranny for sure well I wouldn’t. I’ll sort me superhet out total refurbished for a few days had enough of this one and I know won’t get a headache with it like this unit. Ew you’d think they would at least put test points on it. Blimey I’ve got gear here 70 years old with measuring points in circuit you’d think Morden stuff they’d of thought that. This PA system got for six quid got test points all round the board that’s 1990s.
Anyhow that’s the rant over.
Oh well f..k up there put the mossies wrong way round. What am I doing guys!!!! Heaven knows. Well the transformer is shot for sure getting hardly any AC out of it and the tranny getting hot quick so it’s obviously holding all the load. Oh well. So have a think get a new one probably about 20 quid. Still sit on it.
Have your bits from China come yet. Mine haven’t yet. Looking at the 15000uf psu caps 63v Christ cheapest so far including vat 16 quid Need 8 of em. Transformer 60 quid plus then heat sinks cor. Mounting up. No cabinet yet prob another 169 quid. Unless can find a cheap one of fleabay. Did buy two but cabinets two small ha.
Have your bits from China come yet. Mine haven’t yet. Looking at the 15000uf psu caps 63v Christ cheapest so far including vat 16 quid Need 8 of em. Transformer 60 quid plus then heat sinks cor. Mounting up. No cabinet yet prob another 169 quid. Unless can find a cheap one of fleabay. Did buy two but cabinets two small ha.