I love that light bulb box.
The manual says that the amp can function in class AB or class A. In class A it draws a hefty 90mV.
I wouldn't go that far but that means that the 18mV or a bit less should not be harmful at all.
I'd keep it at that for the moment and hope that everything keeps working.
Hugo
The manual says that the amp can function in class AB or class A. In class A it draws a hefty 90mV.
I wouldn't go that far but that means that the 18mV or a bit less should not be harmful at all.
I'd keep it at that for the moment and hope that everything keeps working.
Hugo
Hahahha i knew you would comment on that one, it's the box from the courrier that brought me the 1st transistors hahahhah.
This model (pm68) is class ab only. The next model pm78 has a class A function. This one does not.
I've set the bias while on the dim bulb so i need to go wayyy lower im thinking 5mv on the dim bulb or even zero and then readjust while on regular mains.
Another good thing since i adjusted the bias is that i was able to get the dc offset down from 30mv to 1mv i could not do that before.
Another note is that the bias keeps dropping over time.. i started at 15mv and im down to 11.2mv (on the dimbulb)
Trimmer pot ohm value is at 27ohms
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Oh, didn't know that, thanks.
DC offset is perfect, good sign.
It's just strange that the bias goes down instead of upping.
Fingers crossed..
Hugo
Well that sounds good
The bulb will have an effect and the bias current will be higher for a given setting of the preset when used on full mains and so always be sure to turn the bias back down before finally adjusting.
It has been on the dim bulb for 1+ hour now.
Both channels drop the bias after a while.
For example from 5mv it goes to 2.3mv.
Volume is at minimum, tone board bypassed, input at cd and no load is connected.
What do you think guys? Is it time to take it of the dim bulb?
One important note:
If i try to start it up with the dim bulb while the bias is set to about 15mv on each channel it cannot start. The power supply clicks on and off forever while trying to charge the capacitors.
If i want to start it with the dim bulb i have to turn the trimmers all the way to almost zero bias so that it can draw the current needed to charge the big capacitors
Both channels drop the bias after a while.
For example from 5mv it goes to 2.3mv.
Volume is at minimum, tone board bypassed, input at cd and no load is connected.
What do you think guys? Is it time to take it of the dim bulb?
One important note:
If i try to start it up with the dim bulb while the bias is set to about 15mv on each channel it cannot start. The power supply clicks on and off forever while trying to charge the capacitors.
If i want to start it with the dim bulb i have to turn the trimmers all the way to almost zero bias so that it can draw the current needed to charge the big capacitors
That all sounds promising.
The bulb may be playing a part here... have you a higher wattage bulb like a 100 or 150 watt?
I'll say yes as long as you feel there is enough range in the trimmer to ensure the bias is low enough when on full voltage. I think it should be OK because you had zero bias before tweaking that resistor.
If you turn the bias on both channels to minimum as it is now, how high do the voltage rails come up to? If they are close to the -/+56 volt then it should be OK.
You have to try it at some point...
With the bias at minimum and on the dim bulb the voltages are +- 51.7v approximately. If i increase the bias let's say to 15mv the rails drop to about 46-47 Volts each. An the dim bulb lights up brighter but not even close to full brightness so that's a food sign.
I'm going to check again tomorrow though just to make sure. With bias at 19mv this time on the dimbulb and see if it will pass the 1 hour mark again.
The bias begins to adjust after reaching 65ohms and below from the 100 ohm range on the trimmers. So i have plenty of headroom to work with. And with the multi turn trimmers it adjusts very smoothly with generous turning of the screwdriver.
I'm going to check again tomorrow though just to make sure. With bias at 19mv this time on the dimbulb and see if it will pass the 1 hour mark again.
The bias begins to adjust after reaching 65ohms and below from the 100 ohm range on the trimmers. So i have plenty of headroom to work with. And with the multi turn trimmers it adjusts very smoothly with generous turning of the screwdriver.
-/+51 volts sounds pretty good and close to normal voltage. It all sounds promising at this point.
Not sure if this has been mentioned before but if you do try it with speakers and the bulb be aware that the rails will fluctuate dramatically as you increase the volume and that can cause unwanted things to happen such as sudden shifts in DC offset... which pulls more current through the load and makes it all worse. In extreme cases the circuit can latch toward one or other rail.
So if you do try speakers and music keep it low volume. If the bulb starts doing a sound to light impression then turn the volume down.
One for tomorrow
Not sure if this has been mentioned before but if you do try it with speakers and the bulb be aware that the rails will fluctuate dramatically as you increase the volume and that can cause unwanted things to happen such as sudden shifts in DC offset... which pulls more current through the load and makes it all worse. In extreme cases the circuit can latch toward one or other rail.
So if you do try speakers and music keep it low volume. If the bulb starts doing a sound to light impression then turn the volume down.
One for tomorrow
Sounds like you have it working now and behaving as expected. Do not like those alligator clips
So what changed? what caused the initial smoke outs?
Got the sim running missing R3330,3332 base stoppers 10 ohm for the outputs. got the service manual from hifiengine
thx for pics to add to the collection.
So what changed? what caused the initial smoke outs?
Got the sim running
missing R3330,3332 base stoppers 10 ohm for the outputs. got the service manual from hifienginethx for pics to add to the collection.
The alligator clips are trash but they are the only ones i have. They cane with the multimeter and they are screw on additions to the regular probes. In time i will buy better ones.
What caused the fault in the first place? No idea!!!
I WILL NOT put speakers on. In fact i will not get it off the dim bulb anyways. I will just be testing for a couple of days and make measurements in order to learn more.
I installed LTspice and ran many simulations which helped me a lot! Thank you so much Mooly!!
I will order new original transistors from mouser, retest and once i'm sure then i will try to use it normally as part of my regular setup.
I simply don't trust it at all. And i am in no mood to desolder everything again and start over.
I'm tired with this thing. I have replaced the output transistors 4 times already. Wasted countless hours and a lot of money (about150€ in parts total so far not counting the new transistors i will buy from mouser) on testing and replacing components.
What caused the fault in the first place? No idea!!!
I WILL NOT put speakers on. In fact i will not get it off the dim bulb anyways. I will just be testing for a couple of days and make measurements in order to learn more.
I installed LTspice and ran many simulations which helped me a lot! Thank you so much Mooly!!
I will order new original transistors from mouser, retest and once i'm sure then i will try to use it normally as part of my regular setup.
I simply don't trust it at all. And i am in no mood to desolder everything again and start over.
I'm tired with this thing. I have replaced the output transistors 4 times already. Wasted countless hours and a lot of money (about150€ in parts total so far not counting the new transistors i will buy from mouser) on testing and replacing components.
Hahaha tell me about it... well it's not all bad though... i learned A LOT. That's priceless to me. And i'm still scratching the surface.
I will probably use these transistors i have now to build my own amplifier at some point it would be fun to design one from scratch and play around with it.
Anyways once i have something good to report i will post here!
Thanks for everything!! 😁😁😁
I will probably use these transistors i have now to build my own amplifier at some point it would be fun to design one from scratch and play around with it.
Anyways once i have something good to report i will post here!
Thanks for everything!! 😁😁😁
I am finding differences in Mooly sim schematic vs the service manual, minor stuff however but bulk of the work is done... stiil need to run more sims
what gets me is they rate it at 90W/ch/8 ohm using +/-56V and one pair of 100W BJTs, know wonder it blows up.
Best to be putting in 200W bjts and if you can add another set add them two, but you could get away with 150W devices if you use 2 pair.
what gets me is they rate it at 90W/ch/8 ohm using +/-56V and one pair of 100W BJTs, know wonder it blows up.
Best to be putting in 200W bjts and if you can add another set add them two, but you could get away with 150W devices if you use 2 pair.
I will be installing 2SC5200-O instead of 2SC5198 and 2SA1943-O instead of 2SA1941.
Those are 150w transistors. The only problem is that they are bigger. But there is enough available space to install them.
Each channel has 4 of those. 2 pairs in parallel. So i should be good. We will see when they arrive. I will post pictures and results
Those are 150w transistors. The only problem is that they are bigger. But there is enough available space to install them.
Each channel has 4 of those. 2 pairs in parallel. So i should be good. We will see when they arrive. I will post pictures and results
ThanksGot the sim running
I had one of the images from earlier in the thread open to keep looking at while flicking back and forth between LT and the image (lol)
Two pairs in the earlier diagrams. I must add those...
Excellent
Οκ good news.
I finally mastered the courage to take it off the dim bulb.
I have it running idle for about 90 minutes now.
I initially set the bias to 18mv. The more it was warming up the more the bias kept creeping DOWN on both channels.
Now that the cooling blocks are quite warm but not hot to the touch, the bias fluctuates very SLOWLY from a range of 15.8mv up to 18.3mv
When one channel creeps the bias up the other creeps the bias down. And at some point they equalize at 17.5mv. Then the other channel goes up and the other goes down.
In theory i still am in range for the factory bias of 19mv +-3.
But I don't understand that behaviour.
I finally mastered the courage to take it off the dim bulb.
I have it running idle for about 90 minutes now.
I initially set the bias to 18mv. The more it was warming up the more the bias kept creeping DOWN on both channels.
Now that the cooling blocks are quite warm but not hot to the touch, the bias fluctuates very SLOWLY from a range of 15.8mv up to 18.3mv
When one channel creeps the bias up the other creeps the bias down. And at some point they equalize at 17.5mv. Then the other channel goes up and the other goes down.
In theory i still am in range for the factory bias of 19mv +-3.
But I don't understand that behaviour.