I decided to buy a few broken ML23 mainboards including output transistors. Like that I can get old Motorola output transistors and I get huge amount of spareparts. Thanks Roar!
I have the feeling that we have already solved the problems that you described in this long journey, but we will see it when I have replaced those two output transistors.
I have the feeling that we have already solved the problems that you described in this long journey, but we will see it when I have replaced those two output transistors.
Hello Handel you may have to replace all MJ15024/15025 , Check Iceo approx with ohm meter scale 20 Meg min, discard all under 5 Meg normal temp 20-25 C, the amp can run with one complementary pair > Check all MJE15030, MJE15031 for Iceo should be over 100 Meg. MJ15024/15025 here on mouser are $6.35 (10) each shop around other suppliers
Hi Rumen,
Intriguing info's. Could you explain why and also where you got this information.
Thanks in advance.
Hans
Hans,
can I try the amp without one pair of output transistors or does it make any sense? Or should I wait for those two?
can I try the amp without one pair of output transistors or does it make any sense? Or should I wait for those two?
That is good to know but I will skip that, I have connected/disconnected the amp now more than enough times.😱
Next question; On Wednesday I should receive the spare boards and also 5 pairs of output transistors. Is there easy way of measuring which pair is the best for my amp? I have been reading from the transistor matching but still not sure how to do it.
I was also able to get rid of the jumper wires in the few output transistors which had the contact problem. The board looks now clean again.
The more output transistors, the less the importance of matching.
But when selecting, by far the most important parameter is the Vbe.
With your multimeter measure the VBe diode voltage in circuit of your existing transistors, separately for NPN and PNP and be aware that NPN and PNP have different values.
Then select from the replacement NPN and PNP transistors the ones with a Vbe that comes closest to what you measured.
Hans
Hello Hans,
I am back in business as I have received the boards and output transistors.
I replaced the broken pair of transistors and put the lamps between the VCC regs. I powered the amp with the pot in minimum(safest) position and I had immediately around 85V drop in the lamps which was evidence another short somewhere.
I started measuring the voltages for you from different areas like before, but when I tried to measure the current between WH.1 and WH.5 suddenly lamps shut down and the voltage drop very small like 1V over the lamps. Could it be that when amp was getting warm the short disappeared?
Anyway, now I can adjust the 50V over the lamps with the pot!
It will be interesting to see what happens after the cooling down and turning the amp back on...
Do you need the same measurements as before with the 50V drop over the lamps?
Handel
I am back in business as I have received the boards and output transistors.
I replaced the broken pair of transistors and put the lamps between the VCC regs. I powered the amp with the pot in minimum(safest) position and I had immediately around 85V drop in the lamps which was evidence another short somewhere.
I started measuring the voltages for you from different areas like before, but when I tried to measure the current between WH.1 and WH.5 suddenly lamps shut down and the voltage drop very small like 1V over the lamps. Could it be that when amp was getting warm the short disappeared?
Anyway, now I can adjust the 50V over the lamps with the pot!
It will be interesting to see what happens after the cooling down and turning the amp back on...
Do you need the same measurements as before with the 50V drop over the lamps?
Handel
Good to hear you are making progress.
I’ll look at your image with voltages first thing in the morning.
Hans
Everything looking fine in your image, except the output voltages.
Probably the output must be 15.5mV instead of 5.5mV, but it can never have a lower voltage as the other side of R20 with 8.4mV.
What you experienced is exactly what I was afraid of, you still don't seem to have solved the intermittent problem.
Without lamps you would probably have killed the amp.
So first of all, was your OL-2 board connected ?
Second question, what lamps are you using, still the 40W 220V ?
The reason I ask is because when you use 110V lamps you have an immediate optical feedback with a sudden 85 Volt on them, I think you will desperately need them now.
So you will have to find the cause of your problem. Obviously there is nothing broken, so it must be a bad contact or a hair crack in a trace.
Two things you can do:
1) Push and pull on many spots of your PCB and cabling trying to invoke the problem. When it occurs, the lamp should go on !
2) Use an freezer spray aerosol to find the troublemaker.
Hans
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I made a mistake in the output voltages. Values were right but vice versa; 8.4mV in the output side and 5.5mV on the transistors side.
OL-2 board is connected and everything except the input cable from OL-2 to the mainboard. The lamps are 40W 220V. I will start looking for 110V lamps.
I pushed and pulled the amp but lights did not go on. I will get the freezer spray somewhere.
OL-2 board is connected and everything except the input cable from OL-2 to the mainboard. The lamps are 40W 220V. I will start looking for 110V lamps.
I pushed and pulled the amp but lights did not go on. I will get the freezer spray somewhere.
I noticed also that when I am switching the amp off there is much brighter light immediately after that, which means higher voltage. It lasts about one second. When switching on there is no similar behaviour.
What exactly do you mean with brighter light.
Are the 220V lamps emitting light at 80 Volt ?
If so, you don't have to look for 110V lamps.
You should also connect the OL-2 board to the main board.
So make the connection, and see if the same happens with the light at switch off.
The OL-2 board should act act as a current limiter.
Hans
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Yes, I can see very little light in the lamps with current setup, so I was also wondering if the 110V lamps are really needed.
I also found one disconnected line from 0.22R to the output transistor. It is very tricky to solder. Need to fix it tomorrow but that cannot be the reason for the intermittent problem.
I also found one disconnected line from 0.22R to the output transistor. It is very tricky to solder. Need to fix it tomorrow but that cannot be the reason for the intermittent problem.
I have not seen anymore the short. Every time I turn on the amp it goes to the right state immediately. I have been using the freezer in many places and pushed and pulled the board and cables but no more brighter lights can be seen.
I soldered the input cable to the mainboard, but it did not help for the brighter light after turn off.
There is now current flowing in all the 0.22R resistors. I noticed that most voltage drops over the resistors are between 1.5-1.7mV but the resistors connected to new transistors the voltage drop is a bit higher 1.9 and 2mV. Is that an issue? Remember that I still have the lamps between the VCC unregs.
I soldered the input cable to the mainboard, but it did not help for the brighter light after turn off.
There is now current flowing in all the 0.22R resistors. I noticed that most voltage drops over the resistors are between 1.5-1.7mV but the resistors connected to new transistors the voltage drop is a bit higher 1.9 and 2mV. Is that an issue? Remember that I still have the lamps between the VCC unregs.
When the OL-2 board is connected, you can remove the lamps and adjust the idle current to 275mA.
Then measure carefully all 0.22 resistors again, that will give a better view on things.
But never remove the lamps without the OL-2 board.
Hans
Yes, I could set the bias to around 275mA!😎
But I still have concerns about the new positive side transistor. All the 0.22R resistors should have around 10mV drop as I compared to left channel. This new transistor has around 40mV drop in that specific resistor and the rest around 3mV so it seems that this transistor is not very good match for the old ones.
The transistor in the negative side is much better match.
But I still have concerns about the new positive side transistor. All the 0.22R resistors should have around 10mV drop as I compared to left channel. This new transistor has around 40mV drop in that specific resistor and the rest around 3mV so it seems that this transistor is not very good match for the old ones.
The transistor in the negative side is much better match.
You are right, this is too much, I'm surprised by this large difference.
You have two options,
1) try another transistor with a higher Vbe, or
2) disable both new transistors by removing their 10R base resistor.
Removing the 10R resistors may sound weird, but as long as you don't need 700Watt into 2 Ohm, you can still safely drive 700*(5/6) = 580 Watt.
So for normal use this is more than adequate.
Now reduce the idle current to 275*(5/6) = 230mA to keep the current per transistor the same 45mA as before.
Hans
Hans,
I changed the problematic transistor and quess what, now they match nicely!
Here are voltages over 0.22 resistors(mV):
positive side:
10,9.6, 11.3, 11.6, 9.4 10.5
negative side:
8.3, 11.4, 9, 8.8, 13.3, 9.7
output dc voltage 4.8mV.
I have no idea how did I put so bad transistor at first.
Handel
I changed the problematic transistor and quess what, now they match nicely!
Here are voltages over 0.22 resistors(mV):
positive side:
10,9.6, 11.3, 11.6, 9.4 10.5
negative side:
8.3, 11.4, 9, 8.8, 13.3, 9.7
output dc voltage 4.8mV.
I have no idea how did I put so bad transistor at first.
Handel