Hello!
I got a faulty bass combo amplifier MarkBass Little Mark 250. But it appears to be a little more complicated than those I worked with before.
In general, I'm at a dead end, hoping for help from experienced forum members.
Symptoms:
The midpoint between the output darlingtons floats at idle relative to the ground (in the negative direction), when the difference reaches about 1V, the DC protection circuit is triggered and turns off the speaker.
While the amplifier is cold, it floats a lot. As it heats up, it stabilizes (it heats up decently, up to 60-70C degrees at idle) and works normally for a long time. Only sometimes the relay clicks for a second.
All supply voltages are normal, I unsoldered all the electrolytics, ESR is normal, does not show leaks...
Where to dig next?
How is this middle point adjusted there?
The purpose of some parts is a complete mystery to me, such as T3, T4, T6 for example...
I've drawn the schematic of the amplifier part of the combo:
I got a faulty bass combo amplifier MarkBass Little Mark 250. But it appears to be a little more complicated than those I worked with before.
In general, I'm at a dead end, hoping for help from experienced forum members.
Symptoms:
The midpoint between the output darlingtons floats at idle relative to the ground (in the negative direction), when the difference reaches about 1V, the DC protection circuit is triggered and turns off the speaker.
While the amplifier is cold, it floats a lot. As it heats up, it stabilizes (it heats up decently, up to 60-70C degrees at idle) and works normally for a long time. Only sometimes the relay clicks for a second.
All supply voltages are normal, I unsoldered all the electrolytics, ESR is normal, does not show leaks...
Where to dig next?
How is this middle point adjusted there?
The purpose of some parts is a complete mystery to me, such as T3, T4, T6 for example...
I've drawn the schematic of the amplifier part of the combo:
The circuit has unity gain at DC, so unless for example C4 is bad, or the input transistors are damaged,
the output DC offset is self-adjusting. Maybe the heat sink grease is dried up.
T3 is a current source, T6 is a driver (as is T5), and T4 is a current limiter.
the output DC offset is self-adjusting. Maybe the heat sink grease is dried up.
T3 is a current source, T6 is a driver (as is T5), and T4 is a current limiter.
That's possible as well, but often the inputs get zapped from hot plugging, and degrade.
The bias diodes could be flaky, inside the cases of the outputs.
But check the heat sink grease to see if it's dried out.
Also first try cleaning the bias adjust pot (set it back to where it was afterward).
The bias diodes could be flaky, inside the cases of the outputs.
But check the heat sink grease to see if it's dried out.
Also first try cleaning the bias adjust pot (set it back to where it was afterward).