I've just rebuilt this amp with new output transistors. I've also replaced fuse resistors, touched up solder, replaced bias/offset trimpots etc. I've replaced the caps in the power supply, power amp, and preamp.
It plays music, but it is impossible to get the DC offset stable.
This looks like one of those weird designs where the preamp and power amp are combined? Any other design oddities?
Are there some mods I can do to get this thing to settle down? Tweaking filters?
It plays music, but it is impossible to get the DC offset stable.
- The offset adjustments are extremely sensitive, and adjusting one channel affects both channels, so I have to go back and forth while watching both channels.
- Offset keeps wandering. It starts at over a volt when I turn on the unit and slowly descends over a few minutes.
- There is a ~50-100mV of offset difference depeneding on whether or not speakers are selected
- Offset jumps about 200+mV when I switch to tuner. Seems there is no highpass filter on the tuner output and I'll have to add one.
This looks like one of those weird designs where the preamp and power amp are combined? Any other design oddities?
Are there some mods I can do to get this thing to settle down? Tweaking filters?
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This thing is nuts. Are you willing to disconnect the tone controls?
Then you can configure the feedback like a normal amp, with a large series capacitor to ground on the ground leg.
But also I would replace the bias and offset pots.
Then you can configure the feedback like a normal amp, with a large series capacitor to ground on the ground leg.
But also I would replace the bias and offset pots.
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Was it a working unit before fiddling about with it? Or was there a problem you were trying to address?
Mike
Mike
It wasn't working, it had blown outputs.
I can ask the client if they would miss the tone controls.
My circuit knowledge isn't up with that of many on this board, but I knew enough to go "WTF" when I looked at the schematic. (I'm a fixer not a designer)
I did some searching on the model and found a few threads where people had similar difficulty with the offset but had apparently gotten it stable. But who knows how stable.
Bias and offset trimpots have been replaced.
I can ask the client if they would miss the tone controls.
My circuit knowledge isn't up with that of many on this board, but I knew enough to go "WTF" when I looked at the schematic. (I'm a fixer not a designer)
I did some searching on the model and found a few threads where people had similar difficulty with the offset but had apparently gotten it stable. But who knows how stable.
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What do the offset pots vr05 and vr06 wipers connect to? If there's interaction between them I'd suspect something there possibly...or maybe bad FET's?
Mike
Mike
The tone control network is AC coupled into the feedback loop and should not cause any offset issues. The DC feedback is via the series connected very high value resistors R57, R59 and R61.
Blown outputs... old designs like this can be super critical on transistor type and often rely on 'old and slow' for stability.
Scope the output and check your offset really is a DC problem and not asymmetric HF instability.
Blown outputs... old designs like this can be super critical on transistor type and often rely on 'old and slow' for stability.
Scope the output and check your offset really is a DC problem and not asymmetric HF instability.
When I lived in Edmonton, a neighbor gave me one of these all blow'd up. Besides replacing the existing outputs, I added another stage, a quasi set of 2N3773's on an added heatsink with ~2 Ohm base resistors. I was/am appalled with the tone controls and the bad switches etc, but it still works ~30 years later. If the bass control affects the offset, replace the associated electrolytic caps C27, C28 Bipolar 47uF 6.3V. These are notorious for failing anyway. If the amp failed, there was probably the supply voltage on these 6.3V caps, and they should have diode clamps.
All electrolytic capacitors in the power amp/preamp and power supply have been replaced.
I used 2SC5200 and compliment for output transistors.
Ft of original outputs is 20Mhz and replacements are 30MHz. That shouldn't be a problem, right? If it was, I'd normally expect AC stability issues not DC offset issues, right?
I used MJE254/MJE243 for drivers.
I'm wondering how much of the DC I'm seeing is from the power amp its self vs before it. Since there doesn't seem to be any DC blocking.
I used 2SC5200 and compliment for output transistors.
Ft of original outputs is 20Mhz and replacements are 30MHz. That shouldn't be a problem, right? If it was, I'd normally expect AC stability issues not DC offset issues, right?
I used MJE254/MJE243 for drivers.
I'm wondering how much of the DC I'm seeing is from the power amp its self vs before it. Since there doesn't seem to be any DC blocking.
If you turn the volume to minimum you remove any influence from anything before the pot
I would assume that -23 volt rail to the offset presets is stabilised somewhere (Zener maybe) but that part of the circuit is off the edge of your image.
You really need to prove it though by looking with a scope.
I would assume that -23 volt rail to the offset presets is stabilised somewhere (Zener maybe) but that part of the circuit is off the edge of your image.
Going by other threads on this unit, difficulty adjusting offset is normal for this unit. Not sure if it is normal to this extent. Either it is, and these units always end up with high DC offset. Or there was an existing design issue made worse by different parts.
Yeah the zener voltage is steady. And I just changed them to be sure.
How could engaging the tuner throw off the DC offset with the volume control all the way down?
Client is okay with removing the tone controls if needed.
How could engaging the tuner throw off the DC offset with the volume control all the way down?
Client is okay with removing the tone controls if needed.
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I can't see how it can... but if that is what happens then you have to look further.
Measure the DC voltage at these two points. It should be 0.000 volts DC. Switch the tuner in and out and check it does alter at all.
Someone else brought me the same model for a basic service, so I was able to compare and confirm the other one doesn't have this issue. The offset adjustment is still very sensitive, but the channels don't affect each other, and changing inputs doesn't make a difference.
Back to the original unit.
I'm getting about 2.5VDC on the volume control. This goes down a few hundred mV when the tuner is selected. There is no DC voltage here on the new unit.
It seems to be a power supply issue. The positive regulated rail is supposed to be 12.5V. It starts low then goes up to about 15. The voltages across the 2 24V zeners are wrong. Getting about 21.5V across one and 25V across the other at the moment. Had the same issue with the original zeners then I changed them to new ones. And I just took them out of circuit to test them and they both get about 23.5V across them when I connect them across my DC power supply in series with a 1k resistor and set the supply to 30-35V.
So the zeners are okay but one has a higher voltage across and one has a lower voltage. I can understand how there could be a lower voltage, but how could a working zener possibly have a higher voltage across it than it is rated for?
Back to the original unit.
I'm getting about 2.5VDC on the volume control. This goes down a few hundred mV when the tuner is selected. There is no DC voltage here on the new unit.
It seems to be a power supply issue. The positive regulated rail is supposed to be 12.5V. It starts low then goes up to about 15. The voltages across the 2 24V zeners are wrong. Getting about 21.5V across one and 25V across the other at the moment. Had the same issue with the original zeners then I changed them to new ones. And I just took them out of circuit to test them and they both get about 23.5V across them when I connect them across my DC power supply in series with a 1k resistor and set the supply to 30-35V.
So the zeners are okay but one has a higher voltage across and one has a lower voltage. I can understand how there could be a lower voltage, but how could a working zener possibly have a higher voltage across it than it is rated for?