A rather clever design that is inline with our design philosophy , those that like it will love the symef ensemble more
check if the M3 power output module has proper conection to GNDs, I had similar phenomena in my C372
Yeah… something is up. All I did was remove the module, solder two leads to pin 4 and pin 8 of the TL082 and then put the module back in. I’m back to it going into protection.
I was at least able to measure the voltages on the IC though, on pin 4 I’m seeing a +45.8 Vdc and on pin 8 a +63.4 Vdc. So extra voltage is getting there, and I’m guessing I’ll see positive rail voltage on the output. The differences between the voltage on pin 4 and pin 8 is 18v.
I’m going to be replacing the outputs regardless once I’m finished with this because I know that the next time I take this board off the heat sink the remaining leads on the outputs are going to snap. Since I am replacing them anyway should I go with the original 2SC5200/2SA1943 or go to MJL1302g/MJL3281g? Same Ft, but the Onsemi having better specs and I’ve read over are better devices with better SOA and such. I also might need to source new ribbon cables after this is done, they don’t appreciate being installed and removed dozens of times.
Edit:
There isn’t much in that portion of the circuit. What would cause the wrong voltage on both pins, would D401 and D402 both be bad? I can’t see both zeners failing. Could it possibly be a bad ground?
Dan
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I will certainly check that, thank you.
Just a quick edit, but using continuity on my meter measuring from a random screw in the amp I have good ground at P303 and P304 as well as at thr 8 pin connector that goes to the binding post board for the output. I also measured for ground on the 10 pin ribbon cable, but I could only measure on the cable side that attaches the main board on the bottom of the chassis, not where it goes into the amplifier board. Just no way of reaching it. Bit I did have continuity to ground and about 42 ohms to pin 8 which should be right.
Dan
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Well I’m an idiot. When I powered it up, I forgot to have this signal cable installed.
I would figure since that’s just for the audio signal that would not make a difference really, but also I really don’t know. Clearly it does. With the cable installed I’m seeing 0.65 Vdc on pin 4 and 5.33 Vdc on pin 8. So that small signal pigtail clearly connects a portion of the board to ground.
I wouldn’t think so, but could anything have been potentially damaged by powering it up without that small cable installed that would be kind of a poor design choice if so, but it is back to not coming out of protect. And clearly I don’t have the +/- 18v there.
Dan
I would figure since that’s just for the audio signal that would not make a difference really, but also I really don’t know. Clearly it does. With the cable installed I’m seeing 0.65 Vdc on pin 4 and 5.33 Vdc on pin 8. So that small signal pigtail clearly connects a portion of the board to ground.
I wouldn’t think so, but could anything have been potentially damaged by powering it up without that small cable installed that would be kind of a poor design choice if so, but it is back to not coming out of protect. And clearly I don’t have the +/- 18v there.
Dan
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What's the voltage drop ( = current) across R455 and R458? That would be a telltale.
I predict much lower across R458 than across R455.
What's the opamp output connected to?
Edit: the voltages and curents in this circuit fragment depend basically on the current flowing from the opamp output. Without knowing the load (the part you left out), you cannot make any meaningful statement about it.
Jan
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I appreciate a suggestion. I wish this amplifier was service friendly. Every time I need to get to the back side of the board to solder I do more and more potential damage. I’m wondering if I should put a socket in and check the zeners. I don’t really know where to go from here which is why I’m here asking and hoping those more knowledgeable than I can help me figure this out. I wish I would have been able to measure the voltage at the IC when I only had 400 mV on the channel.
Dan
I'm not sure where you are up to now but you must have the -/+18 volt on the opamp. Also measure ground voltages as if there are any floating they will show up. Pick something like an input RCA ground as a reference point to measure from.
I would imagine the 'natural' offset of the amp is very low normally. By that I mean the offset with no servo in place (R653 removed or lifted). The servo acts via the 680 ohm onto a 21 ohm resistor and so the servo range is very limited.
I would imagine the 'natural' offset of the amp is very low normally. By that I mean the offset with no servo in place (R653 removed or lifted). The servo acts via the 680 ohm onto a 21 ohm resistor and so the servo range is very limited.
The key is of course do a lot of measuring and only a minimum of soldering, after making absolutely sure the part is defect ....
Jan
Jan
Update: I had checked the rail, voltages at the large lugs going to the board, and they looked good, but I realized I had not checked for the +/- 72v at the small 10 pin ribbon cable. It’s difficult to get to, but I remembered that I had picked up this small extension probe.
I measured and on the +72v pin I saw exactly 72v. On the -72v pin I only read -2.6v. So I wanted to check the working channel and it had a -93v on pin 2! Way too high.
Here is the schematic of that portion of the power supply. Just in case that amplifier channel was throwing off the “good” I went ahead and removed the channel altogether. Now, with the one good channel in the amplifier it powers up and does not go into protection, good! But I checked those voltages on the ten pin ribbon and I was still seeing + 72v and -94v. So even the “good” channel has an issue, but that’s in the power supply.
With the left channel still removed I’m still getting +72v and only -2.6v so now, I guess I need to get the power supply situated first, on both channels, and then move on to the amplifier boards. It’s going to be a bit of a pain though because I have to remove two boards and take out every RCA connector.
I really appreciate you all keeping me motivated instead of giving up. But what a pain lol.
Dan
I measured and on the +72v pin I saw exactly 72v. On the -72v pin I only read -2.6v. So I wanted to check the working channel and it had a -93v on pin 2! Way too high.
Here is the schematic of that portion of the power supply. Just in case that amplifier channel was throwing off the “good” I went ahead and removed the channel altogether. Now, with the one good channel in the amplifier it powers up and does not go into protection, good! But I checked those voltages on the ten pin ribbon and I was still seeing + 72v and -94v. So even the “good” channel has an issue, but that’s in the power supply.
With the left channel still removed I’m still getting +72v and only -2.6v so now, I guess I need to get the power supply situated first, on both channels, and then move on to the amplifier boards. It’s going to be a bit of a pain though because I have to remove two boards and take out every RCA connector.
I really appreciate you all keeping me motivated instead of giving up. But what a pain lol.
Dan
I appreciate it. I will go to that as soon as I get the power supply taken care of. I’m guessing the +/- 72v that goes through the ribbon cable powers this IC.
Thank you for the tip. I most certainly want to go that route.
Dan
There is a chance that the -94V is too high because the load is lower with one channel removed. Although the difference to -72 is unusually high for that to be the case. But keep it in mind.
Jan
Jan
That is absolutely key to successful fault finding.
It is all about gathering evidence and with the minimum of disturbance, and then often when you do fix something difficult you often find that the evidence was there all along. I've always enjoyed fault finding, I imagine its a bit like forensic science in solving crime.
This is going to be my next mode of attack, let me know if it doesn’t seem like the best of ideas.
I am going to remove the remaining amplifier channel and then remove the two circuit boards above the large power supply board. I remember I had to do quite a bit of work on that board as there were resistors that were extra crispy and such. And then going to power it up without the amplifiers installed and hook them back up until I have a positive and -72 V at both headers. I know at least with the left channel that it sits at 72v with nothing hooked up to it. Once I have the positive and -72 V I will install the amplifier boards again and see where I’m at.
Dan
I thought I would go ahead and share more of this amplifier since I have to take it apart. I was mistaken, there are actually three boards layered above the main power supply board, as well as a single board that goes along the front of those boards in order to distribute the supply voltages
Here is the left side of all of the inputs and outputs.
And here are all of the right as well as some input circuitry.
When they built this, they used these small pieces of plastic that isolate the grounded RCA sleeves from the back panel. The white ones are just fine, but I’m guessing something in the dye of the red ones has caused the plastic to deteriorate. They literally crumble in your hands, most of them are now powder. Even with those isolation sleeves, the RCA ground has a dead short continuity to the chassis, so are they needed? I’m hoping they are only for looks.
Here is the next board down, by the way, I replaced all of the surface mount capacitors, most of which were open, with
through hole.
Now the very bottom board I want to get to. Amplifier is so much of pain that I have to completely remove the back panel in order to get to this bottom board. You can see that I replaced some resistors along with every single capacitor. I’ll go ahead and power it up in this state to see what voltages I am measuring at all the places I measured before.
If there are any recommendations as to where to look, please let me know. Whenever I rebuild a vintage piece of equipment, I always replace the zener diodes. I have a bunch of Vishay 2%ers, so I am going to replace all of them regardless. But as was recommended, I won’t do that until I’ve done some digging to see what is causing the issues with the supply voltages. It’s much easier to test components out of circuit, but I will do what I can with the devices and circuit. I wish the schematic had voltages marked in a bunch of different spots so I could have a reference.
Dan
Here is the left side of all of the inputs and outputs.
And here are all of the right as well as some input circuitry.
When they built this, they used these small pieces of plastic that isolate the grounded RCA sleeves from the back panel. The white ones are just fine, but I’m guessing something in the dye of the red ones has caused the plastic to deteriorate. They literally crumble in your hands, most of them are now powder. Even with those isolation sleeves, the RCA ground has a dead short continuity to the chassis, so are they needed? I’m hoping they are only for looks.
Here is the next board down, by the way, I replaced all of the surface mount capacitors, most of which were open, with
through hole.
Now the very bottom board I want to get to. Amplifier is so much of pain that I have to completely remove the back panel in order to get to this bottom board. You can see that I replaced some resistors along with every single capacitor. I’ll go ahead and power it up in this state to see what voltages I am measuring at all the places I measured before.
If there are any recommendations as to where to look, please let me know. Whenever I rebuild a vintage piece of equipment, I always replace the zener diodes. I have a bunch of Vishay 2%ers, so I am going to replace all of them regardless. But as was recommended, I won’t do that until I’ve done some digging to see what is causing the issues with the supply voltages. It’s much easier to test components out of circuit, but I will do what I can with the devices and circuit. I wish the schematic had voltages marked in a bunch of different spots so I could have a reference.
Dan
I just measured those voltages again with both boards removed as well as the upper input boards removed and the right is exactly the same, +72 and -2.5 as well as the right channel, +72 and -94.
I did not understand what was going on at first because I was measuring no DC voltage on the right channel, but that’s because I use the same grounding point. I was seeing 36.5 vac to ground at both supply points on the ribbon cable. I wonder why they have the grounds isolated from each other, I’m sure it’s a common ground amplifier. I measured continuity from the amplifier and the power supply grounding points to the chassis (heatsink).
Dan
I did not understand what was going on at first because I was measuring no DC voltage on the right channel, but that’s because I use the same grounding point. I was seeing 36.5 vac to ground at both supply points on the ribbon cable. I wonder why they have the grounds isolated from each other, I’m sure it’s a common ground amplifier. I measured continuity from the amplifier and the power supply grounding points to the chassis (heatsink).
Dan
You're doing the logical things. You'll get there!
I would give priority to the power supply voltages.
Try to follow the vo;ltages from the rectifiers to the output of the supplies to see where it goes wrong.
Jan
I would give priority to the power supply voltages.
Try to follow the vo;ltages from the rectifiers to the output of the supplies to see where it goes wrong.
Jan
I suspect this transistor to be gone , they usually lly fail in these power supply sections
