There are usually zener diodes somewhere that set the voltage. The drop in the resistor will unfortunately increase as a result - which heats it up more. You end up increasing the value (to maintain the same current at more voltage differential) and the wattage - so it won’t cook the board. It’s really kind of a poor design, considering there is a big heat sink right there (with a fan) to mount pass transistors to.
The “better” grades of op amps are perfectly happy on +/-18V. “22V” op amps are fine on +/-22V if run under lab conditions. In equipment like this, they can see input and power supply spikes or get run in current limit. That kind of abuse is safer with lower supply voltage.
The “better” grades of op amps are perfectly happy on +/-18V. “22V” op amps are fine on +/-22V if run under lab conditions. In equipment like this, they can see input and power supply spikes or get run in current limit. That kind of abuse is safer with lower supply voltage.
What opamp would you go for.?
I've got NE5532 - LM4562. Ive got some old JRC 4558's aswell.
I might put an IC socket there. Try some out.
I've got NE5532 - LM4562. Ive got some old JRC 4558's aswell.
I might put an IC socket there. Try some out.
I suggest removing IC1 as a first step. That will make several issues clearer: do the +/- opamp supply rails become sensible? What supply voltages are presented? That will help guide opamp selection. Do the volume pots then show 0V with suspect opamps absent?
A socket seems like a good idea.
A socket seems like a good idea.
I’m waiting on the new 3.9k resistors now.
I’ll take IC1 out and put a socket in. I’ll test the voltages after.
I can take some photos so we can see the circuit a bit more and see where it all goes. I think there’s regulator diodes there aswell. Those small caps.
BSST.
Did you see the main cap that shorted. Something either got under the cap or it leaked. There was a big moisture stain on the insulation board underneath. I tested them and that one was at 4200uf. The lowest one. Others were mid 5s. That’s what blew that resistor I think. I can take them out and test them aswell. Or lift a leg.
I’ll take IC1 out and put a socket in. I’ll test the voltages after.
I can take some photos so we can see the circuit a bit more and see where it all goes. I think there’s regulator diodes there aswell. Those small caps.
BSST.
Did you see the main cap that shorted. Something either got under the cap or it leaked. There was a big moisture stain on the insulation board underneath. I tested them and that one was at 4200uf. The lowest one. Others were mid 5s. That’s what blew that resistor I think. I can take them out and test them aswell. Or lift a leg.
It's possible the 3.9k resistors are OK. If they were shorted from 85V to ground, the dissipation is about 1.9W, so a 5W resistor is a conservative size.
Try an ohmmeter check directly on the resistor leads for resistance confirmation. This helps to reveal trace integrity problems. Another tip is to test resistance with both meter polarities. If the readings differ, there's a semiconductor in parallel or stored charge in a cap.
I just tested the other Amp.
The exact same problem.
That negative rail is down and there’s +10v on the volume and those wires.
The exact same problem.
That negative rail is down and there’s +10v on the volume and those wires.
Interesting!
I can't help wondering if both amps were subjected to the same calamity? Do you know their history? Is that possible? But what event led to that resistor?
On the other hand, maybe choosing OPA275 was a ticking bomb. But why would failure of the opamp precipitate failure of a 3.9k, 5W resistor? As noted above, if the resistor were shorted to ground, dissipation would be only 1.9W, only 37% of the 5W power rating. It would be nice to be confident of an enduring repair.
I can't help wondering if both amps were subjected to the same calamity? Do you know their history? Is that possible? But what event led to that resistor?
On the other hand, maybe choosing OPA275 was a ticking bomb. But why would failure of the opamp precipitate failure of a 3.9k, 5W resistor? As noted above, if the resistor were shorted to ground, dissipation would be only 1.9W, only 37% of the 5W power rating. It would be nice to be confident of an enduring repair.
I took the good 3.9k resistor out of the other Amp and put it in this one. I had to.
All the problems are gone. Can you believe it.😀🙌
What caused the problem though as it could happen again. I’m wondering what kind of load or use it was getting. The bloke told me they were working and he put them away for a while and then they didn’t work. Protection.
Cheers for all the help so far. 👏
All the problems are gone. Can you believe it.😀🙌
What caused the problem though as it could happen again. I’m wondering what kind of load or use it was getting. The bloke told me they were working and he put them away for a while and then they didn’t work. Protection.
Cheers for all the help so far. 👏
Curiosity compels me--- what opamp are you using? Did the original OP725 opamp survive? What supply voltages do you see?
Congrats and thanks.
Congrats and thanks.
Yea. It’s great. I tested the Volume wires ,no +10v and the IC1 opamp is getting the -20v now. There’s 0v on Pin 1 aswell. It’s the same one I put back in.
All I did before I found that problem ,was replace All the caps. Some of those might have been the cause of the blown resistor. But that big cap might have been involved aswell.
I want to check the BIAS next. I’m waiting on my adaptors. So I can use my oscilloscope leads on the multimeter. The BIAS pots are at about 2 Oclock. I’d assume the BIAS’s are ok but I might aswell check them after all that.
The Amp sounds good though. I’m only using some old speakers from a Sharp midi system but that thing is making them sound really good.
All I did before I found that problem ,was replace All the caps. Some of those might have been the cause of the blown resistor. But that big cap might have been involved aswell.
I want to check the BIAS next. I’m waiting on my adaptors. So I can use my oscilloscope leads on the multimeter. The BIAS pots are at about 2 Oclock. I’d assume the BIAS’s are ok but I might aswell check them after all that.
The Amp sounds good though. I’m only using some old speakers from a Sharp midi system but that thing is making them sound really good.
I’m going to try out a few different opamps in the IC1 spot. When I get the chance. That chip might not be what it once was. The amp has a lot of power ,you can tell ,but I’m finding it a bit on the harsh side. Sounds really good when at a moderate low level. I’m running straight from a MacBook Pro headphone plug at the moment.
Shame I don’t have a scope ,I could scan through and follow the audio signal. Or check that IC1 at least.
I’d like to check if all the transistors on the outputs are functioning as that - resistor might not be the only one that was blown. One channel seems louder than the other. Not much but you can notice it.
Shame I don’t have a scope ,I could scan through and follow the audio signal. Or check that IC1 at least.
I’d like to check if all the transistors on the outputs are functioning as that - resistor might not be the only one that was blown. One channel seems louder than the other. Not much but you can notice it.
I’ve taken the right channel board out and lifted all the resistors I needed to and I’ve tested the resistors and transistors….
So far I’ve found 3 bad drivers and 3 bad transistors.
1 transistors beta is 51. Another is 47. Another 67.( 100 is the target.)
One driver has a beta of 3. Its partner is 7.(high V side) One has a beta of 800. The (good?) one is 276.
So far I’ve found 3 bad drivers and 3 bad transistors.
1 transistors beta is 51. Another is 47. Another 67.( 100 is the target.)
One driver has a beta of 3. Its partner is 7.(high V side) One has a beta of 800. The (good?) one is 276.
Testing at low current can give erroneous readings for beta. Well, not really. On early types the beta WAS much lower in the hundred uA to a mA or two range. Readings in the 40’s-50’s on NPN power types are very plausible. PNPs are usually tighter, even among vintage types. Matching of NPN to PNP over a wide range of current is better among the modern OnSemi MJL versions or Toshiba’s latest 2SC5200 and complement. If your transistors were made in the 90’s your readings are par for the course. Higher end manufacturers used to hand match them. Still do, it’s just easier now.
Readings that far off on typical driver types, however, indicate bad or potentially fake parts. Single digits and readings of 800 are bad either way. They shouldn’t be outside the 100 to 300 range, period - even at “transistor hFE test” levels on a DMM.
Readings that far off on typical driver types, however, indicate bad or potentially fake parts. Single digits and readings of 800 are bad either way. They shouldn’t be outside the 100 to 300 range, period - even at “transistor hFE test” levels on a DMM.