Maybe it's just me, but doesn't it seem like there could be some oscillation going on? The only TL072's that I've seen run warm, either had (a) heavily-loaded output(s), or were oscillating ultrasonically. Do you have a 'sniffer' -- a small cap, 10pF usually, with a small signal diode (Schottky or germanium if you have one) shunt, anode to ground. Gives a small positive indication in the presence of high frequency AC. You really need one if you don't have a 'scope.
Or maybe one of those 10uF rail bypass caps has lost value due to the heat. Even just one of the '072s oscillating would dirty up the sub rail pretty bad.
The designers in Japan often put a fusible resistor in the regulator supply in their consumer electronics. Saves some dissipation for the regulator, gives a handy place to confirm 'normal' current consumption, and adds a small safety margin. Just bend the Input leg up and fit the resistor into its original hole. A 1uF or 0,47uF to ground may be needed for stability. Might still be easier to implement than a bigger heat sink.
Unless I've botched the arithmetic, the LM317 and '337 are being asked to dissipate ~865 and 955 mW respectively -- probably enough to explain those temperatures. But I know what you mean - closed case and all - it'd feel a lot safer if the parts weren't too hot to touch. And there is still a pretty healthy margin between 'too hot to touch', and the 150 C degrees max junction temp most parts are rated.
Cheers
Or maybe one of those 10uF rail bypass caps has lost value due to the heat. Even just one of the '072s oscillating would dirty up the sub rail pretty bad.
The designers in Japan often put a fusible resistor in the regulator supply in their consumer electronics. Saves some dissipation for the regulator, gives a handy place to confirm 'normal' current consumption, and adds a small safety margin. Just bend the Input leg up and fit the resistor into its original hole. A 1uF or 0,47uF to ground may be needed for stability. Might still be easier to implement than a bigger heat sink.
Unless I've botched the arithmetic, the LM317 and '337 are being asked to dissipate ~865 and 955 mW respectively -- probably enough to explain those temperatures. But I know what you mean - closed case and all - it'd feel a lot safer if the parts weren't too hot to touch. And there is still a pretty healthy margin between 'too hot to touch', and the 150 C degrees max junction temp most parts are rated.
Cheers
Totally forgot about the lines labeled +/-18V headed off of the schematic fragment in post #51.I'm on the lookout for a scope, but it might be some time before I find something in this area. I haven't tried changing the bias again, yet.
I have to order parts for a couple other amps, including the same filter & rail caps as this amp uses, so I'm considering replacing all the filter & rail caps, as well as the regulators themselves given their current physical condition/age. Do you think this is a waste of time?
Do you still think it's worth replacing any of the TL072C opamps at this point? If so, do you know if TL072CD is a direct replacement? The SOIC-8 package looks like the correct size.
https://www.mouser.com/ProductDetail/Texas-Instruments/TL072CD?qs=5nGYs9Do7G0niDPig1J2Tw==
Yes. SOIC. You can use a dial caliper to confirm.
It's probably a waste of time to replace the capacitors. They didn't vent and didn't leak so they are likely OK.
I was just curious as to why the TL072s were heating and if you had one. I wanted to see if a new one would run cooler. When you posted the various voltages, the op-amps generally looked healthy. The op-amps work to make the two inputs per channel match and I think they all did within a few hundredths of a volt.
At this point, I don't think I'd change anything. The amp is likely OK. The various things that I asked you to do were simply to try to determine where the current was going. The fact that all 4 channels are operating identically likely means that there is no problem with any of them. Backing down the bias may reduce the load on the regulators. We'll know when you try it.
If you have the choice of a CRT based scope or a scope with an LCD display, I'd recommend the CRT scope. It doesn't have to be new. All it has to do is be a scope with a triggered sweep (virtually all scopes made since the 60's) in good working order. My favorite scope was made in the 80's. I have a Tek 453 that was made in the 60's that works perfectly except may one or two dirty pots and has the finest trace that I've ever seen on a scope. I'm not recommending that you buy a 453. What I'm saying is that an older scope can do the job. You don't need a digital readout, or fancy displays. You simply need something that can show you the signal clearly and an analog (CRT) scope can do that very well.
It's probably a waste of time to replace the capacitors. They didn't vent and didn't leak so they are likely OK.
I was just curious as to why the TL072s were heating and if you had one. I wanted to see if a new one would run cooler. When you posted the various voltages, the op-amps generally looked healthy. The op-amps work to make the two inputs per channel match and I think they all did within a few hundredths of a volt.
At this point, I don't think I'd change anything. The amp is likely OK. The various things that I asked you to do were simply to try to determine where the current was going. The fact that all 4 channels are operating identically likely means that there is no problem with any of them. Backing down the bias may reduce the load on the regulators. We'll know when you try it.
If you have the choice of a CRT based scope or a scope with an LCD display, I'd recommend the CRT scope. It doesn't have to be new. All it has to do is be a scope with a triggered sweep (virtually all scopes made since the 60's) in good working order. My favorite scope was made in the 80's. I have a Tek 453 that was made in the 60's that works perfectly except may one or two dirty pots and has the finest trace that I've ever seen on a scope. I'm not recommending that you buy a 453. What I'm saying is that an older scope can do the job. You don't need a digital readout, or fancy displays. You simply need something that can show you the signal clearly and an analog (CRT) scope can do that very well.
The going rate changes.
You can buy them from ebay but if you try that, email me with any auction numbers that you are interested in. If you post them here, someone else may buy them before you can.
babin_perry@yahoo.com
What's important is that the scope is shown with at least one (preferably two) locked traces. The display has to be bright and sharp. You also have to look at shipping. Sometimes it's more than the scope. Probes aren't important. The ones that sell for 4 probes for$20 work perfectly fine.
$60 for a working 464 would have been incredible. I have a 465B that I paid $500 for about 15 years ago and it has been well worth it. Expect to pay about $150 for the scope then shipping costs.
There are plenty of good scopes so don't limit yourself to Teks.
You can buy them from ebay but if you try that, email me with any auction numbers that you are interested in. If you post them here, someone else may buy them before you can.
babin_perry@yahoo.com
What's important is that the scope is shown with at least one (preferably two) locked traces. The display has to be bright and sharp. You also have to look at shipping. Sometimes it's more than the scope. Probes aren't important. The ones that sell for 4 probes for$20 work perfectly fine.
$60 for a working 464 would have been incredible. I have a 465B that I paid $500 for about 15 years ago and it has been well worth it. Expect to pay about $150 for the scope then shipping costs.
There are plenty of good scopes so don't limit yourself to Teks.
I have some other amps I'm working on, so I shelved this one for a bit. But in the process of repairing the smaller brother Punch 55.2, I took readings off its regulators. I get similar readings to this 600a4, such as:
55.2 at 14.4v B+:
LM317:
In: 29.5V
Out: 14.6V
Adj: 13.3V
LM337:
Out: -13.1V
In: -29.0V
Adj: -11.8V
Nothing seems to be heating irregularly on the 55.2. The voltage differences between this and the 600a4 are almost identical.
Last time I read the 600a4 at 13v I got:
LM317
In: 28.6V
Out: 14.3V
Adj: 13V
LM337
Out: -12.7V
In: -28.6V
Adj: -11.3V
I have a 360a2 for repair as well, so I'll check it too while I'm at it. It seems oddly coincidental that these would have near identical readings if there's a problem. Thoughts?
55.2 at 14.4v B+:
LM317:
In: 29.5V
Out: 14.6V
Adj: 13.3V
LM337:
Out: -13.1V
In: -29.0V
Adj: -11.8V
Nothing seems to be heating irregularly on the 55.2. The voltage differences between this and the 600a4 are almost identical.
Last time I read the 600a4 at 13v I got:
LM317
In: 28.6V
Out: 14.3V
Adj: 13V
LM337
Out: -12.7V
In: -28.6V
Adj: -11.3V
I have a 360a2 for repair as well, so I'll check it too while I'm at it. It seems oddly coincidental that these would have near identical readings if there's a problem. Thoughts?
One person thinks the regulators should be running at exactly equal & opposite voltages. Another person thinks one is being loaded higher than the other, and that there is a problem with the "imbalance of voltage of nearly 2 volts.". One of them I believe is fairly knowledgeable/respected, the other I don't know as well. Neither seem to have much time to go back and forth about it, which I understand.
They should have explained how the regulators work.
As with op-amps or comparators, they use a reference voltage and adjust according to that reference. For these regulators, they are designed to have a 1.25v difference between the output and adjust terminals. They do all they can to maintain that 1.25v difference. Older regulators or lower quality regs will vary more than the 1.25v. The precise offset isn't very important unless you have a circuit where voltage is critical (it absolutely is not for this application).
The two 'programming' resistors are also a factor. The resistors have a tolerance and sometimes drift in value. These make a difference in the way they divide the voltage and therefore affect the final voltage.
If you have the repair tutorial, item #25 on the Audio TroubleShooting page goes into more detail as well as gives a calculator to see how the various components affect the output voltage.
As with op-amps or comparators, they use a reference voltage and adjust according to that reference. For these regulators, they are designed to have a 1.25v difference between the output and adjust terminals. They do all they can to maintain that 1.25v difference. Older regulators or lower quality regs will vary more than the 1.25v. The precise offset isn't very important unless you have a circuit where voltage is critical (it absolutely is not for this application).
The two 'programming' resistors are also a factor. The resistors have a tolerance and sometimes drift in value. These make a difference in the way they divide the voltage and therefore affect the final voltage.
If you have the repair tutorial, item #25 on the Audio TroubleShooting page goes into more detail as well as gives a calculator to see how the various components affect the output voltage.
I went ahead with swapping the power supply caps and both regulators for piece of mind (aka glutton for punishment).
Turning the bias pots fully ccw made no change to the voltage drop across any of the Rx70/Rx71 resistors.
Even with the bias fully ccw, the regulators heat the same - 221°F for the LM317, 209°F for the LM337, 72°F ambient temperature.
Reset the bias via current draw method, starting at .98A idle and turning up bias pots individually for .05A rise, ending at 1.18A idle. Regulators maintaining same temperature.
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