I have a friends Peavey MAX112 bass amp in for repair. If you look at the power supply segment that I posted, 15V zener diode D9, (of the +/- 15 VDC section), shorted out, and C32 blew it's little pressure relief scoring. They also put a pretty good "burn" on the circuit board. Fortunately, it's just cosmetic. No real damage. D9 is a 1N4744A, that's only rated at 1W. Even though the +/- 15V supply is only powering three small op amps, (2x4560 and 1 LM3080), it seems like a somewhat cheesy way to make a bipolar supply.
Mouser was out of the 1N4744A's, so I ordered two of the 5W versions, plus two new caps.
So my questions are:
1. Do you think the upgrade to the 5W version will give a little better reliability and durability?
2. Would I improve the situation by going ahead and adding a true voltage regulator scheme in its place? I enjoy projects and this amp used to be mine. I'm just not sure if I should connect the 78/7915's after that 250 ohm resistor, or connect it differently. The raw 35 volts is right at the upper limit of what those 78xx series like to see.
Any advice would be appreciated.
P.S. This amp does use a nice LM3886 "Overture" amp IC for power. I was surprised to see that.
Mouser was out of the 1N4744A's, so I ordered two of the 5W versions, plus two new caps.
So my questions are:
1. Do you think the upgrade to the 5W version will give a little better reliability and durability?
2. Would I improve the situation by going ahead and adding a true voltage regulator scheme in its place? I enjoy projects and this amp used to be mine. I'm just not sure if I should connect the 78/7915's after that 250 ohm resistor, or connect it differently. The raw 35 volts is right at the upper limit of what those 78xx series like to see.
Any advice would be appreciated.
P.S. This amp does use a nice LM3886 "Overture" amp IC for power. I was surprised to see that.
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Zeners need enough current, as well as not too much current.
A 15V, 5W Zener has a max current of 5W/15V = 333mA.
A 15V, 1W Zener has a max current of 1W/15V = 67mA.
However, if the Zener current is too low, it will be noisy and regulate poorly or not at all.
Generally you want the Zener current to be at least 5% of the max current, and preferably more than that.
A 15V, 5W Zener has a minimum current of 5% x 333mA = 16mA.
A 15V, 1W Zener has a minimum current of 5% x 67mA = 3.3mA.
So using too big a Zener without redesigning the circuit, may cause the Zener to have too little current,
and be noisy or not regulate. The current through the series resistor is the sum of the Zener current and
the load current. Since we don't know the nominal load current, it's difficult to know how well (if at all)
the larger Zener would work.
If you want to use the three terminal regulator instead, the series resistor may need adjustment.
The regulator needs more than 3VDC across it to work, and normally more than that, but too much
DC voltage across it would cause too much power dissipation. Try adjusting the series resistor
so the input DC voltage at the regulator is around 22VDC. That should be enough margin.
Use a separate heat sink for each regulator unless you insulate the tabs from the sink. Make sure
the heat sink remains at a reasonable temperature during operation.
Make sure that the series resistor also remains cool enough, since it has to dissipate power due to
both the DC voltage drop across it, and also the input ripple voltage (which the capacitor after the
resistor causes to be dropped completely across the resistor).
A 15V, 5W Zener has a max current of 5W/15V = 333mA.
A 15V, 1W Zener has a max current of 1W/15V = 67mA.
However, if the Zener current is too low, it will be noisy and regulate poorly or not at all.
Generally you want the Zener current to be at least 5% of the max current, and preferably more than that.
A 15V, 5W Zener has a minimum current of 5% x 333mA = 16mA.
A 15V, 1W Zener has a minimum current of 5% x 67mA = 3.3mA.
So using too big a Zener without redesigning the circuit, may cause the Zener to have too little current,
and be noisy or not regulate. The current through the series resistor is the sum of the Zener current and
the load current. Since we don't know the nominal load current, it's difficult to know how well (if at all)
the larger Zener would work.
If you want to use the three terminal regulator instead, the series resistor may need adjustment.
The regulator needs more than 3VDC across it to work, and normally more than that, but too much
DC voltage across it would cause too much power dissipation. Try adjusting the series resistor
so the input DC voltage at the regulator is around 22VDC. That should be enough margin.
Use a separate heat sink for each regulator unless you insulate the tabs from the sink. Make sure
the heat sink remains at a reasonable temperature during operation.
Make sure that the series resistor also remains cool enough, since it has to dissipate power due to
both the DC voltage drop across it, and also the input ripple voltage (which the capacitor after the
resistor causes to be dropped completely across the resistor).
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Thanks rayma. Great info. Since I already have the 5W's on order, I'll see how they do. If I think there's a problem, I'll order the originals from some other source.
Thanks again.
Thanks again.
NJM4560 have typical current draw of 4.3 ma. LM3080 has typical current draw of 1.1 ma. 2*4.3+1.1=9.7 ma. that is less than the 33 ma rayma postulates as okay on the 1n5352 5W zener.
Peavey has installed millions of these zener op amp regulators. The 1 W zeners that failed in my units were taken out by output transistor failure, with 40-180 v rampaging throughout the front ends. I would postulate that the electrolytic cap (time fuses I call them) shorted then took out the zener. Peavey hasn't used 3080 since the 90's, and that is certainly enough time for even illinois caps made to peavey spec to go bad.
I would consider checking power out max to determine if rail caps are high ESR also. If the unit is yours, I'd change all the little 10-22 ecaps too. They don't fail often, but taking the cover off & putting back on again is worth the cost of these $.20 (in 10000 hour variety) parts. BTW you don't buy 1000 hour or 500 hour caps, do you? I've had to re-e-cap one amp 4 times in my lifetime; too much work now that Newark will actually sell long life parts to individuals. Digikey has long life caps available too, although in 1978 they were selling surplus from a direct mail flyer, not new parts.
Peavey has installed millions of these zener op amp regulators. The 1 W zeners that failed in my units were taken out by output transistor failure, with 40-180 v rampaging throughout the front ends. I would postulate that the electrolytic cap (time fuses I call them) shorted then took out the zener. Peavey hasn't used 3080 since the 90's, and that is certainly enough time for even illinois caps made to peavey spec to go bad.
I would consider checking power out max to determine if rail caps are high ESR also. If the unit is yours, I'd change all the little 10-22 ecaps too. They don't fail often, but taking the cover off & putting back on again is worth the cost of these $.20 (in 10000 hour variety) parts. BTW you don't buy 1000 hour or 500 hour caps, do you? I've had to re-e-cap one amp 4 times in my lifetime; too much work now that Newark will actually sell long life parts to individuals. Digikey has long life caps available too, although in 1978 they were selling surplus from a direct mail flyer, not new parts.
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Thanks indianajo. More food for thought. I guess I'm opening a can of worms by deviating from the design. I went ahead and ordered the proper 1W zener's from another source. I'll just put it back the way it was.
I'm not sure what you mean by "10 - 22 ecaps." I would normally take that to mean 10V, 22uf electrolytic cap. But I don't think that's what you mean since I don't see any of those in the circuit. Could you elaborate?
I originally bought this amp new about 10 years ago. Me and my luthier friend trade stuff back and forth all the time. I think I traded this to him for a guitar. It sits only in a home or shop. Never gigged, and never played at high volume. He just uses it so he can test customers bass guitars. He has my old Peavey TKO-115 on loan right now. But this little MAX112 is way more compact.
Again, thanks all for the helpful information. I really appreciate it.
Artie
I'm not sure what you mean by "10 - 22 ecaps." I would normally take that to mean 10V, 22uf electrolytic cap. But I don't think that's what you mean since I don't see any of those in the circuit. Could you elaborate?
I originally bought this amp new about 10 years ago. Me and my luthier friend trade stuff back and forth all the time. I think I traded this to him for a guitar. It sits only in a home or shop. Never gigged, and never played at high volume. He just uses it so he can test customers bass guitars. He has my old Peavey TKO-115 on loan right now. But this little MAX112 is way more compact.
Again, thanks all for the helpful information. I really appreciate it.
Artie
Peavey sprinkles around 10 uf 22 uf, occasional 47 uf 100 uf 220 uf little electrolytic caps. Very cheap, tend to dry out worse than their rail caps actually. Watch the input caps, they tend to be 2.2 uf non-polar electrolytic. I replace those with ceramic NPO to avoid ever doing that again.
Don't change more than 2 parts at a time between sound checks. I make a lot of bad solder joints, you may too. I change 2 caps, no sound or weird sound, I know just where the problem is - what I just did.
Don't change more than 2 parts at a time between sound checks. I make a lot of bad solder joints, you may too. I change 2 caps, no sound or weird sound, I know just where the problem is - what I just did.