Marantz 300DC. I have one of these amplifiers and inside the amplifier is a small power transformer mounted in the front. The supplies voltage to these really small power supplies, one power supply for each channel mounted up towards the front.
These little power supplies provide a +/- 70vdc to the biasing circuit of each channel. I would imagine the current draw is pretty low. I made some corrections that were made by somebody that worked on it (no idea who did the work) and on power up noticed that the small transformer got pretty warm. Warm enough that the primary winding is now open.
I’m on the search for a transformer, but before I install a new transformer, I want to see what is going on. I intended to hook up a power supply to the amplifier channel so that I can see how much current is being drawn and make sure I get the circuit working otherwise. That way when I put the new transformer in, it’s good to go. It would be nice to see how much current is being drawn so that I might size the new transformer.
This is the amplifier circuit. You can see the +70 V goes towards the top feeding R723, R724, etc. The -70 V is towards the bottom feeding the collector of Q706, R729, etc.
I got it all hooked up as I was wanting to, and powered up the amplifier on a dim bulb tester. The bulbs stayed bright and I heard a “whirring” noise coming from the inside of the power supply. I immediately turned the power off and checked over things and nothing looked damaged at all. I was curious what the behavior was about. I found that the power supply outputs are shorted when they are turned off. I am seeing about 16 ohms across the outputs. Once I turn the power supply channel on the short goes away it course.
I am guessing this is the source of my problem. I thought I would come here for advice before going at it again. I’d really like to not damage this power supply nor the amplifie lol.
The next go, having everything hooked up I’m guessing I should have the +/-70 V turned on before powering up the amplifier? Obviously, they normally turn on at the same time when you’re using the original transformer inside of the amp.
Another thought I had was maybe wiring a diode in series, maybe a 1N4004 or 1N5404. Something to eliminate the short that the amplifier sees. I checked for backfeeding voltage at the points, where the power supply would hook up, but there was no voltage coming from the amplifier itself, which leads me to believe it was just the short it was seeing.
Would love some encouraging advice.
Thank you,
Dan
These little power supplies provide a +/- 70vdc to the biasing circuit of each channel. I would imagine the current draw is pretty low. I made some corrections that were made by somebody that worked on it (no idea who did the work) and on power up noticed that the small transformer got pretty warm. Warm enough that the primary winding is now open.
I’m on the search for a transformer, but before I install a new transformer, I want to see what is going on. I intended to hook up a power supply to the amplifier channel so that I can see how much current is being drawn and make sure I get the circuit working otherwise. That way when I put the new transformer in, it’s good to go. It would be nice to see how much current is being drawn so that I might size the new transformer.
This is the amplifier circuit. You can see the +70 V goes towards the top feeding R723, R724, etc. The -70 V is towards the bottom feeding the collector of Q706, R729, etc.
I got it all hooked up as I was wanting to, and powered up the amplifier on a dim bulb tester. The bulbs stayed bright and I heard a “whirring” noise coming from the inside of the power supply. I immediately turned the power off and checked over things and nothing looked damaged at all. I was curious what the behavior was about. I found that the power supply outputs are shorted when they are turned off. I am seeing about 16 ohms across the outputs. Once I turn the power supply channel on the short goes away it course.
I am guessing this is the source of my problem. I thought I would come here for advice before going at it again. I’d really like to not damage this power supply nor the amplifie lol.
The next go, having everything hooked up I’m guessing I should have the +/-70 V turned on before powering up the amplifier? Obviously, they normally turn on at the same time when you’re using the original transformer inside of the amp.
Another thought I had was maybe wiring a diode in series, maybe a 1N4004 or 1N5404. Something to eliminate the short that the amplifier sees. I checked for backfeeding voltage at the points, where the power supply would hook up, but there was no voltage coming from the amplifier itself, which leads me to believe it was just the short it was seeing.
Would love some encouraging advice.
Thank you,
Dan
I’ve long thought that adjustable bench supplies similar to your Sorensen PS (i.e. with adjustable voltage and current limiting) should be the ideal tool to troubleshoot power amps. If the bridge rectifier works properly, you can often simply connect the bench supply across the the bulk reservoir caps and leave the AC power unplugged.
Can you post the entirety of the power amp with PS included to aid discussion?
Can you post the entirety of the power amp with PS included to aid discussion?
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I figured the same. With this amp technically having two seperate power supplies for each channel I’m guessing with me introducing the short in the bi circuit. It didn’t make the channel very happy.
Here is the circuit as a whole. The two larger blocks that take up the upper and mid left side are the two amplifier channels. Below those is the plug in and transformer. The two smaller blocks in the lower right are the two small power supplies that provide the +/-70 V to the bias circuit. And the mid/upper right side is the meter circuitry.
Here it is a little more blown up.
Does my idea of wiring a diode in series with the voltage line sound like a decent idea? It would eliminate the short that the amplifier sees when it’s powered on, but the power supply channel is off and would also protect the power supply from any potential voltage being fed back, though I don’t think that would really be an issue. I’m guessing the reason the power supply does this is to immediately eliminate any voltage that is present.
Dan
My approach would be to troubleshoot the regulator boards on bench, out of amp. Set current limit fairly low, maybe 20 or 30mA. Connect Sorensen across C803 and C804. Start with supply voltage and raise cautiously. A shorted bridge might show early on.
Monitor regulator outputs as you raise supply voltage—- they should rise smoothly with applied supply and enter regulation if the bench supplies can deliver about 75V.
Good luck!
Monitor regulator outputs as you raise supply voltage—- they should rise smoothly with applied supply and enter regulation if the bench supplies can deliver about 75V.
Good luck!
I will go at that approach, though I’m pretty sure they were okay, but definitely a good idea. I remember that while the transformer was getting very warm these regulator boards were not warm at all, I figured the heat sinks would’ve gotten very warm, but not at all. I didn’t know if this was because there wasn’t excessive current draw from the bias circuit or just a fault with the transformer. But very good thinking, do you think I would need some type of load on the regulator? The Sorensen can provide about +/-76 V. The thing that kind of sucks about this amplifier, not a huge fan of this feature, but it’s self biasing. There is no adjustment for it.
Dan
Perhaps the bridge rectifier has shorted. That could lead to transformer failure without heat sinks getting hot. Bad bridge diodes should be detectable with DVM. Good luck.
The bridge rectifiers on these small regulator boards are made with brand new UF4004s and they are in correctly.
Unfortunately this power supply only has 10 mA resolution, but set them for 30 mA.
Both boards measured the exact same. No current measurement on the negative voltage rail and 10 mA on the positive which is the lowest it can possibly measure. I could use a DMM on each rail to get higher resolution, I have two Flukes that will measure into the microamps. Other than that here are the two regulator boards with voltage on them.
Dan
They were not. I don’t know the history of the amp, but the regulators in one channel were original and the others had some NTE parts. I replaced them with MJE15032/33s. Other than that the silicon was original and I wanted to put fresh parts in. The transformer ran warm on the first power up with original diodes. I didn’t find any faults with these small power supplies. This is why I’m hoping to provide the power that these small PS normally provide with the Sorensen. It can provide the +/- 70V and I can see how much current each channel is actually drawing from them. If it was a lot I’d think the regulators would have gotten warm, maybe the transformer was just faulty???
Is my idea with the diodes a viable one? Just run a single diode in series with the + and the - voltage rail, orienting the cathode the correct direction for the polarity of the voltage. I’d just have the voltage drop of the diode, 600ish mV, eliminate the short the amp is seeing with the power supply channel off and then also protect the power supply. Or is that all just unnecessary and I should power on the +/- 70V first and then power on the amp?
Dan
Hi Dan,
I don’t see any harm in your proposed idea of adding series diodes to the small regulator boards.
Frankly, I’m still trying to sort out the architecture of the amp. It appears to have main, unregulated +/-70V power supplies plus regulated supplies, also labeled +/- 70V for the shear enjoyment of trying to distinguish between the two. 🤨 If I have the schematic properly figured, the regulated supplies power the driver stages, i.e. Q706 and everting to its left; Q711/Q712 and everything to their right is powered by the unregulated +/- 70V rails. Can you confirm?
Assuming this is correct, my suggested plan of attack:
If the cabling/connector arrangement allows, I would investigate one channel at a time. I would unplug AC, and use Sorensen to power the PS board, raising voltage slowly while watching for concerning current draw or concerning current flowing into the output half of the PA. Be sure to test without load applied to speaker terminals. If behavior is benign, repeat for the other channel.
If all is good, then investigate the output section. I.e. no AC applied, use Sorensen to test unregulated half of PA in manner similar to above, with driver section unpowered. Repeat on the other PA channel.
If all transpires as hoped, this would be very encouraging, but I’ll wait for your feedback before venturing further.
Good luck!
I don’t see any harm in your proposed idea of adding series diodes to the small regulator boards.
Frankly, I’m still trying to sort out the architecture of the amp. It appears to have main, unregulated +/-70V power supplies plus regulated supplies, also labeled +/- 70V for the shear enjoyment of trying to distinguish between the two. 🤨 If I have the schematic properly figured, the regulated supplies power the driver stages, i.e. Q706 and everting to its left; Q711/Q712 and everything to their right is powered by the unregulated +/- 70V rails. Can you confirm?
Assuming this is correct, my suggested plan of attack:
If the cabling/connector arrangement allows, I would investigate one channel at a time. I would unplug AC, and use Sorensen to power the PS board, raising voltage slowly while watching for concerning current draw or concerning current flowing into the output half of the PA. Be sure to test without load applied to speaker terminals. If behavior is benign, repeat for the other channel.
If all is good, then investigate the output section. I.e. no AC applied, use Sorensen to test unregulated half of PA in manner similar to above, with driver section unpowered. Repeat on the other PA channel.
If all transpires as hoped, this would be very encouraging, but I’ll wait for your feedback before venturing further.
Good luck!