Need help on Delta 1010 PSU mods

[mtl777]

Hey guys, I hope you could help me with modding my Delta 1010's PSU. Below is a schematic that reflects the latest changes I have done. The circuit employs a voltage doubler that is used to generate +15V and -15V, and two half-wave rectifiers to generate the +5V rails. The 9VAC input is from a wall wart rated for 3.5A that converts the mains 110VAC to 9VAC. That is its rated spec but the actual input voltage I'm reading is 10.95VAC.

My problem is that I'm unable to get a clean voltage into and out of the +15V reg U4. The way I'm checking this is by measuring the AC voltage on the input and output of U4 (I don't have a scope). When I try to measure the voltage, I'm unable to get a reading because my meter fluctuates wildly. But I'm not getting this fluctuation when I measure the AC voltage on the input and output of the other regulators (they are giving me near zero AC voltage). I think this means something is wrong with either the +15V reg itself or the supply voltage going into it. Can someone please help me to sort this out? What are the things that I should check for? What other mods would you suggest to clean it up?

Any help would be greatly appreciated. Thanks in advance!

[mtl777]

Since no one is responding, maybe I should ask more specific questions:

1. What do you think of the value of C14 and C17 (currently 660uF). Are they too small? What value would you suggest?

2. What about the value of C12 and C18 (currently 3400uF). Are they too large? What value would you suggest?

3. Do you think it will improve the power supply if I replace all the UF4002 diodes with Schottky type MBR1100?

Someone help me, please!!!

[AP2]

Hi,
try change c13 at output of u4 with 10uF and .1uF in parallel.
not correct big capacitance at output of regulators (autoscillation)

regards

[mtl777]

Originally Posted by AP2

Hi,
try change c13 at output of u4 with 10uF and .1uF in parallel.
not correct big capacitance at output of regulators (autoscillation)

regards

Thanks, I'll try that! The .1uF should be a nonpolar, film or ceramic, right?

[AP2]

Originally Posted by mtl777

Thanks, I'll try that! The .1uF should be a nonpolar, film or ceramic, right?

yes, ceramic 0,1 (100nF) is good

[mtl777]

I want to experiment with changing the value of the capacitors C14 and C17. But first, I need to know what exactly is the function of these capacitors. Can someone please explain what these capacitors are doing in the circuit? Please be easy on me as I have only a little knowledge of electronics. I'm just handy with a soldering iron.

Next question is the choice of the best capacitor for C14 and C17. I'm guessing that low ESR and high ripple current rating are some of the important considerations, and of course the voltage rating of the capacitor. I measured the voltage across C14 and it's only 13.6VDC. Therefore, I think that any voltage rating from 25V and up would be fine. I am currently looking at the following capacitor options:

Panasonic FM 820uF 25V, 0.018R Impedance, 2470mA Rated Ripple
- or -
Panasonic FM 560uF 35V, 0.018R Impedance, 2470mA Rated Ripple

I want to know which one of these would be the better choice. So the impedance and rated ripple of these are the same, the voltage rating is adequate for both, and the only question is the capacitance value. All things being equal, should I choose the one with the higher capacitance? Is higher capacitance a good thing and an important consideration for the function of this particular capacitor?

Thanks, and I hope someone could please kindly help me with this question!

[Mooly]

C14 and 17 provide "DC" isolation and allow the the voltage on the end of the caps to develop... can't think of a better word.

Without them D2 and D3 would appear as a "short" to the 9vac winding, one conducting on positive half cycles and one on negative.

Caps... weird values you have chosen. I would use at least 2200uf for all. Having said that a lot depends on what current you intend to draw from the rails. Higher current = higher capacitance.
It's the voltage across C12 that matters... you need at least 18v for the reg to function correctly. Those 43 ohms are wastefull, any more than a few milliamps load current and the voltage to reg will fall dramatically. If you insist on resistors then I wouldn't go over 1 ohm non inductive type. C15 and C20 then need around 100uf cap in parallel with them.

[mtl777]

Originally Posted by Mooly

C14 and 17 provide "DC" isolation and allow the the voltage on the end of the caps to develop... can't think of a better word.

Without them D2 and D3 would appear as a "short" to the 9vac winding, one conducting on positive half cycles and one on negative.

Caps... weird values you have chosen. I would use at least 2200uf for all. Having said that a lot depends on what current you intend to draw from the rails. Higher current = higher capacitance.
It's the voltage across C12 that matters... you need at least 18v for the reg to function correctly. Those 43 ohms are wastefull, any more than a few milliamps load current and the voltage to reg will fall dramatically. If you insist on resistors then I wouldn't go over 1 ohm non inductive type. C15 and C20 then need around 100uf cap in parallel with them.

Hi, thanks for replying! The value of C14 and C17 were even smaller before - only 470uF. Please see below the schematic of the original before I did any mods. So you mean I could use 2200uF or even higher for these caps? How about 4400uF? Is that too high? What's the maximum I can use?

I think the reason why the original designer put only 470uF for these caps is because of the heat. Increasing them probably reduces the ripple, causing an increase in the voltage going into the +/-15V regulators due to the cleaning of the ripple. In fact, I noticed that when I changed them to 660uF the input voltage to the regs increased by 1 volt. This increased the heat on the regs.

I've been getting too much heat on those regs that I had to put large heat sinks on them. Despite that, they were still very hot. The +15V reg would run at 63 deg.C and the -15V at 75 deg.C (measured by putting the temp probe on the large heat sink that I added). I measured the input voltages going into the regs, and the +15V reg was getting 25.7V, the -15V reg was getting 25.9V, one of the +5V regs was getting 13.1V, and the other +5V reg was getting 13.5V. The input voltages were just too much. I think the regs need only V output + 3 and anything in excess of that is just dissipated as heat. The heat was causing the capacitors to deteriorate quickly. That is why I put those resistors in, to reduce the input voltages, and thus the heat.

The way I calculated the additional resistors was by first putting a 1 Ohm resistor and measuring the voltage across the resistor while my Delta 1010 was loopback playing and recording a test signal on 8 channels (the Delta 1010 is an audio interface that has 8 analog inputs and outputs) to simulate full load. The test signal was generated by running 4 instances of RMAA, each instance playing and recording a stereo pair. So the first instance was playing a test signal on outputs 1 and 2, and recording them on inputs 1 and 2. The second instance was playing a test signal on outputs 3 and 4, and recording them on inputs 3 and 4. And so on with the third and fourth instances. This went on continuously while the voltage was being measured.

After getting the voltage across the 1 Ohm resistor, I calculated the current. Then based on the current that was calculated and the new input voltage that I would like to have, and the difference between the old and the new input voltage, I was able to calculate the appropriate resistor value to use. I came up with 43 Ohms for R4 and R5, 16 Ohms for R6 (I just realized the 27 Ohms shown on the schematic is a mistake), and 27 Ohms for R23.

After putting in these resistors, the input voltages I am now getting are 19V on the +15V reg, 19.6V on the -15V reg, 8.6V on one of the +5V regs, and 8.8V on the other +5V reg. With these resistors in place, I am no longer afraid of increasing the value of C14 and C17 and the resulting increase in input voltage that that would cause.

You mentioned that "C15 and C20 then need around 100uf cap in parallel with them". Does this apply only if I use 1 Ohm resistors instead of the values that I calculated? What cap value should I put in parallel with C15 and C20 if I'm going to use the resistor values that I calculated?

Thanks again!

[Mooly]

Caps used like C14 and C17 are prime suspects for faults... this type of circuit crops up frequently in varies guises.
Altering their values to tweek the voltage isn't good practice really... and as you say with the regs, you had way to much voltage applied.
No easy answer as it's all a compromise... and resistors will get hot too.
If you use any resistors, even 1 ohms then I would add 100uf or so at each reg input.
It maintains a low impedance as seen by the reg.

Edit... there's no "maximum" value as such. 2200 is low impedance at line frequency and a sensible size. I wouldn't go over that.

[mtl777]

Originally Posted by Mooly

Caps used like C14 and C17 are prime suspects for faults... this type of circuit crops up frequently in varies guises.
Altering their values to tweek the voltage isn't good practice really... and as you say with the regs, you had way to much voltage applied.
No easy answer as it's all a compromise... and resistors will get hot too.
If you use any resistors, even 1 ohms then I would add 100uf or so at each reg input.

So, not just C15 and C20, I should add a 100uF in parallel with C24 also?

And what about on the input of U23? Should I add a 100uF across the input and ground of U23? BTW, why is there no capacitor like C24 on the input of U23? It seems strange. Is this a design error?

Originally Posted by Mooly

Edit... there's no "maximum" value as such. 2200 is low impedance at line frequency and a sensible size. I wouldn't go over that.

I'll use 2200uF then. Actually, I will parallel two 1100uF caps to make 2200uF. That will distribute the heat to two caps and will be a good thing because C14 and C17 are also prone to get hot. Due to the cramped space, however, I can only fit 25V-rated caps here. Is 25V good enough? (I currently measured 13.6V on these caps but I'm not sure if that might increase if I change them to 2200uF.)

I really appreciate your help. Thanks a lot!