Yaqin CD3 / Darkvoice 336SE Mods (Coupling Caps)
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I finished wiring the Darkvoice today and powered up for the first time in well over a month.
Voltages at the first and last filter cap, respectively, were 198VDC and 131VDC. Voltage at the heaters was 6.7VAC. Voltage at the output caps was 83VDC. That all checks out just fine with how it originally was.
Headphones (Denon AH-D2000s) were plugged in the whole time. There was no appreciable hum, which is a good sign considering they are 20-ohm headphones.
I plugged a source in. There was an audible pop upon plugging in the left channel. There was not the same pop when plugging in the right channel. Still no appreciable rise in the noise floor.
I didn't get to feed a signal to the amp before I realized that the Mills MRA12 resistors in the power supply were smoking and the black coating was bubbling somewhat. I immediately powered down, removed power, removed the source, and began inspecting. The power supply resistors were very hot to the touch, but everything else seems to check out just fine. I don't see any visible signs of damage anywhere, and there were no audible signs of failure.
So, does anyone have any ideas why the MRA12 resistors would be running so very hot at what was confirmed to be under 200V passing through them? That would be a very large amount of current passage, which should only happen if there were a short-circuit somewhere.
I have pictures, but I'm having some trouble getting them onto my computer. I can't, by simple inspection, identify anything on the power side that would cause that behavior.
edit: Here are pictures. I'm thinking of just ordering some more 25W Vishay/Dale Wirewounds to replace the MRA12s. I'd still very much like to identify if there's a problem here, though. Smoking resistors are typically no good. -
Powered it up again today to the same effect. Voltage drop across each power supply resistor was ~33V. They're 200Ohm resistors rated for 12W, so that shouldn't be enough to cause them to go up in smoke, right? Or am I doing math wrong here?
V/R = I
33V/200Ohm = 0.165A
V*I = P
33V * 0.165A = 5.445W
On the plus side, there is sound in both channels - I got a chance to pass a signal through before the resistors started smoking again. Is it possible they are just bad in some way and should be replaced? Or did I mess some kind of wiring up? It certainly isn't drawing enough current to trip the 1A fuse at the inlet. -
Ugg... That's not fun... a 12W should be more than enough, especially since I bet the previous resistors were of a lower wattage rating. I would double check the circuit and make sure there is not a loose wire or connection, so something shorting in the circuit.
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Ugg... That's not fun... a 12W should be more than enough, especially since I bet the previous resistors were of a lower wattage rating. I would double check the circuit and make sure there is not a loose wire or connection, so something shorting in the circuit.
I tried to be pretty meticulous with my wiring, to the point that wires' insulation isn't even touching stuff around it. It's all solid-core, bent to shape. It's just annoying that something as simple as a power supply resistor is the only obvious hiccup.
I'll poke around inside a bit and compare it back to the pictures I took of the original wiring. If there were a genuine short, wouldn't it be a lot more eventful than a little extra current passed through two resistors?
I need to test it, but I'd be interested to see if the resistors getting hot affected their behavior and it was all just downhill from there. I ordered 25W Vishay resistors to replace them regardless.
Original resistors were 200 Ohm / 10 Watt -
Well, I finally finished with the faceplate. The headphone jack is in place and everything. The only other change is that I've put a second 100uF Mundorf E-Cap in place.
Now I have even more dramatic problems. Voltage at the first filter cap now seems to be ~250V, voltage drop across the power supply resistors is similarly raised, voltage at the output caps is measured at zero, and the amp doesn't actually amplify and output signals at this time.
Since the only notable changes are the extra output caps and the headphone, I will have to undo one to see if anything changes. The obvious choice would be the output caps, since the wiring to the headphone jack is pretty difficult to screw up. I'll remove them tomorrow and see if anything changes.
My fear would be that the transformer is somehow affected - that is the obvious conclusion based on the markedly higher voltage. And that would be very bad. -
I replaced the power resistors with some higher wattage ones. The dilemma now seems to be that all of the voltages in the amp are markedly raised (240VDC at first filter, 98VDC at coupling cap, 8.4VAC on heaters). Those are all up ~20% from what I would consider normal.
Actually, before I even submitted this, I decided to test the line voltage at the inlet. 144VAC. That's ridiculous. What the hell kind of socket outputs 144VAC? That would certainly explain the high output from the transformer - the input voltage is well above standard. -
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144VAC from the wall?!?
Yup, verified in several different outlets around the apartment. Between 144VAC and 146VAC.
Guess it's time to contact the power company. Isn't standard tolerance 120VAC +/- 10%? -
WOW! :eek: Watch out as that could cook any of the electronics at your house. I would unplug everything you can because most electronics would see that as over voltage. Do you have a power center feeding your audio equipment?
Usually I think the wall voltage lives around 112V at my house, so you clearly have a problem, and a call to the power company is needed. -
WOW! :eek: Watch out as that could cook any of the electronics at your house. I would unplug everything you can because most electronics would see that as over voltage. Do you have a power center feeding your audio equipment?
Usually I think the wall voltage lives around 112V at my house, so you clearly have a problem, and a call to the power company is needed.
Yeah, my audio equipment is protected. Everything near my desk is just on a power strip instead of a power center; the strip hasn't indicated over-voltage yet.
Kinda disappointing, because this means I can't even turn on my headphone amp to test to test things without exceeding the rating on the filter caps. -
Nice to see the modding still going nice and strong. Any latest update?
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What did the power company say about your higher voltage service?
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Nice to see the modding still going nice and strong. Any latest update?
Everything is wired inside, but I still have it open and sitting on my desk because of the voltage problems I've been having. It seems to measure just fine (albeit a bit high, from the high line voltage in) when there is no source plugged in, but plugging a source in makes it act funny: voltage drop across power supply resistors goes up significantly (~35-40V before to ~75V after), and it plays only in one channel. I'm a bit worried to keep it on for too long if the heater voltage is truly above 8VAC, like how it has been measured recently.
Regardless, here's some picture of the insides.
The one thing I'm not sure about in my wiring is on the potentiometer. It is a 4-pin per channel one; the ones that aren't used are loudness taps. After that I referenced this (link). Pins 2 and 6 are left in and right in, respectively. Pins 3 and 7 are left out and right out, respectively. Now, I have the jacket of the RCA plugs connected to pins 4 and 8 (ground), but their left/right orientation is swapped (i.e. right channel is connected to pin 8, left to pin 4). I did that out of convenience for how long the wires were and figured it wouldn't matter, since they share a common ground point; was I wrong in that assumption?What did the power company say about your higher voltage service?
They said they checked at the meter and it measured 122V for each phase and 244V phase-to-phase. I checked at the socket and it measured 141V. Not sure what to take from that - maybe my multimeter is acting funny. -
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Power up, measures just fine (again, a bit high on the voltage.
Plug source in, left channel first, loud pop in right speaker and appreciable rise in noise floor.
Plug right channel in, no pop or extra static.
Voltage drop across power supply resistors rises considerably.
Voltage at third filter cap measures only 75V.
Voltage at output caps measures 40VDC for right channel, 0VDC for left channel.
Sound only plays out of right channel.
Resistance from Left RCA in to Right RCA in measures between 169k and 185k, depending on position of volume knob.
RCA jackets measure continuous (common ground at potentiometer).
Touching RCA pin to either jacket of the RCA inputs or actually plugging it into the right channel input results in sound in right channel. I can't find a way to get sound out of the left channel; plugging a source into the left channel results in nothing. With no source plugged in, voltage at output caps is measured in both channels.
From that, can you draw any conclusions about if there's a short somewhere in the wiring? It has to be on input side of everything; I don't know how it could be anywhere else. -
Maybe double check your soldered cap polarities. Check resistors. Not much else to say but could double check that you have soldered wires to the right connection solder pads?
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Maybe double check your soldered cap polarities. Check resistors. Not much else to say but could double check that you have soldered wires to the right connection solder pads?
The only electrolytics I swapped were the 220uF/250V ones. I already rechecked all of those. Resistors are all fine by inspection and measure correctly. I can't find anything obviously wrong on the coupling cap PCB except. I was thinking it was something on the potentiometer, but I dunno. -
I was just thinking, obviously there is something wrong with there being a measured resistance between the two channels. I tested the old potentiometer, and no such continuity existed between the signal inputs even though they shared a common ground. Here is a picture of the old potentiometer and PCB, all labeled:
For example, I measure 85kOhm between Channel A input and Channel A ground on the old potentiometer, but there is no measured continuity from Channel A input to Channel B ground, even though Channel A ground and Channel B ground (and indeed, everything on the PCB labeled ground) share a common ground plane.
So, what did I mess up on the wiring on the Noble Potentiometer I put in its place? I mentioned it above, but just to reiterate:blairfrischx wrote: »The one thing I'm not sure about in my wiring is on the potentiometer. It is a 4-pin per channel one; the ones that aren't used are loudness taps. After that I referenced this (link). Pins 2 and 6 are left in and right in, respectively. Pins 3 and 7 are left out and right out, respectively. Now, I have the jacket of the RCA plugs connected to pins 4 and 8 (ground), but their left/right orientation is swapped (i.e. right channel is connected to pin 8, left to pin 4). I did that out of convenience for how long the wires were and figured it wouldn't matter, since they share a common ground point; was I wrong in that assumption?
So standard pinout for these pots is
1: Channel A loudness tap (not used)
2: Channel A input
3: Channel A output
4: Channel A ground
5: Channel B loudness tap (not used)
6: Channel B input
7: Channel B output
8: Channel B ground
What I currently have is:
1: Not used
2: Right channel input
3: Right channel output
4: Left channel ground
5: Not used
6: Left channel input
7: Left channel output
8: Right channel ground
I need to find a way to switch pins 4 and 8 - again, I had them that way out of convenience because I snipped one wire too short - but I can't possibly see how that would change anything. I can't see why the two potentiometers should behave differently with a similar continuity between their ground pins; the old one had a shared ground plane on the PCB, the new one has a shared ground point identical to how the ground plane of the old PCB was connected.
I should probably be sleeping. I have a final in just under four hours. -
This is long overdue for an update. However, there isn't much progress to announce.
To the best of my ability, I can only elicit that somewhere inside the amp there is a short between left and right channel. This could explain the high rise in current draw when a source is plugged in and the sound only ever coming from the right channel.
For the moment, I will be using a different headphone amp (PS Audio GCHA), but I really want to figure this one out. I love the sound of tubes on headphones too much to let it die. That, and there are $175+ in parts sitting in this amp.
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Bummer on the effective lack of progress, but keep on working on it!
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Checking in again, any further updates?
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Checking in again, any further updates?
No, I've had surprisingly little time to work on it this summer between my job and my family moving. It's just been sitting in my closet for the past two months. :mad:
I haven't even had time to swap parts on my Focal 807V crossovers yet, and that is a simpler, more accessible project. It's just a matter of time until I have a weekend to sit down and break out the soldering iron again!
