M70 LED meter mod

[caution]

Having it follow the system rail makes more sense, I think I'll give that a shot before I tear it apart.

I set up the op amps the same way the guy explained in his project notes, so I guess I was under the impression I was doing what you are supposed to in order to run an op amp in a single rail configuration. So you're saying I'm actually tricking the op amp into a dual supply setup with only rail, and that there is another way to run in true single rail mode? How would that work? It's a full wave detector, so it makes sense to me that it would need a virtual ground in order to see the positive and negative peaks in the signal.

 

[Superduper]

I have not given this much thought as you have. My questions were merely out of curiosity as to why you chose to rig this circuit up this way.

Forget the voltage divider thing. I was just thinking out loud and upon further consideration, since the current load of your circuit is unknown, it may be unpractical since a voltage divider is very load dependant and a shifting load could shift the ratio. The shift of course could be reduced if the divider powers the base of a driver transistor instead. I only mentioned it since looking at that circuit, 1. without the output being driven by a transistor, and 2. with the load going through a 10k resistor it didn't appear as though that rail needed much current. The most that this rail could source anyhow would've been 1.2mA through that resistor before loading down the circuit. In any event, a proper zener would be more precise and 6-v zeners are available, just not as common, and not available in all watt sizes.

As for your comment that you omitted the 10k resistor, not sure what you meant. Was it left out and that leg of the circuit open, or was it closed? I am presuming that you left it out, and that leg of the circuit open, otherwise, the zener is probably toast if that circuit was closed. Without the resistor to limit current flowing to the zener, it would have to shunt all the power from your LM7912 to ground once that diode reaches zener voltage.

Oh, and one more note.... although your meter is powered by the regulator, the boombox might not be. You might want to study the M70 circuit a bit more. System voltage on the M70 powered under AC is probably going to be more than 15V, maybe as much as 18V and it's an unregulated power supply. Will your signal input represent that and will it make a difference? Just food for thought.

Lastly, maybe I missed it but why did you want to add the peak detector circuit again? Didn't the 3916 alone work properly as a VU for you? To my understanding, the LM3916 does not require rectifiction like an analog meter would and works properly with AC supplied directly to pin 5.

 

[caution]

I'll toss the zener back in there. I'm thinking the load on this virtual ground is low enough not to matter, it's just driving a bunch of noninverting inputs in the 3916 and in my op amps. I'd like to come up with something that uses parts that are easily available. I scavenged the 5.6V zener from a dead box and surprisingly was closer to 6V than a couple dozen different values in a box at the surplus gizmo store nearby.

That is correct, R12 is gone, no connection to +12V. All I can figure is that it was somehow going through one of the ICs and developing a path to power there. It was reading about 5.2V, with the 10K resistor, it was about 5.4V. I was using a 750 ohm resistor because it was closest to the 720 ohm load the project I followed used, but that Analog link had values closer to 10K. When I tried that and it still worked, I figured that could be safer than only 750 ohms.

Adding protection on the line level inputs is not something I considered, let alone know what I'd need. I'm already using DC isolation capacitors right at the inputs. If it's just about accuracy during a transient incident and doesn't have a chance at damaging the circuit, maybe I can save from adding more parts? I'm not sure how risky it is without protection.

As for the peak detectors, yeah, the 3916s just display raw audio in real time, so you end up getting a real strobelight effect that doesn't keep the meters as bright and looks annoying. Most homebrew meter videos on YouTube look like this. Besides, the datasheet suggests it, and it's cheap to add Looks almost just like a couple of my boxes here when I compare them, so I'm happy.

 

[caution]

Here's a video of the difference between the peak detector output without a capacitor vs. with, Also, the chip is also capable of being strung together for longer meters, and dot mode too, which looks pretty flashy too!

http://youtu.be/6pzDhZAxdus

 

[Cpl-Chronic]

Looks like you are eliminating the obstacles, 1 by 1 & I admire your skills. My 1 question again is how will you meld your custom LED system to the music search leds or are you going in a different direction? My original assumption about the project was you wanted to make the RED original LEDs dual purpose but you indicated a different technique & I couldn't quite follow your previous explanation.

 

[caution]

Yeah, you were right about not being able to tap into the circuitry already there, so I tried to add a really small LED next to each of them but as I was doing that I found that if I inserted the LEDs I got with the meter kit into the original holes and let them sit against the board, they fit perfectly behind the original LEDs after bending their leads up out of the way. So, When you try using MMS you'll get yellow light through the red ones, sort of an orange color. I had to grind down the two outer ones a bit though.

The annoying part is that normally the MMS LEDs are just barely stuck through the PCB (you can barely see the metal tip sticking through the other side) and soldered to the same side of the board that the pads are on. They sit about a half inch off the board so soldering them at the factory would not have been an issue, but since my new ones sit flush against these pads, I had to reconnect everything on the other side where all the components are.

 

[Retroresto]

Kick Ass project! I'm buying one if you ever get it to production. I think it would really dress up my box.

 

[SLO]

Wow your guys technical jargon is overwhelming, but quite impressive. I have absolutely no idea what caution, Superduper and cpl chronic are talking about, but it sounds very tech savvy. Good work, I think?

 

[Cpl-Chronic]

Caution & SD are beyond my skill set but I do have the basics down & my roadie experience pays off in the fabbing & modding areas...

Cpl

 

[SLO]

Caution & SD are beyond my skill set but I do have the basics down & my roadie experience pays off in the fabbing & modding areas...

Cpl

You get down on those hi tech led mods though, caution and SD speak in some electronic hi-tech foreign language I've never heard before. They're in a whole other league no doubt. I gotta respect their knowledge and know how

 

[caution]

Wow thanks guys I really appreciate it, can't wait to do another mod. Gettin' close!!

 

[caution]

Decided to go with standard jumper headers for all connectors, and tacked on a couple of male headers on the back of the main board. I'll check some connections with a meter tomorrow and see if it works with some LEDs on a breadboard.

 

[Cpl-Chronic]

Sweet idea with the header connetors. I use 3 & 4 pin fan extension cables for that purpose. You can probably slide the headers right onto the LED pins as quick connects.....


Cpl

 

[caution]

Yeah, I had some other ideas but being one piece and detachable were the most important. I had all wires plugged in as I added the power rail and zip ties it so they'd be spaced correctly.

I only had a couple of missing solder joints and a tiny bridge to ground, fixed those and it worked on first powerup.

https://www.youtube.com/watch?v=nVThC0Wa0LM

 

[caution]

I let it run for a while to see how hot it would get, and it gets uncomfortabe if all 11 LEDs are on full for an extended period of time, even pulling out one LED cools them down, and adding 1K series resistors to each LED on one side for comparison was a little bit helpful. The current readings are about right, 10mA each and about 4mA on the center LED. Depending on which takes less time, I may still make it so you can turn them off, or redesign it to run cooler. It's sandwiched in there good so I can't let it get too hot.

Anyway, good enough for now, even got the LED kit installed, although I had asked for white, I guess I could live with blue, I just figured it would be too much color. What do you guys think?

https://www.youtube.com/watch?v=u-eZX7G6k0I

 

[Superduper]

Oops, I had more than one order and I must've got confused if I made it wrong. Sorry about that bro. However, to be completely truthful, blue is by far, the much more popular color. Blue also carries better across the dial than the white which tends to be brighter at the ends and dims as the light carries across. I think others will agree that the blue is a kick-A color. However, if you decide you prefer white, PM me and we will work it out.

BTW, the meter mod turned out great! Got to do that to mine now, haha.

 

[blu_fuz]

Wow this turned out great! Nice job. I never thought my M70 would fall into a future members hands and had so much cool added to it!

 

[Superduper]

Joe, it was yours? Imagine what you could've done with it, hahahaha.

 

[Cpl-Chronic]

So, if I get the gist of this right, you basically bonded the Yellow LEDs to the original RED LEDs of the search system so that the light from the yellow ones shine through the RED plastic of the originals? I was thinking along the same lines so that you could have dual purpose while keeping the circuits separate from each other.....& the look is still stock original.....


BAM!!! That is DOPE SON!!!

Cpl

 

[blu_fuz]

Joe, it was yours? Imagine what you could've done with it, hahahaha.

Yes! Lol