800B VU Meter LED Conversion

Founded in 1973, Soundcraft first made its mark with the launch of the Series 1, the first mixing console in a flightcase.

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800B VU Meter LED Conversion

Postby badbrad » Tue Nov 15, 2011 2:02 pm

So I decided to convert the bulbs in my 800B's VU meters to LEDs based on all these forum topics I've been reading.

I wanted to size the resistors so that the correct current will flow through the LEDs. I have no idea why I am so particular about stuff like this. I read a post by Mr. Williams that said to use 1/2Watt, 1kOhm resistors in line before each LED and so this was my starting point and confirmation that I was on the right track since I really trust his advice.

I measured the voltage supply to the master left/right VU meters to be 33Vdc on my console. That doesn't seem right but I do not have schematics for that part of the console to check this and the Fluke doesn't lie (usually). I took some continuity measurements around the VU meter bulb terminals and wires and figured out the circuit from there. It appears that the 2 bulbs in each meter are wired in parallel but the meters are wired in series to each other? I may have screwed up right there because that doesn't seem right either so I assumed a new circuit which makes more sense from a design point of view. Anyone with more knowledge of this please feel free to chime in (Gareth??).

Anyway, this is how nerdy I can be! All I wanted to do was size the resistors appropriately and make sure to buy beefy enough ones that can handle the heat dissipation. Check out my nerdy DC circuit analysis below. It could be very wrong for all I know so don't take this as the word of the Soundcraft lord...for I am not.

800B VU Meter DC Ckt Analysis.JPG
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Re: 800B VU Meter LED Conversion

Postby GJC Designs » Tue Nov 15, 2011 5:02 pm

I'll take a closer look in the morning.........

A couple of things to consider are the efficiency of the LEDs. Some will require a lot of current (20mA to give a decent light output), whilst other much more efficient LEDs will only require 1mA or 2mA. Until you have selected your LEDs for colour and brightness and tested them, the major number crunching is all theory.

33V for the illumination supply is about right if the supply is across the audio rails. I'll need to check this when I get in to the office tomorrow.

For a starting guide, if you base your initial LED drive on 5mA and 33V, the resistor to start with is 6600-ohms... 6K8 is the closest standard value. Power rating will be 0.165 watts...... a standard 0.25 watt or 0.5 watt resistor will be OK. The latter will not get as warm.

I'll return tomorrow.
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Re: 800B VU Meter LED Conversion

Postby badbrad » Tue Nov 15, 2011 5:05 pm

I really hope someone who knows their electronics chimes in on this because I would like to know if my thinking here is right. I basically used this little project as an excuse to refresh my memory on basic DC circuit analysis (KCL & KVL) but in reality I am a mechanical engineer and probably should not be getting to far ahead of myself before I modify my board...so I don't set fire to my house!

This posted right as Gareth's post showed up. I know this is a simple topic but I am really excited to be on the right track!
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Re: 800B VU Meter LED Conversion

Postby Jim Williams » Wed Nov 16, 2011 12:07 pm

I used the incandescent colored white LED's from:

www.superbriteleds.com

Sand them to difuse the light. I bent them down to shine down to the meter face. I used 1k 1/2 watt resistors. Works like a champ as the color matches the olde time look but the light is very even without hot spots that happen with the bulbs.

To review, replace the 100 ohm resistors with 1k, 1/2 watt. Observe polarity. Sand leds, bend them down. Re-assemble and enjoy, you won't ever need to do this again.
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Re: 800B VU Meter LED Conversion

Postby badbrad » Thu Nov 17, 2011 2:15 pm

Jim, I couldn't find anything on their site that specifically said "incandescent colored white LED" but I found these:

http://www.superbrightleds.com/moreinfo/component-leds/5mm-warm-white-led-35-degree-viewing-angle-7000-mcd/264/1191/

Are these the same ones you used?
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Re: 800B VU Meter LED Conversion

Postby Jim Williams » Thu Nov 17, 2011 6:59 pm

Those are it. If they are still too brite for you, use 2.2k ohms.
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Re: 800B VU Meter LED Conversion

Postby qdkj21 » Fri Dec 02, 2011 11:46 pm

I know this is a popular conversion and after spending two hours replacing four VU Meter bulbs on my A&H System 8 yesterday I'd love to convert it to LED's sometime.

As I understand it, this is not as simple as it should be because you have to pick the correct color LED (apparently white is TOO white), color temp and then make sure the voltage and resistance issues are taken care of.

I've got a service manual for the desk, but it just tells you how to replace the bulbs not any of the specs on them other than an A&H part number. A&H tech support did tell me the bulbs are 8 volt "wire ended" bulbs.

I'd love to hear from people that did this conversion and what you did, I realize I probably wont get specific details regarding conversion of a System 8 but I'd still like to hear from owners of other old VU meter equipped consoles on how their conversions went.
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Re: 800B VU Meter LED Conversion

Postby JohnGalt48 » Sat Jun 13, 2015 3:56 pm

Hi,
New member here who has just acquired a fairly nice 1600...and right off the bat, digs up an old thread!
Of course, all the meter lamps are dead on my board, so I'm wondering if the information on the procedures and the LED replacements mentioned in this thread also apply to the 1600 consoles. I checked the Soundcraft documentation, comparing the 800b manual with the 1600 manual, and the description of meter lamp replacement looks identical.
I just wanted to confirm that the 800b used the same meters as the 1600 before I start ordering parts.
thanks
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Re: 800B VU Meter LED Conversion

Postby GJC Designs » Sun Jun 14, 2015 2:32 am

Hi John,

Welcome to Analog Recording Console Forum.

The 1600 can be described as an 800B with more facilities, so yes, the meter bulb replacement procedure is the same.

Cheers,
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Re: 800B VU Meter LED Conversion

Postby JohnGalt48 » Sun Jun 14, 2015 9:16 am

Thanks, Gareth. Now, I'll get right on ordering those LEDs!
I'm assuming the resistors get installed in series with each individual LED directly on the front of the meters?
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