Quantum QM 128 faders

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creaturesleeper
gimme a little kick & snare
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Quantum QM 128 faders

Post by creaturesleeper » Wed Jun 25, 2014 4:16 pm

I've been slowly working on this board. It has the optional patchbay and a fucking mess of wires...I think it was attached to a 4 track at one point anyway I seperated it from all that and just tryed to get it to pass a signal non of the VU's registered anything..Im using the osc.tone from my otari into channel one. I opened it up and tested voltage on most of the chips and they seem good so I doused all pots switches and faders with Crc and got all but two channels to register. I know at least one of the faders is bad...it doesnt go down all the way. I'd like to replace them all...it sat in a garage for years. What are some options for this...quality wise? It has Jensen input trannys on a side note..nice to find in there :-)

Jim Williams
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Post by Jim Williams » Thu Jun 26, 2014 8:35 am

I've restored/modified a few of those. The faders can be replaced with 60 mm P+G, but they aren't as long and you will need to drill an extra hole or fasten one of the screws to the fader slot.

Every 128 I've worked on had the original power supply scrapped. It's not built well and the transformer causes hum leakage. I used an externally mounted Power One HDD-15-5AG 5 amp linear supply and added a small 48 volt supply. I used a 5 pin XLR for the supply cable assembly. The -15 switch was changed to a phantom power switch. I added an extra hole and mounted a reverse log 10k pot to create a variable +20 to +60 db mic preamp gain pot. Add some good IC's, remove the electrolytic coupling caps and it starts to sound really good. Warm white LED's replaced the meter bulbs.

One of those modified Q-128's sits in Hyde street Studios in San Francisco.
Jim Williams
Audio Upgrades

creaturesleeper
gimme a little kick & snare
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Joined: Wed Oct 21, 2009 8:56 pm

Post by creaturesleeper » Thu Jun 26, 2014 9:40 am

I was wondering if the power supply is any good...I have the manual with full schematics and voltages seem low. What did you replace the IC's with? I read about a few but you have you have to disconnect the buffering around the originals and strap a cap across them? I'd like to learn more and do this myself. Any help would be awesome!

Jim Williams
tinnitus
Posts: 1135
Joined: Sat Jun 03, 2006 8:19 am
Location: beautiful Carlsbad, CA
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Post by Jim Williams » Thu Jun 26, 2014 11:34 am

I used National LME 49720's. Those draw far too much current for the whimpy stock supply.

Some gain lowering of the output opamps was also done, the discrete transistors were changed to On Semi 172/182 pairs and the biasing diodes were relocated across the bases. Yes, this is very technical rework, not for the DIY crowd unless you have those chops.
Jim Williams
Audio Upgrades

creaturesleeper
gimme a little kick & snare
Posts: 83
Joined: Wed Oct 21, 2009 8:56 pm

Post by creaturesleeper » Thu Jun 26, 2014 5:14 pm

I have built some tube amps pedals and mics and recently fixed my Otari capstan motor...not chip stuff but but I think I'm capable and it would be a valuable learning experience. I understand if you dont want to but if I sent you schematics would you bewilling to pen in the modifications? I could send some cash your way. Thanks much for the above info :-)

creaturesleeper
gimme a little kick & snare
Posts: 83
Joined: Wed Oct 21, 2009 8:56 pm

Post by creaturesleeper » Thu Jun 26, 2014 11:09 pm

This one already has an external supply but I suspect its not great

creaturesleeper
gimme a little kick & snare
Posts: 83
Joined: Wed Oct 21, 2009 8:56 pm

Post by creaturesleeper » Sun Sep 13, 2015 12:47 pm

I am getting back to this board after focusing on tube stuff for quite awhile. I pulled one channel strip to try to get it going good and if I can do that will probably do em all. The power supply is good for now. None of the channels pass a signal. On this one isolated channel its obvious the LM318 is no good. Its the first Opamp after the Input transformer. I ordered the modern day replacements for all the chips on the channel...I'll post back here when I get it going and decide if I want to tackle the stuff Jim Williams mentions above...the channel strips are very simple except for the buffering. Obviously I would upgrade the power supply for those higher current chips

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