Quantum Audio Labs QM-8A Restoration - First Step?

[fazer]

Those on the channel cards look like 100uf to me. Are the 1000/2000 on the power supply?

Edit: I see there on the power supply. You can put your 1000 in and check ripple. The stabilizer circuit probably takes care of the ripple. You can always put extra 1000 ufd on top in parallel if need be. Like others say just replace all of the electrolytics

 

[velo]

Looks like some mods. Nice WIMA. Unclean rosin but it helps with the archeological dig.

 

[velo]

Call me a novice but does this look right? This is how I received it except for the new electrolytic capacitors.

From what I remember of AC->DC power supply there's a diode bridge rectifier there. Yellow is coming from the transformer and red (common) from transformer center tap.

Silver wire looks like it is soldered to the same board trace as the transformer center tap so silver is also common.

White wire is soldered to a point in the diode bridge. That puzzles me given that it seems to bypass the rest of the circuit.

I read 0V across the silver/white wires leaving the power supply circuit. Did someone wire to the bridge to bypass a broken circuit and then get it wrong? Or did they bypass the step down to 16V to use 25V?

I don't get it.

EDIT: See next post.

 

[velo]

I can see a red/purple/black wiring headed to a 4-pin DIN out the back. This appears to branch to at least the VU board with the buss output op amps. So this looks like a mod to use external PSU.

But what is this thing directly tapping the transformer? It looks like it is soldered to the rectifier but I've confirmed that it is soldered to the input coming from the transformer.

At least now I can see a somewhat consistent wire coloring scheme for the voltage rails:

Red = +16V
Purple = -16V
Black = Ground

No wonder this thing wasn't functioning when I plugged it in.

 

[velo]

Found another problem--broken pin on the on-board MPS-U01 transistor.

So I tore open an old MOTU896 I had lying around, hoping to find 16V rails, and holy smokes, I got lucky and I got sound!

Frankenstein's monster is alive! Muah ha ha haa.

The first channel/bus combo I tested sounded rather crunchy but respectable low-end, especially torquing the EQ. This is good. The first application is to mix two buses of three kick drum mics each.

Bus 2 VU meter pops to 100% and stays there. I've not replaced caps yet.

All 50 op amps are mounted in sockets. Thank you QAL.

 

[velo]

I fixed a busted VU meter and pulled out the entire power supply transformer, board, and cabling thinking I would put four output stage transformers in that space instead among various experiments with discrete op-amps.

I'm going to replace the unobtainable AC lamps with some LEDs and a wall wort.

With the LEDs lighting the VU meters I'm thinking

RADIO SHACK COLOR LIGHT ORGAN EFFECT !!!​

Are you picking up what I'm laying down here? ;-)

 

[disjectia]

Catching back up to all this, nice work on the PSU - glad you've got sound!

Yes, the socketed op amps make life significantly easier. I rebuilt all the mic pre circuits as 312s, and replaced the LM318s in that position with discretes (various 2520s) in all but 2 channels (waiting for some new op amp kit options from CAPI). There's three LM301s per channel card; the first is a buffer (I believe) which comes after the post mic-pre/pre fader patch point (I see in yours there's a jumper on the rear bypassing the patch point). The first EQ op amp in mine is an MC1709CP, followed by another LM301 (I think those WIMA caps on yours are modifying the EQ circuit) Then there's one more LM301 which serves some function I never quite figured out in driving the 4 busses and the two echo sends; that op amp position serves no function in channel 7 and 8 as far as I can tell.

I also rebuilt the output buss card as API 325 boosters with 1731 op amps driving four 2503 xformers (for each buss output). Kinda hard to capture in pics, but those are mounted to the top of the case in mine, part of the reason I built a wood enclosure to give myself an access panel on the bottom and a bit more room to add xformers etc, everything is socketed with molex connectors.

The ceramic caps throghout seem to be somewhat temperature unstable; I replaced all of them. Terry added unbalanced direct outs for each channel, I can show you where to make that connection on the channel cards if you want.

Pretty sure those VU bulbs are still available and they sure cast a nice glow.

 

[velo]

Catching back up to all this, nice work on the PSU - glad you've got sound!

Yes, the socketed op amps make life significantly easier. I rebuilt all the mic pre circuits as 312s, and replaced the LM318s in that position with discretes (various 2520s) in all but 2 channels (waiting for some new op amp kit options from CAPI). There's three LM301s per channel card; the first is a buffer (I believe) which comes after the post mic-pre/pre fader patch point (I see in yours there's a jumper on the rear bypassing the patch point). The first EQ op amp in mine is an MC1709CP, followed by another LM301 (I think those WIMA caps on yours are modifying the EQ circuit) Then there's one more LM301 which serves some function I never quite figured out in driving the 4 busses and the two echo sends; that op amp position serves no function in channel 7 and 8 as far as I can tell.

I also rebuilt the output buss card as API 325 boosters with 1731 op amps driving four 2503 xformers (for each buss output). Kinda hard to capture in pics, but those are mounted to the top of the case in mine, part of the reason I built a wood enclosure to give myself an access panel on the bottom and a bit more room to add xformers etc, everything is socketed with molex connectors.

The ceramic caps throghout seem to be somewhat temperature unstable; I replaced all of them. Terry added unbalanced direct outs for each channel, I can show you where to make that connection on the channel cards if you want.

Pretty sure those VU bulbs are still available and they sure cast a nice glow.

Yeah, a URL posted here led me to a site for the bulbs. I placed the order and they never showed and there's no response from them. Gotta look to see if I was charged although I can see a few other suppliers out there.

 

[velo]

Catching back up to all this, nice work on the PSU - glad you've got sound!

Yes, the socketed op amps make life significantly easier. I rebuilt all the mic pre circuits as 312s, and replaced the LM318s in that position with discretes (various 2520s) in all but 2 channels (waiting for some new op amp kit options from CAPI). There's three LM301s per channel card; the first is a buffer (I believe) which comes after the post mic-pre/pre fader patch point (I see in yours there's a jumper on the rear bypassing the patch point). The first EQ op amp in mine is an MC1709CP, followed by another LM301 (I think those WIMA caps on yours are modifying the EQ circuit) Then there's one more LM301 which serves some function I never quite figured out in driving the 4 busses and the two echo sends; that op amp position serves no function in channel 7 and 8 as far as I can tell.

I also rebuilt the output buss card as API 325 boosters with 1731 op amps driving four 2503 xformers (for each buss output). Kinda hard to capture in pics, but those are mounted to the top of the case in mine, part of the reason I built a wood enclosure to give myself an access panel on the bottom and a bit more room to add xformers etc, everything is socketed with molex connectors.

The ceramic caps throghout seem to be somewhat temperature unstable; I replaced all of them. Terry added unbalanced direct outs for each channel, I can show you where to make that connection on the channel cards if you want.

Pretty sure those VU bulbs are still available and they sure cast a nice glow.

What'd you do with the old 75101s? Were you not happy with them or just knew what you wanted a priori?

What is the purpose of repurposing the phones mix pots into mic gain pots? Is that only possible or necessary with the mic pres' CAPI 2520/EA2622 mods? I guess it makes sense to have a mic input gain control! ...like almost everything else in the world.

 

[velo]

"output bus as API 325 booster with 1731 op amp driving a 2503 transformer"

This is what I'd like to do with my output bus. I think it fits where I am in my education. I'll leave the 75101s there for now so I can fully appreciate what I'd do next.

Are there any 1010 circuit board or other circuit board mods necessary to drive the transformers? I think I see extra caps in your photos on that board near the op amps.

 

[disjectia]

Ha, I'm sorry about that bulb link - I ordered some extra bulbs from that site and am in the same boat, but I'm seeing some other options on eBay etc, I''ll let you know if I find reliable source.

I should mention that I'm attempting to use my QM8A as a mini console in an old fashioned sense for a (basically) analog setup, hence the extent to which I've rebuilt the circuitry/routing; depending how you imagine using yours, you may not need/want to go as all out. As a caveat to that, I will say I'm happier with the sound now than I ever was by JUST swapping in op amps (though with the amount of time/money I've poured into it there's a strong psychological incentive to feel it's better now!)

I really just wanted to try the 2622 input xformers based on CAPI descriptions of their sound; they have slightly higher turns ratio than the 75101s (1:7 vs 1:5 wired in parallel as they are in this circuit) so I guess the theory is they hit the op amp a little harder and contribute to some magic? They're not cheap so I only did channels 4-8 and left the 75101s in 1-4; I still need to do some blind-listening test comparisons to see if I can really hear a difference when gain matched, but my highly unscientific opinion so far is that channels 7/8 - which have 2622s input xformers and CA-0252 op amps - sound the best (most vintage/sweet midrangey) to me. I have the removed 75101s, still deciding whether to sell them.

I wasn't using any of the monitoring circuit, so after much deliberation decided to just tear it all out and repurpose those pots as mic gain (though actually I replaced the pots themselves); I've found having variable gain on the mic pres to be an asset, but I like things to live right on the edge of distortion. The QM8A was designed for broadcast use, so setting the sweet spot of the gain and fixing it (with an additional -15dB pad option) made sense but having used it that way for years I definitely find it much more useful in a musical context to have control over the gain (you should be able to do this even without fully rebuilding the mic pres, but it will take some circuit modification - more on that in you're interested).

I repurposed the -15db pad switch as an EQ in/out switch which is also very helpful, so I can toggle the EQ and quickly decide if changes I've made are better or worse.

I'll find some time in the next few days to post my documentation about the existing circuit as well as my mods; I'm not a trained technician and schematics can still sometimes take my brain a minute to digest, so my approach was a bit goofy but it got the job done and should be helpful. It'll be interesting too to compare values of resistors/caps etc to what's in yours, I already see some differences that make me think yours might be a slightly later edition.

 

[velo]

Ha, I'm sorry about that bulb link - I ordered some extra bulbs from that site and am in the same boat, but I'm seeing some other options on eBay etc, I''ll let you know if I find reliable source.

I should mention that I'm attempting to use my QM8A as a mini console in an old fashioned sense for a (basically) analog setup, hence the extent to which I've rebuilt the circuitry/routing; depending how you imagine using yours, you may not need/want to go as all out. As a caveat to that, I will say I'm happier with the sound now than I ever was by JUST swapping in op amps (though with the amount of time/money I've poured into it there's a strong psychological incentive to feel it's better now!)

I really just wanted to try the 2622 input xformers based on CAPI descriptions of their sound; they have slightly higher turns ratio than the 75101s (1:7 vs 1:5 wired in parallel as they are in this circuit) so I guess the theory is they hit the op amp a little harder and contribute to some magic? They're not cheap so I only did channels 4-8 and left the 75101s in 1-4; I still need to do some blind-listening test comparisons to see if I can really hear a difference when gain matched, but my highly unscientific opinion so far is that channels 7/8 - which have 2622s input xformers and CA-0252 op amps - sound the best (most vintage/sweet midrangey) to me. I have the removed 75101s, still deciding whether to sell them.

I wasn't using any of the monitoring circuit, so after much deliberation decided to just tear it all out and repurpose those pots as mic gain (though actually I replaced the pots themselves); I've found having variable gain on the mic pres to be an asset, but I like things to live right on the edge of distortion. The QM8A was designed for broadcast use, so setting the sweet spot of the gain and fixing it (with an additional -15dB pad option) made sense but having used it that way for years I definitely find it much more useful in a musical context to have control over the gain (you should be able to do this even without fully rebuilding the mic pres, but it will take some circuit modification - more on that in you're interested).

I repurposed the -15db pad switch as an EQ in/out switch which is also very helpful, so I can toggle the EQ and quickly decide if changes I've made are better or worse.

I'll find some time in the next few days to post my documentation about the existing circuit as well as my mods; I'm not a trained technician and schematics can still sometimes take my brain a minute to digest, so my approach was a bit goofy but it got the job done and should be helpful. It'll be interesting too to compare values of resistors/caps etc to what's in yours, I already see some differences that make me think yours might be a slightly later edition.

As for my intentions, I'm just trying to kick ass in all matters great and small.

Translation: The more I can chop and customize and drive things into distortion and back again, the better and I cannot imagine a world without iron given that I was born before we walked on the moon.

 

[velo]

Probably at this point, with the caps replaced and power supply circuit problems identified and substituted, this post is toast. Mission accomplished. This creature from the 70s still works. From this point forward I guess I'd be talking mods and start new threads if I can contribute anything.

 

[Jelly]

Interesting - the internal power supply had already been removed from mine by the time I got it.

I had a tech named Terry Wusnack work on it a few years back; he's in LA (where I live) and is a former QAL employee (happy to pass his contact if you want). He gave me an external, +/- 18V DC switching power supply - pic attached, these are pretty cheap - and ran the meter lamps off a separate 28V AC power supply. The bulbs are Chicago Miniature CM334 GE334.

I honestly might just recommend following suit and moving to external supplies - the one Terry gave me works fine (although I did eventually build myself a linear one) - I remember him also saying that using external PSU helps reduce noise.

Attaching manual here if you don't already have it; it's the one circulating on web, certain schematics are definitely not correct for this model but at least portions of them are accurate - not sure about PSU as I never had reason to check that.

I can eventually get you accurate channel card/buss schematics, though they'll be hand drawn

I repaired a QM-168 seven or so years back that I still use today. I remember finding a guy with a small stash of 80mm P&G linear faders for a ridiculously cheap price that I slugged in mine to work like log faders. Terry asked me for the lead on the faders in exchange for some replacement custom pots. I gave him the the guys contact and Terry ghosted me:/

 

[Jelly]

The QM-168 I have is probably a bit different(I think the xmfrs are cheaper than yours for one), but I remember needing to add a buffer to the meter drivers to cut down on distortion. I also swapped a few of the LM301s(not all) to bring the noise down along with adding line input transformers from a 8 channel to balance the inputs. The faders felt horrible on mine, and they're 80mm(yay), along with several corroded pots, but it was literally given to me from a guy the was keeping it in his chicken coop, so it wasn't stored very well...I also added a phase and phantom power switch to each channel(the phantom was global)along with some padauk trim and a leather armrest to add some vibe.

 

[velo]

The QM-168 I have is probably a bit different(I think the xmfrs are cheaper than yours for one), but I remember needing to add a buffer to the meter drivers to cut down on distortion. I also swapped a few of the LM301s(not all) to bring the noise down along with adding line input transformers from a 8 channel to balance the inputs. The faders felt horrible on mine, and they're 80mm(yay), along with several corroded pots, but it was literally given to me from a guy the was keeping it in his chicken coop, so it wasn't stored very well...I also added a phase and phantom power switch to each channel(the phantom was global)along with some padauk trim and a leather armrest to add some vibe.View attachment 122372View attachment 122373

Nice job. The stock PSU there looks beefier than mine was. Larger reservoir caps and more rugged heat sinks.

I see the mic input transformers not on the channel strip boards?

 

[disjectia]

Holy shit, that thing's amazing - our choice of wood trim is very similar, looks great. If you ever wanna sell it let me know ; )

My faders also felt terrible, that's actually part of what lead me to the full teardown - wish I had a P&G hookup, but as you know 80mm replacements are impossible to find. The stock faders in mine are Duncan Slideline 10Ks and fortunately I found they're actually fairly easy to take apart and clean/relubricate; with a healthy amount of fader grease they have pretty decent feel now and no scratchiness etc.

I think the channel cards are slightly different, QM168 has more EQ options, but I'm curious which LM301s you replaced and what with?

The independent echo sends in that model also nice, I was able to modify mine with stacked pots to accomplish the same

 

[Jelly]

Nice job. The stock PSU there looks beefier than mine was. Larger reservoir caps and more rugged heat sinks.

I see the mic input transformers not on the channel strip boards?

Those are the line input transformers I added from an 8 channel quantum mixer. I had extras so I balanced the echo send and return for my EMT 140(which I added balanced outs to) along with the 8 bus outs.

 

[Jelly]

Holy shit, that thing's amazing - our choice of wood trim is very similar, looks great. If you ever wanna sell it let me know ; )

My faders also felt terrible, that's actually part of what lead me to the full teardown - wish I had a P&G hookup, but as you know 80mm replacements are impossible to find. The stock faders in mine are Duncan Slideline 10Ks and fortunately I found they're actually fairly easy to take apart and clean/relubricate; with a healthy amount of fader grease they have pretty decent feel now and no scratchiness etc.

I think the channel cards are slightly different, QM168 has more EQ options, but I'm curious which LM301s you replaced and what with?

The independent echo sends in that model also nice, I was able to modify mine with stacked pots to accomplish the same

Thanks! We do have similar taste. I'll let you know if I decide to sell it, but I put so much work into it I doubt I ever will. The faders in mine I think were originally slideline as well, but they were completely plastic and looked different than yours. It's good you could repair yours. The 168 also doesn't have inductor eqs, so I didn't end up replacing all the damaged pots for the eqs to cut back cost on the repair(half of them work). I run a hybrid setup and tend to do most of my eqing and monitoring ITB. The master section is also not working as is, but again I use it as a hybrid setup so I haven't needed/wanted it...until now. I bought a Otari MX-70 1" 8-track that I refurbished to pair up with this console in which I would definitely want the eqs and monitor section completely working(the MX-70 is also white in case you don't know). Unfortunately I had to sell the MX-70 to pay for a new transmission. But I still plan on getting another machine in the future so I'll probably open it up sometime soon and finish the restoration.

As for the op amps, I'm pretty sure I just used 5534s everywhere except I left the LM301s in the 8 bus outs(which are located on the lower edge of the middle 8 channel strips), which on this desk are the pots located under the meter with no individual stereo bus. Now I'm remembering this console was fixed gain as well, and I added mic trims to each channel–an odd design for sure, but this is my idea of fun and now it serves my purposes.

Here's the before picture just for fun. The crap you see on top is chicken poo, ha!

 

[velo]

Thanks! We do have similar taste. I'll let you know if I decide to sell it, but I put so much work into it I doubt I ever will. The faders in mine I think were originally slideline as well, but they were completely plastic and looked different than yours. It's good you could repair yours. The 168 also doesn't have inductor eqs, so I didn't end up replacing all the damaged pots for the eqs to cut back cost on the repair(half of them work). I run a hybrid setup and tend to do most of my eqing and monitoring ITB. The master section is also not working as is, but again I use it as a hybrid setup so I haven't needed/wanted it...until now. I bought a Otari MX-70 1" 8-track that I refurbished to pair up with this console in which I would definitely want the eqs and monitor section completely working(the MX-70 is also white in case you don't know). Unfortunately I had to sell the MX-70 to pay for a new transmission. But I still plan on getting another machine in the future so I'll probably open it up sometime soon and finish the restoration.

As for the op amps, I'm pretty sure I just used 5534s everywhere except I left the LM301s in the 8 bus outs(which are located on the lower edge of the middle 8 channel strips), which on this desk are the pots located under the meter with no individual stereo bus. Now I'm remembering this console was fixed gain as well, and I added mic trims to each channel–an odd design for sure, but this is my idea of fun and now it serves my purposes.

Here's the before picture just for fun. The crap you see on top is chicken poo, ha!

I am thinking of a new phrase like, "from coop to caps."