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

[velo]

I have a Quantum Audio Labs QM-8A that appears to have quite righteous potential however all I get is hum on the program outs as if I touched a TRS jack onto the tip of my finger.

I believe that I had read +/-24V across the PSU output caps at one time. The back-of-unit fuse is not blown.

There was a busted ch.1 gain/fader that I popped back into apparent mechanical happiness.

These are supposed to have lit VU meters. But toggling power does nothing--no light; no VU spark of life.

Should I use my DMM to check caps? Should I be looking for shorts?

Is there anything like a flowchart for basic things to check without getting into specific circuit details? I have no schematics yet.

 

[13engrsapper]

Your first step is to buy the other one sitting here, so you have spares. lol
They are capable little mixers, Richenbach transformers... and maybe inductors. It has been a while since I have looked inside one. Start with everything in the power supply, double-check all the resistors there too. I have the same early version here too. The later version (QM8B) had an external power supply. Next common issue is the faders - which looks like you covered. Internal connectors, especially if it was shipped to you (or anyone)... I think we have the original manual here for them. If I recall correctly they used the early can type Opamps, we may have new replacements here, or upgrades... I just picked up 8000 brand new can-types of various types; they should be here in a couple weeks or less.

 

[velo]

Your first step is to buy the other one sitting here, so you have spares. lol
They are capable little mixers, Richenbach transformers... and maybe inductors. It has been a while since I have looked inside one. Start with everything in the power supply, double-check all the resistors there too. I have the same early version here too. The later version (QM8B) had an external power supply. Next common issue is the faders - which looks like you covered. Internal connectors, especially if it was shipped to you (or anyone)... I think we have the original manual here for them. If I recall correctly they used the early can type Opamps, we may have new replacements here, or upgrades... I just picked up 8000 brand new can-types of various types; they should be here in a couple weeks or less.

Let us see how my 2024 production value transforms my lifestyle a bit more and then I may be in touch about that spare.

Until then, ok I will open it up again and poke around the PSU. I suppose seeing that 24V and giving everything a wiggle will be among my first goals.

 

[13engrsapper]

Is the manual here online? if not I will photograph and post it.

 

[jeffbro]

According to the schematics in the manual you should have +/-16V at the PS output.

The meter lamps are on their own transformer winding. If you have no lamps it would suggest, in order of likelihood: an open mains fuse (yeah, I know you checked), there is an open wire/connection someplace, all the lamps are burnt out, or you have a bad transformer.

 

[velo]

Is the manual here online? if not I will photograph and post it.

I do not see it here.

 

[13engrsapper]

ugh - i mean i don't have it. lol. I will take pics in a bit.

 

[13engrsapper]

According to the schematics in the manual you should have +/-16V at the PS output.

The meter lamps are on their own transformer winding. If you have no lamps it would suggest, in order of likelihood: an open mains fuse (yeah, I know you checked), there is an open wire/connection someplace, all the lamps are burnt out, or you have a bad transformer.

yes these run on bi-polar power

 

[velo]

The meter lamps are seeing 25VAC. Crazy that all 8 are burnt out.

 

[jeffbro]

Not crazy at all, they're 40 years old!

 

[velo]

According to the schematics in the manual you should have +/-16V at the PS output.

The meter lamps are on their own transformer winding. If you have no lamps it would suggest, in order of likelihood: an open mains fuse (yeah, I know you checked), there is an open wire/connection someplace, all the lamps are burnt out, or you have a bad transformer.

 

[velo]

Plugged into the main AC I hear the (60Hz?) hum from the bus output regardless of the power switch position.

I hear a faint pop when I power up.

 

[disjectia]

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

 

[jokeramik]

Change all elctrolytics in the power supply and on every board. After this long time working they aren´t working right. Don´t use switching power supplies. They made a lot of noise on the dc-voltage. Look with a oscilloscope.
Best regards!

 

[disjectia]

Yeah, ultimately I went with a linear supply though it really wasn't until I replaced all resistors with metal film that my ears noticed a significant audible difference in noise; of course, there are better sets of ears out there.

You'll unfortunately need to remove the top/rear panels from the chassis to get access to the channel cards; there's only 2 electrolytics per channel card, so once removed from chassis you can figure out a way to prop the on end (see pic) and you can probably replace the caps without removing every channel card individually (you probably want it on the other end than the one pictured, so you can access bottom of cards). There are a lot of solder connections on the cards; eventually I added molex connectors to each card for much easier removal/future work, but that all takes a lot of time.

For the electrolytics on the buss card you'll have to remove the card from where it's mounted on the VU meters.

I eventually removed the monitor section cabling as I was not using any of that, so did not bother to replace electrolytics on those cards.

 

[velo]

I guess I'll get started with the recapping. I wonder if there's anything else I should do while waiting for parts.

Some of the wiring looks brittle; I could replace some of it.

I do not have the patch cables snake in the back. It might make sense for me to get more RME analog inputs and use the QM-8A for the preamps, especially if I run into trouble with the mixer circuits.

 

[velo]

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

Thanks for that doc! I'm in/near LA sometimes, especially when the mountain roads are closed to thru traffic and I ride my bicycle up to the Mt. Baldy ski lifts. Mudslides notwithstanding. ;-) Regardless, QAL tech. contact info would be great.

 

[velo]

Yeah, ultimately I went with a linear supply though it really wasn't until I replaced all resistors with metal film that my ears noticed a significant audible difference in noise; of course, there are better sets of ears out there.

You'll unfortunately need to remove the top/rear panels from the chassis to get access to the channel cards; there's only 2 electrolytics per channel card, so once removed from chassis you can figure out a way to prop the on end (see pic) and you can probably replace the caps without removing every channel card individually (you probably want it on the other end than the one pictured, so you can access bottom of cards). There are a lot of solder connections on the cards; eventually I added molex connectors to each card for much easier removal/future work, but that all takes a lot of time.

For the electrolytics on the buss card you'll have to remove the card from where it's mounted on the VU meters.

I eventually removed the monitor section cabling as I was not using any of that, so did not bother to replace electrolytics on those cards.

CAPI OpAmp mod there on ch.1-4 ?

 

[velo]

I can see corrosion on some of the electrolytics. Some of the ceramic caps seem a bit degraded. I guess I'll be placing an order.

 

[velo]

I see the schematic has two 2000µF 35V caps whereas my unit came with two 1000µF 50V caps. I was thinking the 2000µF caps would have had a more stable DC result. However I already placed the order for the 1000µF. I am wondering if I should experiment with 2000µF.