Access-312 Build Thread

[Whoops]

Sorry to barge in here but we certainly still have the gar2520 kits. We have 2 pages of items in our discrete kit category. They are on the 2nd page.
https://capi-gear.com/catalog/index.php?cPath=71_72&sort=6a&page=2

Thanks for letting us know Jeff and for correcting me, I didn't notice there was a second page.

So please let me know if I'm correct or not,
for what I understood in your website you offer 2 different 2520 Opamps,

The gar2520 is a 2520 from the mid '70's, Studio Systems/Huntington era
and the CA-0252-DIY is an early 1970's OG 2520.
Both are 2520 but different versions, is that right?

Thank you

 

[jsteiger]

Thanks for letting us know Jeff and for correcting me, I didn't notice there was a second page.

So please let me know if I'm correct or not,
for what I understood in your website you offer 2 different 2520 Opamps,

The gar2520 is a 2520 from the mid '70's, Studio Systems/Huntington era
and the CA-0252-DIY is an early 1970's OG 2520.
Both are 2520 but different versions, is that right?

Thank you

The gar is a resistive current source for the long tailed pair, 9 transistor amp. My 0252 is a transistor current source so 10 transistor amp. The friend that gave me the unpotted one that I reversed said he got it in '71 or '72 at the NAMM show. I'm not exactly sure of the original of Gary's but legend is what you have mentioned.

 

[Whoops]

But both were originally called 2520 but different versions, is that right?

 

[jsteiger]

Both were originally called 2520 but different versions, is that right?

Yessir

 

[EarthTone]

{Deleted comment}

 

[EarthTone]

One of the channels on my unit's phantom power has stopped working. Channel 4, which is the first channel that accepts and distributes power to the rest of the units, in case that matters. It makes a bunch of noise when you turn on phantom though. Dynamic mics still work fine on that channel, crystal clear. Phantom on the other 3 channels is working fine, so it can't be a power supply problem. Any ideas of where to start looking for issues? I haven't opened it up yet... The problem is also intermittent. I got phantom working the other day on channel 4 but now it doesn't work again.

EDIT: OK, it's 4am but I couldn't resist opening it up and taking a look. Nothing appears to be damaged/burned. I was prodding around and noticed that if I lift up the board slightly on the side that has the terminal blocks, the phantom power works and a condenser mic works fine. I am also reading 48 volts steadily on the XLR input pins 1/2. Could this be a broken trace? Or a bad solder joint? I hope it isn't a broken trace... That would truly suck! Would I be able to see the broken trace visually? Man... Bummer. Love this preamp, would love to be able to use phantom on all channels without thinking about it. Like I said though dynamic mics work crystal clear with no problems and the board slight bending pressure (not much just slight pressure) doesn't make any difference with the dynamic mic. On the condenser, if I press down slightly on the board the sound goes away, lift up slightly the sound returns.

I suppose step one should be to pull it out and reflow solder on all joints? Or anyone have any other suggestions?

Depopulating the board and reinstalling on a fresh PCB would be a huge PIA... Hope it doesn't come to that.

Thanks,
Matt

 

[3nity]

I will check for cold solder or screw terminals..sometimes they look tight but they are not....check it for both 48v and ground. Good luck buddy.

 

[EarthTone]

I will check for cold solder or screw terminals..sometimes they look tight but they are not....check it for both 48v and ground. Good luck buddy.

Thanks. Ok I will! I’ll let you know if I find it. Hope so!

 

[BarneyBrougham]

What's up Everyone

I have a 4 channel build in a1 RU case.
Lately i have been getting random drop of signal on 2 channels when I am recording, DI and XLR.
If i pull the cable out and plug it back in, the signal comes back. Often when I turn the unit on for a session it starts like this, unplug replug and it's back on. But the signal drop when recording is random, and I can't make it happen. Wiggling the inputs also doesn't make it better or worse.
I have had a look inside and nothing looks damaged.

Any ideas where to look to fix this?

Thanks so much!!

 

[3nity]

What's up Everyone

I have a 4 channel build in a1 RU case.
Lately i have been getting random drop of signal on 2 channels when I am recording, DI and XLR.
If i pull the cable out and plug it back in, the signal comes back. Often when I turn the unit on for a session it starts like this, unplug replug and it's back on. But the signal drop when recording is random, and I can't make it happen. Wiggling the inputs also doesn't make it better or worse.
I have had a look inside and nothing looks damaged.

Any ideas where to look to fix this?

Thanks so much!!

Are you using 48v to your mic and DI? Hows your ground looking? Check for cold weld spots.

 

[BarneyBrougham]

It has 48v to every channel.
But it happens no matter how i use the unit. Often happens with a 1/4 inch cable into the DI...no 48v on.
Grounding is good i think.
I have been using it for years, and has been sitting in the same spot unopened, or moved.

 

[EarthTone]

I will check for cold solder or screw terminals. sometimes they look tight but they are not....check it for both 48v and ground. Good luck buddy.

Hey @3nity I think I finally fixed this. I ended up reflowing solder on that entire card and that didn't help, unfortunately. I was still getting screeching with light pressure on the PCB with a pencil or whatever, and intermittently it would happen without applying pressure to the card which is of course how I discovered the problem.

What did seem to finally fix it was drilling out the mounting holes on my case bottom to a larger diameter and using washers on the standoff screws, attaching the standoffs to the card before mounting to the case, and then mounting the whole apparatus to the case from there which allows the card to be mounted without stress.

I have no idea what was happening when I was getting those screeching noises but now I can't make it happen at all. My only guess was the PCB was getting stressed somehow or losing grounding or something when it was mounted too tightly in the case and bending a little.

So lesson learned- Always drill out your PCB standoff mounting holes as large as possible, attach standoffs to cards first, then mount in case using washers and screws from the case bottom to ensure PCB cards are not stressed during the mounting process.

I'm still mystified by this and am wary of using phantom powered mics on that channel but for now I've been testing it in my studio alone and it is working great with phantom powered mics as well as dynamics and ribbons (the problem seemed to be phantom related only; I did not have the issue with mics where phantom was disengaged).

Cheers! Such a great sounding project. I will make a 2nd rack of these this year to finish my studio. I've got 26 channels of front end now and the majority of them are DIY pres.

 

[tonycamp]

Anyone know where to find the first page info on this? Schematic would be good if nothing else, thanx

 

[3nity]

I'll try to find something..see if my old laptop will start. If not I'll draw another schematic.

 

[Whoops]

Anyone know where to find the first page info on this? Schematic would be good if nothing else, thanx

Schematic and docs are posted in many different posts in this thread.
they're also in page 31 and page 32

 

[mooksnook]

Hello DIY friends!
I've spent 2 days reading the whole thread, preparing and learning how to build this unit.
I've built many 500 series units but this will be my first 19" rack unit.

So I found no information on this thread about the actual XLR cables except that it should be shielded.

So my question is, what shielded cable should I get and what do I do with the copper shielding inside? Do I connect it to pin1 or cut the excess off?

Is this a good option or are there cheaper equally good variants?
Mogami 2549 #22AWG Balanced Cable. Shielded

Looking forward to the build