API 525 in Stereo (HELP!)

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wilcofan

Active member
Joined
Jun 4, 2004
Messages
41
Location
Calgary, AB, Canada
Hey all,

I need your help. As you may have seen on a thread here or Gearslutz, I'm trying to sort out my own stereo switch for a pair of 525's. They are consecutive serials from 1998.

There is a PDF floating around which points to pin 7 and 8 being part of a switch, but a call to API tech points to the fact this is for pre-1980 525's. Mine, being from 1998, fall under the lot that need pin 6 strapped with a 120 ohm resistor. The proof for pin 6 is actually on the API site, which I'd missed early in my searches. There is a diagram there of the pin assignment with a stereo link slotted in pin 6.

So, you may ask, why am I still stumped? Well, first I am confused by the action I'm seeing with pin 6 strapped. The meters will not track together. Another API 525 user (with a proper 500H rack promising stereo linking built-in) concurs that the meters don't track together in stereo.

That's the first part of my discussion. Why don't they track together and how can it be considered stereo linked if they don't? (other than the obvious that there are two channels!)

Second, looking at the PCB for the 525 I am not sure there is any connection to pin 6 at all! I am nobody to confirm this but looking at it, it really looks like it's an empty pin.

The silkscreening on the pins suggests it's empty too, but API tech says that's because it was common for the modules to use generic cards. Makes sense. But can someone look at these photos and please tell me where there is a connection on pin 6?


(6 from the right on this photo above)


(it's 6 from the left on this photo above)


(pin layout as API promises for current reissues)

Thanks,
Bobby
 
Yes, I believe it is unconnected.

Look at it this way:

7-8 are tied. That's the detector send and return. When you jump a 120 ohm resistor to pin 6 on both modules, you are connecting the sidechains through a 240 ohm junction. Pin six is just a convenient common point.
If this point is past the meter circuit (likely), then you won't see stereo meter movement.

To test, set up controls on both units the same, send a identical tone through both, and look at the outputs. Try changing threshold on one unit, the output of the other should follow.
 
Crusty,

Thanks a LOT for jumping in! OK, now I'm going in circles because the original PDF, that API says doesn't apply to my generation 525, describes 7 and 8! How are you arriving at knowing the sidechain is tied on those pins?

Here's the clip (it's in another thread here too):

Beware! This is supposedly for pre-1980 525's!
I've updated my top post with the pinout as API advertises right now for my 525.

Bobby

P.S. Excellent tip about the VU's being possibly located in the circuit where they won't see the stereo action, that's really going to help me troubleshoot.
 
Hi Wilcofan,

I didn't read your post careful enough. I can see your confusion now!

I was going by the silkscreen on the connector.
Yeah, the legend on the connector you have is way off, and the second connector diagram looks right, correlating it to the actual connector pic.

The answer you got about using generic connectors is a lame one. A generic connector would have no legend on it; they were using up old stock probably, and didn't bother crossing out the changed pins. You might have a couple of the early versions of the later issue.

I still don't see a connection to pin 6 either. If it's a 2-layer board, it should be obvious. Did you strap the pin 6's and try the signal test I described? If it doesn't work then things aren't as claimed. If you do have an early version of the re-issue, it's possible that a resistor needed to be added at the factory to bring the s.c. input to the backplane connector, and it's just missing. Try the test first, and we'll go from there. I don't have a schematic of that version (just the "C" version) but I think we can figure it out :thumb:
 
Did you strap the pin 6's and try the signal test I described?

No, I'm at work all day and it will be the first thing I try next. Even though you and I both can't see a connection on pin 6, there could be one. Obviously I need to rule out that it's NOT working with pin six connected between the units, a conclusion I jumped to by the needles alone not tracking together.

Now that I have an explanation as to why (possibly) the needles will never track together, I can go forward with your audio test.

I'll report back and thanks again.
Bobby
 
To test, set up controls on both units the same, send a identical tone through both, and look at the outputs. Try changing threshold on one unit, the output of the other should follow.

Crusty,

I didn't re-read this so I did a slightly different test which still proved to me it's not linking. I took a kick and fed it to the left channel and a square wave to the right. I strapped pin 6 with a 120 ohm resistor between with a switch. When linked I can get no reaction from the kick on the square wave. It's just a consistent level while the kick is pumping. I should be able to hear reduction on the square wave.

I'm sure there is no connection on pin 6 right now... what next? Am I gonna need a schematic? I mean, a proper one! :?

Thanks,
Bobby

P.S. I just realized it's possible the switch I installed is faulty and I'll confirm that but let's go on the assumption right now pin 6 is empty. By the way API talks about their changing modules and boards, I wouldn't be surprised at all if there needs to be a jumper installed to get pin 6 working.
 
Well, I have the 525C version schemo; not exactly "proper" :grin: , but it should work. The part numbers probably won't match up, but the circuit will probably be the same. Do you need a copy?

You're gonna have to find where that talked-about sidechain junction is located and tap off there. Incidently according to my schematic, the meter is past that point in the signal chain and should respond from either channels signal, once you get it working. I bet that other guy's rack doesn't strap to stereo either!
 
Well, I have the 525C version schemo; not exactly "proper" Very Happy , but it should work. The part numbers probably won't match up, but the circuit will probably be the same. Do you need a copy?

No, I have the 525C schematic. Thanks.

You're gonna have to find where that talked-about sidechain junction is located and tap off there.

Is it just a straight tap off? The diagram for pre-1980 linking suggests there needs to be a break. The pre-1980 model uses pin 7 and 8 for side-chaining but wants a make there for mono and a break in stereo. The pin 7's are strapped and pin 8's are taken out of circuit.

Can you take a look at the 525C and notice this?

Incidently according to my schematic, the meter is past that point in the signal chain and should respond from either channels signal, once you get it working. I bet that other guy's rack doesn't strap to stereo either!

You know, the way he described the stereo behavior I wouldn't be surprised. He said the stereo interaction was "different than SSL". I can understand a flawed sidechaining link, but mine is flat out ignoring the other channel right now.

Bobby
 
Trace the output of the first 2520 (A1). See if it goes right to the top of the "IN" pot or if it goes through a resistor first. If there is a resistor in series there (hopefully), the "IN" pot side of the resistor is the side to bring out to pin 6 of the connector.

The stereo strapping in this case, is just mixing the audio together before going to the control circuits. Not a popular way to do it because it affects your threshold level; a mono signal in both channels will dominate any signals panned more to either channel. Honestly, everyone I know who has these, runs them unstrapped on stereo pairs.
 
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