How to test an Aphex 124A interface - need schematics

I have an Aphex 124A -10 to +4 (and reverse) interface box that I'd like to test...

I cannot find the schematics anywhere. 

How can I make sure this is functioning properly? 

Can anyone help?

I've got a bunch of these things. Like, probably 8 or 10 of them total. I've never encountered one that wasn't working though, I don't think.

What do you suspect is wrong with it? If they don't seem to have enough gain, I think that's how they all are. I always felt that the two knobs on the front should go to 11 if you know what I mean. I believe they are meant to be unity gain, what comes in is what goes out, so the knobs are just attenuators really, to lower the gain, not boost.

What do you think is wrong with yours?

Sorry, should have mentioned the problem.... the levels do not seem consistent.  I have eight of these boxes and I was hoping there was a way to easily test to make certain they were all behaving the same.  Don't have a scope or signal generator, just a couple of multimeters.

Ah I think I understand. I've never noticed any discrepancies in level between mine, but then I never paid much attention. Once's permanently on the tape deck and it's always been consistent left to right. I assume that's what you mean? One side is hotter than the other and you can't trim it out? Or do you mean one box is louder than another? Maybe one's set up +8 and the other's +4? If I remember there's jumpers inside, but I'd have to unrack one to be sure.

One side is hotter than the other and you can't trim it out

Click to expand...

Thanks... will check this jumper switch.  Does this switch pertain to the entire box?  I'll post a photo of the inside.  This could be the answer. 

Circuit board...

jumper switches for each channel....  I popped the top of 2 units,  channels were all set at +4.

test loops...

I took this photo of the owners manual. It's for the old model 124, not the 124A, but I'm sure the circuit hasn't changed much. Hope this helps.

Did you ever get this sorted? I gather that you feel that at least one of your 124A units has enough difference  between left and right outputs that it can't be trimmed out with the two  pots on the front.

If the "term" 600ohm button and the "input ref" jumpers are all the same between all channels then the next thing I would suspect would be a bad solder joint or blown lead on the balanced side. If either pin 2 or 3 had become open then that could account for -6dB for that side, sort of like balanced vs. unbalanced, and some electronically balanced circuits can automatically detect this condition and account for it, but only if the unused side is shorted to ground, and I'm not even sure if this is one of those kinds of circuits (Aphex is know for using clever tricks in their inputs and outputs). I'd trace from the XLRs back and look for opens somewhere around pin 2 and pin 3.

If it comes down to it the next thing might then be to shotgun all of the electrolytics and opamps. Shouldn't be more than $10 or $15 worth of parts, as it's a pretty simple circuit. Wish I could be of more help. Maybe someone with direct experience with this model could chime in.

Thanks for the schematics... and your suggestions.  Should probably add this to the Tech library in the Aphex  folder. 

I plan on replacing the lytics on the unit in question.  But I have gone through all to take a look at the +4/+8 jumper and also to look at all components.  You are saying to replace the opamps too - so I will try that as well. 

One strange thing I found was that the 7915 / 7815 voltage regulators appeared to be in contact with each other (bent toward each other).  Never saw anything like this before.

The voltage regulators should NOT be touching.

I could have this wrong, but I believe that that "tab" with the screw hole is GROUND in a positive vreg, but it's input or output or something on negative regulators! They should be isolated.

If you suspect anything in the PSU, that would be a good place to start. Check the outputs of the regulators (check pinouts of both regulators by downloading the appropriate datasheets), ground your black lead on your multimeter, and test the output pins of each regulator while the device is plugged in  and powered on. Be sure not to short any of the leads on the regulators! Observe all safety precautions!

If all three of your voltage rails are good, then I would suspect something else.

Could you please any of you guys tell me the setting of the Vrs on this board?
I mean: variable resistors , trimmers . there a 4 on board.

my unit seems to be manipulated. would be great!


thanks in advance!

Variable resistors read as follows (still sealed):

VR102 - 14.62K
VR202 - 13.86K
VR104 - 5.32K
VR204 - 5.05K

Most likely they are trims for the opamp gain settings.  Two will adjust the outputs for +4dBm and two for -10dBu.  Since the ones on your board are relatively closely matched pairs, why do you think it has been "manipulated"?

mjrippe said:

Most likely they are trims for the opamp gain settings.  Two will adjust the outputs for +4dBm and two for -10dBu.  Since the ones on your board are relatively closely matched pairs, why do you think it has been "manipulated"?

Click to expand...

thanks so much! . very appreciated!


because they are not matched or similar values.  i think they are manipulated.
also the trimmers have signs .

musika said:

Variable resistors read as follows (still sealed):

VR102 - 14.62K
VR202 - 13.86K
VR104 - 5.32K
VR204 - 5.05K

Click to expand...

Thanks !! great  ,that will  help me a lot

very grateful for this .

cray said:

because they are not matched or similar values.  i think they are manipulated.
also the trimmers have signs .

Click to expand...

Oooops, I saw the numbers in Musika's post and thought those were your values!  So yes, they should be close in value when set correctly.