Help Getting this 500 Series Aphex CX-1 Working Again (w/ 1537A chip)

[jjsound]

Hey y'all. I'm a hobbyist, and I just came upon an Aphex CX-1 compressor. It seems like a cool vintage unit. I saw on the internet that Pin 15 should be cut immediately before throwing it in a lunchbox, which I did. It powers up fine, and I see the meters light up (though some of the LED bars don't light).

The problem I'm having is figuring out where the input signal stops. I run an oscillator through it, and I was poking around with my scope to find the signal in components. I found it in most of the places I expected to, but I'm also not seeing it where I think I need to in some spots.

Again, I'm totally new to audio repair, but I really bought this as a test to see if I could figure it out. Troubleshooting where the problem is has been slowing me down, though. I think I've pinpointed the problem area to the VCA section. I've attached the schematic page for reference.

This unit uses a 1537A chip for its VCA section, but I think in the incoming signal to the chip, there's a problem. It receives signal on one leg that it should see it on, but not on the other. Also, the voltage on some of the pins of 1537 is completely off from what it should be. So to me, that tells me that one of the inputs to the 1537 chip is off. I could very well be wrong, though.

I've also attached the block diagram for reference. I know this is a shot in the dark, as it's probably a pretty rare piece of gear that many people don't use anymore. If anyone has any advice on how to possibly find the fault or where I should really look, I'd really appreciate you looking into this.

Thanks!

 

[nashkato]

had a similar issue an one of these a while back
i also suspected one of the VCA´s which finally wasn´t the problem .
first fault was a bad connection on the connector between upper and lower PCB .
iirc the final issue was a bad Tantalum which i didn´t recognize during recap .

i´d measure supply voltages around the VCA without the VCA in it´s socket .
maybe swapping  VCA´s between it´s positions might get you closer , too
but be aware , that if you have a fault elsewhere you might destroy a good 1537 which are really hard to find and expensive .

good luck

PS : is the optional transfomer installed in your unit ?

 

[Harpo]

... I saw on the internet that Pin 15 should be cut immediately before throwing it in a lunchbox, which I did...

... just to enable the possibility of misalignment, IE the cards -16VDC pin now can connect to +48VDC and simultaneously the cards 0V pin now connects to -16VDC and the cards +16V pin now connects to 0V. When this happens (FI by hot plugging), it will make shure to blow up most polarized parts of the card and/or blow up the psu.

 

[nashkato]

i do hope , at least that´s what i assumed , he only cut the trace  , not to let Phantom power of 500 series pin 15 into the "Meter Out" circuit of Aphex  Lunchbox  pinout .

 

[jjsound]

i do hope , at least that´s what i assumed , he only cut the trace  , not to let Phantom power of 500 series pin 15 into the "Meter Out" circuit of Aphex  Lunchbox  pinout .

Yep! That's precisely what I did. I didn't physically cut the card connection. Sorry for the confusion. These Aphex units were designed with an external meter unit in mind, so Pin 15 is an External Meter Output, rather than the +48v connection.

 

[jjsound]

had a similar issue an one of these a while back
i also suspected one of the VCA´s which finally wasn´t the problem .
first fault was a bad connection on the connector between upper and lower PCB .
iirc the final issue was a bad Tantalum which i didn´t recognize during recap .

i´d measure supply voltages around the VCA without the VCA in it´s socket .
maybe swapping  VCA´s between it´s positions might get you closer , too
but be aware , that if you have a fault elsewhere you might destroy a good 1537 which are really hard to find and expensive .

good luck

PS : is the optional transfomer installed in your unit ?

No optional transformer in this unit, but the plan is to eventually put one in, once I can actually get an output! The schematic includes the transformer, but they at least included parts for replacement for the non-transformer version.

Taking the VCA out and measuring around it is a really good idea. I'll try that for sure. When you mention swapping the VCA between its positions, what do you mean by that? I definitely don't want to destroy the 1537. I've heard elsewhere that they are very hard to find, too. Will definitely be careful in that sense. Thanks for your input!

P.S. When you recapped, did you just recap the larger axial caps? Or did you include the other ceramics as well? I replaced a few that look burnt, one of which was a part of the VCA section.

 

[nashkato]

  When you mention swapping the VCA between its positions, what do you mean by that?

there are 2 or 3 1537´s in there .
take another VCA and see if it behaves the same way as the suspected faulty one .

  When you recapped, did you just recap the larger axial caps? Or did you include the other ceramics as well?

only electrolytics and tantalum

 

[jjsound]

So, I flipped the 1537a positions and measured for voltage again. The replacement 1537 that was in the CX  Meter Detector  subcircuit behaved pretty differently...

I've attached the 1537a voltage ratings sheet as well...

On the original 1537a, only pin 6 shows an input signal, and only pin 13 is spitting out an output. Also, Pin 7 is measuring at 13.61V, but according to the 1537a voltage rating sheet, it should be showing -6.8V!

On the replacement 1537a,  both pins 3 and 6 show input signal, but no pins show an output. Also, Pin 3 and Pin 7 are measuring at 13.61V instead of the recommended -6.8V!

At this point, I'm pretty confused on where to pinpoint the problem.  Did you come across this problem when you were troubleshooting your unit?

 

[jjsound]

Also, one of these 1537a chips gets REALLY hot when the unit is turned on. In either position it is in, it emanates a pretty intense amount of heat.

 

[nashkato]

did you do a recap ?
start with the tantalums and see if there are changes