Studer A820 Autolocator Problem

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Greg

Greg

Well-known member
Joined
Jun 7, 2004
Messages
1,784
Location
New Orleans, LA
I started troubleshooting a Studer A820 autolocator last night.

Symptoms: No digital readout and no lights. Machine does respond (improperly) when buttons are pushed.

The PSU voltages at and inside the autolocator are fine, so it's not a PSU problem. I then located the serial interface board inside the machine and verified continuity of the TR-A & TR-B (transmission lines) and the signal ground. So there's no connectivity problems between the machine and autolocator.

It was getting late so I called it a night. I decided to look at some docs this morning on IC9 (# 75176), the differential bus transceiver, and I suspect that this chip possibly has gone bad. I just skimmed the datasheet and I'm trying to figure out the best way to test. Should I just scope output (to a u-processor) and push buttons on the autolocator. I should see something right? There's also a line from the u-processor to the 75176. I could scope that as well. My hunch is that the u-processor is fine because the parallel remote interface work properly.

Any thoughts? I can provide docs if need be.
 
Greg said:
I started troubleshooting a Studer A820 autolocator last night.

Symptoms: No digital readout and no lights. Machine does respond (improperly) when buttons are pushed.

The PSU voltages at and inside the autolocator are fine, so it's not a PSU problem. I then located the serial interface board inside the machine and verified continuity of the TR-A & TR-B (transmission lines) and the signal ground. So there's no connectivity problems between the machine and autolocator.

It was getting late so I called it a night. I decided to look at some docs this morning on IC9 (# 75176), the differential bus transceiver, and I suspect that this chip possibly has gone bad. I just skimmed the datasheet and I'm trying to figure out the best way to test. Should I just scope output (to a u-processor) and push buttons on the autolocator. I should see something right? There's also a line from the u-processor to the 75176. I could scope that as well. My hunch is that the u-processor is fine because the parallel remote interface work properly.

Any thoughts? I can provide docs if need be.
Click to expand...
Just making certain we are talking about the auto locator, and not the audio remote right?  not where you arm the tracks. 
Does the machine operate normally with the unit removed?
I would look for shorted electrolytic capacitors somewhere in the auto-locator, and serial communications board that it connects to on the machine. 

 
Yes, Greg... the remote where tracks are armed/disarmed (a parallel connection if my memory is correct) works fine. In fact, the machine still works with the autolocator connected. It just does whacky things if buttons are pushed.

As you suggested, I will look for shorted caps before attempting to order ICs. I have not done this.
 
Are you using the original cables, or have you extended them?
I remember having problems like that when we used "through the wall" tie lines, as well as the original, long, supplied cables at the remote end. Cutting the original remote cable in half cured the fault.
 
I'm not sure, but this deck has been in service for at least a decade... so I don't think the cable itself is the source of my woes.
 
Nor did we think the cable was at fault to start with.
When first installed, with the original remote cable and wall ties, everything worked perfectly. Then after, oh I don't know, say about 5 years or so, we started getting the display on the serial remote locking up or flashing oddly.
Much scratching of heads finally told us to plug the remote direct into the machine (cables through the door job). This showed everything working fine. So we thought "Ahhh, dodgy connections on the tie lines" A couple of hours examining and belling out ties later revealed no faults!
So it was back to head scratching.... ???
I suggested maybe the cables had aged in some peculiar way, and promptly got "Poo-poohed" as we Brits say!
But I shortened the remote cable by half, and "Lo and Behold" the fault vanished. No more lock-ups or flashing.

We never did get to the bottom of why.......
 
My feelings leant towards the cable crystaline structure changing with age/corrosion too.
I don't think it was caps losing tolerance, as we scoped the supply at the remote end and it was clean as a whistle.
But thinking afresh, maybe the tie line cables changed capacitance core to core over the years as they "relaxed" due to gravity/other cables on top of them...
 
You should be able to buy a DB9 cable to see.  I bought one on-line some years ago to test my remote.  Make sure you get one with 9 conductors in the cable.  Some RS232 cables have less.  If it is the original cable (grey, flexible, looks like the audio remote's), and it hasn't been cut, crushed, tightly twisted, or sheered in any way, I would doubt that is the problem.  If you have it routed through homemade wiring throughout studio, using several connection points, I would try to get a more direct path.
 
I recapped the autolocator and powered it up from my bench supply. All the voltages inside looked fine. Nothing displays and no lights work, but I didn't have it hooked to the machine. Is it correct to assume that the locator needs information from the machine itself to display anything? I will be heading out to the studio this weekend to look for other problems. I will look at the serial communications board inside and check the cable?

If anyone has additional suggestions please feel free to chime in.

Thanks.
 
The cable is fine.

It turned out to be a bad 75176 (serial communication DIP) in the autolocator. I mentioned in my first post that I felt this chip (IC#9 on the serial comm board in the machine and IC#14 in the autoloc) was suspect  because the studio engineer told me they were hot swapping the 9-pin when the unit went totally dead.

I have a hunch that perhaps some caps were leaky and the unit started acting badly. So they went back there and started fiddling with the connector and knocked out the 75176. So now the unit is re-capped and it has a fresh $0.80 communication chip.

Thanks to all for the help and suggestions! Cheers.
 

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