Midas XL3 Module Fault

[drsound]

We have an XL3 mono input module that is exhitibing what I think is probably a ground fault somewhere. In the absence of a circuit diagram it's proving difficult to track down.

 

The problem is with the bus enable switches and indicators. When we enable either of two particular sends (From memory I think it's 11 and 15 but that's probably not important) all the module LEDs dim. This doesn't appear to have any effect on the audio circuitry but obviously it's not ideal. We have been through all the usual fault-finding checks like checking the edge connector integrity, supply voltages, switch contact resistance etc but to no avail.

 

I stand to be corrected but I'm thinking it's probably a problem with the 0v rail potential rising for the LED supply since I doubt that it's pulling down the +ve rail (or indeed bringing up the -ve rail) as the rest of the desk is unaffected.

 

Just thought I'd post this here before shelling out for a service for what seems like a minor problem on a spare channel. Anyone seen a similar problem or got any thoughts on test points? We have all the usual service kit available for testing - dual rail supplies, sig gen, scope etc.

 

We're also after a service manual or circuit diagram book if anyone's got one they'd like to put up for grabs.

[dbuckley]

I know nothing of the XL3, but in many a desk design the LEDs run between the +ve and -ve rails to avoid clicks

[drsound]

On further investigation I believe you're quite right. It would also seem that there are three driver circuits for the LEDs so I'm narrowing it down slowly but surely.

[Stan Hope-Streeter]

On further investigation I believe you're quite right. It would also seem that there are three driver circuits for the LEDs so I'm narrowing it down slowly but surely.

 

The leds are in series strings, and each switch has a section that shorts out the associated LED. (This is standard practice in most pro consoles to avoid switching transients)

There are three BC327 transistors at the top end of the circuit board. Each is a constant current source, feeding about one-third of the leds on the module. I think Q4 is the one that drives the leds on sends 9 to 16. These transistors run very hot and are prone to occasional failure so it's worth checking them. However if ALL the leds on the module are dimming then the fault must be common to all 3 circuits, so check the resistors R5 (1k5) and R6 (15k) and the diode D1 (1N4148) which are common to all three. Other than that, follow the tracks and look for a poor solder joint somewhere around those constant current supplies.

 

Most problems with the LED's in these consoles are due to broken connections because they are rather exposed to being bashed, in which case the standard symptom is for the whole string to stop working (ie one third of the led's on the module)

 

HTH

[drsound]

Hi there, thanks for this. We did eventually manage to track down the schematics for this. As you rightly say, Q4 drives the LEDs for sends 7 - 16 as a constant current source. On almost all of the channels, Q3, Q4 and Q5 show some bad discolouration around the base on the PCB so they're obviously running very very hot.

 

I'm thinking we might actually have a problem with an LED on the way out presenting too high an impedance thus preventing the constant current source from being able to provide the current. The reason I'm going down this route is that only two of the LEDs cause the problem when they're switched in. We've sourced some spare LEDs so we can give this a pop. Failing that it'll be onto the other elements of the supply as you suggest. We have already been over the tracks and solder joints very carefully but it could still be a problem there.

 

At least it's a simple circuit, working on this is a damn sight easier than most of the mass manufactuered Chinese kit we normally end up repairing!

[Stan Hope-Streeter]

<br />Hi there, thanks for this. We did eventually manage to track down the schematics for this. As you rightly say, Q4 drives the LEDs for sends 7 - 16 as a constant current source. On almost all of the channels, Q3, Q4 and Q5 show some bad discolouration around the base on the PCB so they're obviously running very very hot.<br /><br />I'm thinking we might actually have a problem with an LED on the way out presenting too high an impedance thus preventing the constant current source from being able to provide the current. The reason I'm going down this route is that only two of the LEDs cause the problem when they're switched in. We've sourced some spare LEDs so we can give this a pop. Failing that it'll be onto the other elements of the supply as you suggest. We have already been over the tracks and solder joints very carefully but it could still be a problem there.<br /><br />At least it's a simple circuit, working on this is a damn sight easier than most of the mass manufactuered Chinese kit we normally end up repairing!<br />

 

Yes, those transistors do run very hot, but allegedly they are within their ratings....

 

Best of luck with the LED's

 

I you bought the desk from its original owners rather than through a dealer, it's worth asking them if they can find the original Spares Kit. It may be lurking forgotten in some dark corner of their warehouse. It would be a small cardboard box full of spares that were (probably) never needed, but will be like gold dust to you as the console gets older and original components go out of production and become harder to source.

Also worth asking friends at other companies who have disposed of their XL3's.

 

HTH.

[drsound]

I suppose they've put up with running that hot for 10 years so I can only assume that they're happy enough with life. I don't think they would meet modern standards for circuit design though, they've burnt the screen printing off the PCBs in places.

 

Sadly our desk was one of several bought by one of the larger rental houses in the mid 90s so I doubt that there's much point approaching them for spares. On that subject though, if any Blue Roomers happen to have any bits lying around for these desks from components to whole modules please PM me.

 

The spares issue for older kit has caught us once or twice but there do seem to be reasonable stocks of the LEDs still floating about (TLG208 if anyone's interested). They are however several pounds each with a minimum order quantity of 25 so I've bought some modern equivalents with the same spec at a few pence each to test the theory. If this fixes the problem then we will order the right items which will also leave us with a decent stock of spares. Alternatively we can always do a swap with one of our spare channels to test components.