Bad output on DMX Splitter

[partyanimallighting]

Hi guys,

I have a DMX splitter with a faulty output. This unit is used to split the signal from a DMX lighting console into 4 optically isolated outputs but one of the outputs is faulty. When the splitter receives DMX signal from a console, the green LED's at each output light, signalling proper output. The red LED's also light, whether or not the unit receives a signal. The green LED on the faulty output in question remains lit, whether or not the unit is receiving signal and the red LED does not light at all. So far I've checked voltages at each of the 120VAC/8VAC transformers and I'm getting 10VAC at each transformer, except for the one feeding the faulty output, 8VAC off this transformer. The voltage seems fine, within the range I would think, but I've noticed a slight swelling on the left side of this transformer. I also checked the voltages at all the LM7805 voltage regulators and the 12VDC side on the 7805 that feeds this output comes in at 8.7VDC, all the others are between 9.75 ~ 11.75VDC. Any ideas as to what I should check?

 

[partyanimallighting]

 

[KrisBlueNZ]

Hello again and welcome back

Can you post close-ups of just the faulty section, on both sides of the board. First please read carefully our guidelines on how to take useful photographs: https://www.electronicspoint.com/resources/how-to-take-photos-of-circuit-boards.6/

 

[partyanimallighting]

 

[partyanimallighting]

Hi Kris, I'm glad for the response. I hope the close ups are good enough (they're a little "flashy" because today is really overcast). I do have a spare defective unit that I may be able to salvage parts from so just point me in the general direction if you see anything wrong on the board.

 

[davenn]

what's the larger middle IC ?
the flash is wiping it out

 

[davenn]

I also checked the voltages at all the LM7805 voltage regulators and the 12VDC side on the 7805 that feeds this output comes in at 8.7VDC, all the others are between 9.75 ~ 11.75VDC. Any ideas as to what I should check?

what is the output voltage of that reg in the faulty section ?
it mat be a faulty reg that is dragging its input voltage down

Dave

 

[partyanimallighting]

 

[partyanimallighting]

Hi guys, here's an old pic showing all the IC numbers. Hope it helps.

 

[partyanimallighting]

what is the output voltage of that reg in the faulty section ?
it mat be a faulty reg that is dragging its input voltage down

Dave

The voltages on that reg are 5VDC and 8.75VDC. Hope this helps.

 

[davenn]

IF you have another 7805 do a swap and retest ... particularly its input voltage
if its still lower than the others then I would definitely be suspecting the transformer


D

 

[KrisBlueNZ]

I'd put my money on one or both of the 75176es that drive the DMX connectors. They have no protection components and could be damaged by ESD or some other kind of surge.

A failed 75176 could draw excessive current from the 5V rail. That would explain why the regulator is pulling down its input voltage, and why the indications are wrong. I recommend socketing the replacement ICs.

 

[partyanimallighting]

Hi guys, here's the update. I swapped out the transformer and the LM7805 and the output voltages across all the reg outputs and transformer outputs were all within range but the faulty output was still problematic. So I swapped out the two 75176 IC's that fed that output and one of them seemed to be faulty as the output returned to normal. I tested each of the four 3-pin outputs one after another and all are functional once again. There is one problem however. The green LED (LED2 G on the PCB) on the faulty output (J8 OUT1) still remains lit, whether or not the splitter receives DMX signal in. Under normal circumstances, without a signal in, all the red LED's would remain lit, then, when a signal is received, the green LED's would light up also. As it is right now in it's repaired state, the green LED on that particular output remains lit, with or without signal in. I'm looking at resistor R12 as being faulty. Am I correct?

 

[KrisBlueNZ]

It's very unlikely to be a resistor.

Check very carefully that you didn't damage any tracks when you replaced the 75176es. If not, most likely the LM339 was damaged when the 75176(es) were damaged.

 

[partyanimallighting]

Well from what I see all the tracks are on one side of the PCB so it should be easy enough to check. If I don't find any damage, I'll swap out the LM339.

 

[partyanimallighting]

Hi guys. I swapped out the LM339 and the unit's 100% functional. I couldn't have done this repair without your help. Thanks!!! I'm going to post another little issue in a bit. Hope you can help me out again.