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LC575 Analog Board Troubleshooting

CharlesT

Well-known member
Hello all

I have now managed to "brick" two analog boards in the process of doing recaps. I've had good luck with all my other recaps but these seem jinxed. The first came up properly (logic board and hard drive) but I had forgotten to connect the high tension led to the CRT. This bodged something and has prevented it from coming up since then (won't even turn on, so assume it's in a safety shutdown mode)

The second, the above's replacement, was working fine but "needed" recap so that was done, but lot's of time passed between removing the old caps, then installing the new. In the meantime my geezer brain took another cognition step function downward and I installed all electrolytics in backwards. No go at power up, at which I quickly realized what I'd done, ordered another set, and did it all again. Again no go at power up. None of the 'lytics showed any signs of stress at being in backwards which I attribute to very little power getting onto the board.

So, I'm tempted to go begging for yet another analog board. But, I have a 'scope and good DMM and would like to give troubleshooting a shot. Problem is I have not been able to find any kind of schematic for these things. Secondly, you have to do all the poking around with the board fully installed, which makes getting access to test points pretty difficult

Has anyone ever attempted fixing one of these? Is there any possibility schematics exist somewhere I haven't looked?  Any insight would be greatly appreciated

Charlie 

 

techknight

Well-known member
Hello all

I have now managed to "brick" two analog boards in the process of doing recaps. I've had good luck with all my other recaps but these seem jinxed. The first came up properly (logic board and hard drive) but I had forgotten to connect the high tension led to the CRT. This bodged something and has prevented it from coming up since then (won't even turn on, so assume it's in a safety shutdown mode)


You likely shorted the horizontal output transistor. Check it in diode check on your DMM to see if you have shorts across all 3 pins. If so, replace it. 

 

CharlesT

Well-known member
Thanks techKnight. I actually did check the HOT after the first failure and it seems to be OK, obviously under no load. The badness must have reached farther back so that the switched mode supply no longer does any switching. That's about all I know.

After two bonehead moves on these guys I'm about ready to throw my cards on the table. I've brought up a 2GS monitor from the dead and fixed a couple of mac plus analog boards so thought this couldn't be THAT hard. But I had schematics for those others, and the 575 board is at least three times the complexity if not more. My background is digital and software, seldom more than 5 volt stuff, this flyback stuff and high voltages are another thing entirely. So I've decided to pull a Kai Robinson (on a greatly reduced scale) and strip one of the boards and try to generate a schematic. Even with that, probing around on a powered up one of these things gives me pause given my bonehead tendencies. 

 

techknight

Well-known member
Thanks techKnight. I actually did check the HOT after the first failure and it seems to be OK, obviously under no load. The badness must have reached farther back so that the switched mode supply no longer does any switching. That's about all I know.

After two bonehead moves on these guys I'm about ready to throw my cards on the table. I've brought up a 2GS monitor from the dead and fixed a couple of mac plus analog boards so thought this couldn't be THAT hard. But I had schematics for those others, and the 575 board is at least three times the complexity if not more. My background is digital and software, seldom more than 5 volt stuff, this flyback stuff and high voltages are another thing entirely. So I've decided to pull a Kai Robinson (on a greatly reduced scale) and strip one of the boards and try to generate a schematic. Even with that, probing around on a powered up one of these things gives me pause given my bonehead tendencies. 


Well we all started somewhere. For me personally, I am kind of "full stack" on electronics, I know the analog side as well as the digital side. but I do not specialize in either, just more generalized. 

All I can say is just be careful is about the best advice I can give you. always double check your work as you are doing things, and do NOT rush. 

if the SMPS isnt running, then its likely something to do with the primary side or opto-isolator so check your work there. if it uses a TDA 4605, change it. 

 

CharlesT

Well-known member
Well, I might be closer to 'ending' up somewhere, closing in on 70.  Thanks for the words of encouragement. I've got a set of the TDAs on order, will probably get some optocouplers while I'm at it. I'm gonna treat this as a learning experience, don't really 'need' the LC. It's just that it's the only thing I have with the beautiful Trinitron in it, which I just hated to see going to a landfill.

I saw the thread where Uniserver mentioned Bomarc had these schematics at one point, too bad that source is gone. I've been looking at whatever Sony 14" monitor schematics I can find thinking the drive electronics must be similar, but for example the CDP-1302 looks nothing like the 575 board, so that doesn't look very promising.

Anyway, will keep plugging along 

 

techknight

Well-known member
Well whatever you do, dont send them to a landfill. 

theres always someone out there that can figure out and fix the analog board. 

 

CharlesT

Well-known member
Just so this doesn't languish forever with no resolution...

Per above recommendations, on both units, I replaced the optocouplers and the TDA devices. On the unit that I blew by not connecting the HV lead to the CRT, this had no efect, something else is at issue. I checked all the diodes and transistors that I could and they all seem to check out fine. So I'm at a standstill on that unit, clearly something else blew, no idea where to go next.

However, on the unit in which I stupidly reversed the replacement elctrolytics, I also replaced the optocouplers and TD devices (thank you Digikey!). In this case, SUCCESS!. It's now working again.  That was a relief!  No idea which device was blown, but at the parts costs, I really don't care.

In the meantime, before I did this testing, I saw a listing for yet a third 575 Analog board on Ebay. After procrastinating for over a week, hoping it would go away, it was still there, so I bought it. Not here yet, but it looks like I'll have a spare.  I'm sure if I hadn't bought that one both of my resurrection efforts would have ended up as fails. 

Finally, the reason I got into all this trouble to begin with was because I couldn't get the machine to reliably recognize an LC slot ethernet board.   After all this, with new caps, the machine still won't recognize the ethernet cards. But I give up, it's enough just to be able to see that good old Sony Trinitron 

 
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