Nieman 3M27SF

[lilshawn]

I'm having a little issue with this 3M27SF monitor. it's currently in a big buck hunter machine, set to be running at VGA (640x480) resolution.

it was transplanted from another machine of unknown quality and/or storage conditions and It required some capacitors be replaced in it before it would start up.

Got it all fixed up and loaded the monitor into this new machine and had it run a few days without issue.

 The monitor would then randomly have no sync as shown here after being booted up:



occasionally i could reboot the machine and have it sync back up properly. only to randomly drop out again.

while i was trying to diagnose the issue the deflection whine started to warble and sound not so good... the screen collapsed and the monitor shut down.

Burned out the HOT.

Replaced the HOT now, and the horizontal won't sync properly no matter what (same as above)

my eyes are bugging out on the schematic and i'm trying to figure out where the HOT is getting it's timing signals from.

I'm not sure if i should be looking at the sync circuit or the HOT timing circuit or what. The fact the HOT is blown out lead me to believe something is amiss somewhere, something more than just sync... a monitor should run all week with faulty sync input.

Where to start?

[grantspain]

that looks like a rodotron 666a schematic so i imagine its the same chassis
these can be difficult to work on
when you said you had a collapse,was is vertical or horizontal?

I had these with a few bad caps in the power supply section but for the HOT to blow then i would check the high voltage poly caps on the deflection circuit

that does not look like a sync problem,more like horizontal frequency-check the small daughter board and connectors for bad solder joints,there is also a 31khz horizontal hold pot on that pcb

i must say that my success rate on these chassis is about 70%,which is extremely poor and i really do not enjoy working on them

[lilshawn]

i must say that my success rate on these chassis is about 70%,which is extremely poor and i really do not enjoy working on them

Yeah i hear that. i can usually get them working if i can get the bad caps before the whole system shuts down, but if it's been run into the ground, i'm probably no better than 70% either.

when you said you had a collapse,was is vertical or horizontal?

horizontal...basically. i never SAW the collapse, the system shut down (as it should) and wouldn't power back on again (main B+ overload protection ticking power supply yackitty schmackity) tested the HOT and it was shorted.

I had these with a few bad caps in the power supply section but for the HOT to blow then i would check the high voltage poly caps on the deflection circuit

that does not look like a sync problem,more like horizontal frequency-check the small daughter board and connectors for bad solder joints,there is also a 31khz horizontal hold pot on that pcb

yeah that's what i was thinking. if it was sync, it would just have a scrambled pic....not blow the HOT from over-driving it.

i'll check out the polys and see if anything is amiss.

thanks for the suggestions, i'll post back once i've had another stab at this monitor. 

[Rickn]

Hey Shawn, I do not seem to have enough time to visit the forums or reply lately. I thought we had a conversation by voice or e-mail about this? High probability is the daughter board designated as IC9. It houses the sync separator chip. Sometimes there are other components. yes Grant is right sometimes these problems are dos to fix, but generally speaking we are well over 90% but I do write them off as non repairable when the time and cost is too high!!
Rick Nieman
Nieman Video Displays

[ed12]

do not forget
2 change the fet
and the slow start timming cap
I am away until sat
so I am no where my notes
but like rickn says 90 is about right

ed

[lilshawn]

thanks Rick i did get your voicemail... been out fixing on the road today

High probability is the daughter board designated as IC9. It houses the sync separator chip

i'll check those out too. I'd kinda suspected them. (IC9 and IC5 AKA "mystery potted boards") but not knowing, or being able to tell what they are ...   

best i could figure out was the sync went through the OR gate then into IC5 then off into the wilderness on an adventure back out teeing off with IC9 and then off the edge of the schematic.   

but yeah, i have a few things to test and try.

thanks guys...I'll be back one i've tried again.

[Rickn]

Hey Shawn, here is a little more detail:
Yes sync goes through the OR gat IC which has various pins connected to ground in order to accept either positive or negative sync.Theout put then goes to IC5 which has chip programmed to determine the frequency of sync. It the has an output to various transistors (Q1etc) in the power supply that will tell the B+ circuit. It also feeds IC9 which is the small mother board that houses an La7851 sync processor. Hope this helps.
Rick

[lilshawn]

Alright, i swapped out IC9 from a junked 3m27 chassis I dug up from out pile and it appears to have worked. Time will tell...begin 24hr burn-in.

Before i powered up, i also checked IC6/c45/R68 (5 volt regulated that feeds IC6) all good there. and checked for shorted/open poly caps. seemed okay.

so it seems the sync driver chip potted in this IC9 has gone sideways.

just for future's prosperity, has anyone successfully chemically removed potting epoxy from a component before? if it hadn't been for the fact i had a donor part, i'd be considering repairing this one.

Lacquer thinner? Acetone?

[lilshawn]

Damned this infernal machine. This monitor just will not stay running.

about ready to drag this SOB out to the field behind the parking lot and go office space on it.

the image isn't synced properly again but this time, the image has barrel'd. can't be sure what's going on. maybe B+? anybody know what it's supposed to be at for 31k res?

sorry for crud pics. hard to get a decent one.

[grantspain]

that is showing a 15khz screen running at 31khz i think,can you change the res on the io pcb to something different and then see what happens
b+ info here
http://wiki.arcadeotaku.com/w/File:Rodotron_666_Info_1.JPG