MS8 dead

[lilshawn]

MS8 was in shop, recapped to fix some foldover. went out on location for about 2 months... died. no ticking that I can tell.

brought it back and found I installed C433 backwards. ) oops. dunno how it managed to work all that time with that cap in backwards.  replaced cap obviously... checked the feeder resistors in that circuit and they seem in spec still.

still no picture.

checked B+ at the fuse and it's at 70 volts. I'm getting some kind of high voltage in the tube (getting a discharge), just no picture and no real evidence of flashover when the power is pulled.

trying to figure out this problem, it doesn't help that the schematics available on the net where done with a potato and a pile of the info out on the internet is sketchy at best... having all kinds of conflicting information.

I'm not used to how this chassis works. I'm more of a switchmode kind of guy. Whoever designed this chassis... i dunno man.

any ideas how I can proceed with diagnosing this?

[grantspain]

i magine you isolated the HOT completely and read it
is it a ms8-26,ms8-25 or ms8-29?

[lilshawn]

this chassis is a MS8-29SU

I did check the HOT and found it good.

I found C909 in a questionable state. I wouldn't say the top was bulging... but it was flat and seemed a little odd. that made me question it, as usually the little marks they stamp in the top usually cause the can to be slightly concave. . I had previously replaced it when I recapped it and I did replace it with a good matsushita cap. So I initially dismissed it. I went back and I couldn't get a reading on it at all with the ESR meter so I replaced it.

Replacing C909 did the trick.

Not sure if having C433 in backwards caused C909 to go wonky and fail too as well.

[lilshawn]

of course I would end up with 3 more of these monitors in to be fixed since I have mostly no idea how they work. A fella had a bunch of daytona cabs where the monitors crapped out and of course brought them to us to be fixed.

I will for now focus on unit#2 unit is set for 25k res.

no power up at all. Blown main resistor (R901) tracked to a bad bridge rectifier. - replaced both.

unit also had blown Q902 D901 Q901 and suspect HOT. replaced all items.

unit now has B+ and anode high voltage, but no audibly discernible deflection or screen. B+ extremely High (measured at TP @ 152v) no tube flashover when power is cut.

[lilshawn]

anybody know how this monitor works?

this power system is absolutely ridiculous. I can look past the hideous B+ drive... but it looks like they are using dropper resistors to dump the voltage down for the subsystems...

  far as I can tell pin 4 is the VCC in.

any idea how this works?
any idea what voltage this should be at?

SFA for information... been touching up my schematic in mspaint as I test along so at least it will end up remotely viewable.

[grantspain]

b+ on these should be 94v on 15khz if that helps
i normally dummy load these for testing b+ btw

[lilshawn]

I switched the monitor to 15k for a test. i powered up with a meter on the B+ and the monitor comes on... deflection runs for 1/2 a second then shuts off. B+ is still in the 130's. suspect it's overvolting and shutting down.

removed D905 and the fuse to isolate B+ section.. loaded the B+ with a 75 watt lightbulb... B+ runs around 135 volts.

so the B+ is partially loaded... but something is open causing it to drive high. I'm getting HV so the flyback, HOT, and (I think) IC103 is working because the HA11423 runs a HOT drive circuit.

maybe something open in the vertical drive then? Q401 Q402? (both test okay with a B&K tranny tester, but they can still fail under load at high voltage) maybe xray protection is kicking in?

[grantspain]

i would say the regulation circuit is not correct

[lilshawn]

I replaced most of the stuff in the B+ regulation circuit... loaded down the supply with 75 watt bulb but it's still 130.

but I've recently found conflicting information regarding whether this chassis needs an isolation transformer or not. I figured since it didn't have a regulator... and that the AC comes in, goes through a bridge rectifier and nothing fancy is going on... that it doesn't.  I've been just plugging it directly to the AC.

I've some other posts online mentioning that it does...yet another saying it doesn't as long as it's not connected to a signal, but does if it is.

I'm just going to go ahead and assume now it does, but now... what's all !@#ed up because of running without one.  Guess i'm going back over the B+ again.

[lilshawn]

okay, I've basically replaced everything in the B+ section (3 transistors, 3 diodes.), but it still doesn't regulate properly.

what am I missing here? is my load not enough?

[ed12]

wait wooooooooooo and hold on...
when it gets rectified dose it hit a switcher transformer ?

ed

[lilshawn]

nope. best i can tell it's some kind of astable multi vibrator linear regulator with the B+ adjustment on the feedback arm to adjust the B+

almost an STR regulator made out of discrete parts.

I've loaded the circuit with 40watts up to 175 watts with varying results (130v B+ up to 150vB+)

[ed12]

it is hot
where dose d905 go (flyback) ? or just horz sweep ?

ed

[lilshawn]

it takes off and goes to power an HA11423 deflection IC, I've actually lifted out D905 to just power the B+ and get that sorted out first.

I've removed the fuse (F902) as well. I'm powering my load from the B+ side fuse terminal and TP2 which is the (-) rail from the rectifier and ultimately the chassis ground

I've literally removed every resistor and diode and tested them all out of the chassis... I've also replaced all but 1 of the transistors (Q902)...I stole that one from another chassis because I couldn't immediately source it.

[ed12]

ok my thinking is to ask
which pin / 's ? on the ha11423

ed

[lilshawn]

I've started touching up the schematic as I attempt fixing it... this is the best quality you can get and it's just barely readable.

the B+ isn't oscillating so i'm not getting any output to the vert IC. (output transformer T901 isn't doing transformer stuff because no change in current...no magnetic field...no output.) it's been eliminated from the circuit anyways...need to get this B+ nailed first.

replaced all transistors (Q901/902/903/904) resistors all within a few % of spec.

figured Q902 was shorted again after testing these voltages (12 volt drop?) so I changed it and it sounded like something immediately popped again. (B+ out is 158 (matching the input) now.)

B+ is currently loaded with a 75 watt lightbulb.

[ed12]

is the ic on a sub pcb ?
i ask because i think it is a modified chopper drive

ed

[lilshawn]

replaced a few things (Q901 and Q902 again) and think I got it mostly working. it's currently at about 96 and i'm getting oscillation now cause i'm also getting output at T901

I also lowered my lightbulb load to 40 watts to be extra safe.

[lilshawn]

is the ic on a sub pcb ?
i ask because i think it is a modified chopper drive

ed

yes it's on IC103...but it's not part of the B+ circuit per se.

[ed12]

good i can still see
welp that is where i would be looking
u see from what i can see,that transformer is there to :induce: the b+ to a regulated load
ie: chopper drive
so normaly if i scope that point i would expect to see a lovely saw there
meaning the monitor is up and stable
i do not know if u ever worked on the hatachi pro co 40 ?
but it was the same design in theory

ed