G07 Issues

[SirPeale]

I have two G07 monitors, here's what I did:

Monitor 1 was totally dead.  F901 blown.  Replaced flyback, Horizontal Output Transistor, F901 and recapped.  Came up with a perfect pictures.  Let it bench for a while.

Monitor 2: did not fire it up as the tube was damaged.  Replaced tube with one from a donor TV.  Figured to use the newly rebuilt G07 to converge the tube.  It was working well, and the convergence was even pretty good - but then I thought I'd get crafty and try and converge it some more.  Stupid mistake!  I made it even worse.  Suddenly the whole thing shut off.  On investigating, F901 is blown again!  The Horizontal Output Transistor appears to be okay (black lead to frame, red to case) and I'm getting very high resistance (like 10K or something, it's in the shop and I can't check it from here).

So why did it blow?

[SirPeale]

Revisiting this finally.

I used a fuse holder with alligator clips to "jumper" in a new F901.  The monitor came right up.

I was about to start work on converging the monitor when it struck me that I'd never adjusted the B+ on this beastie.  Metered it - 147VDC!  Ouch!  I attempted to turn it down, and it would not budge.  Turned it off before it could do any further damage.

It's otherwise working (though that may be a reason the fuse blew to begin with) so what could cause the adjustment pot to not work?

[SirPeale]

Regulator failure?

http://forum.arcadecontrols.com/index.php?topic=64474.0

[Ken Layton]

Any one (or all) of the transistors in the power supply section could be bad. In general if the voltage is always TOO LOW and won't adjust the transistor(s) is/are OPEN. If the voltage is TOO HIGH and won't adjust then the transistors are leaky or shorted.

Check/replace X901 and X902. Sometimes these will test good out of circuit but actually be bad under a load. This is where you must measure the transistors voltage in operation.

I'm betting one or both X901 and X901 are bad.

Don't forget to check R908 too.

[SirPeale]

Thanks Ken; I'll check it next time I'm in the shop.

[Ken Layton]

I had one where the B+ was too low (stuck at 100 volts) and would not adjust. Turned out X901 was open.

[SirPeale]

Though I have not had the chance to test anything, being able to source parts JIC is important.

My go-to place is Mouser (though if I can pull one from a junk TV chassis I'll do it) and while looking for the C2688 I note there are two kinds:

C2688YS

C2688STU

The datasheet URL for both parts is the same.

They're cheap enough, I could get them both for .30 each, but what do the extra letters signify?

[SirPeale]

Great...X902 (C1890) isn't available from Mouser.

[Ken Layton]

2SC1890A (superceeds 2SC1890) and 2SC2688 are both available from MCM Electronics.

I believe that's where I bought them from before a few years ago and they were dirt cheap. Buy 10 or more since they are so cheap.

[SirPeale]

Yeah, but I don't need anything from MCM right now.    I've got a whole Mouser order almost ready to go!

edit: added links above.

[Ken Layton]

The "STU" suffix means it's packaged for automatic inserting/production equipment. You don't want that.

Get the "YS" suffix one as it's normal "bulk" packaging which means you can buy just one or however many you want.

[SirPeale]

R908 completely open.  Would have attempted to replace it, but it's a five banded resistor, and I've only memorized how to read the three banded variety.  And of course I didn't have the schematic, so...

(yes, I have the schematic - it just wasn't @ the shop.)

[SirPeale]

I updated my "Tech Page" to reflect the five and six banded resistor calculator. 

http://www.pealefamily.net/tech/

Before I only had  four band calculator.

[SirPeale]

Success!  Replaced R908 and I was able to adjust the B+ to 120.1VDC. 

Still a couple of minor convergence issues, but depending on what game it goes in it'll never be noticed.

I need to work on my monitor test bench.  I've been using an old Capcom Bowl board and can not get it to lock on, even when connecting to both -V and -H sync.  Hooked it to a Golden Tee '98 I'm working on, perfect picture, synced right up.