Fixed: Wells 7500 Clicking, no HV

[Memories_Games]

This is my 2nd attempt at a post.  Somehow I hit some combination of keys that undid about 15 minutes of typing before.  So this will be without some background info.


Chassis  p793, WGM 2775-OUT561H
Neckboard  p719, WGM 2775-UOT561H
Frame has a sticker "replace with k7500 Chassis only"
Tube : Zenith A68AG01X

Many parts have been removed since the game, (Gauntlet Dark Legacy) got mothballed, but I bought new replacements for all of them.

Original problem was a vertical collapse I think, but the thing still ran.  (HV, some picture or at least a line, etc.)

Should have been a quick fix, but no.

Doesn't come up. No pic, no HV.
 Click click click....

Referred to flowcharts (RF Blue Book) and proceeded to go crazy testing the HOT for a short.  Well of course it is continuous to ground, it's mounted on a grounded heat sink.  Duh.

I eventually noticed some sort of insulator on another chassis I have for comparisons.  (Not exact same model, but many of the same parts.)

Um, not duh?

Ah, lets try it with a piece of duct tape behind it.  Hey it's running!

That lasted 15 or 30 minutes.  Sigh.

Pretty sure my new HOT is shorted now.  Base and Emitter are continuous.... No, it has to be bad now.
Why did it go?  Heat? Or something else in the circuit shorted or otherwise straining things?

I pulled the HOT off of another donor chassis, hoping the HOT wasn't he reason that I pulled that particular board.

Now I can't get past the shutdown clicking.

Where it stands now:
I removed the HOT, and did a lightbulb test between the diode leg on that heatsink (looks like a transistor, but has 2 legs and is marked D) and ground.

Light comes on.  TP202 shows 120V.

The last HOT I have been trying, well I'm not sure about it so I ran some diode checks on it.  Maybe it's bad too? (I don't have another new one to compare to.)

  (Red Lead) - (Black Lead)
  Base - Collector .052, B-E .045, C-E .003 (beep), C-B .053

  (Black Lead) - (Red Lead)(
  Base - Collector .053, B-E .050, C-E .003 (beep), C-B .053


I have spent so much time looking for shorts its driving me crazy, but there might not have been one in the first place?  And I likely made a mistake  (or six) and made things worse.

I would appreciate any help to get this thing figured out.

Thanks,

-eric

[qrz]

duct tape will provide electrical isolation but poor thermal transfer characteristics .

in other words, your hot likely overheated with thermal runaway, then shorted.

 plastic encased horiz output xistors are isolated . they should have a thermally conductive substance
mating the device to the heatsink.
most metal tabbed cases also require electrical isolation ( mica insulator-isolated mounting screw )

an in circuit HOT's base/emitter junction will read as a be short due to the secondary winding of the driver xformer ( see schematic )

out of circuit , some may read a low (<150 ohms) due to internal biasing resistors  specific to the device .
when in doubt , refer to device mfgr specs here  : www.alldatasheet.com

qrz

[Memories_Games]

The game incidentally had OTHER issues.  As in the board wasn't booting up.

Today I had a couple of hours ALONE in the arcade so I tackled this project again.
(You'd think we'd be busy on holidays, but nooooooooooooo.)

It took me quite a while to figure out how to even remove the board set, but I finally got it out and separated the three sub-boards.  I noticed a little corrosion on one of the edge connectors which is pretty much what I was hoping to find.

After reseating them all and putting it back together it didn't seem to be any different, when all of a sudden I saw more flashing on the HDD's LED and some characters on the digital display that weren't there before.  Well well, it booted up fully, at least I think so.  Made that familiar Midway bong noise and all.

So I decided to dig up yet another WG to scavenge a HOT off of.  One of my Cruisn's is just sitting there with a bad flyback so here we go.  After pulling the HOT out I quickly verify that the last one I used was NOT testing the same. Ah, very promising, so in it goes.

Powered it up and got a pretty good picture.  Some adjustments on the remote board and almost perfect.  (Seems like the entire picture is leaning slightly, but that is not a huge concern to me.  Should be a easy fix.)

So apparently my repairs (Caps, new vertical IC, etc) were good, but I was simply unaware of the HOT needing to be electrically isolated from the heat sink.  Live and learn.

I have a few new questions though:

There was a small transistor wired in by the HOT and I had to compare another chassis to figure out where it belonged.

Why were these moved to another heatsink at the frame, and what size is that screw?

And what are the gray isolation  squares called?  I'm gonna need more of them and the screws/nuts to eventually repair the donor  chassis'.  Gotta know what to look for and where I can source them.

Thanks!

-eric

[qrz]

the xistor that has been relocated is likely the pin out xistor .
i would think the move was related to thermal performance .....

mcm electronics   800 543 4330        www.mcmelectronics.com
to-3p  mica insulators ( use heat sink compound )       mcm # 21-1170
to-3p  thermal pad     ( no compound required)     nte/mcm # TP0008
several choices available for heat sink compound