Flyback for Samtron mini monitor?

DrCharles

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I've had two of those 5" (maybe 6") Samtron monitors on the shelf for many years. I don't remember what I paid for them but it was much less than $49...

I have a Nichibutsu "UFO Robo Dangar" board that is driving me nuts, and finally decided to build a test rig for the bench so I can get to work on it. I should have done my Internet homework before hooking up the Samtron... connecting it directly to the board was the wrong move, as the horizontal output transistor got so hot that the paint on the heat sink smoked and it actually charred the board underneath it!
=:^O

Amazingly, the HOT (a BU406) survived this abuse, (confirmed by testing it switching 250 Vce, removed from the board of course), but after building the correct interface board including sync inversion, there is still no HV and the CRT filament doesn't glow. This leads me to believe the extreme current drawn by the improperly driven HOT has fried the flyback also.

So - does anyone have a parts chassis with a good flyback? Also, is there any source of schematic and service info for this little monitor?
 
I have discovered that both Samtrons were defective right out of the box. Even with no inputs connected, the horizontal output transistor (HOT) was overheating, and a heavy current draw on the +12 supply. Even so, the HV was about 6 KV and the CRT filament only had a volt or two. Also the boost supply (generated from the flyback to provide supply voltage to the video amplifiers) was only 17 volts.

I disconnected the yoke and the neck board. No change. So SOMETHING had to be heavily loading down the flyback. After a little probing I found that the boost diode (D307, a 1N4007) was shorted on one chassis and very leaky on the other. I changed it and now I have 64 volts on the boost, normal CRT filament, and I can hear the yoke "buzzing". But still no screen illumination! (While messing around on the original chassis I managed to blow the overheated but still good HOT, a BU406. Waiting for a new one.)

The "Screen" control varies that grid's bias from +270 down to +70 volts or so. Does this sound like a normal range?
But the video amps' DC level is only about +40, and in most schematics I have looked at, the first grid is at ground and the video amp drives each cathode. So the grids are still well negative with respect to the cathode... no brightness!

The sub-brightness pot changes bias on a transistor from 3 to 7 volts but I don't see any change on the cathode voltages. So the problem has to be in that area. Without a schematic this is rather a PITA to figure out, though fortunately it's a pretty standard design. Anyway the flyback does not appear to be the problem after all.
 
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I am a complete idiot. Well, partially at least.

I even tried removing the connectors for each R,G,B gun and shorting the pins (i.e. grid and each cathode at same potential, which should be very bright for that color only). Still no illumination, so something must be wrong with the CRT itself.

I turned off the room lights and couldn't see any heater glow... good voltage from the flyback though. Could all three heaters be bad? Pulled the neck board off and measured at the CRT pins, 6.6 ohms cold... ok, it's not the CRT. Then I paid a bit more attention to the neck board. There is no "key" on the base of this particular CRT, and I had re-installed it one position off!!! So I plugged it back in, hooked up the UFO Robo Dangar board, and a nice clear image of the attract screen promptly appeared :) Murphy's Law is still in effect, it seems.

Anyway, it's interesting that both chassis had the same bad rectifier, right out of the box. Might be worth checking if your Samtron won't light up.
 
My test setup

Just wanted to show it off, working ;) A 5" Samtron, perfboard with the sync separator, inverter and contrast pot, a piece of scrap lumber and a PC power supply from the junkbox (all the 5 and 12 volts an arcade board could ever use).

Actually I cheated and found a TTL vertical sync signal on the UFO Robo Dangar board, because the LM1881 sync separator hasn't gotten here from Jameco yet ;)

P3220182.jpg
 
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