16" monitor for an arcade game machine

[Michael Maurice]

I was given this monitor to repair if I could,

It was originally an American 525 line 110V TV set, indeed both VHF and UHF tuners are present but disconnected.

The video is injected into pin 11 of V5, leaving the coil and 4.7K resistor in place.

What I dont know is the video output from the games, computer card, I presume its a standard 1V p/p video output.

However looking at the diagram, what should be on pin 11 of V5 is an 8V p/p video circuit, so I need to make up an amplifier with a gain of 8.

I could use an op amp, fed from a 12V supply, there would be no dc clamping. Has anyone got any suggestions as to how I go about this?

The diagram of the TV and the enlarged video section are below in PDF form.

The red arrow shows where the video is connected to at the moment

[julie_m]

It looks more like .8V to me.

[Michael Maurice]

I did some experimenting last night.

The one thing I missed (as did the person doing the original mod) is that if you look at the output from the vision detector, it is negative with respect to ground.

The video input is positive going wrt ground and tha'ts being fed to the grid of the video amplifier valve whose cathode is at ground.

I then fitted a very humble component between the input and the grid, a 0.1uF capacitor.

Voila! A decent greyscale and locking picture.

Now all I have to do is fix the audio and tidy it up all!

[Tazman1966]

Excellent! I like it when a simple solution presents itself like that.

[Nickthedentist]

Clever!

[Michael Maurice]

I fitted a 2N3055 transistor in position Q301, there was lots of crackling from the speaker then it went off. The resistors in the collector circuit were smoking.

This means that the driver amplifier has failed, but what is it?

I can think of two options.

1. Design a small op amp like a 741 to feed the original 2N3054 out put transistor.

2. Ditch the output transistor and transformer and design a small amp either around a TBA800 or TDA2030

What do you think?

[ronbryan]

Q301 has 157V HT. Seems a bit much for a 2N3055 which I think is 60V Vce max. Try something with a suitable Vce rating.

Ron

[Michael Maurice]

Sorry I meant 2N3054 300V ce

[ronbryan]

Michael

The 2N3054 is no better for that application as Vceo is specified 55V, which is way too low compared to the 157V HT supply to the output stage.

What was the type number on the original Q301 transistor?

Ron

[Michael Maurice]

Ron,

This is what Farnell showed up http://uk.farnell.com/nte-electronic...tor/dp/1611448.

I will try this one which both Cricklewood and Farnell agree on the spec.

By the way, the original 2N3054 did not go short circuit, when measured it seemed OK.

I will disconnect the output transistor, refit the module and see what comes out of it.

[Michael Maurice]

I finally got round to fixing this.

I made up a board which is the same size as the original and using a 741 op amp a 2SC1815 + a few resistors and capacitors.

I replaced the output transistor with a 2N3858 from Farnell, and it seems to be working.

I can always up the gain by replacing the 12K feedback resistor with a 22K, but I rather think its sufficient as the sound will probably be 5V TTL.

[Michael Maurice]

This is now back with its owner.

I even managed to display a 625 picture off a freeview box.

I would guess this set was either late 1960's early 1970's.

It used valves and valve bases that I'd never seen before. 12 and 13 pin bases. The line output valve and boost rectifier were in the same valve.

The monitor came out of an early arcade game and are apparently at a premium. The owner tells me he has spent £100's to get the whole thing working

He tells me that a replacement monitor from the US would cost $500 + shipping!

[cmjones01]

Originally Posted by Michael Maurice

I would guess this set was either late 1960's early 1970's.

It used valves and valve bases that I'd never seen before. 12 and 13 pin bases. The line output valve and boost rectifier were in the same valve.

I guess those would be 'Compactron' valves. I've just refurbished a Tektronix scope which used one in its EHT power supply, which was odd because most of the rest of the scope was built with standard B7G, B9A and Octal valves.

The monitor came out of an early arcade game and are apparently at a premium. The owner tells me he has spent £100's to get the whole thing working

He tells me that a replacement monitor from the US would cost $500 + shipping!

Yes, CRT arcade monitors are a dying breed - they're being replaced by LCD panels in their droves, which I think is a shame. The screen is very much part of the character of the game. I'm happy to hear that this one lives on!

Chris