ADVENT Videobeam

[etype2]

Advent 750 Owners Manual Link: https://visions4netjournal.com/wp-co...advent-750.pdf

[maxhifi]

Thank you very much!

Mine, being an Electrohome, is a little different from the instructions. The projector looks the same, but my screen has a white molded plastic back, with ELECTROHOME molded into it, and a tilt mount, with instructions how to set it to the correct angle by hanging a weight on a string off the top front, and measuring 7 5/8 inches from the bottom.

The screen is in decent shape - it appears to be some kind of aluminum film which is stuck directly to a plastic dish shaped background. The film is bubbling up in a couple places, but I'm just leaving it as is.

Your instruction book helped me to set up the position of the projector correctly. It doesn't completely fill the screen at the top left corner, and the bottom has a slight bow to it, so I think it will need some work.

One interesting thing I noticed, is the pattern generator only works when the unit has a video signal. If you switch to convergence when the unit is on but not connected to any source, it's just a blank screen.

[jbattles]

show us some insides

[Electronic M]

I like that they suggest using it with an early U-matic deck (based in pictures).

[maxhifi]

Tonight turned it on, smelled smoke, then suddenly the picture looks horrible - washed out, no contrast, and blurry.
It looked ten times better yesterday. No idea what burned.

Now I can't set greyscale, even with brightness all the way down, in service mode I can't get the tubes to cut off.

[maxhifi]

To clarify the above post, the brightness is excessive, even with the brightness control all the way down. I am suspecting that the burning smell, was something related to the G2 voltage, since it seems to affect all three tubes equally. High G2 voltage should give this symptom, right? Another thing is, the power transformer gets very hot in this set.

[maxhifi]

Another thing I noticed, is the contrast control basically controls brightness, where previously it functioned normally as a contrast control.

G1 is just a bias control, and appears to be fine.brighness is set by the cathode, and the cathode DC voltage is about 200 or so, depending on the position of the brightness control.

Focus appears okay, the focus output of the voltage multiplier puts out about 6kV.

Checking the G2 has been so far impossible, theres no way to get the chassis out easily.

[maxhifi]

More exciting developments:
Screen voltage can be varied from 500-750V approx, and and the G1 Kine Bias is set to about 10V.

The focus take off point of the multiplier is about 6kV, and high voltage is about 23kV. My high voltage probe often reads low.

Good continuity though focus controls and focus voltage divider resistors.

The chassis can be very easily pulled, and really is just a console chassis. All wires have enough slack it can be operated while the bottom is accessible.Nothing under it appears damaged in any way.

The neck board has power supply voltages screened on to the board, and they measure as they should.

Not having service data, its impossible to know for sure, but after measuring voltages I think my theory that the tube are over biased is wrong, and I should instead be looking to the video amplifier.

[zeno]

Check the cathodes, they are probably low. Typical K volts on
a console is 125-175 ish depending on design & CRT.
They are run off a 200V supply from a FBT winding. Typ a fusable
resistor, rectifier then a 5-10 mfd filter. Sometimes the supply is
stacked on the main 130V B+ so the filter goes there instead of GND.
To find the 200V each jug has a hefty driver transistor. The collector
goes through a 2-3 watt resistor 18 K ohm plus. That is hooked to the
200V. Look at almost any SS color schematic & it will be near the same set up.
As for the G-2 the low end sounds to high. Typ 800V boost on the high
end & the low end 200-300 V. The wiper goes to G-2.

BTW if you can post some nudies for the group of the chassis.

73 Zeno
LFOD !

[maxhifi]

Originally Posted by zeno

Check the cathodes, they are probably low. Typical K volts on
a console is 125-175 ish depending on design & CRT.
They are run off a 200V supply from a FBT winding. Typ a fusable
resistor, rectifier then a 5-10 mfd filter. Sometimes the supply is
stacked on the main 130V B+ so the filter goes there instead of GND.
To find the 200V each jug has a hefty driver transistor. The collector
goes through a 2-3 watt resistor 18 K ohm plus. That is hooked to the
200V. Look at almost any SS color schematic & it will be near the same set up.
As for the G-2 the low end sounds to high. Typ 800V boost on the high
end & the low end 200-300 V. The wiper goes to G-2.

BTW if you can post some nudies for the group of the chassis.

73 Zeno
LFOD !

Cathodes are actually high, at the top end of that range. About 175. The +200V terminal (marked on board) on the neck board is dead on 200V.

I will look into the bottom end G2 voltages being too high, by tracing out that part of the circuit.

Last night I tried the projector again - things have gone from bad, to worse - the picture now is totally losing contrast and brightness. It's an extremely dim, washed out and blurry picture, but if you turn up brightness the raster itself is quite bright.

I managed to track down a paper copy of the service manual, from www.stereomanuals.com - once it gets to me, I will be in a much better position to troubleshoot.

I'm starting to wonder if this is an AGC issue, but there's only an RF AGC control on this set, and I'm using the composite video input.

Will take some photos tonight for the forum - it's a very neat set, nice change from the usual.

I have to say that last night, the "audience" of the home theater was starting to lose patience

[maxhifi]

Another thing occured to me - why don't I just put on a test pattern and look at the CRT cathode waveforms with an oscilloscope?

They should look very similar to the cathode waveforms shown in the Sams for my Motorola Quasar works in a drawer set, no? I suppose I could pull the back off a known good TV, like said Motorola, and see how it compares.

[zeno]

Why not ! But usually a problem like this is voltage related. All 3 video
outputs are tied at the emitters to the B&W info. The colors come from the
demod to the bases. To simplify turn off the color & look at a stair case pattern.

BTW there is someone here & on AR selling Canadian service manuals. He may be able to supply you with the Electrohome manual for the normal console set......

LFOD !

Originally Posted by maxhifi

Another thing occured to me - why don't I just put on a test pattern and look at the CRT cathode waveforms with an oscilloscope?

They should look very similar to the cathode waveforms shown in the Sams for my Motorola Quasar works in a drawer set, no? I suppose I could pull the back off a known good TV, like said Motorola, and see how it compares.

[maxhifi]

Originally Posted by zeno

Why not ! But usually a problem like this is voltage related. All 3 video
outputs are tied at the emitters to the B&W info. The colors come from the
demod to the bases. To simplify turn off the color & look at a stair case pattern.

BTW there is someone here & on AR selling Canadian service manuals. He may be able to supply you with the Electrohome manual for the normal console set......

LFOD !

I got notice the Advent service manual is in the mail - will be here within a week or so I'd assume.

I really look forward to getting it, so I can check what the voltages are supposed to be.

Either that or I can keep using it - it seems to get worse each time I use it, maybe it will fail altogether and become easier to fix

[maxhifi]

Originally Posted by zeno

Why not ! But usually a problem like this is voltage related. All 3 video
outputs are tied at the emitters to the B&W info. The colors come from the
demod to the bases.
LFOD !

I've noticed that not all TVs are built this way. The Motorola I'm looking at mixes in the monochrome video signal earlier in the chain. Then there's tube sets which apply the monochrome signal to the cathode, and the colour info to the G1 of the CRT. If the Electrohome chassis does mix it in right at the emitter of the video outputs, then that's a good thing. Those three outputs are on a common heat sink, on a floating PC board suspended between the CRT tube sockets by stretched O rings.

The fact that the contrast control was directly controlling brightness, makes me wonder if the DC levels are correct in the video amplifier circuitry.

[maxhifi]

I hooked it up to a test pattern, and an oscilloscope. The brightness and contrast controls do their thing - the brightness shifts the video signal up and down, and the contrast controls its dynamic range. The amplitude is in all cases way too small, it is about a 70V peak to peak signal with brightness and contrast all the way up, with 175VDC on the cathodes. Changing the kine bias and screen controls does affect brightness, but I think the video signal is just too weak to turn the CRT on completely.