Identifying component in Atari 800 (maybe also 400)

[gdement]

Recently I tried adapting the Atari 800's chroma circuit for a 7800 video mod. I couldn't replicate it perfectly though. I'm missing the 10pF capacitor (which I'll be ordering) and also inductor L105.

 

The attached schematic, which originally comes from the official 800 service manual, shows an inductor L105 but doesn't give the value. It appears in the lower right in the attachment. I searched "L105" but of course that doesn't turn up anything.

 

1) Does anybody know the value of inductor L105?

 

2) Also, there are 2 transistors A104A and A104B, but the type isn't marked. I used 2N3904 transistors here. Are those appropriate?

 

Unfortunately I don't own an 800 so I can't inspect it.

=======================

 

So far this 800 circuit is ghosting, but I have a feeling that's because of the missing inductor. I fixed ghosting on an XE-based circuit with it's requisite 100uH inductor, so I think component L105 might be the solution here as well. Unfortunately 100uH doesn't work here, it just kills the color instead.

 

Thanks for any info.

[Divya16]

Recently I tried adapting the Atari 800's chroma circuit for a 7800 video mod. I couldn't replicate it perfectly though. I'm missing the 10pF capacitor (which I'll be ordering) and also inductor L105.

 

The attached schematic, which originally comes from the official 800 service manual, shows an inductor L105 but doesn't give the value. It appears in the lower right in the attachment. I searched "L105" but of course that doesn't turn up anything.

 

1) Does anybody know the value of inductor L105?

 

2) Also, there are 2 transistors A104A and A104B, but the type isn't marked. I used 2N3904 transistors here. Are those appropriate?

 

Unfortunately I don't own an 800 so I can't inspect it.

=======================

 

So far this 800 circuit is ghosting, but I have a feeling that's because of the missing inductor. I fixed ghosting on an XE-based circuit with it's requisite 100uH inductor, so I think component L105 might be the solution here as well. Unfortunately 100uH doesn't work here, it just kills the color instead.

 

Thanks for any info.

 

I have an Atari 800 opened up; if you tell me where to look, I can see if it's marked on the motherboard what type of transistor/inductor. What does uH stand for?

 

And Atari 400 doesn't have composite video.

[sloopy]

it is a plastic piece (most i have seen have been red plastic) about 1/2-3/4 inch high with copper wire wrapped around it. coils resist rapid changes in the signal going through them but allow steady signals througgh with no problem.

 

OP: best bet would be to make one, if you cant find an original. get the gauge of wire used, and number of turns in it and roll your own, the characteristics of them are so variable, that getting an exact measurement are nearly impossible without VERY tight quality control and specific tolerences (i.e. two similar appearing coils like what are in the 800, can vary as much as 20-30% from sample to sample...

 

Also trying different number of turns, and different sizes of wire until best results achieved would probly be even better then getting an original, as the simple fact that your not using the original 800 pcb would change things too... a SWAG would probly be start with 4 turns of 14 guage wire...

 

sloopy.

[rdhays82604]

uH stands for micro-Henrys. Quoted from Wikipedia -"The henry (symbol: H) is the SI unit of inductance."

 

 

I have an Atari 800 opened up; if you tell me where to look, I can see if it's marked on the motherboard what type of transistor/inductor. What does uH stand for?

 

And Atari 400 doesn't have composite video.

[gdement]

Thanks for the replies.

 

I have an Atari 800 opened up; if you tell me where to look, I can see if it's marked on the motherboard what type of transistor/inductor. What does uH stand for?

(uH question answered above)

I don't really know where it would be.. if I had an 800 I'd have to probe around with a multimeter to find them. But these parts are probably somewhat near the video output. The transistor locations might be marked A104 on the board. The transistors themselves should have a small, barely readable part number on them, but probably not on the board. I'm not very worried about those though, as the 2N3904 worked for my XE-based circuit so it's probably okay. It's just odd that Atari didn't label them on the 800 schematic.

 

The inductor is the main thing I'm wondering about, but I'm afraid it might not be marked. It would probably be near the same area, if it's labeled on the board then it would say L105. There might also be some similar inductors nearby marked on the board as L103 and L104.

Problem is, I don't know if these inductors have anything marked on them. My 100uH inductors have a marking but they're bulky shielded types with a shell around them. I'm afraid most inductors might not have anything written on them, so reading the value directly might not be possible.

 

 

OP: best bet would be to make one, if you cant find an original. get the gauge of wire used, and number of turns in it and roll your own, the characteristics of them are so variable, that getting an exact measurement are nearly impossible without VERY tight quality control and specific tolerences (i.e. two similar appearing coils like what are in the 800, can vary as much as 20-30% from sample to sample...

 

Also trying different number of turns, and different sizes of wire until best results achieved would probly be even better then getting an original, as the simple fact that your not using the original 800 pcb would change things too... a SWAG would probly be start with 4 turns of 14 guage wire...

Well, if the value isn't known, then I'll probably try ordering a few different sizes and see if anything appears to work decently, and go from there. I'm hoping somebody might know what the correct value is supposed to be, but I guess if Atari never documented it, it might simply be an unknown.

[sloopy]

my apologies, i was thinking of the L101 Coil, the L105 doesnt list a value because it doesnt have one, it is basically a noise filter... the 7800 probly has them too, if you have an XL or XE machine the have several... it is basically a ferrite 'bead' around a wire with no turns... if you have a 800XL or 130XE handy you can see what they look like, there is one right behind the power connector (right behind on XE, with a resistor between on 800XL) here is a pic of that section of the board...

 

L103, L104, and L105 (and L108) are on the right side in the line of horizontal resistors, look close and you can see the component numbers on the PCB

 

 

sloopy.

Edited August 6, 2010 by sloopy

[gdement]

I see. Thanks for the photo.

I did some reading on ferrite beads and it looks like there's variations in their filtering profile, but I'll probably just get a few and see what happens.

I'm now doubtful that this will do much to the ghosting problem, but I'm glad I read about these things anyway. It looks like a good idea to use a ferrite bead at the output instead of a plain resistor, it might filter a bit of high frequency noise if there is any, and there's probably no downside to it.

Thanks for the help.

[Divya16]

I see. Thanks for the photo.

I did some reading on ferrite beads and it looks like there's variations in their filtering profile, but I'll probably just get a few and see what happens.

I'm now doubtful that this will do much to the ghosting problem, but I'm glad I read about these things anyway. It looks like a good idea to use a ferrite bead at the output instead of a plain resistor, it might filter a bit of high frequency noise if there is any, and there's probably no downside to it.

Thanks for the help.

 

So how does filtering high frequencies using L105 cut off the color and gave you gray output?

[+bob1200xl]

At the color clock frequency (3.58MHZ), a 100uH coil is like a 22K resistor. Might as well be an open circuit... These little ferrite beads only affect really fast signals, 100MHZ or more. They put them in so the computer would pass FCC testing.

 

Bob

 

 

 

 

I see. Thanks for the photo.

I did some reading on ferrite beads and it looks like there's variations in their filtering profile, but I'll probably just get a few and see what happens.

I'm now doubtful that this will do much to the ghosting problem, but I'm glad I read about these things anyway. It looks like a good idea to use a ferrite bead at the output instead of a plain resistor, it might filter a bit of high frequency noise if there is any, and there's probably no downside to it.

Thanks for the help.

 

So how does filtering high frequencies using L105 cut off the color and gave you gray output?