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Electrohome ECP 3500 fails to sync with reversed polarity

 
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johey




Joined: 07 Aug 2022
Posts: 10
Location: Södertälje, Sweden


PostLink    Posted: Sun Aug 07, 2022 12:03 pm    Post subject: Electrohome ECP 3500 fails to sync with reversed polarity Reply with quote


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TLDR; Which part of the Electrohome ECP 3500 is responsible for detecting the vertical sync polarity?

The long story:

I have an Electrohome ECP 3500. Using the dual RGBHS input board, I can drive various RGB sources perfectly well. However, for some resolutions, the projector is unable to find the sync correctly. I have made some conclusions, including to my best theory, it is a sync polarity detection issue.

All 240p kind of resolutions with composite sync works fine. No problems there. The only time I have had any issues is with separate H and V sync, like a PC with VGA card. Some VESA modes don't work, including the default 640x480 at 60 Hz (or 72, 75, 80). Other resolutions may or may not work. 1152x864 in 75 Hz works just fine.

The sync polarity theory I have is deduced from my experience of connecting an Amiga computer with a graphics board. In the driver utility, I have quite many parameters to tweak. I can create basically any resolution and get them all to sync correctly or to fail, simply by ticking or unticking the "vertical polarity" checkbox. The corresponding horisontal polarity has no such effect on the sync.

My best guess, hence, is that the projector fails to detect either active low or active high vertical polarity (I don't know which one) by some reason, but the horisontal sync polarity detection works fine.

Another observation, and this is a strange one to me, is that when I open a screen I know will fail, it first produces it correctly in sync, but after a second it fails. The behaviour is deterministic as it seems. Fully reproducible.

Can anyone please point me in the right direction here? I'm brave enough to open the box and peek around, and I have access to oscilloscope and other potentially useful equipment. I have scanned through the service manual, but cannot really understand which part is responsible for the sync polarity detection. Is it possibly the input board itself?

Thanks for reading my first post in this forum, and by the way, thanks for letting me in!
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kal
Forum Administrator



Joined: 06 Mar 2006
Posts: 17850
Location: Ottawa, Canada

TV/Projector: JVC DLA-NZ7


PostLink    Posted: Sun Aug 07, 2022 12:35 pm    Post subject: Reply with quote

Hi and welcome to the forum!

While many early CRT projectors would specifically require positive or negative sync (I had a Barco 800 like this a long time ago), the ECP series wasn't one of them I believe. See: http://www.curtpalme.com/docs/ElectrohomeECP4501_4500_3500_3501_2500plus_Brochure.pdf
It mentions "• Separate sync is automatically accepted in either polarity"

ECP 3500 will scan to 62Khz (hscan) and 120Hz (vscan) so make sure to stay below those of course.

Hopefully someone who's owned one can chime in. Didn't find much of anything about sync requirements on the ECP.

Some older models also work better with combined H+V sync so you may want to try that. Just use a simple adapter: https://amzn.to/3SxzJIJ

Kal

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robin36mac




Joined: 24 Jan 2009
Posts: 44



PostLink    Posted: Sun Aug 07, 2022 1:47 pm    Post subject: Reply with quote

If you have NVidia drivers, you can create a custom resolution dans select/force the sync polarity you want. Should exist on AMD now too, but I haven't worked with them.

Might be easier, PowerStrip software can do that too.

If you have an oscilloscope, you can also trace the sync signals and observe the polarity you have by yourself. (A t-BNC is ideal to do this effortless)


Robin


Last edited by robin36mac on Sun Aug 07, 2022 3:32 pm; edited 1 time in total
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johey




Joined: 07 Aug 2022
Posts: 10
Location: Södertälje, Sweden


PostLink    Posted: Sun Aug 07, 2022 2:31 pm    Post subject: Reply with quote

Thanks for your suggestions!

I am not sure the ECP 3500 is supposed to sense and adapt to sync polarity, but I strongly believe so. First, the flyer mentioned by kal states that, but that flyer is for the Plus versions of the projectors. I have seen no information of the differences between regular and Plus, and I only find a user manual for the Plus version. Second (more vague perhaps) is the fact that it can determine the polarity of the horisontal sync automatically. It is just the vertical that renders problem.

I am aware of ways of controlling and analysing the sync polarity externally. For instance, I could build a simple sync inverter circuit with a switch. However, I am keen to know if fixing the issue with the projector is an option, or if it is just too complicated to bother. Maybe it is just a capacitor to replace or something.
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johey




Joined: 07 Aug 2022
Posts: 10
Location: Södertälje, Sweden


PostLink    Posted: Mon Aug 15, 2022 6:13 pm    Post subject: Reply with quote

Another failure. I bought a VGA cable for the SEGA Dreamcast, and the same thing happens.

As nobody seems to know anything about this, I guess I need to try building a sync inverter. That's my next project.
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johey




Joined: 07 Aug 2022
Posts: 10
Location: Södertälje, Sweden


PostLink    Posted: Wed Aug 17, 2022 11:26 am    Post subject: Reply with quote

So I build a sync inverter. It made the trick. Based on a 74HC04D I happened to have in a random spares box. Using a switch I can now choose to invert the sync polarity or not. It's a workaround I can live with, but I would love to fix it the right way. Hence, I would still love to know what part of the projector is responsible for the sync polarity detection to have a chance to locate and fix the issue.
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Tim in Phoenix




Joined: 21 Oct 2006
Posts: 4378
Location: Phoenix


PostLink    Posted: Sun Sep 18, 2022 10:23 am    Post subject: Reply with quote

Guys

If I recall, ECPs wanted negative going sync only as that was the standard for RGBS
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