CurtPalme.com Home Theater Forum

[antorsae]

Hi,

I did the HVPS mod replacing carbon resistors with ceramic ones as per HVPS mod in Marquee maintenance tread and I am suffering from the symptom Athanasios describes as "tube intensity anomaly" - i.e. even lowering G2 levels all way to zero, image is lit on e.g. the green tube.

I measured 6 of the resistors whose replacement is 560k and the carbon ones (originals) read anywhere from 670k to 800k. Those carbon resistors drift with age/heat so those resistors.

Anybody has a solution for this other than replacing the 6 resistors with some resistors close to the original (drifted) values?

What does the 3 pots in the HVPS do? (two of them (R35 and R36) are on the base board and have labels 0VP and KVP, the other are on the small daughter board, R53 label KV LOW)

Thanks!

[Nashou66]

I would not touch those unless you have an HV meter and know how to properly set them. I would go with 680k resistors. those seem to work fine for everyone else who had the same issue. I ran my 8500 till a few months ago with those values and all was fine. My other Set was fine with the 560k original values even though those were off too. I guess it depends on the set or HVPS.

Athanasios

[1031]

Like nashou said, 680k seems to best value to go. I have replaced all Ohmite ceramic composition resistor with 2W carbon films at HVPS and had no problems (runned projector about 8 months after that change)
Those two trimmers (side) are for adjusting high voltage (KPV) And second (OPV) is for overvoltage protection ie. point when HVPS shutdown if certain highvotage level is exeedet. I have never looked closer what that third pot. does (r53)
I say that is safe to replace those 560K originals with 2W carbon films.

Edit: here is link to resistor that i used to replace those 560K ones

http://fi.farnell.com/multicomp/mcf-2w-680k/resistor-2w-5-680k/dp/9338330

But any Carbon film that have least 2W power handling and voltage rating 500V or more is ok. Also try to found type that has low temp.drift (Temperature Coefficient)

[antorsae]

Thanks! I understand about the trimmers... NOT TOUCH!

1031: why did you go from ceramic resistors to carbon films? I thought the whole point of the mod was to use ceramic resistors that have less noise and drift... why use carbon films? (aren't those the originals?).

Best - Andres

[Nashou66]

[mp20748]

Carbon film is probably the best to put in that circuit. It's what I replace all high voltage carbon composite resistors with.

DO NOT use Metal Film, as they should never be used in high voltage circuits, and it would be a risk to use any other type of resistor that has not been proven to work in that type (high voltage) of circuit.

The carbon film is a direct replacement, where another type of resistor could work, it might not provide the proper load for the circuit. And that could cause things to be too out of range.


1031 is on the money here, go with 2watt carbon Film. Unless you want to risk damage to your tubes, because that's exactly what happens when the high voltage arcs away at a resistor that was not designed for high voltage circuits.

[antorsae]

Thanks Mike - I'll go with carbon film then. One question: any explanation on the intensity issue? I modded one HVPS with 560k ceramic and it did not have any issue, now this one has... I wonder how the (unmodded) HVPS could have been working correctly with those carbon comp resistors so off....

[Curt Palme]

[Curt Palme]

[tse]

If the third pot does the same thing as the third pot on the Ampro Spellmans then it lowers the switching frequency during low output load conditions. Evidently the converter cannot make a narrow enough pulse when the output current is very low.

At any rate, it is not wise to mess with any of the pots. That is asking for trouble.

Spellman was always concerned with arcs which produce high current surges which can change the value of film resistors. The carbon comp resistors will take a much bigger surge without changing value. They are not stable over time and usually increase in value as the years go by. Probably best to use the carbon comp type when they are in series with the outputs. Maybe not so important in other places.

Scott

[HK-Steve]

[antorsae]

Can you enlighten me as the pros/cons of metal oxide vs. carbon film vs. ceramic?

[Nashou66]

[antorsae]

Nash, even carbon film ones?

[mp20748]

http://www.us-tech.com/RelId/730849/ISvars/default/High_Voltage_Carbon_Film_Resistors.htm

[antorsae]

Thanks Mike!

Very interesting. How do they compare with ceramic wrt drift with temperature?

[Nashou66]

Ok I did a test with my modified HVPS to the the newest HVPS i know of, my 2004 marquee Ultras Longbow.

I compared the two looking into the tube with the 0 IRE up and brightness raised to 71 so i could clearly see any noise.
Both had noise and about the same level. I remember that after I did this nod I compared HVPS to stock and my upgraded one looked better, less noise.
So o thought maybe since these are new they have not developed the noise issue yet, so i opened one up and I see why, the carbon comps in the 560k spots are ot carbon comps but look more like a wire wound type or other material with possibly metal end caps due to the shape of the resistor.

So the NEW HVPS, are pretty well left alone stock.

Then it hit me I have a stock 9500LC here with no mods at all done to it, so i pulled that and popped it in and low and behold it had more noise compared to the other longbow i left alone.

So if you have the older style the carbon film or ceramics I used do much better than the old carbon composition noise wise.
Now with regards to the surges Scott talks about I have over a year with the ceramics and I measured them today and they are still the same, not much change.
The new HVPS resistors were dead on and the 9500LC were way off , one measured 690k.

anyhow here is pics of the new HVPS inards





Now these on the old HVPS had carbon Comps, these look and feel like ceramics or at least have the same cement
casing style.





Athanasios

[antorsae]

Thanks fir the info. Very interesting.

[Nashou66]

What type did your Ultra HVPS have and did the layout look like my Longbows?

Athanasios

[antorsae]

Athanasios - the layout of my Ultra HVPS looks exactly as yours (specifically the R45 and R5 which are in parallel are not stuck together sandwiched between the caps... as they are in a non-Ultra HVPS) but the 6 560k resistors are not like yours, they were regular carbon comps.

[Nashou66]

I wonder what they are? I should send a pic to spellman unless TSE knows for sure?

Athanasios