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--- projects:repair:his_radeon_9600_dual_dvi [2009/01/21 14:54] – admin
+++ projects:repair:his_radeon_9600_dual_dvi [2009/01/21 17:10] (current) – admin
@@ Line 27: / Line 27: @@
 Initially I suspected glitches were caused by jitter on data from video memory. At resolutions below 1600x1200 the video was fine on both outputs. The glitches would only manifest at high resolution above 1600x1200 pixels. So memory timing might be more critical. If that would be the case, cleaner power may solve the problem. So I added extra 100nF decoupling capacitors on top of 4 of the 8 memory chips.
-==== Video memory increasing voltage ====
+==== Increasing video memory voltage ====
-As the decoupling did not change anything at all, I had to look further. A second experiment would be slightly increasing the memory voltage, from 2.50 Volt to 2.55 Volt, by adding a resistor divider at a LM431 reference voltage ic (SOT-23). The voltage increase did not improve anything either.
+As decoupling did not change anything at all, I had to look further. A second experiment would be slightly increasing the memory voltage, from 2.50 Volt to 2.55 Volt, by adding a resistor divider at a LM431 reference voltage ic (SOT-23). The voltage increase did not improve anything either.
 ==== Inspecting DVI transmitter ====
-Because from the two DVI outputs, only the second one had problems, I crossed my mind that a memory timing issue would not be a logical explanation. If it would be, then probably both outputs would suffer. Due to the differences in the DVI output circuitry, it made even more sense that the problem would be in something one of the circuits had which the other didn't have.
+Because from the two DVI outputs, only the second one had problems, it crossed my mind that somehow a memory timing issue would not be a logical explanation. If it would be, then likely both outputs would suffer and not only one of them. Due to the differences in the DVI output circuitry, it made even more sense that the problem would be in something specific one of the circuits had which the other didn't have.
+So I started to measure power supplies for the DVI transmitter, a THine THC63DV164. At the closest capacitor (C1421), I measured 3.3V with about 200mV noise, which was far too much in my opinion.
+===== Solution =====
+To solve the problem with glitches, replace 47uF capacitor C1421 with a larger capacity. I tried several type of capacitors and it seemed that 470uF would be sufficient. The voltage across the capacitor is only 3.3V, so most capacitors will be fine. It's critical that this should be a low-esr type. Smaller values and other kind of capacitors may not filter the power supply enough. You may even improve filtering slightly, by adding a small smd ceramic capacitor (at the bottom) of about 100nF. The easiest replacement capacitor I could find and which would fit as well, was a 1000uF 6.3V capacitor from an older motherboard. The noise I measured was now less then 10mV.

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