@917B.ADF SVGA Adapter (US/EMEA)
06H6915 SVGA Adapter BIOS ROM (from David Beem)
For the chipheads, the Tech Ref Annexes B-7 thru
B-10 have some application notes. Data sheet is 4.8MB,
Tech Ref is 2.7MB. Everybody needs to thank Elar Saar
for guarding these files until the fullness of time.
The CL chipset -by the way- is a
mask-programmed graphic chip with integrated timing
oscillator (no external Xtal) and integrated 80MHz
RAMDAC. Very compact design. A similar chipset is used
on the Server 320/520 MCA/PCI planar. Here they used
the CL-GD5430-QC-D and 2 x Siemens HYB514171BJ-70 (256K
x 16 bit).
Using 1MB SVGA in Model 60?
Wolfgang Gehl wrote:
Flicker in 1MB SVGAI (god Emperor of Microchannel) and others have noticed that the 1MB SVGA flickers. Behold what devious minds can do-
If you check the connections from C31 you will find that the + marked end goes to the V+ input of U9 @ Pin 4, while the unmarked end is tied to +5VDC ... During operation the + end of C31 is at 3.09V ... and the - end at +5 ... with no dynamic offset of any sorts - I'd tested that with the scope.
Shouldn't that be other way round ? There is a "+" marking silkscreened on the board, so the cap is mounted correctly regarding the silkscreen - electrically however it is reversed. And this circuit appears to me as the voltage reference for th CL5428-internal RAMDAC. (They pull down a definite current to give the RAMDAC a stable base voltage for the output drivers).
What happens to a capacitor if you permanently run it at reversed polarity - even though the voltage is rather low ? (the cap is rated 20V) Given it develops some "leakage" and causes a parasitic current to +5V - parallel to the current flow through U9 ? That could explain the brightness changes in the video signal.
I'm going to unsolder the C31 next to see how the card
behaves without it or if it is intended as some sort of
"choke up" capacitor. And what happens if I reverse the
Fixed 1MB SVGA
I replaced the cap on the contemplation card a few minutes ago. This card was known to flicker, quite badly at times, and was pulled from this machine for that reason. I used a new 3u3/16v in the same form factor off a scrap PCB from work.
Before I swapped the card, I laid down on the floor with my handheld DMM and confirmed your voltage readings before powering down the 500. Results; +5.15 VDC on the - terminal of the capacitor, +3.50 VDC on the + terminal. Backweirdness confirmed. The + end is 1.65V _more negative_ than the - terminal.
Swapped the cards. My immediate impression was that the test card produces a much brighter and clearer screen image than the (identical) card I took out. This may be meaningless however, since I did not retest this card in the 500 before I changed the cap.
I have not noticed any flicker so far. I will run it for a while and try to observe it as much as I can. This machine is my APRS server and runs 24/7, so it should be a good test. Type 4 'Y' at 180 MHz.
David Beem provides another data point:
Fixed! I say a dramatic fix in that.
I can reproduce the flickering at will by putting in the
unpatched card; It goes away with the patched version,
even for a lengthy time span. Who would have knew?
To see how this circuit is implemented on the various CL-GD542x MCA adapters go HERE.
Ratings for C31 Replacement
I had a tag-team of David Beem and Dr. Jim working this
> > Capacitance range 2.2 uF to 4.7 uF, voltage of at least 20 V? Or would 16 to 25 work?
Clone designs were using close to a 3.3uF
cap. For voltages 16v is sufficient & a commonly
produced capacitor. I would stay within the range of 2.2
to 5 uF. It is remotely possible that a cap that
is too large may draw excessive surge current on power
up and let some of that delicate silicon smoke loose
inside the chip.
2MB Short SVGA?
Without the help of Jolt, I asked about pumping up the 1MB SVGA:
Pin 141 is right above the trace that goes to R27.
>Please point out the second pair of solder pads for additional memory on the short 1MB.
AdapterId 917b SVGA Adapter