F18a issues with certain games

[matthew180]

i was thinking, probably doest affect it but i have performed a composite video mod which requires component removal, see the PAL section of this guide, this is the mod i have done. would this affect the f18a in certain games?

http://www.coolretroprojects.com/ColecoVision_AV_Mod_Installation_Guide.pdf

 

Is there a schematic of this mod anywhere? I can't tell based on just the photos.

[omf]

I'm still trying to collect some information, but here is something to try in the meantime. Over in the 99/4A group several people have been building their own computers from chips and they are using the F18A. They were having some stability issues and found that they needed to add small capacitors between ground and the /CSW, /CSR, and MODE pins. In one system they only added a capacitor to the MODE pin, in another it was /CSW and /CSR. The values they were using are in the 4.7pf to 120fp.

 

I'm not sure why this would be necessary, the F18A uses a 245 buffer for these inputs. However, mixing IC logic families can always be problematic and it could be something with the specific 245 (MC74LCX16245) buffer I'm using (although I tried really hard to pick one that was designed to work with the ICs used in the home computers of the era.)

i take it these are just ceramic disc capacitors?

 

ill look into this and see what i can get it to do. dont have access to the machine right now but will look in the week when i get home from work, i have some 0.1uf capacitors laying about i could try. just got to get a pinout of the vdp now!

[omf]

ok i have made some very slight progress now

 

for the moment i am using 0.1uf caps because i have these to hand, results:

 

putting the capacitor between mode / ground brings up the black and green text screen

between csw and ground brings up the black and green text screen

between csr and ground seems to do something a little useful, the yellow image attached,

 

it appears to still crash but not as bad (reset works), the moving lines on boulder dash diamonds do not move across with the capacitor attached

 

im going to try a lower value and see what that does

Edited November 23, 2015 by omf

[+5-11under]

0.1 uF is too big. Try ~100 pF.

[omf]

perhaps something like this would be more useful ?

http://www.ebay.co.uk/itm/Miniature-Plastic-VHF-Variable-Capacitor-Tuning-Trimmer-Vishay-BC-Various-/120813273075?var=&hash=item1c21083ff3:m:mopQdr-_4vnXHmdXNiK8YQQ

[omf]

http://www.ebay.co.uk/itm/VARIABLE-FOIL-TRIMMER-CAPARITOR-LARGE-9-200pF-DAU-AUSTRIA-/181936328807?hash=item2a5c401067:g:o9EAAOxynwlTeMTx

[omf]

 

Is there a schematic of this mod anywhere? I can't tell based on just the photos.

im not sure, i haven't seen any personally, ill stick with the capacitor route for now and see if i can get it to work, I think im heading the variable capacitor route as i don't want to be ordering a lot of capacitors to test that aren't needed

 

if i don't get much joy ill look for a schematic

[TPR]

With all these issues you've been having with your system, what about just getting your hands on a new CV unit and re-installing the F18A there?

[omf]

With all these issues you've been having with your system, what about just getting your hands on a new CV unit and re-installing the F18A there?

possible, however the machine that was causing me grief was the the French RGB system, the PAL composite modded was and is fine with the correct vdp chip inserted, so the f18a should work on this machine and it does on a selection of games, its just the other selection of games (strip poker / pong / moon patrol) that ave issues with it

 

but yes, i did consider getting a NtSC machine then hopefully the f18a would function well like yours does

[omf]

i should be able to install the f18a that i got for my friend in his PAL Colecovision within the next few weeks and see the results, i suspect a similar issue as i have tried his f18a in my machine(s) and experienced the same after doing this I will report back

[SiLic0ne t0aD]

I would try it in a different ColecoVision altogether.. one that is working but hasn't been modded for the F18A yet.

[omf]

I would try it in a different ColecoVision altogether.. one that is working but hasn't been modded for the F18A yet.

this will be happening soon

[matthew180]

Sorry for the delay. I don't know the specifics about the caps used in the other homebrew systems, but I suspect they were ceramic caps. They were small though, in the pf range, not uf. In addition to the /csw, /csr, and mode signals, try the interrupt signal too. I'm still not sure why you would need those caps, but maybe the French console used a different family of logic ICs than the U.S. units, or something like that?

[omf]

I would try it in a different ColecoVision altogether.. one that is working but hasn't been modded for the F18A yet.

 

Small update on this issue:

 

last night I modded one of gaztee's colecos (this particular one was a French non scart machine) with a socket and placed the F18A into it and it basically did the same as what i was getting previously. this coleco had not had any modifications already so all I did was get take out the VDP and put a socket in and clip in the f18a (after bending a couple of video memory capacitors over a bit as they were sticking up

 

strip poker was green (again) various other problem games behaved as before, so i had not messed up the installation on mine

 

 

he did bring me a non working USA colecovision to have a look at, when / if i can get that beast working i will attempt the troublesome games in it with the F18A, i'm hoping that because that most people with the f18a are in the US that this will work for me without issues

 

ill probably replace all the support chips again like before, but it did seem to be a bit happier than the previous dead French scart version i had as i get other noise than just a plain tone from the sound chip so it isn't corrupt system ram like before, we will see...