Adam VDP: TMS9928A?

[jltursan]

I'm repairing a friend's Colecovision and it's giving me serious headches. To help the diagnosing process I've even opened my Adam to make some changes over it; so after socketizing the Adam VDP and scrapping the glue of the cooler I've found this little thingie:



I was expecting a plain TMS9928A[NL]; yep, it must be the Texas IC rebranded of course; but the weird thing is that after testing 5 spare TMS9928A I've recently purchased, no one gives me a display, only the original works.

I have two possibilities:

1) It's not a TMS9928A and it's a slightly different chip. It's an NTSC version, I think; I've not checked it as I came from the USA.
2) All the spare ICs are bad or fake (yes, they came from China...).

Does anyone know more about the Adam VDP or even has replaced it successfully?, could it be a 9918?

 

Thanks!

 

 

[jltursan]

Thanks!, in fact, yes, the Adam has a TMS99x8 (as it's suposedly a NTSC machine); but after testing five TMS9928 ICs (suposedly 100% working) in a row without getting output makes me wonder. Also that weird rebranded "Coleco" IC fools me even more, I haven't been able to find a single pic in the web of a "nude" Adam VDP.

 

Looking at the schematics, the ADAM must have indeed a TMS9928 because it has circuits for the separate color components channels (R-Y, B-Y and Y) and not a composite video output like the one in the TMS9918; so yes, the TMS99x8 family are very close each other; but they are not the same and can't be swapped.

 

I'm still wondering what I have in my hands, a piece of useless plastic?, a different IC?

Edited October 3, 2018 by jltursan

[NIAD]

Wasn't the 9918 used for the US/Canadian/NTSC market for the ColecoVision and Adam Computer... hence output up to Composite.

 

And for the European/Scart/PAL market, the 9928 was used... hence output up to Component.

[jltursan]

The 9928 outputs the luminance (Y) and color differences (R-Y, B-Y) and NTSC clocks, the PAL version is the TMS9929 with PAL frequencies; so I guess that mine must be the TMS9928, after all the case shows a "Made in U.S.A" label....

The Adam schematics found here are of very bad quality but the R-Y, B-Y and Y lines can be seen as part of the circuit.

 

The Colecovision I'm repairing has lost his 9928 as I wasn't able to desolder it without stressing a lot the old PCB tracks so I was forced to cut their legs. Once in my hands and after separating the cooler a standard TMS9928ANL appeared, no surprises; but seems my Adam has something unexpected.

[jltursan]

Well, this story has a happy end at least for the Colecovision. It's repaired now!.

 

The VDP was indeed a TMS9928A and the answer of what happened was:

 

1) All 5 chinese ICs where bad (or fake, after close inspecting one of them has "6502" labelled in the bottom side,... just coincidence? )

2) The Adam VDP doesn't work at first cause there were a lot of chained faults in the video circuitry.

 

All in all, it has been a hard job and the Adam suffered a bit. It served as a donor (until new VDP ICs arrived) of the 9928

 

EDIT: Btw, not only TMS9928 were bad, also some SN76489 from the same seller were faulty. A crappy experience!

Edited October 7, 2018 by jltursan