Phonola / Philips VG-8010 ROM, maybe EPROM replacement

By Argon

Paragon (1126)

02-09-2020, 17:58

Hi.

I have a non-working Phonola VG-8010.
I would like to read out the ROM to see if it's fine.

Does anyone know which ROM type this may be?
Could I read it like a 2732, 2764 or something like that?

Thanks.

By Wlcracks

Hero (572)

02-09-2020, 18:04

27C256

philips 8010

By Argon

Paragon (1126)

02-09-2020, 20:30

Wlcracks wrote:

27C256
philips 8010

I get "pin detect error" on pin 1 when I try to read it as 27C256 (I tried NEC, Philips, ST, AMD, ...).
I'm using a TL866-II plus.

If I disable "pin detect" then I can read the ROM.

By Wlcracks

Hero (572)

03-09-2020, 06:01

Put it back intro the msx, and check the level on the pin.
I think its a programming pin, so for a rom its not used. If a bytecompare with a openmsx matches, or a readout of you rom works in openmsx , i guess its ok.
What is wrong with the msx? Does it generate a |INT 50 or 60hz on the vdp? If it does the rom is almost sure ok.

By Argon

Paragon (1126)

03-09-2020, 10:25

When I power it up I get a white flash and then black screen using the RGB output. On RF output I just get a black screen nothing else.

Multimeter:
- Voltages are OK on PSU and computer.

Oscilloscope:
- activity on CPU seems fine.
- activity on ROM seems fine.
- activity on RAM I'm not 100% sure, not easy to measure in this computer because of the little daughterboard.
- activity on TMS9129NL is present, but also not 100% sure, read below.

TMS9129NL:
It gets very hot, so might be defective?

Next steps:
I will first try replacing the 4416 RAM, I have spares.
If that doesn't work I will try to find another TMS9129NL.
If that doesn't work... not sure yet :-)

By Wlcracks

Hero (572)

04-09-2020, 14:44

My human-memory is failing on the 8000 but if the RAM memory is located in slot #3 you can try an external RAM memory cart. My VG8020/00 was failing on a ramchip but inserting an external ram (msx1 compatible ram mapper) fixed it. Locating the broken ram chip was pretty intense.

Fun RGB in on this MSX1

By Argon

Paragon (1126)

04-09-2020, 16:36

I will try a memory cart.

Replacing the 4x 4116 on the little daughterboard hasn't solved the problem.
I'm thinking the problem will probably be the TMS9129NL, but finding a replacement for that will not be easy :'(

By Wlcracks

Hero (572)

05-09-2020, 05:37

Does caps led work?
Does the sound output work, clicking keys or ctrl + g = beep

Did you check the |Int on the vdp? If the interrupt is pulsing and the sound works the system is working and the video hardware could be failing.

I didnt check forsure but the daughter board is an expansion to 32k, there is also main ram on the main board. Vg8000 has 16k w/omthe daughter board, vg8010 32k

By Argon

Paragon (1126)

05-09-2020, 08:44

CAPS does not work, also no beep with CTRL+G

By calderone

Resident (36)

04-12-2020, 22:01

Hi Argon, I have some experience now when troubleshooting my Philips VG-8010, I believe it's the same machine.
I had encountered several problems and some were very strange. It took me 8-9 months to figure it out.
Some say this is an easy machine to work on but I experienced the opposite.

First, your ROM is actually a 23256AC PROM and can be replaced with a 27C256 EPROM. Write the correct ROM image in it and it should work. I confirm ;-)
But I believe it is not the ROM ... and it is NOT the VDP, even its 'hot', let me explain.

This is what problems my VG-8010 had (black screen):

1) reset circuit didn't work. So check it on yours.
I had to replace IC670 (74LS04N) to make it work again.

2) I also tested all memory chips (desoldered) but all were fine (RAM + VRAM) !!
But I noticed a problem with one of the memory multiplexter, so I had to replace it as well.
= IC705 (74LS157)

3) Then about the VDP (TMS9129NL), it is probably not the problem, I ordered several from Aliexpress and it did not solve any problem.
I also started this tread to proof they were good:
https://www.msx.org/forum/msx-talk/graphics-and-music/functi...
And they were all good !! Some gave more heat than others, but they all worked ! (I added a heatsink)
So, if needed I can sent you one to test, but probably that's not the problem.
Even when you don't see a /INT signal on your VDP, it does not mean it is bad. I did'n see the /INT signal on my VG-8010 but my arduino test showed it worked, so I dived in deeper. On the MSX it's just an initialization problem.

4) After all previous steps, I finally had some (short) luck, I saw a blue BASIC screen when handling the board. So I believed it was a broken trace somewhere.
So I checked all traces, over and over again, but could not find any clue. See also my comments in this tread:
https://www.msx.org/forum/msx-talk/hardware/vg8000-and-vg801...
Then I had some more luck (accidently). When touching, or unplug, plug the Z80 CPU from its socket. I saw the initial screen more frequently.
So I began to believe it was something with the CPU. Replacing it did not solve it. Also replacing its socket did not solve it.
The solution was to remove the socket and solder the CPU straight on the board.
BANG, the MSX worked without a problem, every time !! Strange....

By Wierzbowsky

Guardian (3650)

05-12-2020, 21:02

That is indeed strange. I also recently restored Philips 8020 that failed to start. When measuring data and address lines' resistance vs ground, I noticed that 2 data lines and 1 address line were showing unusually low resistance comparing to other lines. It was in the range from 1k to 10k. I desoldered the suspected chip (LS367) and tested it in TL866 programmer/tester. The chip was shown as working. However, when I tested it in another tester (custom-made RAM/ROM/LOGIC tester), the chip was detected as faulty. Looks like the TL866's test was not testing all the features of the chip. The chip was replaced.

Then I desoldered the ROM and read it. No differences were found comparing the dump to the 8020 ROM file, but the low resistance on the data lines disappeared. I replaced the ROM chip and the computer started to work.

Bottom line: it's not always black and white when you repair something. And not all testers can be trusted.