VG8235 joy port 1 troubles

Prophet (3199)

13-03-2017, 19:36

Good day everyone, I'm having problems with my VG8235 and its joy port 1: unless a joy / pad is pemanently inserted, the port is sending button 1 + button 2 push all time. With pad conected, no problem, it works fine and I can play normally using pad or keyboard.

It seems some kind of shortcircuit, but I've resoldered all pins to mother board and problem still remains, so it could be some voltage failure in other point.

I'd like to note that the pad that was connected previously was a chinese megadrive pad adapted to MSX using a NOR chip (I don't remember exact number) and suddenly stopped works. I've opened it and has some kind of strange white dust above all the chip, could the joypad damage the motherboard?

Thanks.

By Jipe

Paragon (1614)

13-03-2017, 19:45

exist 2 models of 8235 and the board are different, see under the MSX and post the serial number ex : 00 19 20 39

By Louthrax

Prophet (2464)

14-03-2017, 12:04

Well, if you connect a "normal" joystick to your MSX joyport, it should not change anything from the eletrical point of view as long as you do not press any button or direction: in this case, no wires are connected at all by the joystick (that's not the case if you use a Megadrive / Joymega joypad, or more sophisticated joysticks with autofire or leds...).

So probably you have a shortcut on the PCB between the button pins (pin 6 and 7), and the output pin (pin 8). First check continuity on the PCB between pins 6 and 8, and 7 and 8, resistance should be 0.

My guess is that inserting the joystick applies pressure on the PCB and prevents this shortcut.

By AxelStone

Prophet (3199)

14-03-2017, 14:37

Thanks people, this night I'll try some tests. I checked continuity and I'm almost sure there were normal, perhaps I did a mistake, I'll try again.

By Jipe

Paragon (1614)

14-03-2017, 16:36

if your 8235 model is 20 or 39 check LF23 , Q1 2SC2603 and R16 10K resistor

if your 8235 model is 00 or 19 check LF72 , Q8 2SC2603 and RA9 22K resistor pin 7 of RA9

By AxelStone

Prophet (3199)

14-03-2017, 22:32

Jipe wrote:

if your 8235 model is 20 or 39 check LF23 , Q1 2SC2603 and R16 10K resistor

if your 8235 model is 00 or 19 check LF72 , Q8 2SC2603 and RA9 22K resistor pin 7 of RA9

Wow so detailed information . Do you have perhaps service manual for VG? It should be great if you can upload it

By Jipe

Paragon (1614)

15-03-2017, 11:39

the ultimate link for service manual
http://msx.hansotten.com/manuals-and-guides/
go to the Philips VG 8235 ;)

By Repair-Bas

Paragon (1174)

17-03-2017, 07:50

Very simple, the engine or gate array is broken.

By sd_snatcher

Prophet (3659)

17-03-2017, 15:50

@AxelStone

It seems that the built-in pull-up resistors of the S3527 for the buttons 1 and 2 have gone bad. Use a multimeter to measure if the pins 52 and 53 of the S3527 still have 22K resistance to GND.

If they don't have 22K, try to add one 10K pull-up resistor for each. The easiest points to solder the resistors will be the output pin of the LF20, and the output pin of Q1. Solder one pin of the resistors at each of those points, and the other pin to the closest/easiest +5V spot.

By AxelStone

Prophet (3199)

17-03-2017, 19:57

Thanks again for your suggestion, I've been really busy and I couldn't try anything still (too much work ), but I've a lot of things to try

Repair-Bas wrote:

Very simple, the engine or gate array is broken.

What's that and how it's fixed?

By Repair-Bas

Paragon (1174)

19-03-2017, 11:32

Those are smd ic's. Hard to change.
100 pins and 80 pins to solder