Got a JVC hc-7e worked at first but now refueso to power on

Rookie (23)

16-03-2016, 19:59

First off this is my first post i like to say Hello.
I got a JVC hc-7e MSX computer look in great shape connected it and worked fine for an hour or so.
Then when i power it off and on i got a blue screen for less than a second and then black using RGB.
Caps lock key don't power on or off.
Tried to get a service manual but unable to find one.
Any ideas where to start ?.

Thanks!

Por Wierzbowsky

Guardian (3597)

16-03-2016, 23:10

As usual - check all voltages coming from the power supply. Try to insert game cartridge and see if the game starts. Check the power supply's fuse (just in case). Check voltages on all RAM and VRAM chips. Check for 3.58mHz on pin 6 of Z80.

Por Sebastian Blanco

Rookie (23)

17-03-2016, 02:16

OK tested the psu first i got this from the 5 pin psu connector this where metered keeping the msx powered on.
blue -11.77v
yellow 0v
orange 11.60v
blk ground
red 5.0v

on the psu i can confirm orange says +12 and blue -12 the other wires have just the color write on the pcb.

Por tvalenca

Paladin (747)

17-03-2016, 16:18

If your computer uses 8x 4116 DRAMs on VRAM bank you lack -5V voltage to make them work. Didn't found any PCB pictures to confirm this theory.

Por Wierzbowsky

Guardian (3597)

17-03-2016, 16:30

I could not find any service manuals for this computer. Tvalenca gave a good hint - If your computer has 4116 chips, they indeed need -5V on pin 1:

If the yellow cable was supposed to supply -5V to the board, then this may explain why the computer is not working - because you got zero volts there. If your computer lost -5V and was on for some time, several 4116 chips could have been damaged and you will need to replace them. I usually replace all of them to make sure they are all working (and putting new ones in sockets for easy replacement). If some DRAM chips get hot, it's a good indication that they may be broken.

Please make a good picture of the power supply and the board, this may help to diagnose the problem.

Por Sebastian Blanco

Rookie (23)

17-03-2016, 16:38

The yellow wire have metered the connector using the multimeter and the yellow wire seem to get high to 1 v when i press on the microswitchs from the cartrige slots when doing this the computer turn off and other voltages are cut i think is used to disable the psu when you open the covers to prevent demage when one try to instert a cart without powering the msx down.

Gonna also check the voltages on the ram chips and the take pictures.

Por tvalenca

Paladin (747)

17-03-2016, 17:42

So the yellow wire is the PSU inhibit wire, and it is supposed to be 0V when PSU is working (note that when you press the microswitch the yellow wire has 1v and all the others has 0v)

The next step is to measure voltage on supply pins on some ICs. you can have a working PSU, but its voltages aren't getting to the chips. If all chips are powered correctly, you'll need at least an logic probe (oscilloscope desirable) to further investigations.

Por Sebastian Blanco

Rookie (23)

18-03-2016, 02:01

Ok guys the ram ICs are quite differente and most of the ics on the board i was unable to get a pinout or even get what they are exept the easy ones like de ttl logic gates z80 etc

Por Wierzbowsky

Guardian (3597)

18-03-2016, 02:33

Ok, what I see is that your computer has 8 x 8118 chips (1 bit 16k) - this is 16kb VRAM. It also has 8 x 416 chips (4 bit 16k) - this is 64kb of RAM. Start checking the voltages on these chips.

The 8118 chips must have -5V on pin 1, +12V on pin 8 and +5V on pin 9, GND is on pin 16
The 464 chips must have +5V on pin 8, GND is on pin 16

Also measure if there's a short circuit between GND (pin 16) and any power pins on all chips.

BTW, some pictures are bad, hard to see the details. For example I can't see any regulator that produces -5V...

Por Sebastian Blanco

Rookie (23)

18-03-2016, 03:00

Ok trow the multimeter at the 8118 chips and find only +5 on pin 8 but no -5 nor +12 on the other pins
Also was able to find this schematic but contradicts that statement im not sure if is correct.
Gonna take some better pictures thank you very much

[img=http://s12.postimg.org/d2j2pzynd/8118.jpg]

Por Wierzbowsky

Guardian (3597)

18-03-2016, 03:05

Ah, interesting - the 8118 chips have only +5V on pin 8, I confused them with other 1 x 16k chips that require 3 different voltages. Well, then voltages are ok. Next is to check if there's 3.58mHz on pin 6 of Z80.