Diagnostic on dead Sony HB-F700F

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By Louthrax

Prophet (2076)

Louthrax's picture

01-03-2015, 01:44

Hi all,

My good ol' Sony HB-F700F MSX2 just died today. Well, at least it's not in a very good shape anymore. I'll try to fix this, but maybe someone could point me where to investigate first ?

Here are the symptoms:
- Black screen. The TV switches to AV and the display blinks a bit when I set the power on, but it remains black.
- No CAPS/LOCK led light. I wait for some seconds after boot, but pressing CAPS does not switch the led...
- Power coming out from the main power-supply looks good : +5V, +12V, -12V.
- No sound.

I managed to re-boot it 2 or 3 times last week by just leaving the machine off for some time and powering on again, but now there's not any sign of life anymore... Also, I never had any problem when the machine was still working (no signs of future problems like resets or VDP problems or anything).

If any of the MSX doctors here may have a diagnostic, that would be great !

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By mikoto

Supporter (16)

mikoto's picture

01-03-2015, 11:35

Does the electrolytic capacitors on the board look good? If there aren't that many it would probably not hurt to replace them all. Should not be an expensive job. Replace the ones in the PSU aswell.

Are any of the chips socketed? Try removing and reinserting them again in that case.

By Louthrax

Prophet (2076)

Louthrax's picture

01-03-2015, 11:57

Thanks Mikoto !

I've not disassembled the whole computer yet, but I've seen no leaking on the capacitors. Do you know which ones are the more likely to fail first (there's a bunch of "big" capacitors in the Sony, some smaller ones too) ? Most of the chips are directly soldered on the Sony (maybe the BIOSes roms are not but I'll need to disassemble everything to access that).

Very-noob eletronics question : is there a way to test a capacitor without unsoldering it ? I know that these capacitors might be the plague for our vintage stuff in the future years, better get used to fix that now :-)

By mikoto

Supporter (16)

mikoto's picture

01-03-2015, 12:32

No there is no simple way to determine if a capacitor is good or bad except the rare cases when they have more or less exploded :-)

They can not be tested without removing, not in a sure way. Perhaps if you have a good service manual and knows what the signal efter the capacitor should be. Though capacitors that run hotter then the rest ususally fail faster. But it is good practice in these cases to just replace the lot so you can be sure that there is no problem with that. Then it is easier to go to the next step it that does not fix the problem.

By gdx

Prophet (2911)

gdx's picture

01-03-2015, 14:00

The broken electrolytic capacitors are often inflated on top. If a capacitor is slightly concave change it.
The solders age badly. Solder them all again with flux.
Have you inserted a cartridge during power turned ON? The VDP of HB-700 doesn't like it.

By CX5Mer

Champion (322)

CX5Mer's picture

01-03-2015, 14:09

I'm doing a course on building, repairing, and modifying PCs. Their simple advice about capacitors is that if the capacitors look swollen on the top or sides, then they're no good. I've also seen videos about repairing tracks on gaming console motherboards, such as https://www.youtube.com/watch?v=fzy7JUu2CiE .

For MSX repairs you can try http://www.bas-ditta.info/ .

Another repair option, although not specifically MSX, is http://www.mutant-caterpillar.co.uk/shop/ .

I hope this helps.

By Jipe

Paragon (1366)

Jipe's picture

01-03-2015, 15:25

remove cartridge if exist in slot and retry
it's a original MSX2 or upgrade ?
then reset have you the click in K7 relay ?

By mikoto

Supporter (16)

mikoto's picture

01-03-2015, 22:41

Electrolitic capacitors can also dry out, this happens gradually and does not necessarily give any difference to its outward appearance, this is very dangerous and the main reason I say if you suspect that even one of the capcitors is bad then exchange all of them cause you don't know how much time the other have left. It's a cheap fix and good training that does not hur even if it does not solve the problem :-)

By RetroTechie

Paragon (1563)

RetroTechie's picture

02-03-2015, 05:51

No CAPS LOCK response usually means the cpu doesn't get a clock signal (in the MSX, dead VDP may cause that), or there's something preventing the Z80, ROM or RAM from working normal. 2 things would be useful here:

1) A known-good external RAM cartridge (doesn't matter how much RAM).
2) A known-good cartridge, that you know to start making sound if you just power on an MSX with it (when it runs some intro-demo or similar).

Insert those 2, connect a speaker to an audio output of the machine, power up, and listen.
Sound as expected? -> Problem is in video output circuitry, or RAM chip(s) defect.
No sound? -> ROM / CPU / clock signal / power supply related problem.

When there's EPROMs in the machine: they may loose their contents after a few decades, starting with single bits that fall over. oO But that's just random speculation, like capacitors that look normal. Smile

Other than that: connector problems. Un- / re-plugging internal connectors, pull / re-socket IC's may help.

By l_oliveira

Hero (520)

l_oliveira's picture

03-03-2015, 16:31

The CPU can be blown also.

Unfortunately, any blown pin or even a broken trace on the bus traces will prevent the machine from starting.

To properly troubleshoot the computer you need some tools.

You follow a flow of things to check and that's why you need the said tools. Here's the "generic" flow of checking:

1st- Power: Check if all voltages are correct (need volt meter for this)
2nd- Clocks: Check if the clocks are being generated properly (need logic probe, oscilloscope or frequency meter for this)
3rd- Visual inspection of traces (need ohm meter with continuity check for testing traces continuity)

If you determine that there's no obvious faults, then you start testing key system components in this order:

VDP
ROM
RAM Memory
CPU

This order is not really critical. I honestly have not seen many cases of broken VDP, for example. But I have seen CPU blown many times. Even more times RAM, including VDP RAM. But VDP RAM fault won't stop the computer from starting. It will just make the graphics on the screen look weird.

By Louthrax

Prophet (2076)

Louthrax's picture

28-10-2016, 21:18

Back on this after nearly 2 years now (OMG, time's flowing so fast).

So I did a full-recap of the upper PCB, except for one .47uf-50V capacitor (had none in stock), and still nothing comes out of my poor HB-F700F.

There are 2 or 3 capacitors on the lower PCB that I also did not change, but they are located on the FDD & keyboard interfaces, so I do not think they are critical here.

Also tried to boot with an external RAM cartridge plugged-in, no change.

So I should probably check the clock signals as suggested, but I have no logic probe or oscilloscope.

Inspecting the ROMs content might be interesting, but they need to be unsoldered from the PCB in order to be checked (darn !).

So I'm thinking about either donate the unit to Bas at next Nijmegen (if he's still interested by that ?), or try to find a cheap or faulty Sony HB-F700 unit alone (trying to exchange the boards with mine to get something working).

EDIT: I also noticed that the backup battery / capacitor has started to leak. So I unsoldered it and cleaned the tracks. But this can not prevent the MSX from booting right ? Or can it ?

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