Sony HB10 / HB20 Hardware issue + fix

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By ericb59

Paladin (844)

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11-03-2017, 08:27

Did you ever get problems with some cartridge on the HB10/HB20 ?

We have found an hardware issue than make some dysfunctions in some circonstances.
Issue can cause a strange mirroring that makes the ram appear in different slots.

A you can see on, the above part of the service manual,
there is a pullup resistor on each signal /WR, /RD, /IORQ, /MREQ
but not on /RESET ! Did sony missed it ? and why did they add a diode on /RESET ?

The fact is when there is no cartridge in the computer, /RESET level is at 5v.
But when you insert a cartridge the /RESET signal down to 3v.
This is a real issue.

fix :
Like we draw on the schematic, add a 4.7K pull up resistor on the /RESET signal, and you will fix the problem for ever.
(Thanks to Eugeny Brychkov and Metalgear2 from MSXvillage for identified the problem)

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By gdx

Prophet (2857)

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11-03-2017, 10:43

Thank you for sharing.

Can you take a picture of the board with the soldered resistor for people who do not understand a diagram?

By mars2000you

Enlighted (5479)

mars2000you's picture

11-03-2017, 10:37

I guess it's the same problem as what's explained in the Wiki:

Quote:

==Quirk on port A8h==
The models HB-10B, HB-10D and HB-10P and some very early versions of the Sony HB-20P|HB-20P have a quirk on port A8h that makes RAM bank 3 appear both in slot 0 and in slot 3. This mirroring effect is emulated in openMSX for the Sony HB-10P, it can be very annoying as it can play havoc with games that need more than 16kB RAM, using the 16kB RAM mirror to store system variables and assuming that all the ram is in the same slot 0 as page 3.

This configuration can cause poorly written software to crash. Nevertheless, you can run these games if you type following command before loading:

OUT &hA8,240

For cartridges, the trick is to plug it after booting using a slot expander that has switches to disable each slot and prevent them from auto booting ('''WARNING'''!! Do not insert a cartridge when power is ON. It can damage the machine). Then type the following instructions on BASIC after re-enabling the slot:

OUT&hA8,240: DEFUSR=&H7D75: ?USR(0)

Alternatively, you can use ROM loaders like Execrom, LoadROM or OPFX /X. Since they start the game from MSX-DOS, this problem will be automatically solved as DOS already selects the proper slots for the RAM.

By ericb59

Paladin (844)

ericb59's picture

11-03-2017, 10:52

mars2000you wrote:

I guess it's the same problem as what's explained in the Wiki:

no it's another issue.

By sd_snatcher

Prophet (3019)

sd_snatcher's picture

11-03-2017, 13:57

@ericb59

You're right. The /RESET signal on the slot is an output signal. Probably the cartridges that are having problems are MegaROMs, "memory-mappers that work on the MSX1" and slot expanders, as they require the /RESET signal to be properly initialized. It has nothing to do with the internal RAM mirroring.

Sony engineers seem to have added that diode there to protect the chipset against badly constructed cartridges that try to reset the MSX via slot, which is incorrect and will indeed damage many machines (i.e.: the NMS-8245). I haven't seen any commercial cartridges that do this, so it's more like a protection against hobbyist experiments.

A simpler solution for the HB-10/HB-20 would be just to replace the diode with a 100R resistor. This method was used by Panasonic for the TR machines. The resistor acts as a fuse.

This method isn't as good to protect the chipset against a reset coming from the slot as de diode+pullup, but it's the same level of protection that you have in any TR anyway.

Thank you so much for reporting this problem. I have both a HB-10 (JP) and an HB-20P, and will fix them both. I noticed that my HB-10 is indeed very temperamental with cartridges that have built-in slot expanders or memory mappers and was thinking that the slot connectors had some problem.

By ericb59

Paladin (844)

ericb59's picture

11-03-2017, 14:15

@GDX : yes i will put a picture soon.

@SD_snatcher : Yes some Megarom may have problems, but also 64K ROM (Like Uridium). But also Beer HD Cartridge, some SD card cartridges and FDD Cartridges.

You're right SD_Snatcher. Replacing the diode by a 100R resistor may fix the issue too. (thanks)

By gdx

Prophet (2857)

gdx's picture

12-03-2017, 01:43

If someone finds a way to fix port #A8, this MSX will become a nice little machine.

By ericb59

Paladin (844)

ericb59's picture

16-03-2017, 15:31

By zeilemaker54

Master (243)

zeilemaker54's picture

16-03-2017, 22:03

gdx wrote:

If someone finds a way to fix port #A8, this MSX will become a nice little machine.

About the hb10p, and I am no hardware expert, would this be as simple as disconnecting pin 9 of ic9 from the s3527 (cas3) and connect this pin (pin9 of ic9) to 5v ?

By sd_snatcher

Prophet (3019)

sd_snatcher's picture

09-04-2017, 22:19

Curiously enough, replacing the D22 with a 100R resistor for the /RESET fix eliminated *a lot* of noise from my HB-10 image! Probably the line was oscillating wildly and caused a lot of noise. This effect didn't happen in my HB-20P though. It seems to have a newer revision of the mobo.

As a bonus, I discovered another problem that needs to be fixed. R22 should be 1K, but is 10K both in the schematics and in the HB-10. This causes the PSG sound to saturate in many songs, leaving the impression that some capacitor has gone bad. It seems that Sony themselves noticed the problem, since my HB-20P already had the correct value.

But the 1K resistor makes the PSG sound volume quieter and Sony didn't bother to adjust the volume of the mixing resistor. To fix this, replace R15 with a 4K7 resistor and you'll have perfect mixing volume too. SCC and FM cartridges will sound just right.

In a nutshell, the sound fix is:

  • Check if R22 is already 1K. If it isn't, replace it with an 1K resistor.
  • Replace R15 with a 4K7 resistor

Warning: do not try to salvage the old resistors, as this motherboard has very fragile tracks. Cut the pins of the old parts, melt the solder and remove the remainder of the pins with a plier. Then carefully clean the holes and install the new resistors in place.

Another fix is to apply a good thermal paste on the 7805 voltage regulator. This will make its heat transfer a lot better.

By ericb59

Paladin (844)

ericb59's picture

22-04-2017, 09:29

NEW !

Port A8 hardware fix for the HB20.
In 30 years nobody tried to fix the HB20 faulty A8 port.
My dear friend Metalgear2 (from MSXVILAGE) found an easy way to fix this issue by adding a 4.7K Resistor on IC 9 (74LS08)
Cut and bend Pin 9 of the 74LS08 and solder a 4.7K resistor from pin 9 to pin 14 of the 74LS08.
That's it... Computer is fixed !

Full tutorial here (in french):
http://mymsx2.free.fr/montages/FIX_A8H_HB20/fix_port_a8h_hb2...

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