SNES multicart worth fixing?

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Fisher
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SNES multicart worth fixing?

Post by Fisher » Mon Mar 01, 2021 7:44 am

Hello.
I've got a SNES multicart and would like some help to know if it's worth trying to go ahead and fix it.

Here is a picture of the board:
WhatsApp Image 2021-03-01 at 07.56.57.jpeg
A "naked" picture of it:
WhatsApp Image 2021-03-01 at 10.05.22.jpeg
A picture of the back:
WhatsApp Image 2021-03-01 at 10.05.21.jpeg
A small ASCII diagram with the ICs identified:

Code: Select all

   +------------------------------------------------------------+
   |  U6 _  _  U1 _  _  U2 _  _     _____     U3 _  _  U4 _  _  |_
   |__  | \/ |   | \/ |   | \/ |   /     \      | \/ |   | \/ |   |
      | |    |   |    |   |    |  /       \     |    |   |    |   |_
    +-+ | D1 |   |74LS|   |74LS| /  Epoxy  \    |74LS|   |74LS|     |
    |   |9625|   |139 |   | 32 | |  Blob   |    |157 |   |174 |     |
    |   |    |   |    |   |    | \   ROM   /    |    |   |    |     | 
   ++   |    |   |    |   |    |  \       /     |    |   |    |     |
   |    |____|   |____|   |____|   \_____/      |____|   |____|     |
   |__________                                            __________|
              |                                          |
              |__________________________________________|   
              
Unfortunately, I couldn't draw a nice schematics of it but did a small ASCII of what I've found with the multimeter. Probably it has lots of flaws:

Code: Select all

            74LS139               74LS32
            +--V--+              +--V--+
      /CART |1  16|   74'139 (5) |1  14| VCC
        BA6 |2  15|          /WR |2  13|
74'174 (12) |3  14|   74'174 (9) |3  12|
            |4  13|   74'157 (9) |4  11|
  74'32 (1) |5  12|   74'174 (2) |5  10|
            |6  11|          ROM |6   9|
            |7  10|          GND |7   8|
        GND |8   9|              +-----+
            +-----+

            74LS157                      74LS174
            +--V--+                      +--V--+
74'174 (15) |1  16| VCC           /RESET |1  16| VCC
        BA1 |2  15| GND         74'32(5) |2  15| 74'157 (1)
        A15 |3  14| BA2               A0 |3  14| A5
        ROM |4  13| BA1               A1 |4  13| A4
        BA1 |5  12| ROM              ROM |5  12| 74'139 (3)
        BA0 |6  11| BA3               A2 |6  11| A3
        ROM |7  10| BA2              ROM |7  10| ROM
        GND |8   9| 74'32 (4)        GND |8   9| 74'32 (3)
            +-----+                      +-----+ 
Interestingly, U6 seems to be some kind of CIC clone, since it has similar connections.

I've tested all the ICs, except the CIC clone, on an old tester and all are fine according to it, except the 74LS157.
I've got some other old 74'157s but all of them tested badly, none worked.

Can someone please explain to me if there's a way to start this multicart with a single fixed game, without using the circuit to change them?
Thanks in advance.

Edit:
Did a "quick and dirty" draw with Eagle.
Hope it helps.
SNES Multicart.png
SNES Multicart.png (10.19 KiB) Viewed 2701 times

lidnariq
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Re: SNES multicart worth fixing?

Post by lidnariq » Mon Mar 01, 2021 12:35 pm

Latches A0-A5 (74LS174N) on a write (74LS32N,A) to $408000-$7DFFFF (74LS139N,A).

Bits work as follows:

If A4 was high, further bankswitches are prohibited (74LS139N,A)

If A5 was high, ROM address is [A15, A16, A18|Q0, A17]
If A5 was low (or on power-on), ROM address is [A17, A17, A19|Q0, A18]
... Is there any possibility that you made a mistake and 74LS157 pin 2 is actually BA0?

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Fisher
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Re: SNES multicart worth fixing?

Post by Fisher » Mon Mar 01, 2021 12:55 pm

lidnariq wrote:
Mon Mar 01, 2021 12:35 pm
... Is there any possibility that you made a mistake and 74LS157 pin 2 is actually BA0?
I checked again and yes, 74LS157 pin 2 is BA0.
My mistake, sorry.

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Re: SNES multicart worth fixing?

Post by lidnariq » Mon Mar 01, 2021 12:59 pm

Cool, that makes much more sense.

A5 selects between LoROM (low) and HiROM (high). (the '157)
A0 high selects a 256KB game; A0 low selects a 512KB game (or a 256KB game with unrelated things in the upper 512KB) (the '32)

You could replace the 157 with a bunch of jumpers, just pick all A inputs or all B inputs.

A1-A3 select one of eight 512KB banks, so the ROM there should be 4MB.

The Menu boots by default into the bottommost 512KB LoROM.

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Fisher
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Re: SNES multicart worth fixing?

Post by Fisher » Mon Mar 01, 2021 6:31 pm

Unfortunatelly looks like the epoxy blob is broken.
Thanks for the information.

These things are very difficult to fix!
I tried the diode test trick and found problems with A9 and A10, but the 1K resistor trick didn't work.

Luckly I saved another one by putting a 470 Ohms resistor on D7.
20210301_070658.jpg
Man, that's weird!
Any chances that this "fix" can damage the console?

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Ben Boldt
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Re: SNES multicart worth fixing?

Post by Ben Boldt » Thu Mar 04, 2021 1:23 am

How did you figure out 470 ohms? I would be afraid of it, that seems pretty low. You might be better off using a bidirectional buffer for D7 like 74HC245. Then all of that current can be isolated from the CPU. This may not be a good sign how long the blob will last.

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Fisher
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Re: SNES multicart worth fixing?

Post by Fisher » Thu Mar 04, 2021 2:37 am

Basically, it was by trial and error.
I tried a 1K resistor first but it failed after working for a while.
470 Ohms was the next smaller one I could find.

I know even the 1K resistor probably is too low.
Most circuits I see with pull-ups/pull-downs are 10K~33K.

I'll study how I can implement your idea of putting a buffer on it.
It seems the best thing to do.
Thanks.

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Fisher
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Re: SNES multicart worth fixing?

Post by Fisher » Thu Mar 04, 2021 3:47 am

Adding the "fixed" schematics.
Maybe it can be useful to somebody, who knows...
At least it seems to be correct now. :-)
Fixed schematics
Fixed schematics
teste.png (10.27 KiB) Viewed 2512 times

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Ben Boldt
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Re: SNES multicart worth fixing?

Post by Ben Boldt » Thu Mar 04, 2021 7:31 am

I am not familiar with this -- is the blob just a ROM, in which case the data bus is just 3-state in 1 direction? That might simplify things buffering it. Did you look at it with a scope before and after adding the resistor? It would be interesting to see exactly the effect from the resistor and compare it to D6, D5, etc.

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Fisher
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Re: SNES multicart worth fixing?

Post by Fisher » Thu Mar 04, 2021 8:15 am

Sure that would be pretty interesting.
Unfortunately, I don't have access to a scope. :-(

I've read somewhere that it's possible to make something similar to a scope with a capture card or even a sound card, but I'm afraid that its frequency would not be high enough to take a good look at it.

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Re: SNES multicart worth fixing?

Post by lidnariq » Thu Mar 04, 2021 12:06 pm

Ben Boldt wrote:
Thu Mar 04, 2021 7:31 am
I am not familiar with this -- is the blob just a ROM, in which case the data bus is just 3-state in 1 direction?
Yeah. It seems that one of the common failure modes of these ROMs is one of the data bus drivers get toasted and it can only pull up or down.

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Fisher
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Re: SNES multicart worth fixing?

Post by Fisher » Thu Mar 04, 2021 2:11 pm

lidnariq wrote:
Thu Mar 04, 2021 12:06 pm
Yeah. It seems that one of the common failure modes of these ROMs is one of the data bus drivers get toasted it can only pull up or down.
Another failure mode is similar to this but on the address bus, wich I have no idea on how possibly I could try to "fix" since the resistor workaround doesn't work.

There are also some that pass the diode test on all pins but simply don't work.
I guess the best thing to do other than discard it is to remove the epoxy blob by heating it and solder an EPROM/Flash with the desired game and call it a day.

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Ben Boldt
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Re: SNES multicart worth fixing?

Post by Ben Boldt » Thu Mar 04, 2021 6:12 pm

Fisher wrote:
Thu Mar 04, 2021 8:15 am
Unfortunately, I don't have access to a scope. :-(
Oh wow, that surprises me a guy like you wouldn't have one. Have you ever looked at USB oscilloscopes that hook to your computer? I haven't tried one myself but something like this is pretty inexpensive and would probably work well:

https://www.aliexpress.com/item/32793431623.html

hantek.jpg

lidnariq
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Re: SNES multicart worth fixing?

Post by lidnariq » Thu Mar 04, 2021 6:20 pm

Sadly imports are stupidly expensive in Brazil, both in flat currency but especially relative to cost of living. Hence why Fisher goes to such impressive efforts to fix things.

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Re: SNES multicart worth fixing?

Post by Fisher » Fri Mar 05, 2021 2:29 am

Ben Boldt wrote:
Thu Mar 04, 2021 6:12 pm
Oh wow, that surprises me a guy like you wouldn't have one.
Oh yeah, I would love to have one.
It would sure make things easier if I knew what's really happening or at least could better describe what's happening.
lidnariq wrote:
Thu Mar 04, 2021 6:20 pm
Sadly imports are stupidly expensive in Brazil, both in flat currency but especially relative to cost of living. Hence why Fisher goes to such impressive efforts to fix things.
Yes, Lidnariq is right.
Most things I do, or try to do, are made with thrashed/discarded parts of electronic devices.

Let's just say that if life was a game, I would be playing it on the hard mode.
That's not too bad, since I see some people around who would be playing it on the very hard mode or even on mania.

Thankfully you guys have helped me alot, not just now but in many occasions in the past and I'm very glad.

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