Myarc cards for sale/repair tips

[+InsaneMultitasker]

31 minutes ago, webdeck said:

Interesting.  I used to have a reset button on my Geneve that routed to the front of the PEB, but I snipped it off when I had to send in the card for repairs.  I foolishly didn't document how it was connected, but based on the blobs of solder left, it looks like the wires were connected to both sides of C12, and there is a resistor on one of the wires, so pressing the button shorts the resistor across C12.  I could be wrong, however.

Where is C12 on the board?   Should be easy to check continuity between the 9995 and gate array to verify whether or not it is tied to their RESET pins. I'll look through my notes for this variant. What is the resistor value?  2k? 

 

The mod I reference earlier was probably shared by Myarc since it involves an undocumented gate array pin.  Of course, that doesn't make it the best implementation! 

[webdeck]

Oh, the capacitor was covering part of the number - it's C127 which is right below the resistor you indicated.  But as I'm looking from the back side of the board, I'm now second guessing what the points I'm looking at are connected to, so it may not be C127.  I've circled them in yellow in this photo.  The resistor is still attached to the momentary switch, which is buried inside the power supply section of the PEB, so I would need to take everything apart to see what value it is.

 

Regardless, I'm going to go with your method as it's easier as all I need to do is wire up a new switch and this time I'll just stick it to the back of the card and not tear apart the PEB.  What point should I use for ground for the other end of the switch?

 

[+InsaneMultitasker]

36 minutes ago, webdeck said:

Oh, the capacitor was covering part of the number - it's C127 which is right below the resistor you indicated.  But as I'm looking from the back side of the board, I'm now second guessing what the points I'm looking at are connected to, so it may not be C127

Ah, gotcha.  Check continuity between the + side of C127 and 'bottom' leg of R62.  I think you'll find them connected electrically.  The spot you circle looks familiar to me as an old connection spot for the reset.  The capacitor's other lead should be connected to ground.  IIRC, using the capacitor points fell out of favor due to proximity of other through-holes and risk of future short, which morphed into using a header or direct mount switch.  

 

You can find another ground spot or use the capacitor lead as was done in the previous install.  You can also scrape off some of the foil where you mount the switch and solder directly to the ground plane.

 

If the resistor is a best practice for the reset, versus a direct 'short', I would keep it in the circuit.

 

Maybe someone with appropriate  knowledge can/will chime in on the use of the resistor in this reset application.

 

Edit: After you solder the switch into place, at minimum verify that you don't have any shorts with adjacent components or through-holes, depending on where you install it.

[+InsaneMultitasker]

Components on the schematic for reference

[webdeck]

54 minutes ago, InsaneMultitasker said:

Ah, gotcha.  Check continuity between the + side of C127 and 'bottom' leg of R62.  I think you'll find them connected electrically.  The spot you circle looks familiar to me as an old connection spot for the reset.  The capacitor's other lead should be connected to ground.  IIRC, using the capacitor points fell out of favor due to proximity of other through-holes and risk of future short, which morphed into using a header or direct mount switch.  

 

You can find another ground spot or use the capacitor lead as was done in the previous install.  You can also scrape off some of the foil where you mount the switch and solder directly to the ground plane.

 

If the resistor is a best practice for the reset, versus a direct 'short', I would keep it in the circuit.

 

Maybe someone with appropriate  knowledge can/will chime in on the use of the resistor in this reset application.

 

Edit: After you solder the switch into place, at minimum verify that you don't have any shorts with adjacent components or through-holes, depending on where you install it.

Okay, I tore open the PEB.  Here is a photo of the resistor.  It looks like brown, black, black, gold which means it is only 10 ohms, correct?

[+FarmerPotato]

On 11/29/2020 at 6:20 PM, InsaneMultitasker said:

Ah, gotcha.  Check continuity between the + side of C127 and 'bottom' leg of R62.  I think you'll find them connected electrically.  The spot you circle looks familiar to me as an old connection spot for the reset.  The capacitor's other lead should be connected to ground.  IIRC, using the capacitor points fell out of favor due to proximity of other through-holes and risk of future short, which morphed into using a header or direct mount switch.  

 

You can find another ground spot or use the capacitor lead as was done in the previous install.  You can also scrape off some of the foil where you mount the switch and solder directly to the ground plane.

 

If the resistor is a best practice for the reset, versus a direct 'short', I would keep it in the circuit.

 

Maybe someone with appropriate  knowledge can/will chime in on the use of the resistor in this reset application.

 

Edit: After you solder the switch into place, at minimum verify that you don't have any shorts with adjacent components or through-holes, depending on where you install it.

You don't want a direct short to ground. Putting a resistor in series, protects against a rush of current (that can exceed the rating of the el cheapo switch you got out of the bits and bobs box.)

 

https://www.eejournal.com/article/ultimate-guide-to-switch-debounce-part-3/

 

Take a look at the reset switches there (there are a LOT of them but the capacitor-resistor-resistor type is adequate for the 9995 reset pin.)

 

If you want to improve on it.. put the signal through a LS14 (Buffer with Schmitt Trigger).

I spent a couple hours making a nice reset button. I thought of making it a tiny PCB, that way it could be shared.

 

[+FarmerPotato]

On 11/29/2020 at 7:15 PM, webdeck said:

Okay, I tore open the PEB.  Here is a photo of the resistor.  It looks like brown, black, black, gold which means it is only 10 ohms, correct?

You want to keep that resistor. 

 

[+InsaneMultitasker]

1 hour ago, FarmerPotato said:

You don't want a direct short to ground. Putting a resistor in series, protects against a rush of current (that can exceed the rating of the el cheapo switch you got out of the bits and bobs box.)

 

https://www.eejournal.com/article/ultimate-guide-to-switch-debounce-part-3/

 

Take a look at the reset switches there (there are a LOT of them but the capacitor-resistor-resistor type is adequate for the 9995 reset pin.)

 

If you want to improve on it.. put the signal through a LS14 (Buffer with Schmitt Trigger).

I spent a couple hours making a nice reset button. I thought of making it a tiny PCB, that way it could be shared.

 

Neat article!   If I read it correctly, the Geneve schematic shows the 9995 (and 9901) reset signal coming from the gate array.  I'm not sure what other magic the gate array is doing before it passes the reset along.  

[+FarmerPotato]

12 minutes ago, InsaneMultitasker said:

Neat article!   If I read it correctly, the Geneve schematic shows the 9995 (and 9901) reset signal coming from the gate array.  I'm not sure what other magic the gate array is doing before it passes the reset along.  

I guess from your post about the magic keypresses, that the gate array decodes those, and pulls RESET down. 

 

Were you concerned about soldering and damaging the gate array, or just interfering with the output of the gate array? For the latter case, that limiting resistor is very wise (even 10 ohm. though I would go larger)

 

By soldering to the back of the board, even at the CPU pin, you are still going to  interfere electrically, but solder heat is much further away.

 

Now I want to know more about the schematic.

[+InsaneMultitasker]

12 minutes ago, FarmerPotato said:

Now I want to know more about the schematic.

Here is their current location on WHT, although somewhere along the line someone unpacked the ZIP I released, so it isn't obvious which files were released together: https://ftp.whtech.com/Geneve/schematics/ 

 

My concerns are two-fold: (1) the underside of the board presents pins that are nearby to one another and over the years, I have had to fix a number of Geneves where wires on the back side eventually shorted other pins, so caution must be exercised for this mod and (2) the recent question of whether the direct short was appropriate, so that the reset mod can be documented properly versus just "what might have been done back in the day". 

 

 

[+FarmerPotato]

2 hours ago, InsaneMultitasker said:

Here is their current location on WHT, although somewhere along the line someone unpacked the ZIP I released, so it isn't obvious which files were released together: https://ftp.whtech.com/Geneve/schematics/ 

 

My concerns are two-fold: (1) the underside of the board presents pins that are nearby to one another and over the years, I have had to fix a number of Geneves where wires on the back side eventually shorted other pins, so caution must be exercised for this mod and (2) the recent question of whether the direct short was appropriate, so that the reset mod can be documented properly versus just "what might have been done back in the day". 

 

 

Those are concerning enough.

 

Today's crazy idea:

 

Use the increasingly hobbyist-friendly flex-board services. Make a flex PCB that goes under the 9995 (but above its socket), connecting to just the VCC, GND, RESET, NMI pins. All conductive traces face the plastic shell beyond that point.  Bring the flex ribbon out to the front of the card. A conventional PCB, just big enough for two buttons, is permanently mounted onto a hole in the Pbox. The flex ribbon attaches here. This tiny PCB implements a safe reset and NMI switch. 

 

 

[webdeck]

On 11/30/2020 at 6:52 PM, FarmerPotato said:

You want to keep that resistor. 

I connected a new switch with a 220 ohm resistor and it is working.  Thanks!

[+FarmerPotato]

1 hour ago, webdeck said:

I connected a new switch with a 220 ohm resistor and it is working.  Thanks!

Hooray!

[Shift838]

All,

 

@GDMike sent me his Geneve to make sure it was bootable before he purchased a GBS8200 and GBSVGA Genie v2.0 from me.

 

I immediately recapped and replaced the voltage regulators, heatsinks, etc and updated the boot EPROM to v1.0.  Mike already upgraded the Geneve with the 32k SRAM and battery replacement upgrade.

 

When I originally tried to boot it I would immediately get 'BAD CPU RAM FOUND' message.

 

I pulled all the 41256 DRAMs and tested them with a Arduino based DRAM tester (project HERE), I found more than 1/2 of the DRAMS were bad and verified they were in fact bad in my Geneve.

 

I replaced all the 41256 DRAMS with new NEC D41256C-10 (all 16).

 

Found the 12mHz crystal was extremely loose, I could almost pull it out).  Unsoldered the rest of the way, cleaned up and resoldered it.

 

I now try to boot it and sometimes there will be cyan dots at various locations on the initial boot screen.  sometimes it will try to boot from my SCSI device or DSK1 and freeze while loading MDOS.  I know it's trying to load MDOS from my SCSI hard disk as I see the SCSI splash screen.  Other times it will give the 'BAD CPU RAM FOUND' message again.  I went so far to pull all the new 41256's and retest and they all passed.  

 

Anyone have any suggestions of what to look at next?  could it be a crystal issue?

 

 

[GDMike]

 the TMS 9995 added 256 bytes of on-chip RAM, mapped to main system memory. Using on-chip RAM to store Workspace data and other program data resulted in noticeably better performance due to significantly reduced memory access time.

 

 

sniff...I think the cpu is bad..dang it all!!

Is it socketed?

 

https://www.cpu-world.com/CPUs/TMS9995/index.html

 

Edited December 17, 2020 by GDMike

[Shift838]

16 hours ago, GDMike said:

 the TMS 9995 added 256 bytes of on-chip RAM, mapped to main system memory. Using on-chip RAM to store Workspace data and other program data resulted in noticeably better performance due to significantly reduced memory access time.

 

 

sniff...I think the cpu is bad..dang it all!!

Is it socketed?

 

https://www.cpu-world.com/CPUs/TMS9995/index.html

 

yes it's socketed.

[+InsaneMultitasker]

If you are using the extender card, check the voltage at the DRAM after receiving a bad cpu message. Confirm 5v.

Check continuity between DRAM chips. Some pins are not common to all chips.

Possibly bad DRAM / VRAM socket

For many of the DRAM chips to go bad points to possibly removing the card out of the PEB or rocking it in the slot with the PEB still on at some point in time.

[Shift838]

33 minutes ago, InsaneMultitasker said:

If you are using the extender card, check the voltage at the DRAM after receiving a bad cpu message. Confirm 5v.

Check continuity between DRAM chips. Some pins are not common to all chips.

Possibly bad DRAM / VRAM socket

For many of the DRAM chips to go bad points to possibly removing the card out of the PEB or rocking it in the slot with the PEB still on at some point in time.

Will be doing this later this evening, need to download the schematics to check which pins are common. 

 

 

Edited December 17, 2020 by Shift838

[GDMike]

possibly removing the card out of the PEB or rocking it in the slot with the PEB still on.

.....

yeah, this wouldn't be very smart for anyone to do.

I suppose someone could possibly not have known that the PEB was on prior to insert.. but,  surely something would fail I'd assume if you didn't blow a fuse too somewhere.

I guess adding a fuse somewhere on the card could help, but you can't stop stupid either. ?

Boy, Im sure hoping for the best outcome.

 

[+dhe]

Wait until you see the mess made of cards jumpered for PC power supplies make when plugged in to a standard PBOX....

 

[+Ksarul]

2 hours ago, dhe said:

Wait until you see the mess made of cards jumpered for PC power supplies make when plugged in to a standard PBOX....

 

That mess is never a good thing to look at. . . .

[GDMike]

Just now, Ksarul said:

That mess is never a good thing to look at. . . .

I think you may be seeing to much of that too. Bwhahaha

[+InsaneMultitasker]

2 hours ago, dhe said:

Wait until you see the mess made of cards jumpered for PC power supplies make when plugged in to a standard PBOX....

 

that's not funny. Mess doesn't do the situation justice.

 

 

[Shift838]

6 hours ago, InsaneMultitasker said:

If you are using the extender card, check the voltage at the DRAM after receiving a bad cpu message. Confirm 5v.

Check continuity between DRAM chips. Some pins are not common to all chips.

Possibly bad DRAM / VRAM socket

For many of the DRAM chips to go bad points to possibly removing the card out of the PEB or rocking it in the slot with the PEB still on at some point in time.

I have run my test with the below results.

 

All chips have tested 5.1v between VCC and VSS 

 

See attached photo

 

Continuity Test 

 

All 16 41256 DRAMs: pins 1,2,3,4,5,6,7,8,9,10,11,12,13, and 16 are all connected together.

 

pin #14 is connected to pin #14 on the DRAM chip immediately to the left or right of itself within chip column #1 and #2

pin #15 is connected to pin #15 of the DRAM chip directly above and/or below itself.

 

 

Edited December 18, 2020 by Shift838

[Shift838]

I decided to try and replace the TMS9905 to make sure there was not an issue with the processor.  Still no change and still receive the BAD CPU RAM FOUND message.