Dead Sidecar Support Thread

Hardware questions and modifications

Dead Sidecar Support Thread

Postby DarkStar2032 » Sun Jun 28, 2020 1:17 am

A thread for troubleshooting and repairing dead and dying expansion sidecars.

Need help?

Got advice?

Got schematics?

Post here!
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Re: Dead Sidecar Support Thread

Postby DarkStar2032 » Sun Jun 28, 2020 1:24 am

To start off.

Anybody got any insights into the Tecmar jr Captain?
I know it's shoddy, but if I can get some technical spec on it, maybe I can get something usable out of it.

Also I have a dead 128k ram sidecar. I've already re-flowed the solder points to no avail and checked the bodge-wires for continuity. Any common fail points I should check next?
Hi-ho, KERMIT the computer file transfer/management protocol here.
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Re: Dead Sidecar Support Thread

Postby prpplague » Sun Jun 28, 2020 7:39 pm

DarkStar2032 wrote:To start off.

Anybody got any insights into the Tecmar jr Captain?
I know it's shoddy, but if I can get some technical spec on it, maybe I can get something usable out of it.

Also I have a dead 128k ram sidecar. I've already re-flowed the solder points to no avail and checked the bodge-wires for continuity. Any common fail points I should check next?


there are two basic issues that i've seen so far with the 128k sidecar:
1) a failed dram. the basic unit has these soldered in so it's hard to diagnose this without being able to test the ICs. the easiest thing to do is perform the 512k upgrade as a starting point to rule out an issue with the DRAM
2) the ICs used to do the address decoding fail so the memory can't be properly accessed. again these are soldered in. after doing the 512k upgrade and still having a failure, you can desolder and replace these ICs with sockets for testing

what kind of symptoms are you seeing? does the system boot but errors out? can you perform the self-test memory cycle?
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Re: Dead Sidecar Support Thread

Postby DarkStar2032 » Mon Jun 29, 2020 4:06 am

prpplague wrote:there are two basic issues that i've seen so far with the 128k sidecar:
1) a failed dram. the basic unit has these soldered in so it's hard to diagnose this without being able to test the ICs. the easiest thing to do is perform the 512k upgrade as a starting point to rule out an issue with the DRAM
2) the ICs used to do the address decoding fail so the memory can't be properly accessed. again these are soldered in. after doing the 512k upgrade and still having a failure, you can desolder and replace these ICs with sockets for testing

what kind of symptoms are you seeing? does the system boot but errors out? can you perform the self-test memory cycle?



No faults from boot or user diagnostic test. It's like it's not even there.

If I'm following you correctly it seems to be an address decoding fail. Correct?
Hi-ho, KERMIT the computer file transfer/management protocol here.
And welcome to NULL MODEM Street.
DarkStar2032
 
Posts: 18
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Location: Northern Minnesota

Re: Dead Sidecar Support Thread

Postby prpplague » Mon Jun 29, 2020 4:46 pm

DarkStar2032 wrote:
prpplague wrote:there are two basic issues that i've seen so far with the 128k sidecar:
1) a failed dram. the basic unit has these soldered in so it's hard to diagnose this without being able to test the ICs. the easiest thing to do is perform the 512k upgrade as a starting point to rule out an issue with the DRAM
2) the ICs used to do the address decoding fail so the memory can't be properly accessed. again these are soldered in. after doing the 512k upgrade and still having a failure, you can desolder and replace these ICs with sockets for testing

what kind of symptoms are you seeing? does the system boot but errors out? can you perform the self-test memory cycle?



No faults from boot or user diagnostic test. It's like it's not even there.

If I'm following you correctly it seems to be an address decoding fail. Correct?


yeah that is what i would look at first...
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Re: Dead Sidecar Support Thread

Postby DarkStar2032 » Mon Jun 29, 2020 6:03 pm

Does anyone have a diagram or part markings I can reference?
My ram sidecars came with nothing.
Hi-ho, KERMIT the computer file transfer/management protocol here.
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Location: Northern Minnesota

Re: Dead Sidecar Support Thread

Postby prpplague » Wed Jul 01, 2020 6:29 pm

DarkStar2032 wrote:Does anyone have a diagram or part markings I can reference?
My ram sidecars came with nothing.


i've not been able to find a schematic for the 128k side car, maybe someone here knows of one...
Dave
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Re: Dead Sidecar Support Thread

Postby JudgeMonroe » Thu Jul 02, 2020 10:46 am

No faults from boot or user diagnostic test. It's like it's not even there.

If I'm following you correctly it seems to be an address decoding fail. Correct?


In troubleshooting my jrCaptain, I've found that "not even there" means it's not plugged in. My sidecar came with a busted AC transformer and a shattered barrel jack so I had to fix those up. Once I corrected the power rail the jr at least counts the sidecar RAM.

If your JR isn't counting the sidecar RAM at all, check the power circuit (check for +5V at the output of the voltage regulator) and that at least one of the banks is switched on via the DIPs.
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Re: Dead Sidecar Support Thread

Postby JudgeMonroe » Thu Jul 02, 2020 3:58 pm

I recently got a PCjr with a jrCaptain sidecar. The only thing good about it is that it was upgraded to 512k, but it doesn't work.

1. The AC transformer and barrel jack were both broken. I replaced the jack and removed the rectifier circuit so I could use a DC transformer.

2. The voltage regulator and its header fell apart during the first operation, so I had to replace it too.

3. Only the first bank of RAM works. With the second bank enabled, the JR counts up to 384 and then halts with an "ERROR A".

It's the last point I've been troubleshooting, but lacking a schematic makes it a bit tough.

I've done the following

- Swapped the bank 1 and bank 0 RAM; no change.
- Randomly mixed the bank1 and bank0 RAM; no change.
- Disabled the Bank 1 RAM; JR counts to 384k and boots fine.
- Used a different 512K sidecar (thanks prpplague); JR counts to 640k and boots fine.
- Disable Bank 0 and Enable Bank 1; JR counts to 128k and halts with "ERROR A".
- Jumper JMP4 (uses 64k instead of 256k per chip) enabled: JR counts to 192k and halts with "ERROR A" (Bank 0 & 1); JR counts to 192k and boots fine (Bank 0 ^ 1)
- Verified continuity of all address lines through Bank 0 and Bank 1.

All of which leads me to believe it is not the RAM, nor the address decoding logic, nor the CPU. Instead, it is something to do with Bank 1 specifically.

I've been trying to figure out what Bank 1 has that Bank 0 doesn't have and it's hard to isolate. All the chips in Bank 0 have their data lines tied to a buddy in Bank 1, and these are tied to a LS245 at U33.

The only thing that stands out is that the RASB signal (pin 4) is not continuous between banks though CASB (pin 15) is. I'm not sure if this is supposed to be so and I can't see evidence for either case from traces on the crowded board.
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Re: Dead Sidecar Support Thread

Postby DarkStar2032 » Fri Jul 03, 2020 5:51 pm

Do you have specs on that AC adapter?
Hi-ho, KERMIT the computer file transfer/management protocol here.
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