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AT&T 6300 won't post

A

AmigaJules

Experienced Member
Joined
Dec 4, 2008
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60
Location
Denton, TX
I recently acquired a pretty loaded 6300.

It has what I believe is a memory board (in addition to 640K on the motherboard) {EDIT} My bad: 640K total with the fully populated memory board. 8087. VDC 750 EGA card. Write-right bus enhancer. 20MB seagate ST-225. 319 display. Mouse. Boxed copy of Windows 2.0. And the full set of manuals, software & documentation.

I purchased it from the original owner who had stored it in its proper orientation.

The battery had leaked onto the case, not the board. There was a very small amount of corrosion around it. I've clipped the battery and cleaned up the corrosion with white vinegar followed by distilled water & isopropyl.

When I turn it on, I get blinking keyboard LEDs but nothing on the display, no beep, no post. I believe I've read that the 6300 does not require a battery for POST, only to maintain clock/calendar.

The PSU voltage to the motherboard is 5.08v, but the CRT voltage measured at the J7 connector on the mono display card is 16.25v. The PSU has the display voltage marked as 12v - is this normal if there's no load from a mono display? Or could it possibly point to a PSU problem? {EDIT} - I'm reviewing the service manual and it looks like the CRT voltage is 15v not 12, so 16.25 is within the 10% tolerances...

Any suggested troubleshooting steps from 6300 experts?

While I wait for replies, I'm going to disconnect HD & controller, remove VGA & RAM card and try to get it down to a minimum config to rule out an issue from one of the cards.

I've pushed all of the socketed IC's into their sockets, but may also remove and reinsert them.

Thanks for any insight!
 
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This motherboard is more & more suspect...

It looks like someone did some re-work on the board at some point.

IMG_7734.jpg

And there are a couple of components missing

IMG_7729.jpg

IMG_7732.jpg
 
It's possible that some components were "robbed" or "relocated" on the MoBo for some purpose, but this also may be a factory-modification (i.e.---engineering change), or something was located/removed in those locations for testing purposes. It would be almost impossible to speculate without finding another identical machine with the same revision of motherboard. Your system may not be booting for the above or other reasons....
 
NeXT said:
The symptoms you are getting are typical of 90% of the PC 6300 and variant owners once their battery has leaked.
Grab yourself a parallel port POST display and join the group trying to figure out what's wrong.
Click to expand...

I'm doing what I can with what I have available at the moment, but I'll keep an eye on that thread and see about picking up one of those cards.

I've removed & replaced all of the socketed chips, except for the 2nd bank of RAM. The machine now has nothing in it but PSU, mono display card & 128K RAM. I set the dip switches for 128K.

Same symptoms.

One interesting (possibly?) thing I noted: if I press the reset button, the keyboard lights flash for about 2 seconds, then one more single blink, then they go dark, but still no beep or POST.

There are 4 22uf 25v radial electrolytic caps on the board, 3 of which have some corrosion on their leads. If I can scrounge some of these around the house, I may replace them.
 
Above I can read that your 6300 has an EGA card. On older board versions that is only possible when modifying the original video card, that means remove one chip (PAL). So if you try to start the 6300 without the EGA card and without that chip, it will not show anything on the original graphics card. With some new board revision it is also possible to disable the graphics by DIP switch https://sites.google.com/site/att6300shrine/tutorials/disable-video-card
 
1ST1 said:
Above I can read that your 6300 has an EGA card. That is only possible when modifying the original video card, that means remove one chip (PAL). So if you try to start the 6300 without the EGA card, it will not show anything on the original graphics card.
Click to expand...

Interesting. That might explain the 2 chips in a bag that came with it? They have a white printed label. One says PL68, the other PA96.

I only have the EGA display, so there's no monitor connected during this testing. I'm just trying to get past the blinking keyboard LEDs & get a beep from the speaker.
 
removed, it was attached to the wrong thread. See post code thread.
 

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I want to confirm that the PC6300's PSU is working under load, before I move on to other components.

I don't have resistors to make a dummy load, but I do have a Seagate ST-225 and an ST340016A (40GB IDE).

The smaller drive is spec'd at .85a @ 5v and .75a @ 12v. The older drive is spec'd at .8a @5v and .9a @12v.

The PSU spins up both drives. When I measure the power at the drive, I get 5.2v on the 5v and 10.93 on the 12v.

I think those numbers are within the 10% spec of the PSU.

Based on this testing, do you think I can conclude that the PSU is good?
 
Here's hoping the best of luck for ya on the troubleshooting - I've got a second 63k in the same boat!

If it helps at all, i should also mention that for the POST card linked earlier you'll want a parallel cable to use with it - trying to just plug it right into a 6300 is somewhat awkward as due to how the parallel port is oriented, you'll wind up angling the 7segs downwards. A passthrough cable will mitigate that, though you can also just jam a mirror under it if need be.
 
Wanted to update everyone that I now have a working 6300!

The original PSU was failing, even after replacing all caps, it would only put out ~2v on the 5v line and ~6v on the 12 v line. I gutted it, kept part of the top circuit board with the screw terminals and stuffed a modified ATX PSU into the casing. I was able to keep the original switch and power connectors, so it's hard to tell it's not an original part.

The 1.43 ROM motherboard gave very inconsistent results with the code reader.

I swapped the ROMs and 8087 chip onto the 1.21 ROM motherboard and after fiddling with the dip switches a bit it worked.

I ran the memory, video card, 8087 & HD diagnostics and all components passed.

Windows 2.0 was installed on the HD.

IMG_1644.jpg

IMG_1646.jpg

IMG_1647.jpg
 
Apologies for jumping in here.

The PSU voltages appear to be within tolerances.

Right now, you've got an EGA card sort of fouling the waters. When I'm confronted with an "He's dead, Jim" board, I try to return it to a near-naked functioning state. In your case, this would involve getting rid of the EGA card and returning the display card to its original functioning state. Without knowing what was done to your system before you received it, it's hard to say otherwise.

There is a bunch of technical documentation here, including schematics and a service manual. FWIW, the schematics include the so-called "Piastro Piggyback" keyboard controller board that has some people puzzled.
 
Hmm, so the PSU's definitely a culprit? Good news for my dead specimen, i was about to pull the MFM drive from it to replace an ailing drive in my otherwise known good model.
 
Chuck(G) said:
Apologies for jumping in here.

The PSU voltages appear to be within tolerances.

Right now, you've got an EGA card sort of fouling the waters. When I'm confronted with an "He's dead, Jim" board, I try to return it to a near-naked functioning state. In your case, this would involve getting rid of the EGA card and returning the display card to its original functioning state. Without knowing what was done to your system before you received it, it's hard to say otherwise.

There is a bunch of technical documentation here, including schematics and a service manual. FWIW, the schematics include the so-called "Piastro Piggyback" keyboard controller board that has some people puzzled.
Click to expand...

If this is in reply to my post, the PSU was definitely bad. Early on in the thread I was getting proper power out, but after additional testing the voltages dropped significantly and wouldn't even spin up an IDE hard drive. I modified an ATX PSU and put it into the original PSU case and now have a working machine.
 
Anybody want an M24 (PC6300) motherboard (PC1050)?

No way to test, but no reason to think it's bad; no battery leakage, all socketed chips missing except L & H BIOS, PAL @ H8 and PROM @ J6.

$25.00 + postage from Toronto; PM if interested.

m
 
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