Self-restarting STf with noisy video

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tzok
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Self-restarting STf with noisy video

Postby tzok » Fri Aug 10, 2018 8:50 am

Iv'e bought an 1040STf for parts, but finally I've decided to try to repair it. This is an early 1040STf with a RED leds and TOS 1.0 GB. Initially it was starting out-of-sync on ST-mono monitor (actually VGA with adapter). I've removed all the chips from their sockets, cleaned them with contact cleaner and reinserted them. I've also cleaned the board with IPA, but it wasn't really bad, just some minor solder joints corrosion. While reassembling I've accidentally put the SHIFTER upside down (the socked was mounted that way). This caused a short on a power rail. I've later checked that SHIFTER in another ST and it is fine. But the ST is behaving unstable. Initially it self-restarts just after the mouse cursor shows up. It also displays some random number of bombs. I've seen 2,3 or 4. After some time it boots to GEM desktop, but the picture is "noisy" (a clock phase problem shows up on LCD monitor like this). On the other hand color modes are just fine, with some minor vertical bar-like shades on the desktop area (but not on the borders). They are visible in the same area even on overscan-enabled demos. Sometimes it restarts when moving a mouse or accessing a floppy (either mechanical or Gotek). But there is a certain "time window" from the start when it works stable. I've run YAART on it and it was running without error for about an hour. After the computer warms up, it starts to restart again.

What I've done so far:
- using two different bench power supplies to power it (I don't have the original power supply),
- replaced all electrolytic capacitors (all removed were ok),
- tested and socketed bus buffers U22, U23, U26, U27 (as I pulled one or two pads doing this, I've tested continuity of all connections around),
- swapped socketed chips (SHIFTER, MMU, GLUE) with a known working ones - no change in mine, "donor" still running fine on mine chips,
- tried freezing main chips when it heated and started to restart - no results.

Most test I was running on a single 5V supply, but 12V is only used for powering the floppy (not all require 12V, newer SONY drives don't need it), and is present on a monitor out. Before I was sucessfuly running 520ST on single 5V (but there was required 12V/-12V from PSU to power UART and MIDI ports, but everything except these ports was working on a single 5V).



I've also noticed that when it reboots the upper row of RAM is somewhat warmer than lower. Also the CAS signal on upper row of chips is nice packs of pulses, while on lower row it looks random, similar with RAS - on upper row there is a constant signal, on lower row, some random pulses are missing. Once something happened while I was probing the lines and pulses at bottom row CAS lines disappeared (CAS were high all the time), so did the noise on a picture.The pulses in each case are 2MHz. I don't have the right schematics for it, it is the C070523-001 board.

Does anyone have any suggestions what to check next?

P.S.
Shouldn't it boot with CAS lines from bottom row disconnected from MMU and pulled high?


tzok
Atari freak
Atari freak
Posts: 57
Joined: Fri Jun 30, 2017 7:22 pm
Location: Bielsko-Biala, PL
Contact:

Re: Self-restarting STf with noisy video

Postby tzok » Fri Aug 10, 2018 6:02 pm

Quick update as a self response:
- it doesn't any more self-reboots, but it wasn't its only issue. The reason for restarts, probably was a bad contact/bent pin in MMU socket.
- The HIGH mode is still noisy and very prone to Vcc fluctuations and noise (changing Vcc by 0.05V shifts the screen left or right by 2-3 pixels),
- I've also tried the 320 × 400 mode and it looked bad:
Image
- after some time of running YAART (30+ passes) I saw strange distortion of a text in status line (test itself looked fine, and still was running without any errors), after holding Alt and going back to desktop (in ST MED) it looked like this:
Image
ST HIGH looks noisy like this:
Image

Anyone have any idea where to look for fault?


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