1040 STF repair

telengard · September 15, 2015

I've been working on my 1040 for a month or two now as it does not work reliably. The symptom is that on a cold (i.e. sat for an entire day without being powered on) it will not boot. I get seemingly random number of bombs or white/black screens. After staying on for about 15-20 seconds the machine works fine and will work for quite a while (maybe 8 hrs or so).

This is what I've done so far:

- Eliminated the power supply from the equation by running off my Rigol supply

- Used coolant on all chips/caps/etc to try and induce the behavior (have done this twice)

- The ST diags all pass

- Ran a RAM tester program for about 3 hours, passed

I did find what looks to be an inductor which is broken, L1. But going by the schematic it seems to be connected to the MIDI port, so possibly not relevant. It is also 3 pins and I have no idea what to use as a replacement, according to the service manual it is a 10uh 10% axial. These seem to be connected to all extern pins on ports, etc. They look a lot like ceramic caps.

I plan on testing the ESR of some of the larger caps next, but would appreciate any ideas or insights.

telengard · September 19, 2015

I changed out the inductor with one from a spare board, it made no difference. However, I was able to get some information by running RAM diags while still "cold". As I mentioned, after a minute or two of being powered on, things work fine. I ran these diags off the diag cart. I have about a minute before the RAM test starts passing.

In one of the pictures it looks like the 8th most significant bit is flipping or sticking high. I'm not sure about the other issue w/ the bad instruction fetch. The address for that seems a lot lower than the other failed run. I also sometimes just get a white screen w/ a red border.

Edited September 19, 2015 by telengard