wpc@druhi.UUCP (ChristmasWP) (08/29/86)
<Summary: here's one way to fix it, are there better ways?> I have an early 520 ST with the new ROMS and the 1Meg upgrade. For the past month (since the ROM upgrade), the system has been crashing frequently. The crashes have come in the form of lockups, spontaneous resets and the disk suddenly appearing to have nothing on it. None of these things were permanently fatal; time, repowering or jarring the ST would temporarily fix the problem. Anyway, after much fooling around, I discovered that the voltage to my ICs was low (about 4.8 volts). I assume the extra bank of RAMs and extra ROMS (it was shipped with just the two) caused excessive current drain. In poking around, I noticed there was a choke in the path of the 5 volts that was not in the developer's kit schematics (L29 or L39 I think). This choke had a DC resistance of about 1/2 ohm and with my system had a voltage drop of about .4 volts under load. My current fix to the problem was to place another choke (of high AC impedence and about 1/2 ohm DC impedence) in parallel with the first. This reduced the DC drop across the inductor and raised my IC voltages to 5.0 volts. The questions- Do all STs have this inductor? What's it for? Any idea of what it's value is supposed to be (both DC and AC impedance)? Am I effecting anything else by changing the value of this inductor? Is my problem really a low voltage coming in from my power supply (I have the older style power supply), or is it that inductor? The system has behaved much better since I made the change, but I'd love to know if there's a better/safer fix! Thanks! Walt Christmas AT&T Denver ...!druhi!wpc
jhs@MITRE-BEDFORD.ARPA (09/02/86)
(For what it's worth, possibly but not necessarily equal to what it cost)... Your fix sounds eminently reasonable to me. The choke might be for RFI and spike protection. Finding a better choke (same inductance, lower series R) might be still better, as shunting the existing choke will reduce the inductance as well as the series resistance. The only penalty then may be the cost of the choke, which is probably not a problem if viewed as part of the cost of the upgrade. Shucks, even buying a heftier power supply would be a cheap way to get a 1MB ST, starting from the current $495 price of the 520! -John Sangster jhs@mitre-bedford.arpa
cms@vlsvax1.UUCP (Chuck M. Sweeney) (09/03/86)
In article <8609021508.AA25295@mitre-bedford.ARPA> jhs@MITRE-BEDFORD.ARPA writes: >Your fix sounds eminently reasonable to me. The choke might be for RFI and >spike protection. Finding a better choke (same inductance, lower series R) >might be still better, as shunting the existing choke will reduce the >inductance as well as the series resistance. The only penalty then may be >the cost of the choke, which is probably not a problem if viewed as part of >the cost of the upgrade. Shucks, even buying a heftier power supply would be >a cheap way to get a 1MB ST, starting from the current $495 price of the 520! > I have had the same symptoms after upgrading my 520 to 1Meg and, hearing (reading) the chatter about problems and weaknesses with the original power supply, was ready to swap it for a new switiching power supply or something. Recently, though, I talked to a hardware hacker type in the East Bay (east San Francisco Bay, that is) who told me that the original power supply, bad though it is, is not where the problem lies. He has done (and repaired) many ST upgrades and his procedure includes changing an inductor on the right side of the ST board (with the RAM at the bottom or toward you). The original coil is rated at about 4.something micro-henries. He replaces that with one of 2.2 micro-henries or less. His claim is that this will prevent the power drop to the RAM that is causing the problems. He also adds an additional buffer chip. I am unwilling to post what I remember of his instructions since I am not a hardware engineer or very knowlegeable of such things and I would mislike being the proximate cause of someone frying their ST. If there is any interest from the net in complete instructions, I will try to get him to write them up and I will post them. I would also be interested in comments from other hardware people about this approach. Sweeney qantel!vlsvax1!cms 415 490 3555