Well, I'm glad you revisited your work and didn't buy that Reverb IC ....

If I were being compensated for working on this, I would recommend replacing all those chips as all exceeded the expected draw.

The description of running hot ... is relative. Certainly running at more than the normal current can be described as running hot. Hot also can describe every degree of burning we know of ... especially when it comes to flesh being burnt. Not too many technicians had thermo-couples with their meters back in the day of manufacturing those chips, but they are common these days and copious notes would be the order of the day.

 

I ordered 7......just in case.

 

It's only step one, but the DRAMS arrived today and I stuck them in their respective sockets and fired 'er up. No smoke! Everything NORMAL!!!!!! God, it's been a long time since I've seen these readings! I'm fully aware I may not be out of the woods, but today's events shows me this was at least a valid issue. Hopefully, later, when I have more time and get it all put back together, with proper logic voltage, I'll be able to run it's internal diagnostics. I'll report back when I know more. Jim

 

I'm still having issues, but my supply v problems are gone. There's no display and no functions from the switch panel (which status is only determined in a working display). I need both these functions in order to invoke the internal diagnostics. I've first poured over the board under magnification for solder artifacts, bridges ect from my repairs and feel pretty good that the board is clean. So next, for this moment, I'm looking for something new that doesn't measure right. Both the switch matrix and the display are fed by a gate array chip. The switch data lines run in parallel with a couple discrete resistor arrays (15K x 8) which are all connected in common to the 5vdd which I'm guessing is providing a voltage to establish hi/low on the data lines, but this is beyond my knowledge. Here's my first question.... isolating the resistor array, measuring each resistor in the 2 arrays in ohms, not under power, they all test fine. Power up and measure each pin to ground in vdc, I confirm 5v on one side of each resistor as it should be, but only about half of the resistors produce any voltage on the other side. A couple only report mv's. A couple about 2 volts. I go back to resistance and there's 15K everywhere. I've done this several time with consistent results. Is this a problem or is this as it should be? Reference IC28: gate array. The two resistor arrays are right above the chip. Note, the factory Service Notes suggest that if there's no display, possible suspect is IC27.I think that's a misprint, but just mentioning it. That's it for tonight. Jim

 

I'll assume you snatched this manual ....

 

That manual has the troubleshooting flowcharts. Unfortunately they are split for ease of appearing on a standard sheet of paper. I haven't invested the time to combine what needs to be combined so a document would be nice and clean.

 

Yes, I have the manual and am aware of (and used in the past) the diagnostic flow chart. It works great, but depends on the display and switch bank to work it's magic and report it's findings. I expect that I probably need to pursue one of a couple directions, both which require more knowledge than I have. 1. Determine if the display (and related module electronics) is working, but because it gets it's signals from the gate array through some rather sophisticated local module electronics, I am not skilled enough to know how to set up a breadboard to emulate signals coming from the Gate Array and maybe cause the display to show even one normal digit or symbol. 2. Determine if the Gate Array (that feeds both the display and gets it's instructions from the switches) is working. I have a scope, but again, without detailed instructions, do not have the knowledge needed to test it. The IC is proprietary and try as I might, I've been unable to find any supporting documents on the Internet. Meanwhile, I've checked voltages at all the major ICs as well as the values of many of the caps and resistors. All appear fine. Later today, I'm going to reflow a number of relevant solder joints on the switch board that look questionable, but am not expecting anything special. Also, the manual recognizes the display module, but only says it's not serviceable so no further information is available. For anyone following this, if you'd like a copy of the Service Manual, see post number 10. Thanks everyone.

 

Sounds like a good starting point.

 

I found a guy who's experienced my same issues and found out it was the ROM that contains the low level boot and diagnostics programs that had lost its programs; explaining that if I was getting the one row of bars in the display (which I am), the bar is the display's default error message meaning that it's working, but simply not getting any data to display. If there was a problem with the switchboard (which was fine before all the happened) that the display would still show the default splash screen. It makes sense to me. We'll see. I need to walk away from it again for awhile and let this all digest.

 

Contaminated or lost firmware programs certainly cause a variety of strange problems. I've see a few in satellite receivers.

 

This actually makes more sense to me that suddenly all sorts of unrelated problems that I'd never seen before. Also, its the first scenario that completely matches the symptoms. Besides, whats the typical life of an Eprom? I seem to remember 20 years......its been 33. So, I bought a couple new 27C512's from Mouser for a couple bucks each and dug into my equipment archives and found a programmer and then found the original bin file on the internet. I need to order a cable and some other minor stuff to get the programmer running, but thank God for Amazon. Once it's all here, I'll be able to upload whatever's in the existing IC and compare it to the bin file and if nothing else, have a working backup. I'll report back in a few days.