Saturday, July 29, 2017

Ups and downs with Alto demonstration, finished the Heath C-3 Condenser Checker restoration


The last of my replacement parts arrived today and I soldered everything together. The chain of higher wattage 22K resistors were placed on the selector switch. The octal socket for the magic eye tube was wired up. New tubes were then placed into sockets. Finally, I fired it up to check the voltages and gross behavior. 

The magic eye glowed properly, voltages seemed sane, so I moved on to check some components. The resistance test range was close to the resistor value and can be calibrated by moving the dial on the pot shaft if I wished to. The capacitance check worked well for a .1uf capacitor, as did the leakage test. 

I then put in a 16uf 450V electrolytic and did some capacitance and leakage tests. It worked exactly as it should for both capacitance detection and leakage testing. The tool is now properly restored and ready to use with those suspicious power supply capacitors. 

Restored C-3 Condenser Checker

We attempted to clean the heads on the disk drive, using Kimwipes and isopropyl alcohol. The bottom head is easily visible but seeing the top one involves mirrors and awkward angles. It seemed that the head was okay and they landed okay on the cartridge that originally had the problem.

We then wanted to write our demo image on one of our three writeable cartridges - those that don't have historical data from PARC - but immediately the drive began cycling the solenoids in a loud clicking manner that we associate with head crashes. I immediately switched off the drive and removed the cartridge.

We can barely see what appears to be packed in oxide on the upper head, thus our earlier cleaning was insufficient. Inspecting the pack shows signs of a crash as well, brown rubbing on the oxide in a ring. No aluminum substrate exposed and no apparent ridges, thus the pack is probably still usable once the cause of the crashes is corrected.

We decided to switch the disk tool over to the drive attached to our Alto. This has good heads and we placed one of our good cartridges in the drive to write the demo image. Using the disk tool, however, showed a troubling sign. Rather than seeking crisply and regularly through all 202 cylinders, the drive would advance slowly and irregularly.

I believe this may be a grounding issue where the drive with our head crash is well grounded through the signal cable but the one in the Alto is not. Poor quality signals could account for the failure to 'see' the ready signals that follow each seek. Further, this likely means that the data being written or read would also be suspect.

I did a ReadEntireCartridge and processed the captured image. Comparing this to the image I used to write the cartridge earlier showed many small errors across the disk. Whatever signal fault exists with the tool attached to this drive, it must be corrected to get reliable operation.

We booted the disk image I had written, got a Alto Executive prompt which seemed promising, but found that while most programs seemed to work, SIL did not. We ran the scavenger program which clears up problems on disks by finding mismatches between the disk directory contents and the chaining information in the label records of each sector.

If found quite a few problems, many in SIL related files, and attempted to fix them. It also found problems in the high-res picture we display late in the demo, which was missing the bottom fifth and had an obvious bad stripe in one spot up higher.

To proceed with our demo and other activities today, we switched over to doing a copydisk over the network from the same image placed on the LCM bridge. That worked and we could move ahead with our taping and demo practice.

 We videotaped the demonstration with wide shots in the afternoon. Once some screen closeups are taped, to be used as insets or spliced with the main taping, a complete video can be edited together to share the demonstration on YouTube.

Finally, we worked on the presentation we would give as part of our panel at the Vintage Computer Festival West conference next week. We spent hours tightening it up, rewording and strategizing on how to squeeze it into the slot we have available.


I have a Tektronix 7854 scope, which is a mixed analog and digital scope that timeslices access to the screen between the two sources of display information. It displays characters on screen in the digital mode, but they are distorted while the beam is sweeping from the analog side. The logic should be switching the analog off while displaying digital, and vice versa, but it appears something is going awry.

I began scoping the various signals involved in this timesharing of the CRT. The first three I could measure had an easy to reach set of testpoints at the top edge of the card. The next few are internal or the testpoints are so far down the card that they are blocked by adjacent cards. I pulled the card and stuck some probes on the points I need to monitor, then reseated the card.

All the control signals appear to be switching as they should - at least their relative timing and shape are correct. My next set of tests, after pulling the card again and changing probes, is to see if the vertical and horizontal outputs of the analog section are indeed going to ground during the digital interval.

Tektronix scope being debugged
There is an Intersil DG181 DPST switch that grounds both lines when in the digital interval, but if that is failing then the analog signals will interfere with the sweeps from the digital/character generator section. I will watch the sweep outputs to see if they are non-ground level during the digital interval. This will have to occur tomorrow as it is too late to continue out in the garage tonight.

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