Found bad custom chip part in 7854 scope and replacement part is amazingly available

TEKTRONIX 7854 REPAIR

I can see that the Display board is generating the XYINH signal that should block the analog scope signals from being displayed during the digital interval, however the characters are smearing because the analog signals are not inhibited.

Further, in looking at the way the characters are distorted, it is strictly in the horizontal direction. The top and bottom of the character is never affected, but the left to right placement of the pixels is impaired. That suggests that the flaw is in the Horizontal Amplifier circuitry, which is not acting on the XYINH.

Looking at the horizontal circuitry, I see one place where XYINH is fed into a custom chip that is the channel switch. This chip is available on ebay as New-Old-Stock just in case it appears that problem is there. It will have to come from the island of Rhodes, meaning several weeks of transit time, but is reasonably priced all in all.

When checking out the rest of the scope, I observed that only the B horizontal plug-in is working, the A plug in does not cause traces. Swapping the actual plug-in modules made it crystal clear that the problem is in the horizontal switch circuitry, just where my other problem is manifesting.

The circuitry in question is on board A9 inside the main body of the scope, not on the plug in boards on the side. It is behind the plug-ins, which suggests that access a real pain. However, when I began disassembling I found that it was brilliantly designed for easy access.

Card A9 exposed behind metal plate

Card A9 seen from underneath

A metal cover plate blocks each of four possible card positions, with the card itself designed to fit through the opening exposed. I only had to remove a bunch of coax cables and one small connector, take out two screws and it slide right out.

Card removed through opening

Access from behind plug-ins

The switching chip is right at the beginning of the circuit path on the board, routing two sets of horizontal signals from the A and B plug-in positions, switching with the Benable signal and blanking with the XYINH signal. This is pretty clearly the part that is fried, so I bought the NOS spare from ebay.

Suspected chip circled at left

New Old Stock chip to replace on card

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

HEATHKIT C-3 TESTER 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

ALTO DISK TOOL

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.

TEKTRONIX 7854 SCOPE REPAIR

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.

Fixed triple power supply from a flea market

TRIPLE POWER SUPPLY REPAIR

At a flea market, I came across a triple power supply for a very attractive price. When I brought it home, I noticed that a switch on the first section, which changed the meter between voltage and current readings, was mechanically broken. I also heard some rattling inside.

The switch was a common small switch that I had sitting in my back room, so it was a quick fix. The rattling sound was a heat sink with four power transistors, powering the third section which could deliver relatively high current but lower voltage than the first two sections. I attached it to the standoffs, which was all that was required. 

Testing showed that all sections worked properly, including their current limiting circuits and overvoltage protection. This will be a very useful bench tool that I acquired for $40 plus a few minutes replacing a switch and attaching a heat sink assembly internally. 

Restoration work on Heath C-3 and on a 1401's card reader

HEATHKIT C-3 CONDENSER CHECKER RESTORATION

Most of my replacement parts arrived, including the 1626 tube, which I soldered into place. I do have two problems that will require more part orders. First, since I am reverting the design back to the 1629 magic eye tube from the 6E5, it switches the required tube socket from the six pin part I had to remove and an octal socket for the 1629.

Second, part of the first order was wrong. I remember that when I found the 22K 2W resistors on Digikey, I initially ordered 6, the quantity I needed. However, the web site popped up an alert telling me that for the same price I would get 10, because of the volume break at that number. I approved the change but didn't check closely to see that it had changed the type of resistor to a 1/4W 10K that I don't need.

When I was ordering the 22K resistors today, the same pop up was delivered but I cancelled out of it and manually ordered 10 of the desired part. It was now correct and will arrive later this week or over the weekend.

I am awaiting the new 22K resistors, the octal tube socket and the 1629 tube, after which I can wrap up this restoration and begin using the checker.

IBM 1401 RESTORATION WORK

We have been fighting erratic reading on the 1402 reader that is part of our "Connecticut" system over the past couple of weeks, but today someone noticed that dust was wedged into slots in a plastic disc used with a solar cell to generate the timing pulses for the 12 card rows. With an air compressor and vacuum, the disc was cleaned and our reader is back to proper operation.

We still have a permanent error in core at one address (high in memory, fortunately). We will try varying the core unit voltage in both directions to see if we are right at the edge and failing on the weakest core, or if we have a more pernicious flaw in that core location.

If we can't get the core working, we have a spare 4K core unit that we can swap in. It is a major task, but much easier than disassembling and rethreading a core plane to replace a bad core. 

Wrote demo disk cartridges, practiced VCF-W demo and continued archiving PARC cartridges

XEROX ALTO RESTORATION PROJECT

We met today to work on the demonstrations we will give at Vintage Computer Festival West in a few weeks. Ken and I had built disk images using the LCM's Contralto emulator, which I wrote onto two disk cartridges using the disk tool. With those, we began booting and practicing the demonstrations.

Marc also recorded the demonstrations which he will post to YouTube in a few weeks.

In addition, I returned the batch of disk cartridges to PARC since we had archived them already. I took a new batch of 30 back to Marc's where we can read them all with the disk tool to ensure the contents are preserved for posterity.

I successfully archived four cartidges, giving us two dual-disk Mesa images and source code of Press, but while reading the fifth disk we had a minor head crash at cylinder 184. The disk surface doesn't look too bad so once we clean the heads on the Diablo drive, we should be able to extract the contents.

Failed part discovered in C-3 Condenser Checker, improvements made

HEATHKIT C-3 CONDENSER CHECKER RESTORATION

The main power is set by a voltage divider, with ground at the center node. There is a 47K resistor up to the side which feeds B+ to the 1629 magic eye tube. On the other side, there is a series chain of resistors which total 98K. This gives roughly 1/3 of the voltage to the B+ and 2/3 to the resistor chain that are on the selector switch which determines what negative voltage is presented across the capacitor under test.

The first 22K resistor in the series chain was wide open, which meant that the minus side of the divider was disconnected. This explains the values I measured and is consistent with the design flaw where the divider string uses mainly 22K resistors to drop about 100V per step. This is because the chain of resistors has about 4.5ma running through it.

That puts 1/2W on the resistor, its rated value, but if a capacitor is tested with substantial leakage current then the power on the resistor will temporarily be well above its rated power. Over time, the abused resistors change value or open up like mine did. The replacement components I bought are rated at 2W which gives a lot more margin.

The prior owner of this tester had substituted a different magic eye tube for the 1629 I used. It was a 6V type while the original has 12V filaments. He or she put in some large low value resistors to divide the filament voltage in half. I bought the proper magic eye tube therefore I removed this hack.

The circuit should be back to original design when I finish the restoration, other than any changes I need to introduce to deal with excess HV from the transformer. It should produce no more than 500VAC which when rectified and filtered puts about 450V on the low side filter capacitor, within its margin. Thus, I don't strictly need to introduce by pair of series capacitors to provide a higher voltage handling capability.

Instead, I added resistors in series from the cathode of the rectifier so that they drop the excess voltage. I measured the transformer output at 600V, about 1.2x the intended voltage. The appropriate resistance appears to be 29K which will dissipate about 0.6W. I chose to build this as a series combination of 1/2W resistors, values of 11K and 18K, to lower the power on the two resistors since they now split the total voltage drop.

I prepped the unit for the new parts, removing the old series resistor chain and 47K resistor that all had drifted out of spec or failed open. I also reversed the wiring changes introduced by the prior owner for the substitute magic eye tube, since I will have the proper one to insert.

The two filter capacitors I have are 500V rated units, adequate to the voltages I will see now that I am dropping some of the voltage from the transformer across the new 29K of resistance. These are compact modern electrolytics, which I relocated so that I made use of the ground tabs around the rectifier tube socket. The capacitors sit below the tube socket, as does the 11K + 18K pair of resistors.

I made use of a spare pin on the tube socket that is not connected inside the tube, spanning the 29K of resistance from the cathode pin 1 to the spare pin. The filter capacitor and wiring for the B+ line to the magic eye were moved to the spare pin to effect the voltage drop.

When I get my replacement components, I have to install the series resistance chain of 22K and 11K resistors onto the selector switch. I also need to put on the new magic eye tube grid and bias resistors and wire up that tube socket properly for a 1629. The last step will be to put the 47K resistor that completes the voltage divider, completing the divider that apportions my 650VDC as 210V for B+ and -440V for driving the capacitor under test. When I pop in the new tubes it will be ready to go.

Fixing up an old Heath C-3 Condenser Tester

HEATHKIT C-3 CONDENSER CHECKER RESTORATION

I brought my C-3 over to test the capacitors we removed from the old Alto power supplies, but the unit failed in transit as the magic eye tube wouldn't glow. As the only output to indicate the results of a test, that meant the unit was unusable. 

I discovered several flaky things when I looked into the unit at home. It uses a transformer generating nominally 500VAC through a 1626 tube rectifier to produce both a high negative voltage for testing capacitor leakage current and the B+ voltage of 130 to 200V for the 1629 magic eye tube. It has a resistor that establishes the ground point and therefore divides the DC into the + and - amounts. The two sides have electrolytic capacitors rated at 8uf 475V.

Unfortunately, the transformer is producing more like 600VAC. Instead of dividing 660VDC into -450 and +200, already close to the capacitor rating, it is producing over 600V negative and my B+ was under 100V. I have one problem that is causing the current through the dividing resistor to be too low, which causes B+ to be too low, but that also exposes the other cap to way way more than its rating.

It is very hard to find capacitors at 8uf or higher with ratings of 675V or higher, thus I decided to put two 16uf 450V caps in series and include some high resistance bleed resistors to ensure voltage balance. The target current will be around 0.5 ma in the higher side, so that I will burn only 180mw with that resistor but ensure that two caps divide the voltage evenly. That means I need two 680K 1/4w resistors. The low side will burn about 60mw.

On recommendations from others who have refurbished these, I am replacing the main voltage divider string of resistors on the 'Scale' switch with 2W units as the existing 1/2W parts are close to rating at the high setting and can be damaged when testing caps with high leakage current.

To wrap things up, I ordered new 1626 and 1629 tubes. I will do some cleanup of the unit while waiting for my parts to arrive.