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.

Disk tool working, used to build images for demo at VCF West

ALTO DISK TOOL

I brought the updated logic to Marc's lab, set up the disk tool and successfully wrote cartridges and read cartridges from the Diablo disk drive. We are using the tool to build images for our upcoming demos at the Vintage Computer Festival West on August 5 and 6.

The rest of the day was spent working out the exact software images we needed after developing our high level script for the demonstration. We will show:

• Bravo, the word processor that was written by Charles Simonyi before he left PARC to go write its descendant, Word, at Microsoft. 
• Draw is what it sounds like, a generalized drawing program. 
• SIL is a tool to draw schematics
• Neptune is the ancestor of File Explorer or similar file management software
• Ethernet which was created for the Alto and its use for FTP, network booting, file serving, etc
• Smalltalk, the language and environment developed by Alan Kay to explore object oriented sw
• Games such as Astroroids, Pinball, and others developed by students doing grad work on Altos

Testing complete for relay tester, ready to ship

I have been on vacation for a week, no posts or activity but returned to the workshop today. 

ALTO RESTORATION EFFORTS

We are going to exhibit the Alto at the upcoming Vintage Computer Festival West event, as well as hosting a few panels. I will be on two of them, one concerning the restoration of the Alto and another concerning restoration of the 1401 systems.

This will require development and scripting of demonstrations as well as our material for the panels, but at the same time I want to begin using the disk tool I built to archive the stash of cartridges from Xerox PARC. The tool will also allow us to create disk cartridges to support our demo activity.

PERMISSIVE MAKE RELAY TESTER FINAL TESTING

Having located the proper code and identified a few tweaks to make to the wiring in the box, I began the adjustments today. As I had earlier written, the four or six sets of contacts in a relay could be identified with ordinal numbers starting from either end of the relay, but I chose the incorrect end.

Rewiring those connections will give me a box that works properly with the unmodified code developed by Stan, thus keeping the two relay tester boxes in sync as future changes are made.

I ran the tester through its various functions, using a relay from our spares stock and found that I still have one flaw. I can individually drive the pick or the hold coil in the relay under test. The Arduino pulls down a wire for each that is connected to the dual relay module inside the tester. However, the relay modules only click on when both pick and hold wires are activated, not individually.

We reasoned that this is due to a short between the two wires driving the dual relays. If they are shorted, then one is outputting high and the other low if I try to drive only one of the two coils. This produces an intermediate voltage, not 0 and not 5V, which is too high for the dual relay module to fire either relay. When both are active (low), the wires are down at 0 and the relay module will click on.

I opened the box to locate and correct the short. With that done, everything is working correctly and this is ready to be shipped to TechWorks! for use with their 1402 reader/punch. I wrote up a user manual and will ship that along with the box. Stan is going to set up a laptop to ship with the box, allowing this to be a turnkey test system.

Finished construction of the permissive make relay tester

1402 PERMISSIVE MAKE RELAY TESTER FINAL CONSTRUCTION

My grommet supply arrived today and I finalized the holes after selecting the ideal size grommet for the LEDs. I seem to always underestimate the hole size required to fit in the grommet, but eventually got it right. After installing the LEDs into the cover, I was ready for applying the insulation - a rubber insulator that is brushed on over exposed wire junctions to form a protective coating.

Unfortunately, two things went wrong today. First, my existing bottle of connector coating is partially dried out and almost empty. Second, when I got over to CHM to check for the updated Arduino code that runs this version of the circuit, I couldn't locate it. Stan will need to hunt it down tomorrow.

Frys did have the connector coating in stock, which I ordered for pickup in the afternoon.  After lunch with my wife and a friend, I dropped them off and rode over to grab the purchase at the store. After it was applied, I loaded the out of date sketch.

I can accomplish some testing of functionality using the old versions of the sketches provided by Stan, which proves out ability to pick and hold, rattle relays and test contact resistance. First I did a test with only the USB link powering the Arduino and its relay board. All the LEDs and the relay module fired properly and the sketch communicated with the PC.

Next up was a test with the 20V supply activated in addition to the USB link. That way I can see if the relays pick and hold using a red relay plugged into the socket. The relay magnets did seem to work fine although the first flaw was uncovered. When the tester believed it was using the green relay socket, my LED lit under the red socket.

The next flaw was uncovered when I noticed that my relay module was active initially, although I believe the code thought that both Pick and Hold coils were off. My LEDs light when the drive line is high, but the relay module is off for a high voltage.

I can fix both of these flaws easily. The relay module can be changed to use the NC contact to drive the LEDs and pick/hold coils - when the relay is off, the lamp and coil will be on and vice versa. I could move the two LEDs under the relay sockets allowing the proper one to light. Dealing with the pick and hold LEDs is more complex but I can just swing the pick and hold LEDs onto the actual relay socket pick and hold wires, adding a suitable additional resistor to limit current.

Next up, I added the 5V supply and tried measuring contact resistance on the red relay. I can't check circuits 5 and 6 as these are only on green relays, but I had some interesting readings for the others. I will have to check this with Stan tomorrow when I am retrieving the correct code.

The quality of the contact test readings is mediocre as I don't have the code that makes use of the voltage reference from pin A15, but I could check out operation well enough to prove that my wiring seems sound.

I will haul this into CHM tomorrow to check it out with Stan - who will be hunting for the updated code - and I can work out any wiring changes needed before closing everything up. 

Built RF amplifier to interface digital VFO to Heathkit HW-100 transceiver

ALTO DISK TOOL

I tweaked what signals are displayed on the LEDs and that show on the seven segment display, otherwise no change to the disk tool as it is simply waiting for the chance to test with real drives, disk cartridges and Altos.

RF AMPLIFIER TO MATE DIGITAL VFO TO HEATHKIT HW-100 TRANSCEIVER

The digital variable frequency oscillator I built to improve the HW-100 tube transceiver produces a low voltage output with 50 ohm impedance. I had tried to leverage the existing tube amplifier in the analog VFO to try to boost the signal before it is mixed with the local oscillator in another tube.

The impedance match is wrong and the tube really needs a larger voltage swing, leading to poor results as it sat. I found a kit by W1AFFL that will boost and match impedance to the tube in the HW-100 from the 50 ohm low voltage swing produced by my digital VFO.

I completed the kit in my spare time today and will put it with the rest of the HW-100 project to resume work in the near future. I have to concentrate on the relay tester, 1130 light panel and Alto efforts first.

1402 Permissive Relay Tester construction nearly completed

RELAY TEST TOOL CONSTRUCTION

I bored out the hole for the power cords, inserted the grommets and then tied the cord strain relief inside. Began soldering grounds together, the +20V to the two relay module contacts and the +5V to the 0.1 ohm resistor leading to the bank of resistors hooked to the individual contacts.

Beginning to wire up the insides of the tester

I had some wiring to accomplish between the relay module and Arduino, set up all the grounds from power supplies to Arduino and the two relay test sockets, solder up headers for all the contact wires from the relay test sockets, and finally hook up my four LED units.

The pick and hold coils circuits were wired to the relays, including the snubber diodes that will short the back EMF as the coils are released. I partially wired the two LEDs that illuminate that will show when the pick and hold circuits are energized. The ground connection to the Arduino was installed, but there was still quite a bit more to do by midafternoon, mostly involving wiring stiff wires to small header pin ends.

When dinnertime arrived, I had all four LEDs wired in, the relay module connected to the Arduino, and the only wiring left to do was to hook the twelve contact leads (6 each for normally open and normally closed sides) to the header that connects to the analog input pins on the Arduino.

Early evening brought completion of all the wiring. Two of the header strips were left detached, giving me more working room to install the LEDs, but they only need to be plugged into the Arduino to complete all circuits. I carefully tested everything for shorts while I was wrapping up, so that as it is ready to have power applied.

Final tasks that will wait until Sunday include

• insulating all the bare metal soldered junctions, 
• inserting the proper grommets into the cover, 
• mounting the LEDs into the grommets,
• install the headers onto the Arduino and
• close up everything after testing

Too, I have to visit CHM over the weekend to pick up the current code (Arduino sketch) that matches the circuit. The major change is the addition of a 0.1 ohm resistor between +5V and the individual 33 ohm resistors feeding each contact.

The code adjusts the voltage measured on each contact (powered through its 33 ohm resistor) against the voltage at the junction with the 0.1 resistor. The contacts are connected to ground by the relay and any voltage above 0 at the contact is due to resistance in the contacts.