Heathkit EC-1 Analog Computer repair - Page 1
Forum section: electronics / repair

author: vintcomp2
author_rank: Newbie
timestamp: April 15, 2024, 07:30:21 pm
content: I am going to try to get an old Heathkit Analog Computer working again. The schematic is attached. The unit seems complete, but has not been powered up in a very, very long time. This is my first post about the restoration. I have a background in computers, not electronics (outside of hobbyist arduino/microcontroller level projects). My plan is to restore it primarily to a safe working condition - not necessarily to original condition where/if those contradict.I guess I have one newbie question about the 'chassis' ground. The unit only has a two prong plug. I thought it might be good to replace that with a grounded three prong plug and ground the chassis. However, the schematic shows various connections to the chassis from some of transformer's secondaries. What is your recommendation for this situation?A secondary question is that the unit contains selenium rectifiers, that I have read I should replace. The schematic also shows some of the diodes doubled up, though I'm not sure why this was done. Do you have any recommendations about whether to keep them, or what type of diode to use to replace them.Any additional tips would also be great. I plan on replacing the capacitors, and I have a variac/dim bulb tester to bring everything up slowly. Thanks.

author: Kim Christensen
author_rank: Super Contributor
timestamp: April 15, 2024, 08:09:13 pm
content: Quote from: vintcomp2 on April 15, 2024, 07:30:21 pmThat should be fine, (grounding the chassis) since the secondary side is isolated via the 2 transformers.I would move the fuse so it's first in series with the "heater switch" and protects the hot side.ie: Both the switches and fuse should be in the hot side.QuoteThey are doubled up for increased reverse voltage capability. You can replace a selenium rectifier with a silicon diode and a resistor in series.Some more info:https://w5rkl.com/wp-content/uploads/2022/07/RBSelenium2.pdf

author: MathWizard
author_rank: Super Contributor
timestamp: April 17, 2024, 12:01:01 pm
content: Ok so it's an OP-Amp type calculator, doing some math. I'll have to look harder to see what the oscillator does. But that's pretty cool, I might try and make a BJT or JFET version in LTSpice.Another thing to check would be old resistors, especially the brown style Carbon resistors, their values can drift, IDK how badly, but easily outside of their 5 or 10% tolerance. If any need replacing, just make sure to use ones rated to equal or greater power dissipation.Tubes can wear out over time too, like as the cathode materials get worn away, or cracked or whatever they do. And gasses can slowly come out of the solid parts, and build up inside the tube, and make it harder for the electrons to move. The tubes have some chemical in there, on a part called the Getter, that tries to absorb back these gases. And from the way they look, people that know tubes, could see that type of problem, when the Getter wears out. I think they go from shiny silver, to dark grey or black even.Check out Mr.Carlson's video's, he repairs/restores a lot of old tube equipment.https://www.youtube.com/@MrCarlsonsLab/videos

author: mag_therm
author_rank: Frequent Contributor
timestamp: April 17, 2024, 02:00:06 pm
content: Here are three museums (that I know of,) that have collections of analog computers1) analogmuseum.org. Germany2)thecomputerchurch.org/index.html Computer History Learning Center Parkesburg, PA - with 3 of Heathkit EC1 are in that collection3)computerhistory.org Computer History Museum Mountain View, CAA suggestion would be to contact one of these to check rarity of the EC1, especially if it is factory built, before deciding on modification.Otherwise, definitely change out all those selenium rectifiers before trying to power it up.(They can absorb moisture over time and can release toxic gas upon failure)The whole circuit of the EC1 could easily be replicated on a small pcb. As I see from cct diag posted,, it has only two operational amplifiers.So, it will be capable of solving some 1st order ODE, and a basic 2nd order ODE.To demonstrate the EC1 working, an output device will be needed. in its day, would have been a paper plotter with a time base,or an oscilloscope with long persistence. So any 'scope with storage can be used today.I see that the EC1 instruction book has good description on how to use it. You might be able to find later information, on 1) abovewhen a standardized method converting ODE into block diagram form was used.

author: vintcomp2
author_rank: Newbie
timestamp: April 18, 2024, 12:58:33 pm
content: Thank you for the useful information. I was trying to get an idea for how dangerous operation of the front panel would be.I was a little concerned about the high voltages on the secondary sides of the various transformers being connected to the chassis. With the voltages involved, it seems you could receive a shock from the chassis under certain conditions. I wasn't sure if this is just normal for the time period, and OK to keep as is, or whether it would be safer to try to lift those connections from the chassis if possible and use a different shielded path back, if that makes sense.Here is a (stock) picture of the front panel:

author: wasedadoc
author_rank: Super Contributor
timestamp: April 18, 2024, 01:33:02 pm
content: Quote from: mag_therm on April 17, 2024, 02:00:06 pmNo. Only two opamps (1 and 9) are drawn in full. The others are repeats. See the text "AMPLIFIERS 2ND THROUGH 8TH"

author: mag_therm
author_rank: Frequent Contributor
timestamp: April 18, 2024, 03:42:55 pm
content: ( Thanks Wasedadoc, It is a real analogUE computer)Hi Vintcomp2About the earth grounding. I suggest:to first check with ohm meter that the earth ground symbol on the (-) of the main anode supply ( top left) is actually connected to the metal housing and front panel (The alternative although not likely, is use of a separate isolated chassis or ground bar for the main circuit)The circuit shows the Op-Amp Outputs are taken from the cathode of "B" side (+) 240V via a level shifter consisting of two NE2H (90 V each) and a 100k resistor to a (-) 150 V bias supply This level shifter reduces the voltage on the banana jacks to +60V, provided the NE2's strike.So the student (if at earth ground) could have been connecting the patch leads (exposed banana plugs by the photo) at dc voltage exceeding 60V, if the "High Voltage" switch was accidentally left ON. Maybe that voltage along with the impedance of the NE2's was considered safe enough.In an NE2 fails, there will be (-) 150V on the Op-Amp Output via 100k.Maybe the manual has some safety details?In those days ( I started in 1968), it was normal for the electrical and physics lab student experiments to require "connecting up" or making changes , just relying on an on/off switch. Some experiments had tube and klystron voltages exposed; Some power experiments were at 400V 3phaseAll that has passed.Another suggestion. In 120V countries, ungrounded appliances (2 pin) were normal, however in 230V countries, 3 pin grounded leads were mandated from earlier.So if you can find a UK or EU circuit diagram (showing 230V supply on left side) check if a earth/ground connection is shown to the 3rd pin.

author: mag_therm
author_rank: Frequent Contributor
timestamp: April 19, 2024, 02:31:56 pm
content: Hi Vintcomp2I spent a little time this morning on possible EC-1 failures which would allow higher DC voltages on the banana jacks for the Op-Amp outputs.Failure (to high resistance) of components R10, R14 and the pentode of 6U8 could cause the triode cathode follower of 6U8 to go high.By 6U8 data, positive grid could allow 40 mA to flow with the plate at 250V as shown on circuit.The design has the two series NE2H Neons partly to provide some protection. However they have no current limitation on the high side.With uncontrolled current these can go into an arcing mode ( bright purple) before failure by overheating.The actual overvoltage on the jacks would have to be checked by simulating the above failure modes.Changing the Output jacks to the present shrouded type (as in multimeters) may be possible, but that would still leave the other end of the patch leads with exposed bananas, and by the photos, changing ALL jacks may be impractical.The EC1 has the high voltage switch and green lamp maybe as the only means of protecting the programmer.If attempting to power this up, repair and when using it, I would use a 120:120V isolation transformer with a grounded interwinding screen, and leave the EC-1 chassis floating. That would reduce the possibility of a grounded user receiving shock, except if he was touching the chassis at same time.I have such a transformer here, and used it when restoring valve/tube units. I have a 1959 60W tube guitar amplifier , a FM radio from about 1958, and a shortwave receiver Lafayette HE30 from 1963. I have bolted the guitar amp into a grounded steel rack as a practise amp and it is not to be taken out for use in gigs.The HE30 still has a 2 pin 120V lead. I did not replace the lead with a 3 wire, as the antenna input is grounded to the ham radio ground bar on my workbenchFor the FM receiver I added a new internal isolation transformer and left the 2 pin supply because the chassis is completely contained in a wooden case.And the analog computer that I learned on, was a EAI, fairly new in 1969. It had transistorized op-amps as plug in modules.Hope this info helps

author: vintcomp2
author_rank: Newbie
timestamp: April 20, 2024, 12:47:36 am
content: Quote from: mag_therm on April 19, 2024, 02:31:56 pmThank you for the detailed feedback on some failure modes.Quote from: mag_therm on April 19, 2024, 02:31:56 pmI was able to find a really good articlehttps://www.nutsvolts.com/magazine/article/May2016_Heathkit_Restorationon Nuts&Volts, where an absolute pro rebuilt one. Although he made other parts upgrades to the unit (he didn't worry about keeping it completely stock) - he also left the unit with the two wire plug.Quote from: mag_therm on April 19, 2024, 02:31:56 pmThe information definitely helps. The National Cryptography museum in DC has an EAI Pace on display for anyone out there who wants to see one up close. That machine is a real beauty.