K66 Transistor Ignition - Delco Circuit Module Failure, Diagnosis and Repair

[Dan D.]

Re: K66 Transistor Ignition - Delco Circuit Module Failure, Diagnosis and Repair

Rich, I have not seen the schematic of the Delco module before. Thanks for posting.

This is a classic transistor ignition circuit of the early 60s. Very basic and simple.

All three transistors are PNP, therefore most likely are all Germanium. This surprises me somewhat as Silicon was available by that time. But Delco semiconductor never invested in silicon technology, and apparently had a mindset not to buy any transistors from a competitor. They just let Germanium die a natural death and then went out of business.

I suspect that what is happening is at low cranking voltage the output transistor, TR-1, runs out of beta at high current levels. When that happens the driver transistor, TR-2 then has to supply the current. The driver is a small low current device that can't supply much current, and that is what fails. I used to see this quite a lot back in the day - especially with stereo amplifiers that us kids used to play loud music with. One common design back then was to use silicon in the output stage (usually 2N3055 or equivalent), but kept Germanium for the drivers. The Silicon output was a big step in the right direction, but the same failure mechanism existed with the driver.

Another possibility is that during cranking there will be a huge inductive kick from the solenoid when the starter is released. There are no bypass capacitors or reverse diodes in the circuit to absorb this energy, and again, the driver transistor is very vulnerable. The output transistor is diode protected - it has to be because of the kick of the coil every time there is ignition, but the driver is just hanging there - waiting to be hit.

On a separate note, you should probably sell your old Tek scope, in light of the very nice digital one you have now. You will never need to use the old Tek again.

-Dan-

[Richard M.]

Re: K66 Transistor Ignition - Delco Circuit Module Failure, Diagnosis and Repair

Thanks for that info Dan. My analog transistor theory is a bit rusty these days. Most of my work was digital related from 10 years ago and before.

One other thing I conjerred up the other day when trying to decipher what speed the distributor was turning at when taking waveform measurements.

It finally hit me. I took out my Actron dialback timing light with digital tach. I set the inductive pickup on the coil wire and ran the drill. The Makita is spec'd at 1700 RPM max. The tach drive is 1:1, as proven with a simple telltale mark on the drill chuck turning one rotation and observing the distributor rotor plate turning one revolution. I ran the drill and recorded the RPM, divided by 8 and that's the speed of the distributor for the tests. Then double that for crankshaft RPM. What I wanted to see was the effect of simulated "crank" speed on the output of the Pickup Coil. For reference, at 1700 rpm drill/dist speed, I can only simulate a max engine speed of 3400 rpm.

I've never recorded what actual realtime cranking RPM speed actually is on a real engine test. Does anyone know that answer? Of course it will vary based on battery condition and engine and ambient temperature, etc.

When cranking at a slow "drill" rate, about 300 simulated engine RPM, the amplitude of the trigger input(yellow) is quite small, on the order of only about 2.5 volts. This is likely critical to the input of the TI module input trigger circuit. The lower blue waveform is the amplifier output, so it's working by observation as it's pulsing low. The probe was set to x10 so that's why the amplitude appears lower than previous waveforms.


Here are a few waveforms at higher distributor speeds, up to max 1700 distributor rpm. You can see how the trigger input amplitude rises based on distributor speed, along with more frequent pulses due to the higher distributor speeds.


As another experiment, I set the channel 1 probe(yellow trace) laid across the coil high voltage wire. This was just to see the coil output to the spark plug as a scope trace. The scope trigger was set to the coil input(Ch 2 blue). This is basically just to see the coil primary to secondary switching attributes.


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[Dan D.]

Re: K66 Transistor Ignition - Delco Circuit Module Failure, Diagnosis and Repair

Hi Rich,

Interesting work you are doing. To answer your question about cranking speed, I do not know for sure - never tried to measure it, but I would think it would be in the neighborhood of 200 - 300 RPM. That would be 3.3 to 5 revolutions per second. You could measure that in a car by putting your scope on the distributor wire while cranking and measure the time base that the pulses occur at.

For your test, did you have a spark plug attached to the coil, or was it running open circuit? That would limit the coil secondary voltage to the level when the plug fires.

It would be interesting to measure some actual voltages and waveforms that the transistors are seeing during actual operation. In particular the poor old driver transistor that seems to always take it on the chin. Also during cranking and when the solenoid is released. I would expect to see some transients here.

-Dan-

[Richard M.]

Re: K66 Transistor Ignition - Delco Circuit Module Failure, Diagnosis and Repair

Hi Rich,

Interesting work you are doing. To answer your question about cranking speed, I do not know for sure - never tried to measure it, but I would think it would be in the neighborhood of 200 - 300 RPM. That would be 3.3 to 5 revolutions per second. You could measure that in a car by putting your scope on the distributor wire while cranking and measure the time base that the pulses occur at.

For your test, did you have a spark plug attached to the coil, or was it running open circuit? That would limit the coil secondary voltage to the level when the plug fires.

It would be interesting to measure some actual voltages and waveforms that the transistors are seeing during actual operation. In particular the poor old driver transistor that seems to always take it on the chin. Also during cranking and when the solenoid is released. I would expect to see some transients here.

-Dan-

Dan, Yes a single plug was connected. Since I was using a Delco board, if there was no load it would likely have fried the output transistor. Since I don't have a actual TI car here I cannot do actual cranking tests.

When I get some time I'll do some additional tests and probe the entire circuit path and save the associated waveforms. The scope has built in measurement cursors so I'll try get get accurate timing data too. I have to read up a little more on it's features. It's a very nice scope.

I wish I had another K&B board here to play with.(hint-hint KA) I had one here and it tested fine but now it's back with the car, installed and working great. My friend that own's the car notices a major performance improvement over the antique Delco board. Car runs great at any RPM and starts up in a few milliseconds every time.

Rich

[James G.]

Re: K66 Transistor Ignition - Delco Circuit Module Failure, Diagnosis and Repair

Hi Rich,
Have you ever talked to DAVE FIEDLER at TI SPECIALTY? He has a website. T. I. Specialty is owned and operated by DAVE FIEDLER. Dave is a graduate mechanical engineer from Purdue University. He spent most of his career (26 Years) in the automotive industry, working for Perfect Circle - a manufacturer of engine parts. During his career he was involved in the design and development of piston rings, valve stem seals, and electronic speed controls. In 1997 Dave turned a part time hobby (T.I. Repair) into a full time business and expanded to include work on all types of Corvette Tach Drive Distributors.

BACK IN THE OLD DAYS, we all went to him for service and help. In RICHMOND, INDIANA at

• 1-765-962-4265 (10 AM to 6 PM EST)

[Richard M.]

Re: K66 Transistor Ignition - Delco Circuit Module Failure, Diagnosis and Repair

Jim, Yes several times. He and I had a nice conversation on Monday. I told him about my bench test and I asked him some TI questions. He is a great resource and always helpful.

The first time I had a TI experience was on that Norway 435hp back in 2010 and Dave helped me figure out that we had a bad pickup coil in the distributor.

Since then I've learned much about the TI system. Several years ago I helped Chuck with his black '65 fuelie when we had some intermittent TI issues. A new K&B board fixed that one.

Rich