calling CDI gurus/electronic engineers

[ajwans]

I bought this "rebuilt" CDI unit from a guy who said it had been
stashed in his garage for 20 odd years. I took a peek inside and
there were some notes written on tape under the cover which
indicate it has been apart at least twice for repairs.


2011-06-18 11.13.50, on Flickr

Needless to say the additional components you see were insulated, albeit
with masking tape.

I took a couple of elec eng classes at university but let's just say I wasn't
as focussed as some of the other students ;-)

It seems to be the the Zener diode ZD1 is being bypassed in favour of the
'wire mount' components. I'd like to know what the other components are
being used for and whether I should unsolder the whole thing and replace
ZD1 with the correct component.

The wire mount Zener is 1N1373A.

andy

[Wilhelm]

Is that a blue Permatune box ? Have you tried it on your car? Is it functioning as is? I wouldnt trust it very long of course with that wiring job, but if it is functional you may be able to sort it out if you are handy with soldering, etc. I think somewhere on Pelican someone has posted a schematic and parts list for the Blue Permatune box. Do a search. You may be able to tidy up the kludged up wiring. Oh, what did the notes say regarding the "rebuild"? Can you read the notes?

[ajwans]

It's a OEM Bosch 001 unit. I haven't installed it, car is in rebuild and
I'm still to locate a connector to modify the engine wiring to incorporate
this.

I'm curious whether this is some type of upgrade to the circuit or just
a kludge to avoid desoldering the board to gain access to the ZD/thyristor
package underneath.

andy

[Joe Bob]

Talk to Ingo, 3.6@cox.net resident Bosch CDI guru.

[Wilhelm]

Originally Posted by Joe Bob

Talk to Ingo, 3.6@cox.net resident Bosch CDI guru.

Right. Or if you have about 6 hours or so,...you can check this old thread from the late "EarlyS Man " (R.I.P.)
History of Bosch CDI ... toubleshooting info, parts list changes, and schematics ...

Thanks EarlyS Man !

[Lorenfb]

"I'm curious whether this is some type of upgrade to the circuit or just
a kludge to avoid desoldering the board to gain access to the ZD/thyristor
package underneath."

Hardly an upgrade, total Mickey Mouse! Avoid leaving your driveway with
that unit installed. Call Pelican Parts for that unit, i.e. avoid DIY fixes
like the previous DIYer, as they have multiple suppliers for Porsche parts.

[HawgRyder]

This unit reminds me of the old "don't take the old stuff out...just wire a replacement around it" kludge.
What a mess!!
By all means...check with someone who has posted a schematic online.
Remove any components that are bad.
Definitely remove "added" parts.
Clean everything very well...check for broken traces on the bottom of the circuit board.
Run the traces if in doubt (use a hot soldering iron on the traces from one end to the other)...this will ensure that the trace is continuous.
Bob

[Wilhelm]

Originally Posted by HawgRyder

This unit reminds me of the old "don't take the old stuff out...just wire a replacement around it" kludge.
What a mess!!
By all means...check with someone who has posted a schematic online.
Remove any components that are bad.
Definitely remove "added" parts.
Clean everything very well...check for broken traces on the bottom of the circuit board.
Run the traces if in doubt (use a hot soldering iron on the traces from one end to the other)...this will ensure that the trace is continuous.
Bob

Yes. Good advice for sure HawgRyder !
And as it is an OEM Bosch unit I would say it would be worth repairing and cleaning up. The electronic components aren't expensive.

[ischmitz]

As others said I would call this a poor attempt at repairing a Bosch CDI box. Not sure what these diodes are supposed to accomplish.

One of the challenges when repairing these boxes is that you have to take the PCB out and desolder some 10 odd wires to do so. Then you make the repair and put it all back together only to find that something else is still not working. So you can temporarely solder some flying components to the board (after removing the suspect component from the circuit) to test out a repair strategy. However, leaving stuff in there like that is not something you want to do. Imagine when things get up to temperature (120F - 140F) and the electrical tape glue gets all soft and components start to get exposed. Not a recipe for reliability.

I usually try to stay away from boxes that have been worked on. You really don't know what the previous attempts did to the box and to the components.

Ingo

[wwest]

As everyone already knows the CDI is highly subject to alternator voltage spikes, anything on the "12 volt" supply above ~15 volts and the SCR begins to "self-fire" (tach jumping, etc.). This looks like it might be an input voltage "snubber" to "sink" any incoming voltages above...12 volt zener plus 2 diode drops....~13.2 volts. Might have been used with a ~0.5 ohm 10 watt external resistor.

[Lorenfb]

"As everyone already knows the CDI is highly subject to alternator voltage spikes, anything on the "12 volt" supply above ~15 volts and the SCR begins to "self-fire" (tach jumping, etc.). This looks like it might be an input voltage "snubber" to "sink" any incoming voltages above...12 volt zener plus 2 diode drops....~13.2 volts"

Actually, both statements are incorrect.

1. Neither the early (3 pin) nor the late (6 pin) are "highly subject" the alternator
spikes. The early unit can operate with spikes less that about 22 volts. The later
unit has a regulator circuit which regulates from about 7-8 volts to over 20 volts.
As a result, neither the early nor the late CDI will "self-fire" based on the triggering
circuitry used on both. CDI failure usually occurs when an alt continuously over-charges.
The tach jumping as occurs on the 911SC results from the tach itself being sensitive
to over-voltage, as the CDI does not cause mis-fires nor poor running when over-voltage
occurs.

2. The Mickey Mouse components attached were most likely used to replace the
failed zener diode which protects the power transistor, i.e. As previously posted
one is a 1N1373A (82 volts). That was NOT used to snub the possible input spikes.
Just review the early CDI component layout and this will be apparent.

[wwest]

For #1 my statement was "looks like it might be", so I was wrong.

But my #1 statement was DEAD ON...!!

Show me the regulator circuit/component that prevents "the" capacitor from being overcharged as the battery voltage rises if you would, please. Insofar as I can see the DC to DC upconverter runs open loop.

Tach signal is provided directly from the CDI and when it begins to self-fire due to the SCR voltage rising too high (or to fast) the tach "jumps". If it happens to "jump" too high in frequency the rev-limiter locks up, again a design fault due to excess battery voltage, and the engine now cannot be restarted absent a few seconds with key fully off.

[wwest]

Originally Posted by Lorenfb

"As everyone already knows the CDI is highly subject to alternator voltage spikes, anything on the "12 volt" supply above ~15 volts and the SCR begins to "self-fire" (tach jumping, etc.). This looks like it might be an input voltage "snubber" to "sink" any incoming voltages above...12 volt zener plus 2 diode drops....~13.2 volts"

Actually, both statements are incorrect.

1. Neither the early (3 pin) nor the late (6 pin) are "highly subject" the alternator
spikes. The early unit can operate with spikes less that about 22 volts. The later
unit has a regulator circuit which regulates from about 7-8 volts to over 20 volts.
As a result, neither the early nor the late CDI will "self-fire" based on the triggering
circuitry used on both. CDI failure usually occurs when an alt continuously over-charges.
The tach jumping as occurs on the 911SC results from the tach itself being sensitive
to over-voltage, as the CDI does not cause mis-fires nor poor running when over-voltage
occurs.

2. The Mickey Mouse components attached were most likely used to replace the
failed zener diode which protects the power transistor, i.e. As previously posted
one is a 1N1373A (82 volts). That was NOT used to snub the possible input spikes.
Just review the early CDI component layout and this will be apparent.

Wow...Loren...??!!

You've been working, repairing Porsche CDI boxes all these years(***) and still have little idea as to their common failure mode/cause...?

*** My 78 SC Targa about 2 years ago.

Alternator proved to be faulty, stator windings intermittently shorting, resulting in high voltage spiking upon "un-short" of a stator winding. Installed new alternator but the tach still "jumped" after the engine was run for long enough for things to heat up. Sprayed CDI housing with cooling spray and tach would stop jumping until the CDI (SCR) heated up again.

CDI's SCR failed due to being run too long in over-voltage state.

During the drive from Seattle to ID the tach would start jumping and shortly thereafter the engine would quit. Limped the last 100 miles by alternately running with the voltage regulator connector unplugged, until battery got too low, plug in the regulator until the battery voltage got too high, etc, etc.

[Lorenfb]

"Show me the regulator circuit/component that prevents "the" capacitor from being overcharged as the battery voltage rises if you would, please. Insofar as I can see the DC to DC upconverter runs open loop."

Actually, that's again incorrect. One needs to review closely the Bosch CDI circuit diagram!!!!!!

1. The circuit is NOT the typical DC to DC converter. It is a fly-back circuit where the
output voltage over time is a function of the energy stored in the primary before the 'switch'
turns off, i.e. like the H.V. in the old CRTs. The energy is transferred to secondary which
pump charges the output storage capacitor thru the coil over time and is applied to the SCR.

The output voltage is NOT directly a function of the battery voltage which is the reason
Bosch used a fly-back circuit, i.e. It's better for cold weather low battery voltage starting
not like the typical D.C. to D.C. converter of the MSD CDI unit where the output is DIRECTLY
proportional to the input battery voltage. With the Bosch CDI, the SCR voltage over time is directly
a function of the transformer core saturation current, where I * I * L = V * V * C and V is the
pump charge capacitor voltage, I is core saturation current, L is the inductance of the primary,
and C is the output capacitor. The final pump voltage of the capacitor & SCR is limited by zener
or the RPM, i.e. as the RPM increases the maximum pump voltage is reduced.

The input zener (82V) (check the diagram) limits the peak primary & secondary fly-back
voltage with the primary collector capacitor. The voltage rating of the SCR is >600 volts
and never goes higher than about 475 no matter what the battery voltage is because of
this limit. Early CDIs had zener failures resulting in higher that normal output voltages
at low RPMs, but not the later 3 pin nor the 6 pin.

"CDI's SCR failed due to being run too long in over-voltage state."

2. Never really occurs!!! The SCR rarely fails, and if so, usually the result of shorting
the CDI output. Other CDI components cause the CDI to 'lock-up' when the
temp is high but again it functions again when the CDI cools.

"supply above ~15 volts and the SCR begins to "self-fire" (tach jumping, etc.)"

Totally erroneous! Do a bench test! If it does, there's a circuitry problem other that the SCR.

3. Some 3 pin CDIs trigger the tach from the points, so the points have to bounce
for the tach to have problems. The 6 pin CDI (911SC) triggers the tach from a Schmitt
Trigger (low voltage). Only some 3 pin CDIs trigger the tach from the coil and internal
CDI component failures, other than the SCR being over-voltaged, can cause tach problems.

A. "CDI's SCR failed due to being run too long in over-voltage state."

B. "During the drive from Seattle to ID the tach would start jumping and shortly thereafter the engine would quit. Limped the last 100 miles by alternately running with the voltage regulator connector unplugged, until battery got too low, plug in the regulator until the battery voltage got too high, etc, etc."

A & B contradict one another, i.e. So the SCR didn't fail, but only locked-up and then recovered.
That's because another CDI circuit component/components were not functioning.

Bottom line: Time to do a Pelican search to analyze (if you can) the Bosch CDI
circuit design posted many times on Pelican before making erroneous statements
and mis-leading others, please! Remember, many of the original Bosch CDIs
are still operating after 40 years which speaks to the comprehensive and robust
Bosch design given a wide range of operating conditions, e.g. voltages, & temps.

[HFR_Racer]

Loren and Ingo...

Isn't that second SCR supposed to be tied to a heat sink? Isn't that why it has a threaded base?

[wwest]

Originally Posted by Lorenfb

"Show me the regulator circuit/component that prevents "the" capacitor from being overcharged as the battery voltage rises if you would, please. Insofar as I can see the DC to DC upconverter runs open loop."

Actually, that's again incorrect. One needs to review closely the Bosch CDI circuit diagram!!!!!!

1. The circuit is NOT the typical DC to DC converter. It is a fly-back circuit where the
output voltage over time is a function of the energy stored in the primary before the 'switch'
turns off, i.e. like the H.V. in the old CRTs. The energy is transferred to secondary which
pump charges the output storage capacitor thru the coil over time and is applied to the SCR.

The output voltage is NOT directly a function of the battery voltage which is the reason
Bosch used a fly-back circuit, i.e. It's better for cold weather low battery voltage starting
not like the typical D.C. to D.C. converter of the MSD CDI unit where the output is DIRECTLY
proportional to the input battery voltage. With the Bosch CDI, the SCR voltage over time is directly
a function of the transformer core saturation current, where I * I * L = V * V * C and V is the
pump charge capacitor voltage, I is core saturation current, L is the inductance of the primary,
and C is the output capacitor. The final pump voltage of the capacitor & SCR is limited by zener
or the RPM, i.e. as the RPM increases the maximum pump voltage is reduced.

The input zener (82V) (check the diagram) limits the peak primary & secondary fly-back
voltage with the primary collector capacitor. The voltage rating of the SCR is >600 volts
and never goes higher than about 475 no matter what the battery voltage is because of
this limit. Early CDIs had zener failures resulting in higher that normal output voltages
at low RPMs, but not the later 3 pin nor the 6 pin.

"CDI's SCR failed due to being run too long in over-voltage state."

2. Never really occurs!!! The SCR rarely fails, and if so, usually the result of shorting
the CDI output. Other CDI components cause the CDI to 'lock-up' when the
temp is high but again it functions again when the CDI cools.

"supply above ~15 volts and the SCR begins to "self-fire" (tach jumping, etc.)"

Totally erroneous! Do a bench test! If it does, there's a circuitry problem other that the SCR.

3. Some 3 pin CDIs trigger the tach from the points, so the points have to bounce
for the tach to have problems. The 6 pin CDI (911SC) triggers the tach from a Schmitt
Trigger (low voltage). Only some 3 pin CDIs trigger the tach from the coil and internal
CDI component failures, other than the SCR being over-voltaged, can cause tach problems.

A. "CDI's SCR failed due to being run too long in over-voltage state."

B. "During the drive from Seattle to ID the tach would start jumping and shortly thereafter the engine would quit. Limped the last 100 miles by alternately running with the voltage regulator connector unplugged, until battery got too low, plug in the regulator until the battery voltage got too high, etc, etc."

A & B contradict one another, i.e. So the SCR didn't fail, but only locked-up and then recovered.
That's because another CDI circuit component/components were not functioning.

Bottom line: Time to do a Pelican search to analyze (if you can) the Bosch CDI
circuit design posted many times on Pelican before making erroneous statements
and mis-leading others, please! Remember, many of the original Bosch CDIs
are still operating after 40 years which speaks to the comprehensive and robust
Bosch design given a wide range of operating conditions, e.g. voltages, & temps.

You missed something, entirely so.

The old original point/coil ignition system was also a "flyback" system, switch off the current flow in the primary, the core magnetic field begins collapsing, the secondary voltage (and the primary) begins to rise, rises (to infinity...??) until current flows, continuously rising voltage results in current "jump" across the shortest distance, easiest path, plug, wiring, or even points themselves.

[wwest]

I was probably, more likely than otherwise, WRONG.

Loren, how many CDI's have you repaired wherein you replaced that 10W 82 voltage zener...?

I hereby revise my analysis as follows.

1. The alternator, engine area/local, voltage begins spiking, or regulating to >15 volts.

2. The 82 volt 10 WATT zener begins "sinking" the transformer primary "flyback" excessive voltage, then begins to overheat and eventually fails "open".

3. Now the "flyback" voltage is free to rise above the SCR "self-firing" voltage level (that level declines as the SCR temperature rises) but does not do so except at times, or instances, with battery supply voltage >15 volts.

4. When the SCR "self-fires" and the distributor HV points are too far apart the resulting magnetic field collapse reflects to the transformer primary and also quickly recharges "the" capacitor, putting even more stress/HEAT on the SCR.

5. The more the SCR "self-fires" in the above state the higher it's temperature will rise.

6. Rapid series of self-firing causes the tach to "bounce".

7. Enough tach bounce and the engine over-rev circuit is triggered.

8. Engine over-rev circuit design is also flawed and locks up with over-rev detection and the only way to "unlatch it is to remove power completely via switching the ignition off entirely for a few seconds.

[wwest]

Originally Posted by Lorenfb

"As everyone already knows the CDI is highly subject to alternator voltage spikes, anything on the "12 volt" supply above ~15 volts and the SCR begins to "self-fire" (tach jumping, etc.). This looks like it might be an input voltage "snubber" to "sink" any incoming voltages above...12 volt zener plus 2 diode drops....~13.2 volts"

Actually, both statements are incorrect.

1. Neither the early (3 pin) nor the late (6 pin) are "highly subject" the alternator
spikes. The early unit can operate with spikes less that about 22 volts. The later
unit has a regulator circuit which regulates from about 7-8 volts to over 20 volts.
As a result, neither the early nor the late CDI will "self-fire" based on the triggering
circuitry used on both. CDI failure usually occurs when an alt continuously over-charges.
The tach jumping as occurs on the 911SC results from the tach itself being sensitive
to over-voltage, as the CDI does not cause mis-fires nor poor running when over-voltage
occurs.

2. The Mickey Mouse components attached were most likely used to replace the
failed zener diode which protects the power transistor, i.e. As previously posted
one is a 1N1373A (82 volts). That was NOT used to snub the possible input spikes.
Just review the early CDI component layout and this will be apparent.

Wow...

"..2...components..were most likely used to replace the failed zener.."

Absolutely correct IMMHO...

"..That was NOT used to snub the possible input spikes..."

Wrong, ABSOLUTELY WRONG.

The EXACT use of the 10 watt 82 volt zener is to "snub" the possible input voltage spikes. Snub indirectly, maybe, but the exacting purpose all the same.

[Lorenfb]

"The EXACT use of the 10 watt 82 volt zener is to "snub" the possible input voltage spikes."

Guess again! You might want to look closely at the Mickey Mouse connections.
The unit pictured does have input spike protection (as designed by Bosch) but whoever
started replacing parts has used an inadequate device.

"The 82 volt 10 WATT zener begins "sinking" the transformer primary "flyback" excessive voltage, then begins to overheat and eventually fails "open"."

Not really, as mentioned before, this rarely if ever fails on either the later 3 pin
nor the 6 pin, i.e. both use better devices and better heat sinking.

The Bosch CDI has basically a peak fly-back current based on the
core saturation characteristics of the fly-back transformer which stores
about 3 - 4 m-joules of energy per cycle under normal operating conditions.
This energy is transferred to the collector capacitor (across the zener),
i.e. once the output capacitor is fully charged, and the zener which
dissipates a small amount of power per cycle.

Again, review the diagrams posted on Pelican and for more info read
'Ignition Systems' here: TECHNICAL

Bottom line: Guessing is NOT allowed here!

[wwest]

So, question, if the 82 volt zener were open, not limiting the voltage across the "flyback" transformer primary to ~70 volts, what would be the final charge voltage for "the" capacitor..??

What might be the "target" design parameters, given a good, even excellent, design engineer, for the power handling parameters of that 10 watt 82 volt zener..?

Just what design engineer would have the expectation of the design having to operate and survive continuous operation with the battery supply voltage well above design expectations..22 volts such as you state ?

So, isn't it likely that the zener diode is operating very near it's upper power disapation limit with the battery supply voltage on the "normal" high side, <15 volts..??