Loren does the original porsche dizzy uses a special hall device which pulls the base into negative? Do you have a scope picture of the signal?

Bob

Loren there a two type of hall sensor.
Low over tooth & high over tooth. What technology is Porsche using.

Bob

"does the original porsche dizzy uses a special hall device which pulls the base into negative?"

No. There's no Hall device in the SC distributor. It's a magnetic pickup which basically produces a distorted sine wave. Just use a sine wave generator for testing and your headache will be solved.

Man so much trouble about nothing.

Thanks Loren,
so the magnetic pic up on the dizzy produces sinus peak to peak of 1 volt which goes to 133Hz which is 8000 RPM.

Or is the value higher than 1v peak to peak.

Bob

This is what the output of the variable reluctance sensor looks like (see Jimmcc). The signal has no DC component so 0 volts goes right through the middle. A sine wave would probably work (the 2 Vp-p in my simulation) but I remember reading from someone that fixes these (in France not the local vociferous TV repair entrepreneur) that the negative slope of the signal wasn't always quick enough to trigger the SCR

DC from the level shift input to the trigger does go through the sensor coil but does not show up as a voltage on the signal since dI/dt = 0

The signal is generated using Faraday's law v = N dflux/dt where the flux changes because the iron star wheel completes the magnetic circuit through the coil in the sensor.

The signal amplitude varies with the RPM, i.e. it increases as the RPM increases.
So use a sine wave with a 1.0 volt peak to peak. Since a six cylinder at 6000
RPMs produces a spark every 3.3ms, the corresponding freq is 300 HZ at 6000
and 400 HZ at 8000.

"the negative slope of the signal wasn't always quick enough to trigger the SCR"

Nope! Remember, the input circuit is a Schmitt Trigger which is INDEPENDENT
of the input slope, i.e. That's the reason for using it.

I get smarter now.

A page out of the "Bosch Bible".

http://forums.pelicanparts.com/uploa...1194902084.jpg

Bob

See I knew it was a problem with your induktionsgeber. :rolleyes:

I am going to Germany next week and don't speak a word of German. This is what I have to look forward to.

Is it eine beer bitta?

Loren, Waaren and all who can help & I changed the transistor T1 2N3055 but it was not the problem. I still have the failure.

See my scope pictures measured at the Zener at 12V without trigger.
What happens is at 12V the DC DC Inverter work. when I increase the voltage to 14V. Suddenly the Zenner Voltage cuts back. (20V/div). I get one spark.
It is repeatable without trigger. I have about 85V

http://forums.pelicanparts.com/uploa...1195758831.jpg

Here the picture with 14V and the current goes up to 3A.

http://forums.pelicanparts.com/uploa...1195758942.jpg

Here the pictures with trigger 12V & 14V.



http://forums.pelicanparts.com/uploa...1195759069.jpg

http://forums.pelicanparts.com/uploa...1195759102.jpg

Is this a Zener failure ?

Thanks for the help and have a nice Thanks Giving.

Bob

Bob, you probably have a dying SCR. The SCR becomes conductive (avalanche mode) when you increase the output of the DC/DC converter. This is why you see one spark. Next the frequency of the DC/DC goes much higher because its outout is shortened to ground by the SCR. Replace it and it should work fine again. Read here for more details.

http://forums.pelicanparts.com/porsche-911-technical-forum/378518-interesting-bosch-cdi-box-failure.html

Ingo```

Thanks Ingo.

So you mean my Thyristor is bad not the Zener?

Bob

I changed the Thyristor against an new OEM part. The failure is still there.
Without trigger I do increase the voltage to 14V. Suddenly the Zenner Voltage cuts back. (20V/div). I get one spark.

I will change now the Zener.

Bob

Ingo and all,
the Thyristor & Zener was not the root cause for the problem. Heat and 14.5 V causing the problem as explained before. The current goes up and the frequence changes.

Here all the parts circled red that I have changed.

I will change now the C2 capacitor. Any ID now?

bobhttp://forums.pelicanparts.com/uploa...1195849342.jpg

Bob, I wonder what SCR you put in there for replacement - if its tq is larger than 35usec it won't work! When you say you get one spark when rising the supply voltage can you measure (with a simple DVM) what voltage you see at the SRC right before it becomes conductive. Is it much greater than the 460 volts? If not it is the SCR going into premature avalanche breakdown. Also check with a scope if the voltage is mostly DC. Otherwise maybe the diodes D6 D7 are suspect.

Cheers,
Ingo

Ingo I used a T12N 800COB form AEG Brand.
Your Information with the 35 usec is not correct. Here original datasheets.


See my picture from the board. What do you think ?
New.http://forums.pelicanparts.com/uploa...1195887566.jpg
http://forums.pelicanparts.com/uploa...1195887771.jpg


Greetings Bob

Out of curiosity where did you get ahold of the AEG brand SCR? I was under the impression that AEG stopped to make semiconductors a long long time ago.

Back to your issue. The data sheet says tq = 60uS@125 degrees. I think this is borderline at best. I sometimes see 65 - 68 uS on the scope for the time the anode is negative. This is why I prefer a replacement with tq =35 uS. However, this does not really explain why your SCR turns on whith no trigger signal present when you raise the supply voltage. It should not happen even if you put 16volts or more into the box.

One reason for premature turnon is if dV/dt (the rise rate) of a voltage spike across the SCR is too high. This is called the Miller effect. Think of the voltage spike having to charge the capacitor formed by the junction in the SCR. The resulting current is proportional to dV/dt and if high enough you have static dV/dt breakdown. Assuming your SCR is in good working condition here is what you can do:

1. Check if the snubber network C7/R16 is working properly. It is to limit dV/dt across the SCR.

2. Check the voltage at the anode of the SCR with a DVM and a scope right before breakdown. If you are concerned about noise on the gate simply disconnect it for the test.

Let us know your findings.

Cheers,
Ingo

Dear all,
some information in advance. Measured at room temp.
My voltage at C8 is 433V without no noises on the DC voltage.
When I ramp though the 16.6 Volt level with my power supply. I get one spark.

When the box is getting warmed up this failure occures at 14V. that why I changed R1. The resistor was not stable at temp. I tested it.


I investigated that bosch was using at their 3 pin a 22V Zener and an R1 120R.
So I put and 120R as a replacement to 56R.

Result the single spark problem occured at 20.3 volt without a trigger hocked up.
Any ID what this causes. of cause I reduce the DC Base voltage on T1. But do I change the phase shift between W1 & W2 of the windings?

Bob

This is what I measure between T4 Emitter & R14.

Before the thyristor complete opens I am having an egde on my scope before the pulse comes.

Sorry for the bad pictures. Christmas comming soon.
http://forums.pelicanparts.com/uploa...1195906944.jpg
http://forums.pelicanparts.com/uploa...1195907001.jpg

This happens with triggering the box.
With trigger on it pulls down the output and locks up.

When the box is cold I have a nice spike on the Thyristor,
when its gets wamed up it is cutting out. See scope pictures.

Then it cuts out complete.

bob

[QUOTE=ischmitz;3604605]Out of curiosity where did you get ahold of the AEG brand SCR? I was under the impression that AEG stopped to make semiconductors a long long time ago.

I got one unit from ebay brand new old stock

Bob,

The 22v zener diode is a protection against over-voltage spikes on the supply. It has nothing to do with the DC/DC converter. Z1 (82 volt) is what limits the DC/DC converter output voltage. However, I am confused. Let's get some answers:

1. Are you saying the one-spark without trigger behavior changed because you changed R1 to a good working one or because you changed the value of R1 t0 120Ohm?

2. Do you see T4 producing a spike that fires the SCR? If yes simly disconnect the gate on the SCR and repeat the test. I am trying to isolate whether your signal stage fires the SCR or whether it breaks down.

Also is this single-spark event happening at 20 volts or at 14 volts and is it depending on box temperature?

Next, for the locking up with trigger signall can you measure the dV/dt rate for the SCR? See the scope picture as an example? In this case the anode was negative for 110uS. I have seen that time to be as short as 68uS in working boxes. So under temperature this is what challenges the SCR.

http://forums.pelicanparts.com/uploa...1195928974.jpg

Also, did you check the snubber?

By the way, even new old stock (NOS) is not an insurance for a good working part. This could well have been parts that did not meet one or several performance specs such as tq. If I were you and I wouldn't want to diagnose the problem but shotgun it I would drop in a suitable replacement SCR and check if this fixes the box.

Ingo