6 pin CDI problem under temp.

[HKZ Bob]

[QUOTE=ischmitz;3605019]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?

With an 120 R resiator as R1 the failure mode occurs at higher voltage and it is limiting the current. I assume this is not the problem.
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.

Will do.
T4 2N2905 Emitter disconnect from the gate of the SQR.

When the failure occures the emitter shows this little egde on the scope. The spark started to show an intermittance. Also is this single-spark event happening at 20 volts or at 14 volts and is it depending on box temperature?

With the 120 R resistor is came at 20V with the original 56R it can at 16V no trigger. Let me check if this failure is still there.
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.



Also, did you check the snubber?
I changed the capacitor and the resistor. I have to reasseble it to see an improvement or not.

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.

[

[HKZ Bob]

Originally Posted by HKZ 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.



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

Ingo
This is the output emitter of the 2N2905 at failure mode.


Loren this is what I measure on the Gate of the SCR without a trigger.

[HKZ Bob]

Ingo I change the SCR again and replaced it by an 2N6509.
The problem still occurs.

The problem is still there. I ramp up the voaltage one spark.
As you said I disconnect the Gate and also measured the base on T4. (2N2905) and Gate paraelle.

At 15V no trigger




Now with 16V no trigger. Suddenly I get pulses al over. Like a break through.




When I trigger the box I see multiple trigger next each other.









Any Id what can cause this ?

Bob

[Lorenfb]

"Do you see T4 producing a spike that fires the SCR? If yes simly disconnect the gate on the SCR and repeat the test."

It's not a good testing procedure to leave the gate open, given the need to
isolate the sporadic problem. The best approach is to just short the gate to
the cathode. Then place a .01/600 capacitor accoss the anode to cathode.
If the SCR still "fires" as the input voltage is increased, then the SCR is bad.

[HKZ Bob]

Originally Posted by Lorenfb

"Do you see T4 producing a spike that fires the SCR? If yes simly disconnect the gate on the SCR and repeat the test."

It's not a good testing procedure to leave the gate open, given the need to
isolate the sporadic problem. The best approach is to just short the gate to
the cathode. Then place a .01/600 capacitor accoss the anode to cathode.
If the SCR still "fires" as the input voltage is increased, then the SCR is bad.

The systems fires up without the Gate connected.

So it defenitly get ignited over the Anode. I tried several different Thyristors.
THE SCR is not the root cause.
All of them get ignited over the anode without a trigger connected.

What could be the reason that the SCR gets ignited over the Anode.
I can measure 433Volt on Anode and I don't see any spikes or noises on the load capacitor.

Bob

[HKZ Bob]

Originally Posted by HKZ 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.



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

Loren this is what I measure on Gate without a trigger.
When I ramp up the voltage see what happens.

[Lorenfb]

"It's not a good testing procedure to leave the gate open, given the need to
isolate the sporadic problem. The best approach is to just short the gate to
the cathode. Then place a .01/600 capacitor accoss the anode to cathode.
If the SCR still "fires" as the input voltage is increased, then the SCR is bad."

Follow the above instructions.

[HKZ Bob]

Originally Posted by ischmitz

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.

Interesting Bosch CDI box failure

Ingo```

Ingo,

the symtoms you are describing are correct. Even without a gate & trigger connected CDI produces one spark and the DC/DC converter frequency goes up
and the current rises to more than 3 amps. But it is not the thyristor which causes to go into avalanche mode. I tried 4 types of SCR now.
T12N 800COB /T10N600COB / T6N600COB and 2N6509. I am 100% sure the problem is hidden somewhere else.

But I have no ID where to look. When I disconnect the Anode I can ramp up to 18V and the DC/DC converter keeps it frequence. with the Anode connected I get one spark without the trigger .

Any further ID. Till today I have to changes the 0.015mF & 6800nF capacitor. I need to order a replacement. But that, s not root cause. What is the reason that the DC/DC converter makes the SCR to go into avalanche mode.?

Bob

[HKZ Bob]

Originally Posted by Lorenfb

"It's not a good testing procedure to leave the gate open, given the need to
isolate the sporadic problem. The best approach is to just short the gate to
the cathode. Then place a .01/600 capacitor accoss the anode to cathode.
If the SCR still "fires" as the input voltage is increased, then the SCR is bad."

Follow the above instructions.

I will do it, just to prove your test method, but it is hard to believe that all my SQR are bad.
I tried 4 types of SCR now.
T12N 800COB /T10N600COB / T6N600COB and 2N6509. I am 100% sure the problem is hidden somewhere else.

thanks Bob

[HKZ Bob]

Originally Posted by Lorenfb

"It's not a good testing procedure to leave the gate open, given the need to
isolate the sporadic problem. The best approach is to just short the gate to
the cathode. Then place a .01/600 capacitor accoss the anode to cathode.
If the SCR still "fires" as the input voltage is increased, then the SCR is bad."

Follow the above instructions.

Loren I did it as you said,

and the SCR is not firing. Not even at 20V.

I did it with & without trigger.

Does it means the failure is been initiated through the gate?

Bob

[Lorenfb]

"Does it means the failure is been initiated through the gate?"

Yes, but more tests are required. Next remove the .01 you added
and do the test again, still with the gate shorted to the cathode.

[ischmitz]

Hm, it seems you are finally starting to narrow down the reason for the failure. Without trying to jump to conclusions it looks like either noise on the gate or a bias prematurely fire the SCR. With the gate tied to ground you seem to be able to eliminate this.

I would guess that by removing the 0.01uF/600V over the SCR the result should stay the same. However, without data it's just another damn opinion. So please do that test.

Next you need to find out why the signal stage is noisy (AC) or leaky (DC). Loren (or anyone else), correct me if I am wrong but one should be able to monitor the output of the trigger signal stage (R14/R15) even when it is not hooked up to the gate of the SCR, right. Check if you see a bias (DC) or noise (AC) component when you raise the supply to where the SCR usually fired.

[HKZ Bob]

Dear all & Ingo, Loren,
the second test with gate shorted to kathode without the capacitor showed no spark as well.

I also measured a little 0.35V sawtooth signal at Gate with 15V supply just before the SCR ignited.

Bob.

[ischmitz]

Originally Posted by HKZ Bob

Dear all & Ingo, Loren,
the second test with gate shorted to kathode without the capacitor showed no spark as well.

I also measured a little 0.35V sawtooth signal at Gate with 15V supply just before the SCR ignited.

Bob.

Your message is a little confusing: You say you do not see a false trigger with the gate shortened to GND and not additional cap placed across the SCR. Understood so far. This tells us the false trigger happened because of a faulty signal at the gate. Now that it is tied to GND no signal - no false trigger.

However, where and under which conditions do you measure a sawtooth signal at the gate? Is this with everything connected and the gate not shortened to GND or with the gate shortened to GND and disconnected from the signal output stage? Please be a little more specific.

Cheers,
Ingo

[HKZ Bob]

Originally Posted by ischmitz

Your message is a little confusing: You say you do not see a false trigger with the gate shortened to GND and not additional cap placed across the SCR. Understood so far. This tells us the false trigger happened because of a faulty signal at the gate. Now that it is tied to GND no signal - no false trigger.

However, where and under which conditions do you measure a sawtooth signal at the gate? Is this with everything connected and the gate not shortened to GND or with the gate shortened to GND and disconnected from the signal output stage? Please be a little more specific.

Cheers,
Ingo

Ingo,
I measure this little sawtooth in normal usage, without the short from gate to kathode.

No trigger. 15V Supply.

Everything connected and the gate not shortened to GND

[ischmitz]

...wait a second: Now that I am reading back through your posts I see this image showing capacitor C2 that filters the 6.8V from the Zener diode to be puking electrolytic fluid So it is a reasonable assumption that it's that either dead or has a least lost most of its capacitance.

That being the case the regulated 6.8V will be "dirty" (aka ripple) and WILL cause problems downstream. Please tell me it's not that simple and you have either changed that cap or at least verified the 6.8V supply to the signal stage on your scope.......

[HKZ Bob]

Originally Posted by ischmitz

...wait a second: Now that I am reading back through your posts I see this image showing capacitor C2 that filters the 6.8V from the Zener diode to be puking electrolytic fluid So it is a reasonable assumption that it's that either dead or has a least lost most of its capacitance.

That being the case the regulated 6.8V will be "dirty" (aka ripple) and WILL cause problems downstream. Please tell me it's not that simple and you have either changed that cap or at least verified the 6.8V supply to the signal stage on your scope.......

I measured it. C2 has full capacity. I also added an addtional 22micro farrad paraelle to it.
No change either. I ordered new one and will replace it to be 100% sure.

Bob

[HKZ Bob]

Originally Posted by HKZ Bob

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.



Greetings Bob

The stuff around the capacitor is very hard, like epoxy. It glued the capacitor to the board. When I replaced it is saw no difference.

but can it be that this material under the cap is conductive solid electrolyte.
I cleaned it with a dremel and brush. final test wil follow with the 22mF.

What is the Voltage Range according spec. 10 to 18V?

I replaced the C2 capacitor against an 22mF with some litle improvement
when I replaced it with and 100mF the break through came at 18.8V
with an 220mF at 19.5V.

Regards Bob

[HKZ Bob]

Loren, Ingo I replaced C2 and have the CDI run hot. Same problem appeared.
First I see the current start to go up and down from 2.5A til 2.7A.
Then I get this noise on the Gate.



Then the frequenz goes up. The SQR goes into conductive.
Thats what I see on the scope.



Any Id what to do next.
How about disconecting C4. My Scope is not able to the spike when the Thyristor is getting conductive. I only see this unsober spike. What is the root cause for this?



Thanks Bob

[ischmitz]

Disconnect the gate of the SCR and tie it to GND. Then check your signal stage for where the sawtooth comes from. Check the supply voltage (6.8volts), the Schmidt-Trigger, and the final output stage - something is oscillating.