Originally Posted by HKZ Bob (Post 3604669)

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

Originally Posted by Lorenfb (Post 3606429)

"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.

Originally Posted by HKZ Bob (Post 3604669)

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

Originally Posted by ischmitz (Post 3602881)

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```

Originally Posted by Lorenfb (Post 3606647)

"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.

Originally Posted by Lorenfb (Post 3606647)

"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.

Originally Posted by HKZ Bob (Post 3608297)

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.

Originally Posted by ischmitz (Post 3608333)

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

Originally Posted by ischmitz (Post 3609059)

...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 :eek: 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.......

Originally Posted by HKZ Bob (Post 3604599)

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

Originally Posted by HKZ Bob (Post 3574309)

I use a new Bosch Hall distributor. It is the std. hall device Bosch used for Hall distributors. I drive it with my Makita http://forums.pelicanparts.com/uploa...1194460722.jpg

Originally Posted by ischmitz (Post 3615049)

Wow, that is a lot of mechanics for triggering the boxes. I needed something that runs indefinitely for long-term testing so I came up with a little 555 timer based trigger circuit on a bread board.

http://forums.pelicanparts.com/uploa...1196359316.jpg

http://forums.pelicanparts.com/uploa...1196359619.jpg

It works for both 3-pin and 6-pin boxes. With this design you can choose between a high speed around 6000 RPM and a low speed setting around 1000 RPM. It shouldn't take more than half an hour to put this together. If you want to get more sophisticated you could put another voltage devider and RLC network for pulse shaping onto the 6-pin output to closer simulate the real trigger signal of the 6-pin units.

Any news on your testing?

Ingo

Originally Posted by HKZ Bob

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?

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.

Originally Posted by HKZ Bob

I do see multiple trigger on the Gate.
Gate of the SCR is bridged to Kathode. Is that normal

Originally Posted by ischmitz (Post 3615658)

Bob, your above statements are contradicting each other. First, you reported a couple of days back there was no spark whatsoever with the and without a capacitor over the SCR while the gate was tied to GND. Second, you keep saying that you measure something on the gate while at the same time it is tied to GND. This becomes very confusing.

The gate conection was open. SCR Gate & Kathode shortend. I meaured between R14 & R15.

Let me re-phrase what you are saying: You have the gate shorted to the cathode, in other words C6 is shorted. If that is the case you will not be able to see anything on the gate because it is tied to GND. I assume the latest scope traces shows the voltage at the anode of the SCR. When you raise the supply voltage to 16V and beyond you start to see spikes on the scope, correct?

I measured the output of T4.
The gate conection was open. SCR Gate & Kathode shortend. I meaured between R14 & R15.


If my above description is correct there are only two possibilities at this point. Either the SCR breaks down electrically or you have an isolation problem.

Inspect the mounting of the SCR closely. I don't remember what the last SCR is that you have swapped into your box. Was it the 2N6509? TO220 cases have the anode present on the cooling fin. The heat sink in the box is at GND. If you don't use the proper washers and isolators around the metal screw there will be arcing going on. It most likley happenes between the metal screw and the inside of the hole of the cooling fin. Look for the tell-tale black marks.

Ingo

Originally Posted by HKZ Bob (Post 3615444)

That trigger you see I used in the past perfect. I use now a 2 Volt pp sinus signal. I also bought a function generator. It is still in the mail. Will have it soon.


Regards bob

bob

Originally Posted by HKZ Bob (Post 3615444)

That trigger you see I used in the past perfect. I use now a 2 Volt pp sinus signal. I also bought a function generator. It is still in the mail. Will have it soon.


Regards bob

bob

Originally Posted by Lorenfb (Post 3581732)

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.

Originally Posted by ischmitz (Post 4041289)

I don't think the frequency of the DC/DC converter affects the SCR's ability to comutate. You can see each swing of the half-wave rectifier on the rising voltage at the anode of the SCR as a little riple. I have seen anything between 3 and 4 kHz in different boxes and it did not seem to affect the stability of a given box.

Originally Posted by rick-l (Post 4040356)

HKZ Bob

What are the traces in the scope photos? I think the first one is the high voltage cap but what are the others?

http://forums.pelicanparts.com/uploa...1193934881.jpg

Originally Posted by jmchrist (Post 4041360)

In lock up situation, I observed that the frequency of the converter climbed up to about 10 kHz. This left less than 100 µs to the SCR to go off.


Of course I had also checked that C7 and R16 were OK.
The only major component that I was not able to check or to change is the transformer.
May be it is the real culprit, with an inductance that I found a little too low by comparison with another box (operating correctly).
Now that the box is correctly operating with a BTX30-800 I am not really motivated to try to change the transformer.

jmc