Moderators: Dave Moll, Andrew Davie, Steve Anderson
Harry Dalek wrote:Ok Steve i will look into a probe for this scope...
Steve Anderson wrote:It may be the ZTX558 has a short e-b...what's the grid of the CRT doing? -ve probe on cathode, +ve on the grid. It should swing from a couple of volts negative to almost -75V. If it doesn't try replacing the ZTX558. CRT cold I assume.
Steve A.
Steve Anderson wrote:Hmm...not easy to do this via 'remote control', i.e. I'm not actually there. Have you tried changing the ZTX558? If not, that's what I would do next...even if it 'reads' correct.
I assume you've checked resistors are actually the value they say they are.
Steve A.
Harry Dalek wrote:i am right in saying the voltage across the 5.1 zener diode should be a constant 5.1v no matter what i just the opto led brightness too.
Steve Anderson wrote:Harry Dalek wrote:i am right in saying the voltage across the 5.1 zener diode should be a constant 5.1v no matter what i just the opto led brightness too.
Yep, that's correct.
Steve A.
Steve Anderson wrote:Well, those two results (if correct) say those two transistors are as good as dead. Assuming you've got the pin-out correct and they really are ZTX558s. The hFE (current gain) should be at least 50, probably anywhere from 100-200 or more. If you mixed up the leads and applied the system voltage to them there's every chance that the b-e junction is buggered. The c-b handles up to 400V, but the e-b maximum is 5V - this is quite normal.
Time for some new ones, throw those two away.
Check any new ones for a good hFE before fitting them.
Steve A.
Harry Dalek wrote:Those were new untested Steve i got a 2 batches a year ago i wonder if its another case of bad parts .
Steve Anderson wrote:*Puts elbows on table and cups head into hands* This is the second time this sort of subject has come up in recent times, fake or defective parts. I won't repeat my diatribe of the last time, I'll just say, "DON'T BUY THIS CRAP"......because that's what it is.
In this circuit you can replace the ZTX558 with a MPSA92/93/94 with no other changes being required. Pin-out might be different so check first and test the Hfe before fitting.
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