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Originally posted by J M Fahey
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Sorry but I have nothing.
I downloded the schematic, added some red and blue arrows suggesting test points and uploaded it.
And erased my side of the deal, trusting Forum server.
Now I see only the thumbnail is available, although Forum "memory" remembers that itīs 120 something kB .
Weird but not surprising considering the Forum stored file problems.
But basically:
1) all bipolar transistors work with around 0.65V Base to emitter; positive in NPN and negative in PNP.
So measure all; if you find, say, +3 or +5 or +15V BE in an NPN transistor, then BE is internally open.
If less than 0.4V, itīs unbiased so itīs OFF, check why itīs not receiving its turn on signal.
If 0 then itīs probably shorted or the path leading to that base is open.
If negative, then something is grossly wrong.
Same for PNP but invert expected polarity.
2) All NPN transistors need Collector more positive than emitter to pass current, if not see who feeds it and why voltage is not reaching transistor.
If 0 then itīs probably shorted CE *or* base is being fed a ton or current (why?) .
Again PNP is same but with opposite polarity.
So in a nutshell: we do NOT pull all parts for measurement: itīs slow, can damage parts, *will* damage PCB and best case that covers only *half* the problem.
We DO measure voltages, inject and follow signal, and that LEADS us to the bad parts area.
I downloded the schematic, added some red and blue arrows suggesting test points and uploaded it.
And erased my side of the deal, trusting Forum server.
Now I see only the thumbnail is available, although Forum "memory" remembers that itīs 120 something kB .
Weird but not surprising considering the Forum stored file problems.
But basically:
1) all bipolar transistors work with around 0.65V Base to emitter; positive in NPN and negative in PNP.
So measure all; if you find, say, +3 or +5 or +15V BE in an NPN transistor, then BE is internally open.
If less than 0.4V, itīs unbiased so itīs OFF, check why itīs not receiving its turn on signal.
If 0 then itīs probably shorted or the path leading to that base is open.
If negative, then something is grossly wrong.
Same for PNP but invert expected polarity.
2) All NPN transistors need Collector more positive than emitter to pass current, if not see who feeds it and why voltage is not reaching transistor.
If 0 then itīs probably shorted CE *or* base is being fed a ton or current (why?) .
Again PNP is same but with opposite polarity.
So in a nutshell: we do NOT pull all parts for measurement: itīs slow, can damage parts, *will* damage PCB and best case that covers only *half* the problem.
We DO measure voltages, inject and follow signal, and that LEADS us to the bad parts area.
Thanks so much,,
nosaj
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