Re: Amplifier Problem - ***SOLVED***

On 22 Apr 2008 20:09:23 GMT, John Phillips
<news0804@[EMAIL PROTECTED]
> wrote:

>On 2008-04-22, Don Pearce <nospam@[EMAIL PROTECTED]
> wrote:
>> On Tue, 22 Apr 2008 12:38:19 -0700, "Richard Crowley"
>><rcrowley@[EMAIL PROTECTED]
> wrote:
>>
>>>"EADGBE"  wrote ...
>>>> That's when I found it -- a *microscopic* crack in the trace between
>>>> the emitter of Q208 and its adjoining 3.3 ohm resistor.  The crack
was
>>>> so incredibly tiny that I easily overlooked it--repeatedly.
>>>>
>>>> I took one of my jumper leads and clipped one end of it to Q208's
>>>> emitter and the other end of it to the 3.3 ohm resistor.
>>>>
>>>> I then took voltage measurements.  EVERYTHING WAS NOW ON SPEC:
>>>
>>>Its hard to imagine how the output node (where the two
>>>3.3 ohm resistors join) could be correct (6.4V) without
>>>the path through the lower 3.3 ohm resistor and Q208?
>>
>> Pure coincidence - it was the breakdown voltage of the reverse biased
>> base-emitter of Q207.
>
>Rubbish.   ( Well, possibly :-) )
>
>Q207 was indeed hard off with its base at 0V.  Q208 was not connected
>to the output because of the fault.  The output junction was therefore
>biassed only by the emitter voltage or Q204 (8V nominal) via the 
>15 k resistor.
>
>If the measurement was made with a 20 kohm/V multimeter on the 10 V range
>(200k) then I would expect the potential divider effect to pad that 8
>V down to about 7.4 V. EADGBE actually posted that the node was 7.3V
>(not the nominal 6.4 V of the schematic).
>
>OK, I admit it could have been reverse bias breakdown but I prefer my
>version as it doesn't require that coincidence.  (Well OK I do make an
>assumption about the measurement kit instead ...)

Mmmmm...... 20k/V multimeters are pretty rare beasts these days. I
have an old AVO 8 but I don't think I'd have the confidence to
estimate two decimal places in its voltage readings ;-)

My money is still on reverse breakdown. Trannies are almost as good as
zeners in this configuration.

d

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Pearce Consulting
http://www.pearce.uk.com