saturated's all encompassing inclusive and exclusive bin thread

[saturated]

Today I learned:

Just because a resistor has no voltage drop across it and measures the same voltage at each end with respect to ground

Doesn't mean the resistor is shorted

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[saturated]

What it does mean is:

The circuit is open.

 

[saturated]

And I'm ready to put forth another conclusion:

If I measure xxx volts on one end of a resistor.....and ZERO volts on the other end

Guess what


 

It's time to go ISO

 

[saturated]

These two ceramics are still hot enough to cook steaks on....

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At least small steaks...well done...

So I'm going out on a limb here and proposing that the 8k ohm 10w resistor is shorted to ground.  It measures 170v on one end and zero on the other.
A quick look at the schematic (a forensically enhanced version kindly provided by the Mighty Mick) to me places cross hairs directly on the zener diode.

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So I'm guessing that the zener diode is shorted...idk.

Or perhaps I'm unfairly blaming it. 

 


To be continued...the story of the zener diode that was (possibly) framed

[saturated]

What it does mean is:

The circuit is open.

 

I decided to have a look at this scenario...of course I had seen it and read it in books and kind of unwillingly accepted it.

My point being...take an open circuit...add some resistors in series to the positive side...and your gonna measure the same voltage anywhere along that open series...doesn't matter....one resistor...two resistors...three resistors....there is no voltage drop. 

I had to try it.  I made up a three resistor open series circuit connected to 5v.

Well you guys know what happens and already know this.  But seeing it in practice really firmly eliminated any skepticism. 

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Obviously this is way over simplified but it still is a powerful tool when probing.

 

[g1]

So I'm going out on a limb here and proposing that the 8k ohm 10w resistor is shorted to ground.  It measures 170v on one end and zero on the other.
A quick look at the schematic (a forensically enhanced version kindly provided by the Mighty Mick) to me places cross hairs directly on the zener diode.

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So I'm guessing that the zener diode is shorted...idk.

That's not going out on a limb, that is good solid logic. (assuming you verified the resistor is ok) 

[saturated]

Thanks G1 it's a gorgeous day today with plenty of sunshine so I'm actually fixing to take some good pics of this scope so I can put it on the auction site.  Hope is waning that the ever so coveted green trace will ever appear.
I'm happy that it's been a great learning tool but at this point I don't even know if the CRT is any good. When I first got it there was a slight glow at the small end and it gets dimmer and dimmer.
Also there are a bunch of rusty trim pots then another board has an open somewhere (at the minimum).
A 68v zener is cheap though I might need to see that localized issue settled.
Also I really like that arrow pointing at that node and a diagram of a square wave.
Would be cool to see if that is there with my other scope.
 

[saturated]

Meanwhile... experienced my first Chernobyl.

So this metronome I had removed the bridge rectifier a while back and it tested good...oh well

Put a new one in today.... thought it would be fun to hook my meter to the leads before I clipped em and read ac and DC on the different leads.

So I solder it in...crudely soldered the transformer wires back making sure I put the right ones in order.  Plug it in...I'm gathering up my meter and test leads and I hear woosh and see lots of smoke

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And liquid 

Later I was outside and thinking did I put it in backwards or something...or leave a solder bridge?  But looks like the orientation is correct and the solder reasonable (these pics are post meltdown)

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Im beyond hoping this thing will be fixed but I did want to replace that part.

Meanwhile I put it on my ugly dbt after the fact yes it was glowing.

 

[g1]

Follow the trace from the bridge + lead to filter cap.  Is it going to + or - end of cap?

[saturated]

Yes sir appears to go to

-negative terminal of capacitor

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-and the emitter of the tip31a on the right

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and a resistor

[Jazz P Bass]

The rectifier is installed backwards.

[saturated]

Yes sir thanks I was able to find some old pics of the original part

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[saturated]

Ok guys sorry for the bo bo 

Anyhow removed the old one (no easy task) and got a new one in

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Seems all is good when measuring the (+) pin and two middle pins (~)

(+) pin reads about 40v DC (have no idea what it is supposed to be


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But when I read the middle pins I get like 16v ac but what the heck I get 20v DC

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I think I'll call it a night

 

[Jazz P Bass]

I think your meter is trying to average the AC voltage on the two inner pins.
Try with the meter set to to Volts AC.