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.
 

Quote from: saturated on April 12, 2024, 07:53:18 PMSo 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) 

Quote from: saturated on April 11, 2024, 10:29:56 PMWhat 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.

 

Quote from: saturated on April 09, 2024, 08:49:16 PMThese 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.

You cannot view this attachment.

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

Welcome to the forum Valleyslad

 

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

 

What it does mean is:

The circuit is open.

 

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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Quote from: joecool85 on April 11, 2024, 03:53:39 PMI recommend making sure that the input jack is clean and connected well to both line and ground.  If not, it could be only letting some signal through causing these issues.

I will, thanks.

Hi. Did you manage to make much of an improvement to the amp? I've just picked one up for £20 on marketplace. I'm looking to tinker a little too!