Crate GLX212 preamp screeching sound

[g1]

Yes, R95 probably never got up to temp, I missed the part about "after 15 seconds", sorry. Once it is running right, they should both get hot after a few minutes.
Check D29 like Jazz said, you will need to lift one end to see if the diode is the source of the low resistance, or something else down the line.

[robertro]

Just got back from the FIFA women's World Cup soccer matches here. I've never seen a 10-0 soccer match before!

Thanks for the suggestions. I just looked at D29 and I noticed that one lead looks like it may have partially melted at some point. I'll unsolder a lead and measure the resistance tonight.

[robertro]

So I lifted one lead of D29 and measured resistance. My meter read infinity in one direction and appx 35 ohms in the other. I'm not sure what normal would be on this component.

One of the solder joints seemed a bit loose, so I reflowed it after measuring it, but no joy, same noise.

Must have gotten too much sunshine yesterday - just remembered that DMM has a diode range - will retest !

[Jazz P Bass]

35 ohms measured on any diode, measured either way, indicates that it is indeed bad.

[J M Fahey]

Why don't you use the diode measuring function which gives you the voltage drop across it?

Resistance values don't mean much in a diode, except getting zero ohms either way which means a dead short.

But yours still seems to behave like a diode, so .....

[Jazz P Bass]

At 35 ohms, that is not much of a PN junction anymore.

[robertro]

So I tried the diode function on my meter, and got a value of 1250, but am not sure what this measures. I thought it induced some current to allow a resistance reading?

Should I be doing this with the circuit energized? I had removed the diode from the circuit for my measurements.

[g1]

Ignore the diode for a moment, with it removed, what do you measure for resistance across the pads it was connected to?

[Jazz P Bass]

Black lead on the striped end (cathode)
Red lead on the other end. (anode)
The voltage in the meter will forward bias the diode.
A good zener will read about .700

With the leads reversed, the meter should show nothing.
My Fluke reads O.L. (Open Line)

As yet, it is uncertain why the zener failed (and at 35 ohms resistance, it is bad).
If you still have the diode out, that is a good opportunity to measure the circuit where the diode was.
If That is also a really low resistance, then there is the reason for the zener failure.
So then, the continued short in the circuit must be explored.

Note: if you are unsure of the diode readings, check the one on the -16 volt rail.
You should be o/k testing it in circuit with the power down.

[robertro]

My DMM reads 1 when resistance = infinite, with leads reversed it seemed jump to 165 before settling to 35 ohms, doing a resistance test. It gave 1 and 1250 when using the diode setting, which I assume adds some bias voltage.

I have put the zener back in, and can remove it tomorrow to measure circuit resistance.

Thanks folks, for bearing with me so far....

[The Dude]

When you remove it, be sure to do as g1 suggested and measure the resistance across the pads where the diode was.

[olddawg]

With the power off and the amp unplugged. Just lift one leg of both the + and - 16v rail zeners and compare the two resistances on the diode scale. It sounds like the positive one has failed. Measure the resistance of both rails to ground. There may have been a reason the zener blew.

[J M Fahey]

For God's sake stop talking about "resistance" values in a forward biased diode.
The proper parameter is voltage drop .

FWIW, and considering that R=V/I

a 1N4002 diode passing 20mA will drop 700mV
Meter thinks and display: 0.7V/0.02A=35 ohms ..... which by the way is not failure at all, it's working perfectly.

Another meter passes 10mA to measure: 0.7V/0.01A=70 ohms.
Oh my God!! , this diode has twice the resistance of the other!!!! , call 911 !!!!!!!!
Not at all, is the exact same diode as before.

And so for any current between 0.1mA and 1A, where voltage drop will vary slightly between, say, 0.6V and 0.8 or 0.9V

Why do I call it "slightly"?
Because I find this 1.5:1 variation being incredibly small compared to a 10000:1 variation in current.
By the way, a meter would have shown a 10000:1 variation in so called "resistance"

And to check diode health, the first and foremost parameter is that it behaves as a diode, meaning high resistance one way, low (and depending on current) the other way ... what we have here.

[olddawg]

For God's sake stop talking about "resistance" values in a forward biased diode.
The proper parameter is voltage drop .

FWIW, and considering that R=V/I

a 1N4002 diode passing 20mA will drop 700mV
Meter thinks and display: 0.7V/0.02A=35 ohms ..... which by the way is not failure at all, it's working perfectly.

Another meter passes 10mA to measure: 0.7V/0.01A=70 ohms.
Oh my God!! , this diode has twice the resistance of the other!!!! , call 911 !!!!!!!!
Not at all, is the exact same diode as before.

And so for any current between 0.1mA and 1A, where voltage drop will vary slightly between, say, 0.6V and 0.8 or 0.9V

Why do I call it "slightly"?
Because I find this 1.5:1 variation being incredibly small compared to a 10000:1 variation in current.
By the way, a meter would have shown a 10000:1 variation in so called "resistance"

And to check diode health, the first and foremost parameter is that it behaves as a diode, meaning high resistance one way, low (and depending on current) the other way ... what we have here.

Sorry Juan. That's what I meant. Use the diode scale and yes it measures the voltage drop. Try not to freak out about it. Lol. The forward biased voltage drop of a silicon junction is .7v approximately.

[J M Fahey]

Cool
But I wasn't answering you , rather the general direction this thread is taking.

It's that I just hate seeing a friend needlessly running in circles , in this case, reading a good diode as bad one

As in:

My meter read infinity in one direction and appx 35 ohms in the other.

^^^^^^ good diode

35 ohms measured on any diode, measured either way, indicates that it is indeed bad.

What does "either" mean in this case?
If "both ways" , it would mean a bad diode, but that's not what Robertro wrote.
If "at least one way", if it measures high resistance the other way, the diode is fine.

At 35 ohms, that is not much of a PN junction anymore.

I've just shown how a healthy junction can measure 35 ohms, just pass 20mA through it .

the zener failed (and at 35 ohms resistance, it is bad).

STILL talking resistance? ..... Oh my God !!!!!!!
Proper measurement is forward voltage drop (healthy around 700mV) and reverse Zener breakdown (whatever the Zener is rated for).

with leads reversed it seemed jump to 165 before settling to 35 ohms, doing a resistance test.

Post #24, still talking diode resistance ... am I supposed to ignore it ?
Is that a way to help Robertro?

Take care

EDIT: to measure

reverse Zener breakdown (whatever the Zener is rated for).

, by the way something that the meter can not do on its own, connect anode to ground, cathode to +40V through a resistor, 4k7 or 10k is fine, and measure voltage drop across it, it should be around 16V .
This test has not been made yet.
It measures Zener diode on its own, not in parallel with the rest of the preamp where you might find other problems.
When doing this test, open at least one leg of both large voltage dropping resistors, so you don't needlessly subject the preamp to receiving just -16V or worse, undropped +40V .