Crate GLX212 preamp screeching sound

[olddawg]

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:
^^^^^^ good diode

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.

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

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).

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

Take care

EDIT: to measure , 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 .

Or.. Just replace both zeners for less than a dollar.....

[robertro]

So I removed both D29 and D28 and measured the following:

Resistance between pads:
D29 location: 0
D28 location: 150/450 ohms given by my analog meter depending on which way I connect the leads (my digital meter seems to have problems getting a reading, going from infinity to about 160, than back to infinity)

Resistance to ground:
D29: 150/450 ohms for anode side (no ring) pad, 0 for cathode side pad
D28: 0 for both pads

On diode setting, with black lead of DMM on cathode, each diode gives the same reading, about 1250.

Seems that the diodes are ok?

[The Dude]

Yes, the diodes are probably ok. You have a short on your +16V line. There will be many things connected to that line- op amps, bypass caps, etc. I would reinstall the diodes and see if anything is getting hot. Use your fingers to see if you can detect a certain IC or other component getting hot. A lot of times, although certainly not every time, it's the first IC into the amp that gets roasted from somebody plugging something into the amp that they shouldn't. Also, if there are any auxiliary boards that you can unplug from the power supply, try that first. You may be able to narrow it down to a certain board or area.

[g1]

Yes, the diodes sound ok. There is a short on the +16V line. Try lifting one end of C12 and see if the short is still there.

[Enzo]

Your ohm meter works by applying a low voltage current to the resistor. It interprets the voltage across the part as resistance. Now that works fine in isolation. But when the resistor is connected to a circuit, there will often be capacitors connected to it, and the caps try to charge up from that small meter voltage. What you are seeing is the "resistance" changing as the caps charge up. When you reverse the leads, those caps try to charge up in the other polarity, so you get a reading as your meter discharges what was there, gets to zero then charges up the new direction.


That also explains how a resistor can measure one thing one direction, then another in the other direction. That is usually a sign there is some small residual voltage left in the circuit you are trying to measure. Sometimes there is enough voltage, your meter indicates a "negative resistance".

COuld just be that filter cap shorted.

You have +16 shorted to ground, so we need not power up to chase this. In fact, prefer we don't. The +16 supply is on the power amp board, so disconnect any of the little cables that connect that board to the preamp board up front. Now if the short remains, it must be on the power amp board, and if the short is now absent, then the preamp board has the short. Unless the ribbons are soldered at both ends, in which case we cannot disconnect them.

[robertro]

Yes, the diodes sound ok. There is a short on the -16V line. Try lifting one end of C12 and see if the short is still there.

Tried lifting C12 - makes no change to the readings.

[Jazz P Bass]

On diode setting, with black lead of DMM on cathode, each diode gives the same reading, about 1250.

Seems that the diodes are ok?

I don't want to keep harping on the diodes as it seems you have problems further down the line but the readings that you posted do not make sense.

I have in my hand a 16 volt/ 5 watt zener diode.

On the diode setting of my Fluke 77/BN, with the black lead on the cathode I get a reading of .670.
Reversing the leads there is no reading. (O.L.)

(Not to piss off Juan) Resistance reading: black lead on cathode reads 2.3 million ohms.
Reversing the leads reads nothing. (O.L.)

[robertro]

Yes, I didn't report it, but my DMM on diode setting first gives a reading of 681, then quickly shifts to about 1250, so the initial reading is consistent to what you describe. It's not a high-end instrument - some obscure handheld unit "rapitest electrical tester" that I bought some time ago - no manual, no units, etc.... that's why I keep returning to my antique trust Radio Shack analog meter :-) but it doesn't have a diode setting.

I don't want to keep harping on the diodes as it seems you have problems further down the line but the readings that you posted do not make sense.

I have in my hand a 16 volt/ 5 watt zener diode.

On the diode setting of my Fluke 77/BN, with the black lead on the cathode & get a reading of .670.
Reversing the leads there is no reading. (O.L.)

(Not to piss off Juan) Resistance reading: black lead on cathode reads 2.3 million ohms.
Reversing the leads reads nothing. (O.L.)

[g1]

And both diodes do the same?
And just out of curiosity, on resistance range, what do you get checking both ways on both diodes. Then with your other meter also.

[Jazz P Bass]

And please do not let your fingers get involved in the measurements.

The human body can read around 2 million ohms. (try it!)

[robertro]

Yes, just checked and both diodes do the same.

On the analog meter, R is about 30 ohms and infinity on each diode. On the DMM, I get infinity both ways on the 200 ohm setting.

And both diodes do the same?
And just out of curiosity, on resistance range, what do you get checking both ways on both diodes. Then with your other meter also.

[The Dude]

I'm with Juan on this and I don't believe the diodes are a problem. Especially given that we have a short on the +16V line with the diodes removed. Also, the diode reads infinite reverse biased. If it were leaky, it would not. It is, however, curious that you get only 30 ohms forward biased. I think we have some sort of metering anomaly going on here. Are you sure you're reading the right scale- maybe your missing a K or an M or a decimal?

[robertro]

Thanks for the excellent description!

I removed the blue harness leading to the dsp board, with no change in pad to ground readings.

I want to remove the two white connectors that lead to the front pot/preamp board, but cannot for the lufe of me figure out how they detach. They seem to have a loose u-shaped section that is loose that i could pry off, but it looks fragile and i don't want to break it. Any suggestions?

Your ohm meter works by applying a low voltage current to the resistor. It interprets the voltage across the part as resistance. Now that works fine in isolation. But when the resistor is connected to a circuit, there will often be capacitors connected to it, and the caps try to charge up from that small meter voltage. What you are seeing is the "resistance" changing as the caps charge up. When you reverse the leads, those caps try to charge up in the other polarity, so you get a reading as your meter discharges what was there, gets to zero then charges up the new direction.


That also explains how a resistor can measure one thing one direction, then another in the other direction. That is usually a sign there is some small residual voltage left in the circuit you are trying to measure. Sometimes there is enough voltage, your meter indicates a "negative resistance".

COuld just be that filter cap shorted.

You have +16 shorted to ground, so we need not power up to chase this. In fact, prefer we don't. The +16 supply is on the power amp board, so disconnect any of the little cables that connect that board to the preamp board up front. Now if the short remains, it must be on the power amp board, and if the short is now absent, then the preamp board has the short. Unless the ribbons are soldered at both ends, in which case we cannot disconnect them.

[robertro]

Checked the readings......

On Rx1 scale 30. 30 ohms
On Rx10 scale 20. 200ohms

Inconsistent, but from what others have said, it sounds like a simple circuit is needed to properly test zener diodes, and a resistance test is useful as a crude tool to ensure infinity in one direction and some flow in the other.

I'm with Juan on this and I don't believe the diodes are a problem. Especially given that we have a short on the -16V line with the diodes removed. Also, the diode reads infinite reverse biased. If it were leaky, it would not. It is, however, curious that you get only 30 ohms forward biased. I think we have some sort of metering anomaly going on here. Are you sure you're reading the right scale- maybe your missing a K or an M or a decimal?

[g1]

Yes I think the diodes are fine and we are just seeing variations of meters. I think what threw some of us off was we thought you were getting the low resistance reading with the DMM, which would be very uncommon. For the analog meter it's no big surprise.

Can you post a picture of the connector?
Some connectors for ribbon wires are spring loaded and you push down on one side while pulling up on the cable.