Hi,

Can anyone suggest what I should do next?

There is -42 vdc on the output of this amp (GK 200MB bass amp). See schematic below

I have tested the transistors with a diode test (Q21, 20, 19, 18, 13, 12, D20) and I can't find a short anywhere.
The +/-15v and +/-12v rails are all working.

pretty funny what i just learned

I was making circuits on breadboard and trying to measure voltage drops with my oscilloscope.

I was getting frustrated...crazy whacked numbers...trying to make sense of it..which I could not.

turns out I thought a scope was like a multimeter 

just connect it across the component in question..just like a multimeter...right?

NO 



(of course you already know this)

the ground terminal on oscilloscope probe

drumroll......

spoiler....

is actually a ground.

Quote from: mandu on October 14, 2023, 10:53:46 PMThe picture you posted shows common cathode (connected to the -ve).
The other two are for red or green (or yellow if you power both) goes to the +ve
The limiting resistor may be 1 if wired on the cathode or 2 nos. if wired from anodes.
regards.

Thanks for your help.

one thing I need to work on is checking resistors.  I kinda dont want to because I have to desolder one leg and lift it.

 

Quote from: Den. on October 14, 2023, 06:51:47 PMI'm working on my Peavey amp and I need to replace a few 3mm, 3-leg LEDs. In ordering, I have a choice of common cathode or common anode. How would I determine which I need?

I just found these schematics online; perhaps they tell what the LEDs are.

The picture you posted shows common cathode (connected to the -ve).
The other two are for red or green (or yellow if you power both) goes to the +ve
The limiting resistor may be 1 if wired on the cathode or 2 nos. if wired from anodes.
regards.

ok this evening I wanted to check some voltages on my oscilloscope now that thanks to Mr Mick I have a schematic I can actually read.

I also learned that wires have two ends.  meaning all the while I knew to stay away from the wires on the end of the CRT.
I should have known that the ends of those wires are in the top middle of the following photo.

particularly the larger diameter red ones

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I only became aware of this as happily trudging along taking measurements suddenly saw a large negative number.

I was somewhat pleased yet perplexed that the numbers (to me) looked pretty good...? as if they were reasonably close to those on the schematic.
edit: also the anode of d408 measures 21.8v

so moving on to the board with the ICs I quickly noticed a trend...that is there was no voltage to be had..

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I was a bit uneasy because a couple days ago I was able to get low voltage on those IC pin 14s...now nothing. 

now granted IC 303 is vacant but idk...

Im glad i didnt start ripping out capacitors and stuff.

 

I'm working on my Peavey amp and I need to replace a few 3mm, 3-leg LEDs. In ordering, I have a choice of common cathode or common anode. How would I determine which I need?

That potentiometer is meant to rotate fully around, it is made that way.
That one has no inbuilt stops on the shaft.

well that was informative
it just starts over and over if you keep spinning

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amazing

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but...alas the task of reinserting this is not easy...connected to each terminal are very fine jumpers....

from what i remember i didnt sever them they basically disconnected just by breathing on them

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ugghhh