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Author Topic: Peavey Filter Caps  (Read 1406 times)

Tennerondack

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Re: Peavey Filter Caps
« Reply #30 on: December 03, 2018, 08:00:38 AM »

Thanks Phil, actually I did a search just to identify the type you described. Just trying to confirm by a visual if this is what you were talking about.
I have to get a magnifier to try to get some more info off these tiny things . Presumably that's what you need , correct ?
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Tennerondack

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Re: Peavey Filter Caps
« Reply #31 on: December 03, 2018, 08:21:39 AM »

The both have M ( looks like Motorola style M ) SPS 761 printed on them.
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phatt

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Re: Peavey Filter Caps
« Reply #32 on: December 03, 2018, 09:16:00 AM »

I only mentioned the Transistors to verify if the circuit had any opamp chips.
But don't concern yourself about that right now.
Your main concern is to establish why you have no voltage at that power connector.
So get the meter out and check that 100 Ohm resistor.  ;)

With black probe on common terminal,, Measure voltage on left side then right side of that resistor and report what you find.
From the symptoms you explain that resistor could be open circuit.
Phil.
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Tennerondack

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Re: Peavey Filter Caps
« Reply #33 on: December 03, 2018, 09:34:24 AM »

+ - 65 v both sides.

Edit : just when I thought things couldn't get any stranger, the preamp power supply pin now reads 43 volts. The resistor reads the same on both sides, after a new reading.  The audio pin reads 1.5 volts

:loco
« Last Edit: December 03, 2018, 10:31:37 AM by Tennerondack »
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g1

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Re: Peavey Filter Caps
« Reply #34 on: December 03, 2018, 12:32:47 PM »

Edit : just when I thought things couldn't get any stranger, the preamp power supply pin now reads 43 volts. The resistor reads the same on both sides, after a new reading.  The audio pin reads 1.5 volts

:loco
If these readings are after mounting the board with the screws, that is probably why, it was not grounding before with the board lifted.
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Tennerondack

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Re: Peavey Filter Caps
« Reply #35 on: December 03, 2018, 01:09:32 PM »

Yup gotcha !
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Tennerondack

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Re: Peavey Filter Caps
« Reply #36 on: December 04, 2018, 09:01:36 PM »

I heard from PV this is what they sent.
Now that the power ampseems functional, here is the next issue.
I connect the preamp, can detect 42 volts getting to the board, but get no audio output turning the various pots, whether or not an instrument is plugged in
I have checked all physical connections and used deoxit, etc.
« Last Edit: December 05, 2018, 08:08:03 AM by Tennerondack »
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phatt

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Re: Peavey Filter Caps
« Reply #37 on: December 06, 2018, 06:51:34 AM »

Yes that Schematic looks a lot closer.
OK check for voltage at all the power supply nodes on preamp.
Power runs up the right hand side of schematic so just find those voltage drop resistors on the PCB and check you have supply on all those points.
If they all check ok then start checking if you have signal.

(I'm assuming you have limited understanding of how these things work)

Without a signal injector just use an insulated driver and touch each output while powered up.
You have 3 channels so start at the outputs and work back towards inputs.
Top ch OUT is top right at the junction 2/25 cap and 10k resistor.
if there is continuity then you will hear a crackle through the speaker.

Then work back towards input till you have no crackle.
As you may note Ch1 has 3 stages made up of two BJT pairs and input has 3 BJT.
Notice how each of those 3 stages have a Capacitor on input as well as Output.
They isolate the DC of each transistor stage from the AC signal.
You can safely touch the AC points without damaging the DC inside each pair of BJt's
So the first 3 transistors are DC (Directly Coupled, no caps between them)
but after Q3 of stage 1 you have a Decoupling Cap then the AC signal runs through the middle control before it passes into stage 2.
touching any of those AC points will make a noise if it's working.
Just work along the signal path till you find where the there is no sound.

All of this assumes the power amp is actually working. I'm not sure if that has been established yet?

If not then with preamp disconnected tap the power amp signal input and you should hear a little noise through the speaker.
Better still connect an mp3 player or any audio signal into the power stage to make sure it does deliver clean signal through the speaker. You will have to improvise a connection for that. maybe wise to put a plastic tube over the power pin for that test.

Your preamp is classic discrete design (each stage decoupled) while the power amp section is DC (Direct Coupled) notice how the power amp has no caps between each transistor.
When looking at a schematic you are really looking at 2 separate but interconnected circuits.
There is the DC path (power supply) which runs (in this case) from top to bottom, power rail on top and Common/Ground on the bottom. while the AC path (Signal you hear) runs left to right.
The basic concept is that you have to setup all the DC stuff so that the AC signal gets amplified.
If the DC stuff is wrong you get little or no signal or really bad distortion or nothing.

A simple way to understand Amplification is;
The AC signal floats on a DC voltage potential.
hope that helps a bit.
Phil.
« Last Edit: December 06, 2018, 07:03:40 AM by phatt »
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Tennerondack

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Re: Peavey Filter Caps
« Reply #38 on: December 06, 2018, 08:59:01 AM »

Thanks Phil you are correct I have limited understanding but catch on quickly.

Your help is greatly appreciated. I am gradually learning.
I am going to get a signal injector and have at it.

Many thanks.    T .
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Tennerondack

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Re: Peavey Filter Caps
« Reply #39 on: December 12, 2018, 09:57:41 AM »

Well Phil I return with big  ??? .
I finally got a few minutes. Put the power and preamp boards on top of the case, connected everything fired it up and everything worked. ?!?!
Maybe I jarred the right physical connection.
I' ll take it  :tu: Now I will have to check out sound quality, I may be back.
So far even lucking out, many thanks !
« Last Edit: December 12, 2018, 09:59:50 AM by Tennerondack »
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phatt

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Re: Peavey Filter Caps
« Reply #40 on: December 13, 2018, 04:50:35 AM »

This has all the hall marks of a mechanical fault.
Sounds like you have a loose connection, Gently wiggle jumper cables and see if you can find where the issue hides.
A cracked solder pad on the pcb can also give intermittent drop outs.

Gentle taps or pressure on the pcb with a wood kebab stick can help you find those gremlins.
Cracked solder pads or cracked tracks can be hard to see so
Close inspection in bright Sun Light as well as a magnifying glass will help sniff those issues out.

I once went bonkers trying to find an intermittent issue with a PCB and after hours of head scratching I turned the lights off and then went to turn the amp off.
As I turned off the amp with open chassis I saw a little flash of light under a power Cap.

Bingo,,, a hairline crack in the copper track hard up against the solder pad of the main filter cap.
Almost impossible to see even when looking straight at it as the crack followed the edge of the solder, a slight darker carbon burn was the only sign.

I once read a comment from a teck book that said a large part of electrical problems are in connections, whether they be a plug or solder or wire connection a large part of electrical breakdowns are in connections.
After years of fixing Muso gear I agree. 8|
Phil
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