Hey folks,
Thanks for the help already received on this fine forum.
My latest question : I have an old Peavey 260 Standard that has a good bit of hum in the output so it seemed replacing the ancient filter caps would be a good start , (right ? ).
i am aware by search that the originals are no longer available, 5000 uf 55 volts and that some have used these higher values. Will they work ? Thanks.
May or may not be those caps. Scope the supply, is it full of ripple? Are the connections or solder to those caps good and solid?
5000uf is non-standard value, 4700uf would be the current thing to use. You want to use 6800uf instead? go ahead, it is just a guitar amp, not lab gear.
55v is also non-standard, so 63v would be the thing to use.
But look at the low voltage supplies, are they ripple free? Are the main voltages ripple free? Are the DC voltages wher they ought to be - ie none on the speaker.
Thanks for the info Enzo.
I'll go with what you suggested, I just saw the alternate value suggested in an old post. I'll give the other stuff a try as well.
Thanks !
Update, I don't have a scope but do have a decent meter. I'm reading up on how to use it to check for ripple voltage.
I did go ahead and changed out the filter caps because it was an easy first step as I learn. The old ones discharged a good zap but when the new ones went in everything quieted down a lot,
I do want to learn the test procedures too, I'll be puttering when I have the time.
This old beast sounds remarkably good with guitar, the distortion circuit is more over- drive voiced IMHO, and gets a nice grit w/o being harsh or tinny. It has a short reverb tank that sounds a little funky, but fine in my view.
It's a little thin for bass in my view, but I like a fat tubey tone. Actually I think this old beast would fit right in with the modern voiced bass amps such as class D etc. but with my Harke bass driver in front it sounds great !
" I'm reading up on how to use it (the volt meter) to check for ripple voltage. "
Easy. Peasy. 8|
Set your meter to read Volts ac.
I like that. Simple is good. :dbtu:
Well fellas, as I practiced putting filter caps In a second Peavey amp head I bought to tinker with and learn on, I made the bonehead move of getting one of the cans reversed and when I fired it up it blew the fuse.
I rewired them correctly and everything powers up fine, but the amp has no output power.
I get a very weak signal to the speakers that is very distorted .
Is it likely I just cooked the caps, or that other things got cooked as well ?
Any clues appreciated, and ridicule richly deserved
Find out. The backwards cap may well be damaged. Is it doing the job? Are both power rails up to voltage and clean?
Thanks for the push, now I shall learn. I found a schematic in another thread, I am green as a gourd.
The schematic said + - 39 volts, I see where the transformer feeds the rectifiers and the leads off the f.c.'s on the same connector ?
Where do I read the rail voltage ?
"Rail" just means power supply. SO if this has 39v rails, look for something like 35-50vDC plus side and minus side.
For future, you can get any schematic for the asking from customer service at Peavey.
It seems erratic. A couple of times, it read 48 volts on plus and minus, but then was all over the place.
On a related tangent I have this 150 watt power section from a KB 300 amplifier. Visually it seems a more modern version of the same theme.
If I can connect it to the existing preamp, maybe I would do this. It is dimensionally almost exact.
That almost sounds like a problem with your meter probes.
Could be. I also discovered one of the solder joints on the cap repair failed. I've had to steal fuses to keep other stuff going, I'll retest and be back.
Here is my update :
I got some fuses, fired things up and found I was getting plus and minus 42 v.dc on the rails, I could not detect any ac with my meter.
I have a second identical head which works fine, and I tested around and compared things just to get familiar and everything tested very similarly.
I still have the same weak distorted output.
Here's something I discovered I hope will give you a clue :
On the three pin connector that goes to the pre amp, the pa pin reads -0- the minus voltage pin less than 1volt. If I touch the - voltage pin, I get a weak but audible open ground sound in the speaker,, like when touching a live guitar cord.
I compared it to the working unit, which had + 42 v on the pa pin, and less than 1 v on the - voltage pin. There was no open ground sound when touching the - voltage pin.
"Well fellas, as I practiced putting filter caps In a second Peavey amp head I bought to tinker"
Which model amp are you now working on?
I have a second identical head which works fine, and I tested around and compared things just to get familiar and everything tested very similarly.
I still have the same weak distorted output.
This is from the post previous to yours. Sorry that wasn't clear in the one you quoted. Here is a schematic too.
Thanks for your help.
Well go check you have +/-15 vdc at that pa header block.
Also check the preamp supply is outputting same voltages, at C28 and C29.
Phil.
Wrong schematic. Waiting on PV. Meanwhile, a pic or two
Can you point me in the right direction what to test ?
Quite sure this is it.
I have no voltage to preamp pins .
I doubt the schematics posted are even close to the pictures shown.
Best I can do is ask Q's and a couple of observations;
Your Amp pictured has no opamps on the PCB poweramp board which tells us the schematic is different from the board shown.
I assume the 3 pins on the top left corner are where the preamp plugs into this board?(Please verify *Where* on the board you are taking measurements)
If so one will be preamp signal input, one circuit ground/common and the other will be power for the preamp.
If there are no opamps on the preamp PCB then likely the preamp runs on single supply which would run TO92 type BJT's or similar.
Also how many wires come from the main power Tx? (I can only make out 3?)
Phil.
Thanks Phil,
There are three wires coming from the transformers, two red , one each to the the + - rails, and one yellow to the filter caps.
The connector for the pre amp cable is on upper left hand part of board. Pins are power, ground, and signal top to bottom.
I have include a couple of new pics of that section of the board, there are two transistors I believe are what I understand you to be describing.
I have lost track of what we are fixing.
Your three wires to the preamp are indeed ground, signal, and power. Power is 40v or so? That goes onto the preamp where it is stepped down to lower voltages for the transistor circuits. If nothing is working there, see if it has lost that lower voltage.
We are fixing an early PV Standard 260, ca. 1972.
I'll get back with more, not home @ moment. Thanks Enzo.
I have zero voltage on the pa pin.
Not sure where the naming of that pin as 'pa' is coming from? Maybe it is marked on the board like that?
Anyway, I think it would be better to call it the 'preamp power supply' pin to avoid confusion.
So there is 0V coming out of the power supply/power amp board at the pin that feeds the preamp?
We need to find the correct power amp schematic for this amp.
There is probably a resistor that knocks down the main +V supply a bit before it gets sent to the preamp.
Also, in one of the pictures you can see through the board that there are ground traces that rely on the mounting screws to connect the ground to the chassis. If the boards are not mounted with the screws, they probably will not work right.
Thanks g1
I can't find a proper schematic anywhere.
I took the "pa" cue from PV schematics of later revisions.
I'll take your suggestion for "preamp power supply" thanks.
The board is labeled " 130W Power" on the solder side. I've searched high and low as well as contacting PV is and come up dry.
I will remount the board, I'm pretty sure the pin was showing 0 voltage before I removed it.
Thanks, T
It looks like that larger 2 Watt resistor at top right corner (Brown-Black-Brown-silver = 100 Ohms) is connected to the top power pin (the hollow one)
Again I'm guessing but the other side of that resistor is connected to what looks like the positive supply track which runs across the top edge.
So if you have full supply voltage on one side of that resistor and little or none on the power pin side then that resistor is likely dead.
Easy to check with power off,, read the Ohms across that resistor it should be around 100R.
Phil.
Quote from: Tennerondack on December 02, 2018, 08:52:40 AM
Thanks Phil,
There are three wires coming from the transformers, two red , one each to the the + - rails, and one yellow to the filter caps.
The connector for the pre amp cable is on upper left hand part of board. Pins are power, ground, and signal top to bottom.
I have include a couple of new pics of that section of the board, there are two transistors I believe are what I understand you to be describing.
Regards the preamp and TO92 transistors.
TO92 is the case type,, there are many types.
Your big power transistors are TO3 type.
Your preamp likely has many small signal transistors and a fair bet most will be similar types.
Just google to92 transistor and you will get a page full of many. ;)
Phil.
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 ?
The both have M ( looks like Motorola style M ) SPS 761 printed on them.
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.
+ - 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
Quote from: Tennerondack on December 03, 2018, 09:34:24 AM
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.
Yup gotcha !
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.
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.
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 .
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 !
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