Solid State Guitar Amp Forum | DIY Guitar Amplifiers

Solid State Amplifiers => The Newcomer's Forum => Topic started by: saturated on March 18, 2023, 04:24:46 PM

Title: are rows of knobs (pots, potentiometers) like old Christmas tree lights (series)
Post by: saturated on March 18, 2023, 04:24:46 PM
I have an amp (Peavey Audition Chorus) that used to play and intermittently cut out (silence) and a while back it cut out for good.

I was thinking about checking continuity on each volume, tone etc pot

 :)
Title: Re: are rows of knobs (pots, potentiometers) like old Christmas tree lights (series)
Post by: phatt on March 18, 2023, 05:08:58 PM
Hi and welcome,
Q does the amp have an effects loop?
If so plug your guitar directly into the RETURN socket and if you can hear sound then

bridge the Send and Return with a short lead and if that resolves the issue then the switch in the return socket is faulty.
It's a very common problem.
Do that first then tell the outcome, if no go then we look deeper.
Phil.
Title: Re: are rows of knobs (pots, potentiometers) like old Christmas tree lights (series)
Post by: g1 on March 19, 2023, 01:49:02 PM
Also try with headphones.  The phones jack usually also has a switching contact that disconnects the speaker.
Title: Re: are rows of knobs (pots, potentiometers) like old Christmas tree lights (series)
Post by: saturated on March 19, 2023, 08:37:58 PM
thanks guys I have tried a little bit of different things....trying to upload a pic of the back
I need to figure out how to do that and get back to you.
but all I have on the back is three jacks...footswitch, preamp out 1, preamp out 2


thanks again
Title: Re: are rows of knobs (pots, potentiometers) like old Christmas tree lights (series)
Post by: Tassieviking on March 19, 2023, 11:01:15 PM
This might be the schematic for it, but I think it might be for a head unit and not a combo as it has speaker jacks and no speakers on the schematic.
Is it the small combo with 2 x 6-1/2 inch speakers you have ?
I think it has 2 x 10watt output so 20watt total ?
Title: Re: are rows of knobs (pots, potentiometers) like old Christmas tree lights (series)
Post by: Tassieviking on March 20, 2023, 09:00:45 AM
Do you have another amp that does have send/return jacks?
If so, run a cable from the Peavey Preamp Out to the return jack on the other amp.
That will check if the preamps work in the peavey.
Title: Re: are rows of knobs (pots, potentiometers) like old Christmas tree lights (series)
Post by: g1 on March 20, 2023, 01:40:12 PM
Quote from: Tassieviking on March 19, 2023, 11:01:15 PMThis might be the schematic for it
Yes that is the correct schematic, this is just the combo version but electronics are same.
Title: Re: are rows of knobs (pots, potentiometers) like old Christmas tree lights (series)
Post by: saturated on March 21, 2023, 09:33:30 AM
thanks again guys
I tried running both preamp out to an old working Crate 15 combo and it was silent.
I am in this for the long haul so I am working on making a signal input like 1 khz connected to a guitar cable for testing.
then for trying to track the signal I might make an audible guitar cable plus capacitor to amp type signal tracer or maybe a cheap scope.

 thanks for the help
Title: Re: are rows of knobs (pots, potentiometers) like old Christmas tree lights (series)
Post by: Tassieviking on March 21, 2023, 11:26:49 AM
I recently bought a Signal Tracer/Injector Pen V6.3 from https://www.diyguitarpedals.com.au/shop/index.php?main_page=product_info&cPath=1&products_id=728.
I haven't tried it yet but I believe it is a signal injector and an audio amp all in one. It injects a signal on one probe and then picks it up and makes it audible on the other probe, should be easy to see where you loose the signal.
Depending on where you are you can also buy one from TH Customs who makes them.
 https://diy.thcustom.com/shop/signal-tracer-injector-pen-v6-2/
There is a Utube video of how to assemble and use it
Or just make something similar yourself.

Have you made sure that you have power going to the PCB in the amp ?
Do you have -9V on pin 4 and +9v on pin 8 of the 4558 OpAmps ?
Measure each one to ground, it should be 19v between pin 4 and pin 8.
Please check that you have the correct voltages on the PCB board.
Do any resistors look burnt ? , any capacitors swollen up ?
Remove the PCB and look at the copper tracks underneath, check all solder joints with a magnifying glass to see if they look bad, dull, etc.

I would start with a quick check of voltages followed by poking the components with a plastic pen or chopstick while strumming the strings, this might find a bad connection or solder joint even if you can't see it with the naked eye.
Remember to use something plastic to tap the components with and be very careful with any wires connected to the mains voltage.Fuse, main switch and transformer are deadly to touch, but like I was told when I was an electrical apprentice "It Only Hurts Until You Pass Out Or Die".
Cheers
Michael
Title: Re: are rows of knobs (pots, potentiometers) like old Christmas tree lights (series)
Post by: saturated on April 01, 2023, 08:12:07 PM
thanks for the homework assignment I was going to check in and say I bid on a scope earlier today but didnt win. 

well....Im off to find pin 4 and pin 8  :)
Title: Re: are rows of knobs (pots, potentiometers) like old Christmas tree lights (series)
Post by: saturated on April 01, 2023, 08:20:51 PM
also I noticed a nice wooden adjustable chassis holder in a D-Lab video I thought he had bought it but a vendor sent it to him.
I want to put something like that together sure would be nice
Title: Re: are rows of knobs (pots, potentiometers) like old Christmas tree lights (series)
Post by: saturated on April 01, 2023, 08:49:47 PM
well I googled 4558 pin out, removed the chassis and turned it on and measured the pins 4 and 8

I need to find my mini grabbers because I was slip slidin away like Paul Simon worried I was going to short something out.

anyhow
pin 4 to ground -9.36v

pin 8 to ground -0.64v

pin 4 to pin 8  -8.73v

all measurements rechecked a few times

btw my chip or whatever had these markings

731 A B
RC 4558P

with a Texas Instruments? logo
and thank God for that circle mark I guess that helped me know which way was up...TM Richard Pryor

 :)
Title: Re: are rows of knobs (pots, potentiometers) like old Christmas tree lights (series)
Post by: saturated on April 01, 2023, 09:07:28 PM
sorry forgot to add...the speakers were disconnected during the testing...dunno if that matters
Title: Re: are rows of knobs (pots, potentiometers) like old Christmas tree lights (series)
Post by: phatt on April 01, 2023, 10:21:59 PM
Well you have no +9 volt rail.
Now go check you have +9Volts at the output of Q8 the 7808 reg chip.
The output leg is on the left (looking at the layout) it goes to C58.
If no +9V then likely the chip is fried.
Phil.
Speaker is not needed for this test.
Title: Re: are rows of knobs (pots, potentiometers) like old Christmas tree lights (series)
Post by: saturated on April 02, 2023, 09:49:16 PM
guys...im sorry
in my haste I didnt notice there were three RC4558Ps

because I was at work today and wanted to see where I had been on the schematic and look at what mr Phatt discussed and then i saw them.  now I am at home and see them on the pcb as well.
my apologies.
some good news though maybe it looks like two of them are in sockets
 :)
so let me look at the schematic and try to figure out which one I took measurements on.

thanks again
Title: Re: are rows of knobs (pots, potentiometers) like old Christmas tree lights (series)
Post by: saturated on April 02, 2023, 09:51:00 PM
also now going back I see where mr TV did say measure each one....doh  :'(
Title: Re: are rows of knobs (pots, potentiometers) like old Christmas tree lights (series)
Post by: saturated on April 02, 2023, 10:41:34 PM
update
all three 4558 have nearly identical readings
the two socketed 4558 have -9.4v on pin 4 and -0.64 on pin 8
the 4558 that is not socketed has -0.64v on pin 4 and -9.4v on pin 8

all three measure about 8.8v pins 4 to 8

now moving on to 7808...the good news I think I found it...the bad news is it is underneath about a gallon of silicone

I cant get to the legs at all...I could probably lift the board out and measure that way....

thanks again
Title: Re: are rows of knobs (pots, potentiometers) like old Christmas tree lights (series)
Post by: Tassieviking on April 03, 2023, 09:26:20 AM
You can measure the output of the 7808 by measuring the voltage across
C58, it should be approximately 8.5 to 8.7 volts.
The diode going to the centre leg of the 7808 regulator increases the voltage at the output by the amount of voltage drop across the diode, about 0.5 to 0.7 volts.

You can measure the voltage going into the 7808 regulator by measuring the voltage across capacitor C57.
 C58 is just above the 7808 IC and C57 is next to the fuse on the right hand side.

You should have 17.5 volts across C57, if you don't then check the diodes CR20 and CR23. Also check the AC voltage from the transformer, red wire to red/yellow wire.
Title: Re: are rows of knobs (pots, potentiometers) like old Christmas tree lights (series)
Post by: saturated on April 04, 2023, 09:37:04 AM
yes sir thanks that is a monumental (to me  :) homework assignment but will be a great learning experience

I will report back with results
Title: Re: are rows of knobs (pots, potentiometers) like old Christmas tree lights (series)
Post by: saturated on April 07, 2023, 02:29:27 PM
ok im back...the three transformer wires...red/yellow with red on each side...I get 13.6v a/c testing each side to center

on the big c57 I get 17.6v dc

on the little c58 I get 0.646v dc

then I tracked down cr24 and got dizzy trying to remember arrows and cathodes.... bands.

I messed with it quite a while got lots of different readings...but all under 1v...time for refresher course

hth btw I will look to subscribe or contribute as gratitude for the help I have received

regards
Wesley
Title: Re: are rows of knobs (pots, potentiometers) like old Christmas tree lights (series)
Post by: phatt on April 07, 2023, 11:03:03 PM
Then Q8 (7808) is most likely stuffed.
Phil
Title: Re: are rows of knobs (pots, potentiometers) like old Christmas tree lights (series)
Post by: Tassieviking on April 08, 2023, 02:57:24 PM
I also think that the 7808 chip needs replacing.
I would check if the7808 gets hot first, just in case there is a short somewhere in the 9v circuit that brings the voltage down.

There are 3 jumpers that you can lift that feeds the 9v tracks on the PCB, if you lift them then all that is left in the circuit is C58, then lift one leg on C58 and check the voltage as well.
Its just a process of elimination in case you have to wait to get  the 7808 from a shop, you can eliminate all other problems while you wait.

Remember that the LM7808 is an 8v regulator and there should be 8v from leg 2 to leg 3 on that regulator, the output voltage is raised to somewhere between 8.4v to 9v because there is a voltage drop over CR24 that raises the voltage from 0v to near 0.4v to 1v on leg 2 of the 7808.
If you shorted CR24 then you should get 8v output from the 7808 on leg 3.

You could replace the 7808 with a 7809 if you can't get a 7808 easily, just short out CR24 and you should get 9v out.
Title: Re: are rows of knobs (pots, potentiometers) like old Christmas tree lights (series)
Post by: saturated on April 09, 2023, 02:38:06 PM
thanks guys
I havent ordered any components yet but aside from the physical dimensions is something like this ok

Linear Voltage Regulators 3 Pin 1.5A Fixed 8V Pos Vltg Reg

I want to do as much testing as possible to learn

regards
Wesley

btw Happy Easter or whatever your endeavour
Title: Re: are rows of knobs (pots, potentiometers) like old Christmas tree lights (series)
Post by: phatt on April 09, 2023, 07:17:59 PM
As TassieV mentioned replace with the more common LM7809 as the LM7808 might be harder to find. Just bridge CR24 with a wire or remove it and replace it with a wire link.
google LM7809 and check the pinout is the same as the 7808,, should be the same but wise to check.
Phil.
Title: Re: are rows of knobs (pots, potentiometers) like old Christmas tree lights (series)
Post by: g1 on April 09, 2023, 10:29:30 PM
I suppose a 7805 and a 3.9V zener would work also. :)
Title: Re: are rows of knobs (pots, potentiometers) like old Christmas tree lights (series)
Post by: Tassieviking on April 10, 2023, 12:52:08 AM
Quote from: saturated on April 09, 2023, 02:38:06 PMLinear Voltage Regulators 3 Pin 1.5A Fixed 8V Pos Vltg Reg
That sounds like a L7808, if you give a us link to it or a picture we can be sure.
Any L7808, LM7808 or KA7808 in a TO220 package should work, make sure pin 1 is input, pin 2 is ground and pin 3 is output.
As far as I know the LM78## regulators are rated at 1 Amp and the L78## regulators are rated at 1.5 Amp, all the ones I have seen are In, Ground, Out (1,2,3).

Do not get the smaller 78L08 one in the TO92 package as it is only rated at 100mA and is wired differently.
Title: Re: are rows of knobs (pots, potentiometers) like old Christmas tree lights (series)
Post by: Tassieviking on April 10, 2023, 01:15:32 AM
If you want to do as much testing as possible there are a few more tests you can do, the fact that you have 0.646v across C58 at the moment could indicate there is a shorted track or component somewhere in the circuit.
It might also be that the 7808 regulator is stuffed but CR24 is lifting the voltage to 0.646v.

I would short CR24 and remeasure the voltage across C58 to see if it changed.
 
I would remove the links I put an X on in the picture in post 21.
Lift one link at a time and then measure the voltage across C58 to see if it made any change, if you lift a link and the voltage goes up to 8-9v then there is a short somewhere that that links goes to.

There is a small chance that C58 has failed big time and is shorting out, but you can usually see if there is something wrong with a cap and they don't usually short out.

You can remove the 3 links and lift a leg on C58 and then measure the voltage on leg 3 of the 7808 regulator to make sure it is the regulator.
You would measure from leg 3 to ground.

I still think Phil is right and the 7808 needs replacing, but doing more tests can not hurt, all you can do is gain more experience.(and have more fun doing it).
Mick
Title: Re: are rows of knobs (pots, potentiometers) like old Christmas tree lights (series)
Post by: saturated on April 10, 2023, 10:18:04 PM
well guys I kinda inadvertantly jumped the gun.  I wanted to remove the "silicon" blob encompassing 7808 only to discover it was armor piercing resistant hot glue.  so I resorted to some medieval methods ie soldering iron and picks etc to get all of that crap off and out of there.
meanwhile I wanted to short that diode and measure but it was getting late and it was so tiny I was trying to think of how I could achieve it....then I was wanting to lift those links as advised so I knew I would have to turn the board over...which I did...and then that poor 7808 had suffered so much abuse I just desoldered it and put the board back like it was. 
go figure in other places the hot glue just peeled right off.

anyhow the marking on the device is

"MC (MC logo)

7808CT
QQ8606A"

thanks for your help
Title: Re: are rows of knobs (pots, potentiometers) like old Christmas tree lights (series)
Post by: g1 on April 11, 2023, 02:18:29 PM
While you have the regulator removed, measure resistance across C58.
UA7808CKCT are cheap and readily available at places like mouser or digikey.
Title: Re: are rows of knobs (pots, potentiometers) like old Christmas tree lights (series)
Post by: saturated on April 11, 2023, 08:45:27 PM
yes sir will def be checking that.  glad to see you recommend using mouser or digikey....was tempted to go elsewhere.
if there is anything else I should get from there let me know I hate to spend $7.99 on a $3 order.  maybe some of the other peavey-esque chips..just to have on hand.
I ordered a function generator today....that should make me learn the signal path
anyhow thanks for all of the help
Title: Re: are rows of knobs (pots, potentiometers) like old Christmas tree lights (series)
Post by: saturated on April 11, 2023, 09:30:54 PM
resistance across C58 about 3.5kohm
Title: Re: are rows of knobs (pots, potentiometers) like old Christmas tree lights (series)
Post by: g1 on April 12, 2023, 12:21:06 PM
Quote from: saturated on April 11, 2023, 08:45:27 PMglad to see you recommend using mouser or digikey....was tempted to go elsewhere.
if there is anything else I should get from there let me know I hate to spend $7.99 on a $3 order.
Any kind of shop supplies or tools you might need.  Solder, solder wick, heatshrink, test clips, pliers, cutters, etc.

Quote from: saturated on April 11, 2023, 09:30:54 PMresistance across C58 about 3.5kohm
No shorts or excess load on the +9V rail indicated.  Might have been that just the regulator went bad.
Title: Re: are rows of knobs (pots, potentiometers) like old Christmas tree lights (series)
Post by: saturated on April 13, 2023, 05:28:23 AM
hello friends
Im thinking I made incorrect measurements or something..maybe there is an insufficient ground...(I dont know anything...just guessing with my limited knowledge)
anyhow since 7808 seems to test ok out of circuit I will put it (or replacement) back in and test more points.
I really have a feeling this is going to be something really stupid like a deficient contact....ground...switch...jack...idk.

thanks again you guys rock !!
Title: Re: are rows of knobs (pots, potentiometers) like old Christmas tree lights (series)
Post by: Tassieviking on April 13, 2023, 09:38:55 AM
If you are not very used to reading schematics or looking at PCB overlays, especially larger ones where the tracks are very close it might be a good idea to print the layout on paper and use highlighter pens to mark out ground, +9v, -9v or any part of the circuit you are tracing.

Use your computer to crop and print the area you are working on, or maybe go to a library or somewhere and print it out at A3 size.

I always use the earth from a nearby component, in the Peavey that would be the part I coloured green, the minus side of C58 should be ok.
On a PCB with so many links you might have a bad solder joint where you might loose the earth signal going to the part you are testing.
Very often the centre tap of the transformer is connected to the earth, or 0V.
That would be the red/yellow wire from the transformer in this case.

Once you mark out the power rails and the traces you are following it becomes easier to follow the the layout.
Peavey Audition Chorous.png
Title: Re: are rows of knobs (pots, potentiometers) like old Christmas tree lights (series)
Post by: saturated on April 19, 2023, 10:05:00 AM
well guys apparently hell has frozen over because I ordered some 7808ct voltage regulators
they had different letters and stuff and i didnt understand the differences so i got a few different ones.  digi key had a $4.99 ship for light orders so I jumped on it

in honor of hell freezing over....I leave you with this

Title: Re: are rows of knobs (pots, potentiometers) like old Christmas tree lights (series)
Post by: saturated on May 01, 2023, 08:26:40 PM
well guys i have been super busy but i finally soldered in a new 7808CT

and measurements are promising

input
IMG_20230501_192012938.jpg

output

IMG_20230501_191925101.jpg
 

 :)

Title: Re: are rows of knobs (pots, potentiometers) like old Christmas tree lights (series)
Post by: saturated on May 01, 2023, 08:29:52 PM
input

IMG_20230501_192012938.jpg
Title: Re: are rows of knobs (pots, potentiometers) like old Christmas tree lights (series)
Post by: saturated on May 01, 2023, 09:26:32 PM
well you guys fixed it

IMG_20230501_202250038.jpg


YOU GUYS FIXED IT!!!!!!!!!!!!!!

SSGUITAR #1 !!!!!!!!!!!

CANT THANK YOU ENOUGH!!!!!

 :)  :)  :)  :)  :)  :)

Title: Re: are rows of knobs (pots, potentiometers) like old Christmas tree lights (series)
Post by: saturated on May 01, 2023, 10:01:34 PM
sorry to ask two more questions to the SSGUITAR GODS

(I humbly bow down to them)

1.  does speaker polarity matter?  I have blue and yellow on one and red and black on the other

2.. do I need to put a little heat sink on that part we just installed.

you guys #1!!!!
Title: Re: are rows of knobs (pots, potentiometers) like old Christmas tree lights (series)
Post by: phatt on May 02, 2023, 06:02:15 AM
Good work, Glad to hear it's working again. ;)
Re speaker, Either way is fine, it only matters when you have more than ONE speaker THEN you have to make sure the polarity is IN Phase.
When more than one speaker the sound still works and no damage is done while out of phase but you tend to notice a loss of Bass.

When I met my wife years ago she had a stereo unit and she said her friend had the exact same unit but it had much better bass so I checked and sure enough the speakers were wired out of phase,, swapped the wires on ONLY One and bingo she then heard the Bass that was missing.

Yes adding a slip on heat sink can't hurt.
Phil.