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May 30, 2023, 04:42:47 AM

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are rows of knobs (pots, potentiometers) like old Christmas tree lights (series)

Started by saturated, March 18, 2023, 04:24:46 PM

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also now going back I see where mr TV did say measure each one....doh  :'(


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


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.
There are no stupid questions.
There are only stupid mistakes.


yes sir thanks that is a monumental (to me  :) homework assignment but will be a great learning experience

I will report back with results


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




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.
There are no stupid questions.
There are only stupid mistakes.


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


btw Happy Easter or whatever your endeavour


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.



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.
There are no stupid questions.
There are only stupid mistakes.


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).
There are no stupid questions.
There are only stupid mistakes.


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)


thanks for your help


While you have the regulator removed, measure resistance across C58.
UA7808CKCT are cheap and readily available at places like mouser or digikey.


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