I was given a pair of small practice amps last week, one was a Samick SM-10.
I searched the Net for a schematic and some information,-----Nothing----.

I wanted to know what I had before I started re-assembling it, it was pulled apart.
This was the hardest circuit I have ever traced, and strangest for a budget amp.
I have traced it as well as I can for now, but I think I will do it again in the future just to see if I can find any faults in my tracing.

It has a strange setup with the Volume pot and Overdrive Pot.
I have no clue yet what the Overdrive Socket does, when I get it working I will try shorting it out to see if it kills the Overdrive I think.
Serial Number is 94060140, 1994 model ?

Here is the schematic I have traced at this moment:


 

The Schematic on https://www.schematicsunlimited.com/e/epiphone seems a little bit clearer.
Look for the "epiphone regent 2050r" file.

https://www.schematicsunlimited.com/e/epiphone/epiphone-regent-2050r-amplifier-schematic

This is a good place for finding schematics.

Ops...sorry if I raved on a bit too much in my last post about earthing.
I hope I didn't offend Enzo or anyone else here.
I guess I am a bit sensitive about safety when it comes to electricity.
I have had to revive work mates after getting zapped, not a nice thing.

Some quick reading, and playing with the calculator will help to understand maybe ?
https://www.learningaboutelectronics.com/Articles/Low-pass-filter-calculator.php#answer4
There are several calculators on this site for easy learning, just have to look hard for them.
Another good place to learn:
http://sim.okawa-denshi.jp/en/opampkeisan.htm
http://sim.okawa-denshi.jp/en/Fkeisan.htm

Quote from: Enzo on April 11, 2022, 12:54:41 PMIf double insulated, no need for earth.   Just my opinion.

I don't think I have ever seen a guitar amplifier that is in a plastic box, with plastic pot shafts and knobs.
I think any older amplifier, especially  from the last century, should be earthed if at all possible.
When an appliance is directly connected to your body (strings), there is a potential of electrocution.
Even if the appliance in question (amp) is correctly manufactured we don't know who has modified it, or used the correctly rated parts in any repairs made to it.
Not all power transformers have the right insulation between the primary windings and the secondary windings, when a transformer malfunctions it can short the mains voltage to the low voltage rails and then you end up doing a weird little dance if you are on the other end.
Thankfully it only hurts until you pass out.
I could rave on about the "handy man" installed power outlets I have come across in the last 45 years working as an electrician, how I have seen someone use a wire coat hanger as cable to connect a second power point beside the first one, or thin speaker wire,etc.
I must have found dozens of power outlets where the neutral and earth wires have been swapped, or active and neutral swapped.

Anyway, if it has a metal chassis then I earth it, if any metal you can touch is connected to the circuit (like guitars, microphones etc) then earth it.

If I was a performing artist I would carry a small power point tester with me, and I would most likely have an earth leakage protecting plug on the lead to my amp, and any other equipment that plugged into a power outlet.
At least in Australia you can get plugs that will protect against electrocution that you can fit on the end of a power lead.

Here is a crazy thought Jimi, what if you put the 5010 amp and the 5205 amp side by side, and wired the pre-amp of the 12 watt reverb to the power amp of the 30 watt ?
Go from pin 1 of IC2 of the 12 watt and go to C8 of the 30 watt.
Just to see how it would run with the 30 watt amp attached to the 12 watt pre-amp.
I think I will just make a picture up so it is easier to understand. =)

I would not be using the reverb tank you have, I think it should be a 8FB or 8EB tank, you have a working tank so measure the DC resistance of that one and work it out from there.
If you use a tank that has an input that is too low you could burn out the Op-Amp.

I personally would replace the 2 wire mains cord with a 3 wire earthed one.
You could get electrocuted if the mains transformer shorts out if there is no earth wire going to it. But that is just my feelings on the subject.

Eliott from ESP has a good article on "Care and Feeding of Spring Reverb Tanks",
https://sound-au.com/articles/reverb.htm
At the bottom of the page he has a list of part numbering of reverb tanks, Table 4.
In the table he lists the bobbin colour that match the impedance for that coil.
So if you have say a red bobbin its a F type coil with an impedance of 1475 Ohm.
I don't know if this is always the case, especially with older tanks but it might help.

This might help a little bit:

In my opinion, and it is only my opinion based on my understanding.
If you ignore the LED's,R13 and C9 would be a low pass filter, the LED's will clip the signal when it reaches the voltage to make the LED's conduct.
Look at the LED's while using the amp, full gain and the switch on overdrive and if the LED's light up its working as a clipper.
Q1 changes the gain of the op-amp, it puts R5 in parallel with R6.
If you want more clean head-room, you could add a germanium diode in series with each LED.
That would slightly increase the clipping voltage.
You could try different led's or different diodes in series with the led's to experiment until you are happy with the sound.
I think removing the LED's might not be a good idea, too much signal might get through.

Which amp model do you have ?
It appears that there might be 2 models using that circuit, the watts available depends on which power transformer is used.

This is circuit as I traced it from the pictures, very similar to the online circuit for the 5205 with a few small differences.

The C12 in parallel with R17 is actually C16

The tone controls are connected slightly different

The power rails on mine are 24 Volts, ignore this as its approx 21 Volts.
KiCad only has 15 Volt or 24 Volt symbols and I haven't made any 21 volt ones yet

Your Bridge rectifier is different number then Marshall drawing, but both are 1.5 Amp.

That's all the differences I found from the original schematic.

Michael

Quote from: Chorch Cluni on March 29, 2022, 06:39:13 PMIt's a gift from my mum, so, you know... sentimental value, I guess.

Well...a gift from mum changes things, mod it is.
Yes, add the reverb circuit to the PCB, while you are doing this you should add IC sockets to all the op amps.
Swapping op-amps will be easier, and sometimes this will change the tone or the distortion.
There are a lot of mod pages out there for the Frontman 15 if you google it.

The best mod is usually changing the speaker, also try using it with the back cover off, partly closed etc.
I have read that someone had great sucsess with a MOD speaker and 2 planks across the back, leaving a gap in the middle.
Is it possible to fit a larger speaker ? If you can go up a size it might be more efficient/higher dBA as well as nicer sounding.( you still need room for a reverb tank )
I don't know the amp at all, but if you can locate the caps in the filters maybe stick some sockets in so you can swap the caps till you find a better tone.
I mean low pass and high pass filters.
You could do the same with the resistors in the gain circuits, sockets and try till you are happy with the results.
Here is the service manual url: https://elektrotanya.com/fender_frontman-15_frontman-15r_sm.pdf/download.html
You cannot view this attachment.

The reverb tank might be harder to work out, Insulated input and earthed output, but the model number might be harder to work out exactly. The numbers people are floating around don't match what the DC resistance indicates exactly.
Fender Part No: 041397  Reverb Spring 1BB2E4A Reverb Pan
Someone measured the DC resistance as Input 28 ohms DC, and Output 229 ohms DC
That would indicate to me the input could be B or C, output B
If you mount the tank on the floor below the speaker go for a 8BB2C1B (8CB2C1B ?)Tank

A lot of people ask what mods they should do, not many answers as it depends on what you want yourself, nobody knows this.
If you ask specific questions like how to get more Bass, less treble etc. you are more likely to get more answers. People smarter than me know just what to do to to change high/low cut filters, increase/reduce gain and all that stuff.
Good luck and happy modding

That power chip (TDA2050) can drive a 4 ohm load, so you could theoretically run another 8 ohm speaker in parallel with the inbuilt speaker.
You will most probably find out the hard way that your power transformer will not be large enough, you can tell by the smoke pouring out of the amp.
If you did not melt the transformer you might blow some tracks up on the PCB if they are not large enough for the extra current if running 4 ohm loads.

But why would you even consider using the internal speaker which you said is crap, if you have a good sounding speaker cab you can plug it into ?
Why would you want to add the crap sound with a nice sound ?
(Unless you are using it to scare the crows of your veggie garden)
 Just get a nice external speaker cab, or replace the internal one with something good.

I would get a nice speaker cab, one you would want to keep for your next great sounding amp, build your gear up with quality one step at a time.

The price you might spend to install the internal reverb might be better spent on a nice reverb pedal, especially if you can make it yourself.

Here are my KiCad files for the Marshall 5010 PCB78-2

The heatsink I would use would be :https://au.element14.com/aavid-boyd/505403b00000g/heat-sink/dp/1313918?st=heatsink%20to-3
Or just make one out of aluminium yourself
The Schematic does not match the board files in KiCad as I changed it slightly to make the PCB, I removed the transformer, switch etc.
I think it is the best I can do for now.
 I might make a smaller PCB in the future with the Jack sockets off the PCB, and add an effects loop and maybe reverb as well.
I will not be making this PCB myself but it is fairly close to the original.
The strange capacitors are a mixed footprint so you can use Tantalum, Electrolytic or WIMA box type for the 2,2 uF caps.
The PCB will accept Alpha 24mm pots like the original, or Alpha 16mm pots (The holes will be lower on the front panel with 16mm pots.
Michael

That is....sparkly
Those are very big glitter
I think I will use an aluminium panel and plain old gold spray paint can when I make my Marshall front panel.

This is a test panel I sprayed with the cheapest satin gold paint I could get at the hardware store