I could not see the schematic, so I will post the ones I have if someone is interested.
Photos would be very nice.You cannot view this attachment.You cannot view this attachment.You cannot view this attachment.   

while I got you here I am really struggling with trying to understand how to be safe with oscilloscope grounding.
I was going to put a two prong adapter on the cord to unground it but maybe that is not a good idea.
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It has been a long time since I used my scope, and my scope is a real CRO.
I usually use one ground clamp that I attach to the 0v rail, as long as the 0v rail is grounded.

All my probes have a removable ground wire so all I am left with is the probe itself, I leave one probe with the ground wire attached.
My old "cathode ray oscilloscope" is nothing like the modern units most people have today, turn it on, have a cupper, use it.

What model are we talking about ?
I have only one Mark Bass service manual, the Little Mark 2.

I have had a good look for this schematic but i can't find one either, are you going to repair this one yourself?
I think this might be a very simple circuit as they sold very cheaply when they came out, it might not be hard to trace out.

If you can open the amp up and take photos of both sides of the PCB we might be able to trace it out for you, the more pictures the better.

I would imagine this might be a good project for people to build and also it sounds ok for a small amp, it might even be ok as a small stomp box size practice amp that you can plug into a speaker cab.

Cheers
Mick

These might have them,

https://www.esr.co.uk/components/transistors.htm

I think the BC337 would be ok as well

The biggest fault with dishwashers is that something small gets in the drain pump and jams the impeller, just remove the item and it's all good.

The second biggest fault is that someone uses normal dishwashing detergent in them and they over suds and then overflow, this causes a small float switch under the machine to cut the machine out, and usually the drain pump runs all the time.
This happens a lot when people have had a party and some good samaritan decides to load the dishwasher and start it for you.
The best way to kill the bubbles is to add some hair conditioner into the machine, it works really fast and that's what most Hotels do.

I once did a house call and found the drain pump blocked, I stuck my hand into the dirty water and I pulled a used hypodermic needle out of the drain pump. I was not impressed.
Another time I stuck my hand into the dirty water and found they had filled it up with hydrochloric acid to try to unblock the machine, that burnt.

BC184, 184B, 184C are nearly the same but their Hfe values are different.
Hfe is basically the gain factor of the transistor, you want to put in what you take out.

BC184LC has the legs in a different configuration so if you put one in the same as a BC184 is in there it will not work.
BC184 has the legs CBE and the BC184LC has the legs ECB.

You should really get the closest matching BC184 you can get for TR1 and TR2,
I used some 2N3094 transistors when I made some Marshall 12w amps.

You can still get the BC184 if you shop around, but I would stay away from Ebay and any Chinese sellers just because there are a lot of fakes around.

If you are in the USA you can try Tubes and More.
https://www.tubesandmore.com/products/transistor-bc184-general-purpose-92-case-npn

I have one original schematic, but there are several versions made so numbers might not match.

It might be you have a different style of PCB, possibly an earlier or later version of the same amplifier.
Marshall would release the same model but with several different layouts but basically the same circuit, they did however completely renumber all the components with several new revisions.

If you have the same circuit you should have 2 47nF capacitors in there, C14 and C15.
Can you find 2 47nF capacitors in there ?
Can you find a 25uF capacitor on the PCB ?, there should only be one.

If you can take some pictures of the top and bottom of the PCB I will see if I can trace your version out for you properly, the more pictures the better.
If I can see the top and the bottom and then compare where the traces go to what component it's fairly easy to do.

Cheers
Mick

It looks ok, they need a big overhaul before you would know if they could be saved.
So it's 120V AC in and you can get 0-140V AC out at 3 amps, say 400 watts.
They would be very nice on a workbench, but they would have to be enclosed with a lead for input and a power outlet for outlet voltage if you are not a qualified technician just for safety.

Me, I would salvage them and use them, but I know whats involved having been a licensed electrician since 1980.

The main worry I can see is rust in any steel parts as that might have caused an internal short in the windings, all the rest can be cleaned and inspected very gently.

It might be cheaper to buy  a new unit with the time and effort (and money) it will take to restore them, but I still would.

Mild soap and a toothbrush would be my starting point, some WD40 to loosen up any seized nuts and bolts and slowly pull them apart.
Some electrical cleaner on the electric parts, rub the windings that have been bared for the carbon brush to run on with something slightly abrasive, you can get stones made for polishing up commutators in motors that are perfect for that.
(I often clean contacts with an eraser made for ink, especially old PCB's before soldering)

Reassemble and bake in an oven to remove moisture and test the insulation with a HV megger, all done.

I would say they are too far gone, but when I zoomed right into the copper windings it looks like the varnish is still good.
They look like nice rheostats that might be ok after a full restoration, but it depends on the metal having survived in the core.

If they clean up ok they could be re-dipped in varnished then tested too se if they are ok.

Rheostats are very useful for bringing up the mains voltage nice and slow when testing electronic gear.
I have a small one and a large one myself.

What are the ratings on them ?

I would guess its in the first couple of transistors in the preamp, TR1 TR2 TR4 TR5 is the most likely in my opinion.

It would be easy to see if it is in the power amp or the preamp, if you turn the gain up all the way and leave the volume pot really low do you have distortion ?
If the amp distorts at low volumes  then it is being generated in the preamp, if you get a clean signal with full gain and low volume then its most likely distorting in the power amp section.

The preamp section goes up to the volume pot, and the power amp starts on the output of the volume pot, C18 is the first component in the power amp.

Cheers
Mick

Quote from: saturated on September 12, 2023, 01:02:11 PMmade it sound like a chainsaw

That's funny, there is a PCB for a HM2 clone pedal build and they named it the "Swedish Chainsaw".
https://www.diyguitarpedals.com.au/shop/index.php?main_page=product_info&cPath=35&products_id=565

What sort of breadboard are you using ?
If you haven't got a solder-less plug in breadboard I would highly recommend you get one.
https://www.taydaelectronics.com/hardware/breadboards/830-point-solder-less-plug-in-breadboard.html

As you get further along you are going to get more and more components and gear, I would recommend you find a good value place to buy your components in bulk.
I use Tayda to get most of my supplies as they are cheap and I only build stuff for home use, if I was going to sell my gear I might buy more expensive components but I find Tayda is more then good enough for me.

I buy resistors in lots of 50, capacitors in lots of 10 or more, diodes usually 50.
I build stomp boxes and amps for family mostly so I find that amount will keep me going for a while, if I bought the components locally I would pay more for a bag of 5 resistors then lots of 50 from Tayda.

There are bound to be more good value shops around but I will stick with Tayda myself as they send the components in nice little zip-lock bags.

I usually buy a fair amount at a go because of shipping costs to make it cost effective.

Cheers
Mick
 

I have absolutely no idea myself where the distortion is produced, I would need to put a scope on the amp to work it out, but I don't have an amp.

I would guess the transistors are over-driven and possibly clipping the supply rails so the top of the sine-wave is squared off ?
It would be easy to simulate in a spice program but I have never used a spice program so I can't work that one out either.

Hopefully someone smarter then me will jump in with the answer.

The "there are no stupid questions, only stupid mistakes" was something I was told when I was an apprentice electrician by someone who got too close to 11,000v AC and had the left side of his face burnt with the ear melted off, that stuck with me ever after.
Stupid questions can be embarrassing, stupid mistakes can be deadly when playing with electricity.

I would describe the voltage regulators as an "IC", just a regular IC with 3 legs.
IC = integrated circuit meaning the chip contains an assortment of parts that perform a function.
There are many components that are contained in the same shape as regular transistors, you can get anything from resistors, diodes up to more complex circuits all inside a regular sized component shell.