HH MA150 blowing mains fuses

[g1]

Sorry I misunderstood, I thought power supply and amps were all on one board and you simply disconnected the main caps.
Carry on. 

[Melv1]

Hey no probs G1.
:tu:

[Melv1]

Bit of an update. I have checked the tracks on the board and some of the tracks have had repair work done and are a bit shabby, one even has a wire soldered to it and is soldered further down the track to make continuity. However, that same track is shorted to it's adjacent track where an attempt at repair work has been attempted. Hope everyone's following this lol.
    Anyway, one track is connected to the Collector of Q7 and the Base of the main output Q2. The other track is connected to the Ground wire terminal in which one of the filter caps connects to and the other end to R32. I'm pretty sure they are shorting to each other where the attempted repair is, as there are a few dodgy blobs of solder, but it's hard to see them touching even with a magnifying glass. How I've established a short between the two tracks is I did a resistance test between the two tracks and there is continuity. Am I correct in assuming there shouldn't be or are these connected to each other further down the line anyway? Either way I'm going to try and desolder the dodgy area to separate the tracks there. This just could be the whole problem! 

[Melv1]

Well I've done all I said above and no traces are touching each other now. So I tested it with the lamp limiter and the bulb lit up Once again I'm stumped! Can someone give me any ideas? Those two traces I was talking about in my previous reply are now getting a 33ohm resistance across them so no longer directly shorting together but are connected some how, does that seem right?

[phatt]

The other track is connected to the Ground wire terminal in which one of the filter caps connects to and the other end to R32.

Hum?? That's not what the schematic shows.
The Centre Tap of the main filter caps (which is circuit common) goes to *G* on the PCB board.
To get to R32 it has to pass through R31 first. The junction of R32 and R31 is the speaker negative.
R31 is a 5 watt OR1 (.1 Ohm) which is a *current sense* resistor (shown as S on the pcb connection)
These circuits have 2 types of feedback,, being Voltage FB and Current FB, R32 is the current FB

The speaker neg is lifted off ground (circuit common) via that CS resistor and the output sockets have to be isolated from ground. If those sockets get grounded back to circuit common in some cases it can burn out the power stage.
Others here will know far more but meantime if you can post some hi res pictures of what you have in front of you then it may speed up the process. :tu:
Phil.

[Melv1]

Thanks for your reply Phil. I will check those traces on the board again against the schematic and make sure everything is as it should be, I may have followed them wrong. Also thanks for explaining what each of those resistors do especially the R31 as I was wondering what that S stood for. I will have a good look at it in a couple of hours time.

[phatt]

When working between schematic and PCB it's easy to get lost,, It took me quite a few years to master the art and I still get lost with more complex circuits. :duh :duh :duh One trick is to turn the PCB around so you are not looking at it from the same direction all the time. this forces the brain to take a different view of the track layouts and can help avoid the same mistakes. same for Schematics turn them upside down and often you find it's not what you thought. and take a break if you get lost and the brain starts to fry with frustration. xP
Phil.

[Melv1]

Wow! Good advice Phil, I will certainly do that as I do struggle and get lost at the mo. I do have an NC in electronics, but that is only the basics really. But I do have a massive appetite to learn. Infact most of my learning has came from books and the internet, so I will certainly take on board what you've said. Thanks.
PS. I will try toput some photos on later.

[phatt]

Don't forget Rod Elliot's pages which are aimed at folks just like you who are keen to learn.

http://sound.whsites.net/index2.html

Look at project pages for some explanation of how these things work.
There are many other pages on subjects that will help you grasp how to trouble shoot amplifiers.  It's a massive site with a massive amount of info.
Rod helped me out a few times and got me out of a few jams even though I have never purchased any of his projects. :dbtu:
Phil.

[Melv1]

Thanks Phil, I will defo check that link out when I've got time.

[Melv1]

I still have the same problem folks. Any ideas at all please?

[phatt]

The reality here is that you have just shot gunned a whole lot of components without FIRST establishing what is at fault. That approach actually makes it harder to find the real issue and now you may never know the cause? :duh

Put the lamp limiter on the amp and measure some voltages.
Start by reading the 2 supply voltages (which will be lower while lamp limit is on) and measure the DC voltage at the speaker terminals?
Maybe start with a 40 to 60 Watt bulb which will glow bright but keeps it all from blowing up in an instant.
If R28 is burnt then it's likely blown Q7 and Q2.
If R22 and R24 have burnt then Q4 and Q5 may have a problem.
That whole middle section around Q4 and Q5 is the current limit circuit which is supposed to save the amp from overload but just like all electronic circuits,,, what happens if the backup system fails?

That works for intermittent shorts and overloads but NOT meant for continuous shorts. :trouble
If you do not know the history of the unit then it may have been flogged to death with a low Z speaker load and finally blew up. Sadly a lot of muso folks often have very little understanding of the limits of their equipment and as long as it SEEMS to work and makes sound it must be alright. yeah it runs really hot and blows a fuse every now and then but it keeps going :loco :loco :loco

So get some pics and report the speaker voltage.
Phil.

[Melv1]

Okey doke phil, will do.Thanks for your advice. Although I wouldn't say I've just shot gunned a load of components. I did actually use a structure as I always have when I've successfully fixed other amps. I always first check the power supply and then the signal chain. But like I say this just instantly blows fuses so I first checked the obvious and took it from there, and then discovered all the blown components listed earlier, so replaced them. I don't really see what I've done wrong there, or what I could've done differently?

[phatt]

Yes you replaced some parts which may well have been faulty but without first establishing the fault it just instantly produced the same fault and blew the new parts.
These are DC amps and if something is not right then as above,, instant destruction. :trouble
A DC (Direct Coupled) circuit means that ALL the transistors are directly coupled so if one blows then more often than not it will take out other parts,, as you noted it had at least 3 burnt resistors. You can't reset these suckers without a Variac or Limiter of some kind and bring them up to voltage and current softly.

90% of power amps all work in a similar way, they are just High current opamps.

The old Valve circuits were AC coupled,, meaning they had a DC blocking capacitor between each stage so each stage was isolated from the others. Any faulty valve was just replaced and all was good,, but DC SS Amps are a very different beast.

When working properly these SS Amps will keep the input and output as close to 0VDC as possible  hence the middle of the split supply is Zero volts DC which should also be the DC at speaker,,expect +/- 100mVDC at the speaker when working right.
When a transistor blows it often pulls the output to one of the supply rails and the output will sit at Pos or Neg rail voltage and burn out the speaker VCoil.

You have not mentioned the ua741 opamp and if one of those Zener diodes have blown it would be a major issue. They regulate pins 4 and 7 at +/-15 volts.
I'm only a hobby geek so others here will have far more experience.
meantime fire up through the limiter and measure voltages.
spk out,, supply,, and 741 pins.
Phil.

[Melv1]

You are a top man Phil, and that was some really useful info, thankyou. Yes, funny enough, most amps I've worked on successfully have been Valves, as I love the old valve amps. I'm not so clued up on ss so that is a good explanation for me. When I found those dodgy components, I presumed I had found the fault, I guessed one had failed due to just old age and then took the others out, but clearly there is another problem. I really do appreciate your help Phil, so please keep it coming. Also, sorry about the delays in some of my replies, I am rather busy at mo, but really do want to get this thing back up and running again.