HH MA150 blowing mains fuses

[Melv1]

Hello everyone, this is my first post on here. Sorry for it being long winded, but I want to try to give you as much info as poss.
  I would appreciate some help/guidance for my HH MA 150. The problem is, it blows both fuses instantly. I have checked the output transistors, they are fine, and I even disconnected them from the main board in case they were shorting to the chassis some how, but fuses still blown. I am confident filter caps are good as I disconnected the power leads, (the wires from the caps to the main board) and this time the fuses didn't blow. So I assume everything upto there is fine. I then checked all components on the main board, There were many problems, most transistors were faulty,I replaced Q2,3,4,5,6,7 (  these were all faulty when checked out of board too), also many resistors were out of spec and there had been a burn out around  R 22,24,28, one of those 3 was totally burnt black but I can't remember which one. I replaced R4,13,19,20,22,24,26,27,28. I was confident the problem would now be solved, but no, the fuses still blown. I also checked all diodes and they are fine. Any help would be very much appreciated as I am now stumped.

[J M Fahey]

I used to have that schematic some 3 computers ago, it´s probably on some old hard disk.
If anybody has it, please post it here, otherwise "R4,13,19,20,22,24,26,27,28." and others mean nothing to us.

In any case start building a lamp bulb limiter, you´ll need it , search this very same Forum for it.

[Jazz P Bass]

HH MA150 schematic:

[Melv1]

 JM Fahey, you are right the numbered resistors are no use without schematic, doh sorry about that, I actually have the schem too and didn't think to put it up. Also thanks Jazz P Bass for posting it. Any ideas at all. Also yes JM Fahey I have read about the light bulb tester, I will build one, but am a little concerned that if I use it, there is still current getting to the rest of the board etc, wouldn't this still do some damage to components?

[Enzo]

That is the point of the bulb, most of the voltage is dropped across the lamp under failure conditions.  It certainly puts less current through a circuit that just plugging it into the wall.

[galaxiex]

Grogg say "lightbulb limiter goooood" 

[Melv1]

Ok, thanks Enzo and Galaxiex, and indeed Fahey, I realise how the lamp works and that there is a lot less current going through the circuit, I was just concerned that it still may burn something out after a while. However if you boys say it's ok then I'm with you all the way and will defo use the lamp limiter. I really appreciate all help given here. I have the lamp limiter and am keen and ready to go!

[Melv1]

Help!!  :loco

[Enzo]

I don't have the head to wrap around it at the moment, but do not assume it has to be a pad part, a broken trace or solder joint can be just as damaging to its function.

If it isn't the power supply, then the amp circuits are bad in some way.  If the bias circuit opens, then both polarities will turn on at once, blowing fuses.

[Melv1]

Rightio Enzo, for now then,I will check all traces and solder joints on the power supply board and wait patiently for you or other replies. Thanks for checking in.

[g1]

I am confident filter caps are good as I disconnected the power leads, (the wires from the caps to the main board) and this time the fuses didn't blow.

This leads me to the exact opposite conclusion, that one or both caps was causing the fuse to blow.

[Enzo]

I might be wrong, but I read that as the caps are in the power supply, which he disconnected from the "main board" which I interpret to be the amp circuits.  So if I am right, the caps and power supply alone will hold a fuse, but connect the amp circuits and it blows.

[Melv1]

That's right Enzo, When I refer to the "main board", I mean the board with all the transistors on, Q1 to Q7 on the schematic. So I disconnected the wires that lead from the filter caps to the board (I de-soldered them at the board). The fuses didn't blow when I did this, so I'm pretty confident everything is good to that point at least.
      G1, I am confused why you say that "leads you to the opposite conclusion", as the fuses are before the caps, so if the caps were shorted, surely the fuses would still blow?   

[Enzo]

I THINK g1 was imagining you pulling the transformer wires off the power supply.

[Melv1]

Yeah probably. Sorry if I don't use the correct terminology here and there, I am  English and guess you good people are American? Therefore I may explain things a little differently.