Rega Maia Stereo Power Amp blowing one fuse, where to start testing? (beginner)

Deaf Cat

Member
2009-01-15 10:09 pm
Hi I wonder if you may be able to point me in a direction at least to start problem solving?

Basically there are two fuses one for left ch. one for right ch. (I think) then the transformer stage the rectifier stage and smoothing caps. This little section takes up about 1/2 the amp's box, then there is a row of 4 transistors (2x Sanken 2sa1216 & 2x Sanken 2sc2922) that split the amplifier box pretty much in half, then there is the rest of the board full of little electronic components.

The smoothing caps and rectifiers are new. The fuse still blows.

I am thinking:

Remove the transistor emitters from the board, on all the transistors, and then power up.
Am I right in thinking if I do this and the fuse does not blow all the power supply section of the amplifier is okay, and it is the rest of the board that will need more investigation?

Many thanks for your time :)
DC
 
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In hard to find faults I sometimes resort to removing output transistors and feeding back VAS output into LTP.
This gives me something I can power up and test voltages on.

I did this with a fault on a Maplin 50 watt amp.
I eventually got to one transistor where voltages were wrong.
Turned out someone else had tried to fix it but had put in a pnp transistor instead of an npn. To make me finding it even harder the transistors number had been erased off it.

As others have said the light bulb in series with the amp is a good technique.
On power up the light bulb should go bright momentarily then go out.
If it stays on you have a short somewhere.
I have fixed tons of amps and its usually the output transistors going short that are the problem.

Having said that sometimes the output transistor(s) going can take out the drivers too.
A multimeter on the transistors will tell you which ones have gone most times.
 
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Deaf Cat

Member
2009-01-15 10:09 pm
Thanks for your replies much appreciated!

The 4 main transistors seem to be fine as far as I can work out
they give CE of:
0, 12MOhm, 0 12MOhm
and then reverse polarity of meter:
12MOhm, 0, 12MOhm, 0

I am just starting to work towards checking all the other little transistors in the left and right channels :-0

At least I know board is factory original ;-)
 

Deaf Cat

Member
2009-01-15 10:09 pm
Ah thank you for the tips, I have been a little unclear how the light bulb tester works, your pic explains :) but I have a fuse and that just blows might I not harm the circuit if I put a light bulb there instead :-0. hehe maybe my fuse is my replaceable light bulb as that glows bright to start with :-0 I am not sure if I would be comfortable testing the circuit when it is powered as I may accidentally touch something and short it or even short me :-0

Just trying to understand the main transistors, don't know if your able to help me understand where/what I'm thinking?
Would I be along the right lines thinking that:
Base is supplied by the power amps input signal.
Collector is supplied by the big smoothing caps,
Emitter is the input/base signal with the collectors power to give it a boost,

So if I disconnect the emitters from the board the input/base and powerboost/collector circuits will still be on line and if a fuse does not blow then there is a problem somewhere amongst all the other power signal output components on the board... yes?

The relay on the speaker outputs does not have a chance to click on as the fuse blows pretty much instantly.
When I have disconnected the supply to the left channel (that blows the fuse) so input circuits and the right output circuit are supplied the fuse does not blow but the speaker output relay still does not click on, so no speaker output.. I guess it needs left and right to work?

Yes I have asked for the schematic and have been told that any servicing or repairs have to be done by the manufacturer and no one is allowed schematics apart from the manufacturer repair man..