PA Amplifier MWL-5E/200 Repair Advice 1

[Legoman]

Hi,

this is my first post, so please bear with me, if this is far from perfect.

I am currently attempting repair of a PA Amplifier MWL-5E/200 and at the same time try to
at least half-way understand the background of the design.

The Amp has several pre-amplifiers / equalizers for microphone inputs, a main amplifier section (where the magic smoke has escaped)
and an output transformer to allow  connection of 50V / 70V / 100V transformer based speakers or normal 4 ohm / 8 Ohm speakers.

As no schematic has been available, I read back most of the circuit of the main amplifier board.
(I am still working on some part of the protection circuits driving the output relay.)

1.) Repair attempt

I cleaned off the burn marks, checked most components and marked up in the schematics what seems ok and what is definitely broken.

Please check / comment attached:

Schematic_Amp_Final_Stage_defects.pdf
 Schematic_Amp_Final_Stage_defects.pdf (1839.18 kB - downloaded 80 times.)
Schematic_Amp_Input_Stage_defects.pdf
 Schematic_Amp_Input_Stage_defects.pdf (102.46 kB - downloaded 62 times.)
PCB.pdf
 PCB.pdf (1626.39 kB - downloaded 52 times.)

I’d like to have your help / comment regarding the following:

- What is likely to be the cause of the damage?
What could have been the starting point?
   Prior to the damage, it has been reported, that the RCD has been set off when the Amp has    been switched on. However there are some outdoor circuits on the same RCD and the    behaviour could not be reproduced….
   Later the glass tube fuse of the Amp has blown and been replaced by some user with a    higher    rated one leading to black smoke….

- Which components would you exchange even though they seem ok?
   I ordered replacements for everything broken and the components directly next to the    burning marks.

- What other parts need checking? Could it be that the (seemingly) intact input section drove the
   rear part into desaster?

thanks a lot for all help.

If any info is missing, please let me know.

Johannes

[Audiorepair]

Replacing the blown fuse and switching on again probably did most of the damage.

[Legoman]

Thank you for your feedback.

Yes, I cannot understand why a larger size fuse has been used. The fuse usually blows for a reason and latest after a same
type fuse blows again the reason should have been found....

Anyway, what do you think regarding replacing other parts to be on the safe side? The Amp has quite some years of operation (my guess is >20 years)
and usually people recommend cap replacement. However the caps I measured seemed ok and are brand type. (Rubycon, Jamicon)
I am not sure if the Panasonic ones I have are actually better / longer life.

Currently I am repairing the burn holes in the PCB with Epoxy, then I will drill new holes for vias to be pressed in. I guess that could have been prevented
if the fuse had not been replaced...

However it's the first time I repair an Audio Amplifier, so all feedback / advise is welcome.

Johannes

[Audiorepair]

I would just replace ALL the transistors and diodes.

You cannot be sure  whether any surviving did not have huge currents put through them, or whether they were actually the cause of the initial problem.

And also someone has properly messed with it by putting one or possibly several over rated fuses in the vain hope it won't go bang again.
Or they may have gone in there and tried to meter stuff and shorted something out whilst turned on.

So all bets are off assuming anything is ok.

The capacitors may well have survived, are C20 and C32 really destroyed?




As an example, 2 out of the 4 output transistors are dead.  It's more than possible one shorted, then this put loads of current through the 2 on the other side, before one of those 2 finally melted and shorted before the other one did, finally blowing the fuse.
So which one of the 2 dead ones shorted first?

[Legoman]

Thanks a lot for your help.

C20 and C32 have been completely burned to ashes, some with the connected resistors.

I managed to find a picture of the same PCB on the internet, so I attached a screenshot of C20 / C32 on my PCB along with the picture I found.

My PCB:


Sample picture:


Am I reading correctly 221 --> 220pF on the C20?

You are absolutely right about the high currents, I have been surprised that there has not been any visible damage to the 2SA1943 / 2SC5200 current path / PCB traces. However the R54 / R53 / R76 resistors died (open circuit).

I will replace all transistors as you suggested.
I guess that even if they test ok with a low voltage/low current component tester it doesn't really prove what they will do when facing higher voltage / current / heat...

Today replacement components arrived, so the next days I will start measuring the remaining components and then start replacing.
I will keep posting.

Thanks again for your help.

[Audiorepair]

I have pulled transistors out of audio amps that test good on a component tester, but a replacement fixed the fault.

A component tester is really only good for testing for bad components, it can and does give false positives.