40w amp repair

[shpongler]

Hi guys,
I have a 40w amp here belongs to a friend, I haven't powered it up yet as there seems to be some damaged/missing components. I can see that there is a broken resistor and missing transistor. Can anyone tell me what values these are? And if you think I need to do anything else for repair from just looking at the pics

Thanks!

[teckniqs]

Looks you've probably got a blown output transistor for the resistor and driver device to go black like that.

[shpongler]

Looks you've probably got a blown output transistor for the resistor and driver device to go black like that.

Thanks teckniqs , I'll replace both transistors if I can source them

[MiXiN]

As far as I know, the Transistor that's missing and has scorched the PCB is a 2SC1947. An MRF237 or SD1127 are good replacements and can be found much cheaper than the Mitsubishi 1947.

If you want an eBay link for a seller of cheap legit' Motorala MRF237, just ask. Be careful if you buy "willy nilly" on eBay as there's a lot of shit copies about.

When I 2SC1947 is used in this Amplifier, the input is meant to be 1W, when omitted I believe the input should be 4W maximum.

I'm unsure as to the value of the Resistor, so you'd have to refer to a schematic for that.

[shuffy]

On the face of it, it looks like the amp has been overdriven, but that said, it's hard to imagine how that 22R resistor has gone pop without there being other artefacts visible on the board; perhaps it's been connected up incorrectly.

I think the driver transistor on that board was a C1947. That's a BLW60 in the final. You can test the BLW60 in situ by disconnecting the components from the base and collector (far easier than removing the transistor) as you might not have popped it. They are getting quite scarce now and it's going to be a pain to change it. If you can't source the C1947 you might get away with SD1127 and you could also use MRF237 which would be my preference. Be aware though that there are variations between these devices both in pinouts and also in what the can's connected to!

[shuffy]

Ah, Mixin got there as I was writing. But yeah totally agree.

[MiXiN]

No worries, Shuffy.

Here's a link to the guy who sells real Motorola MRF237 Transistors (1989 dated) by the way. A really nice guy to deal with. http://m.ebay.co.uk/itm/LOW-POWER-TRANS ... 1162331319

In a 5W Stentor kit I recently built, these Transistors easily achieve 6 Watts.

[shpongler]

Thanks guys, I'll give it a go replacing the damaged components,thanks so much for your help. I'll try The MRF237 and 22r and see what happens. Hopefully the BLW60 hasn't popped.

[sinus trouble]

Pretty much summed up by the other comments!
The only thing i would add is a good bench type PSU with current limiting is a life saver in these situations!
Best of luck!

[radium98]

Parts list:

R1=5.6

R2=5.6

R3=470

R4=22

R5=33

R6=33

R7=15

R8=15

R9=330

this what you want BUT do u have a layout for it pcb layout ?
R4=22 transistor is C1947 4w 175mhz 12,5v 4amps

[MiXiN]

There must be a PCB layout for this somewhere, but if there is it's nowhere to be found on the tinterweb. Or, perhaps it was only for sale fully built & never in kit form so no need?

I have this exact PCB here, & there may be a full components list with it, but there's definitely no guidance as to where to place the parts on the board so I just set it aside.

[sinus trouble]

@ Mr Mixin
Stephen never supplied me with any layout or components lists! Just a basic setup guide!
It seems selfish of him? But he was right! As mentioned many times on this forum, Amplifiers are not for the 'Rookie' and every amplifier needs testing with expensive equipment no matter how good its build quality!