Testing a fried Marantz SR50 - any tips?

[Spasticteapot]

I have a Marantz SR50 I got for free. It's toast, but I think I can fix it, and the output FETs are about 3$ each, anyway.

The diode rectifier bridge was blown (I replaced it with a RadioShack bridge), and while the control section seems OK, one of the channels (if used) instantly engages the protection relay.

Now, for a dumb question - what's the best way to test the individual transistors? Do you think an output transistor is blown (an easy fix), or one of the preamp transistors is blown (a hard fix)?

 

[XEAGLEKEEPER]

I would tend to be suspect of the output devices on the channel that triggers the protect circuit . Check out this site.for troubleshooting tips.
http://sound.westhost.com/troubleshooting.htm

 

[Spasticteapot]

Hmm...

I'll check the output MOSFETs. Any tips for doing so, or should I just blow 6$ and hope it works?

Finally, will my amp go "boom" if I remove the output transistors and try the headphone jack?

 

[XEAGLEKEEPER]

what is the part # on the fets? Chances are slim that you will do anymore damage with outputs removed provided you make sure that you don't leave any solder bridges ie. shorted traces after removal.

 

[Spasticteapot]

The numbers were:
A1263
C3180

Look like the A1263 is a Toshiba transistor - and hence, liable to be a fake.

 

[XEAGLEKEEPER]

Those are audio transistors not mosfets. If you have a dmm 1.)remove the transistors from the circuit . 2.)measure from base to emitter and from base to collector and from emitter to collector .3.) now reverse the test leads and make the same measurements they should measure similar to a diode. High resistance one way and low resistance with the leads reversed. If any combination reveils a zero ohms "bingo" LOL

 

[Spasticteapot]

Those are audio transistors not mosfets. If you have a dmm 1.)remove the transistors from the circuit . 2.)measure from base to emitter and from base to collector and from emitter to collector .3.) now reverse the test leads and make the same measurements they should measure similar to a diode. High resistance one way and low resistance with the leads reversed. If any combination reveils a zero ohms "bingo" LOL

D'oh!

Will do. Anyone know where I can find replacements?

 

[XEAGLEKEEPER]

You asked .

 

[XEAGLEKEEPER]

http://www.newark.com/
Newark lists them bit doesn't have them in stock. Try a search on the net.

 

[EchoWars]

Digikey sells Toshiba 2SA1962 and 2SC5242 that will work fine. Overkill for your amp, but they'll be tougher to blow up.

 

[Spasticteapot]

Digikey sells Toshiba 2SA1962 and 2SC5242 that will work fine. Overkill for your amp, but they'll be tougher to blow up.

Is there any real difference between the two?

I'm presuming I should replace all four, but is any matching necessary, or should I just cross my fingers and install?

And, finally...with beefier transistors, would the amplifier in any way benefit while driving four ohm speakers?

Also, the 2SC5242s are non-stocked items.

 

[Audiofirst]

Very interested in this...My SR-50 is also fried. I powered it up and it continually blew the mains fuse. It powers up ok without the output transistors (Hooray!) but one of the large resistors gets smoky fairly quickly. I'd really love a schematic for this unit, any help would be excellent....
Blake

 

[Audiofirst]

anyone??

 

[Audiofirst]

I fixed it, blown 2SC 1265 output transistor

 

[Burnedfingers]

I fixed it, blown 2SC 1265 output transistor

I hope you replaced ALL the outputs and not just the bad ones.

 

[Audiofirst]

Initial tests were done by disabling the good channel and swapping the power transistors across. I have a full set of new ones coming. Managed to get a SM on line, had to pay but worth it. Thank you for replying burnedfingers.

 

[Audiofirst]

Oh wow this thread is over ten tears old now - and that SR-50 is long gone having worked well. I have another one now - same issue. One channel fried. Replaced that side, ran for half an hour before another 'pop' and lights out. I think the other channel has probably taken it out. I'll replace the full set this time and see what happens. More testing to do.

 

[jaycee]

If you have damaged output transistors, you should also suspect the driver transistors before them have been damaged - quite commonly the case. Also make sure you are re-adjusting the idle current as specified in the service manual

 

[Ian Finch]

If you recently bought these replacement output transistors from a Fleabay etc. seller, its likely since another 10 years have now passed, that these obsolete style parts are now fakes and will fail at realistic power levels anyway. That's possibly why they they worked for some time until the system warmed up, then the usually undersized and weakly mounted chip fails when called on to deliver a reasonable amount of power.

It's better in the long run, to fit current style replacement output transistors in a TO3P or TO247 package. These may be better performers too but will require a new mounting bolt hole and more than a little tinkering with with leads. However, if the parts can be adapted sensibly and without damage, you can at least have a reliable amp. as a result. Otherwise, there's not much that can be done without genuine replacement parts.