soundstream mc245 blowing speaker fuses

[marko]

one of my channels is blowing a 3amp speaker fuse soon as you put one in, for a split second there is sound then pop! it's the end channel as per pic, apart from this channel all the other channels function correctly including the sub channel, any ideas?

nothing appears to be blown although the speaker cable loks to have got a little warm where it touches some resistors..

[1moreamp]

Blown outputs Marko

[marko]

is it unusual to have blown outputs in just one channel? i havn't stripped the amp down yet but they look a similar size to those found in m50's etc.. i bought the amp as a fully working unit.... yeah right!

i'm the same marko off pg forums unless you already didn't realise! picking up those 3 ref300's this weekend

gonna be a soundstream month i think

[1moreamp]

Originally posted by marko
is it unusual to have blown outputs in just one channel? i havn't stripped the amp down yet but they look a similar size to those found in m50's etc.. i bought the amp as a fully working unit.... yeah right!

i'm the same marko off pg forums unless you already didn't realise! picking up those 3 ref300's this weekend

gonna be a soundstream month i think

Marko who? lol lol lol and I still say blown outputs We can talk SS over at the other place if you like

The fuses are in the supply rails to the outputs, so they blow when your Darlington's are shot. I suggest replacing both for good measure. Then check your DC offset. It should be better on the repaired channel then the rest....LMK,,, C

[marko]

i posted on here as didn't know if talking ss would be taboo on there

they are so simple though and look a joy to work on, my 1st soundstream project!

[marko]

took the board out today to investigate and could see no signs of blown outputs, i'm guessing they could still be blown, there's 2 for each output so will need at least 2? is there a simple test i can do to to comfirm blown outputs?

gee i've got a lot of stuff to do now, a mate has given me another zpa to have a look at with suspect blown outputs and i just got a eq215x with blown earth tracks for rca's

[pugdogs]

Hello,

Yes, indeed you have a blown output channel as Mr. C mentioned.

The fuses are in the Negative supply rail to each channel. I'm not exactly sure why they did this but that is where they are located. The output transistors are TIP101 and TIP105's according to my schematic. (Dated 1991) This was a very common failure. There is only one P-device and one N-device per channel. (Darlington Output Transistors) Replace BOTH devices with matching date codes.

Be VERY careful when removing and replacing the main PCB! The chassis DOSE NOT have threaded mounting holes! They used sheet metal screws that can strip-out very easily if you are not careful. We lost a ton of heat sinks over the years even mounting them in production. DO NOT OVER TIGHTEN THEM! Remeber, these old amps used Mica insulators too. After your PCB installation, measure the voltage from the chassis to the output rails. You should only see a few mili-volts. It is very common to find that there many be a small piece of metal that may have come lose that will short from the Collector of one the output devices to the chassis. (heat sink)

We went to threaded heat sinks after I complained for many years before the original Reference series was released. After that...it was a no-brianer! Go figure. LOL!

Good luck to you on your repair!

pugdogs

P.S. Thanks C!

[h3ad]

marko, to confirm blown outputs, that is if they are blowing fuses, amp is protecting as soon as power is applied etc... a simple test is to get a multimeter at set to ohms, and test beween all 3 legs (left to right, middle to right and middle to left).
the reason blown outputs blow fuses is because they internaly short out. so if ANY of the legs read really low (less than 100ohm) between them, that transistor is a gonner.
transistors dont ALWAYS short when they blow, but from my experience, most do.

another thing to note, ALWAYS replace the pair, even if just the 1 is blown, and even better is to replace all outputs in the amp, because if 1 channel blew, generally the other channel(s) have seen the same abuse.

happy fixing
h3ad

[marko]

thread from the dead!! i've learnt a lot since i started this topic and fixed 3 ref300's as well!!

i did buy a continuum to fix up but got stung big time on ebay and it's nothing more than junk

@cecil, hope all is well, will send you an email shortly to see how you are

[pugdogs]

Wait a minute...

Replace ALL output devices? Not necessary. Only replace the ones on the bad channel unless you have lots of money and time to do it all.

It's up to you. Having professionally fixed countless amps for car/home/pro audio amps over the passed 25+ years, I tend to fix what is broken. A full restoration is up to you.

pugdogs