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[Rabbs]

I am working on a Marshall G 50 CD, the output IC (IC6) is blown and unreadable for part # and R65 is burned, is someone can give me part #s or a schematic for this unit It would help me a bunch.
Thanks
Rabbs

[teemuk]

The output IC is LM3886. R65 (if in series with power indicator LED) is 4.7 kilo-ohms. Schematic can be found from:
http://www.about-guitar-amps.com/free_guitar_amplifier_schematics.html
You need to register to that site to access the pages but it is free.

[Rabbs]

Thanks for the info. I'll see if I can get the schematics form there.
Again
Thanks

[illpanteraa]

 hey  there i know this is an old thread but i too need some help in this series of amplifier what  i have is the g 100 r cd head and it has some burnt out resistors  the R94 ,R123 ,and  R90  now these are all close to the heat sink  i  think i can be able to replce these my self  if some one could point me in the right direction on the values  when i buy the parts  i do have  a pic of the board if needed     this is a head i picked up cheap so i can experiment  on it http://i80.photobucket.com/albums/j163/illpantera/sdfgdfggfgfgfgfgfgff.jpg

[J M Fahey]

Hi illpantera.
I guess not only the resistors, but also some or all of the transistors will need replacement.
Start by searching for series lamp or bulb limiter here and build one, and the schematic, or at least post the list of the transistors mounted there.

[illpanteraa]

yes i will try this as for the transistors  go     the 5 on the heat sink are as follows (C35 19a  97p)     (K D525  Y 940)      ( K  D1415 927)
  (K B595  Y 935)    (A1386A 98P) these are from left to right i appreciate the quick response  as u can see on the picture of the heat sink there is some burning of the second transistor http://i80.photobucket.com/albums/j163/illpantera/104_0373.jpg                                                                        http://i80.photobucket.com/albums/j163/illpantera/104_0377.jpg          the bulb limiter will be made tonight

[illpanteraa]

well after tonns of research  i found the schematic  got the relpace ment  of thr resistors   and will be replacing all 5 transistors  on the heat sink  now my only delima is finding the  which way the resistors arte placed on the  board they are burnt  so badly i cant see   which ways the colurs placed  is there a general rule for this

[J M Fahey]

Resistors work both ways.
Afterreplacing everything, doublecheck, measure insulation from transistor cases to ground or chassis to check any misplaved mica insulator and the like, disconnect the speaker, turn every control to "0" and only then turn amp on.
The lamp pimiter shoild blink a second, while tha capacitors charge, and dim to almost invisible, barely orange or less.
Good luck.

[phatt]

Hi can I just say,
Without trying to dampen the enthusiasuim of all the budding home fixit folk.

These Discrete components are part of a High current DC coupled Poweramp
and generally (in my limited experience) if the output devices are blown then novice replacement of such will yield instant vaporization of $cost and hours of hard work.

DC amps are not just the Output Transistors
If the amp has already taken out the power transistors then it's highly likely the *Cause of the problem* is in the *VAS* stage. (Voltage Amplifier Stage)
This commponent is often a single transistor sitting further back in the circuit and usually takes the full Rail to Rail DC voltage.

Without the knowledge and equipment needed to test for problems it's going to take a lot more than a soldering iron and enthusiasm to fix it.

I recommend a lot of research before attemting to just swap a few burntout components.
Yes you could be lucky but I've messed around with a lot of these troublesome DC power stages and I've been caught out a few times. Even with the light bulb idea if there is instability in the VAS then the moment you think it's fine and you reconnect a speaker,, bang.
  
Some of the better qualified folk may wish to comment here because my explaination may just add confusion.
Phil.

[illpanteraa]

well got to thinking  i went back and checked every transistor in the amp  and found  4  more

[phatt]

well got to thinking  i went back and checked every transistor in the amp  and found  4  more

http://sound.westhost.com/project03.htm
Have a look at the schematic on the above page as it's about as simple as it can get, Only 7 BJT's and 50/60 watts.

The single BJT (under the diodes) is the VAS.

The reason I mention it is simply because nearly all modern SS amps are based on this basic concept, So if you can develop some understanding of how this one works then you have a resonable chance of understanding the more complex circuits.

More than once I've just cut the original poweramp in amps like yours and built a new power stage, as some are hard to get parts for and often an off the shelf kit or chip amp works just as well or better.
My 2 cents worth. Phil.

[illpanteraa]

hey phatt    im gonna replace these bad transistors  then go from there and i have taken a look at your links and studied them as far as i can understand  if  what im replacing doesnt work then i will have take  larger measures and start building my own output stage  i have all the time in the world for this project  thank you for your input  pleas keep on helping me