Marshall 3315 Solid state attempted repair questions - Page 2 - diyAudio
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Old 13th September 2013, 08:00 PM   #11
Tarzan is offline Tarzan  Belgium
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Location: Genk
Before placing an order for new transistors, check if they are faulty.
Google for a method to check a transistor with a multimeter.
Only then order new parts.
And while the transistors are out of the circuit, check the diodes and then the remaining resistors.
If one is showing an odd figure, desolder one side and measure again.
Note all defect components on the schematic.
The chance that R23 and R24 (0.33Ohm) are open is very real.
Replace all defect components.
Use the lamp protection circuit to "protect" the amp at switching on.
Do not attach a load and measure if there is any dc on the output.
If not; measure the current (bias) over R23 ans R24.
Both values should be very close and can be adjusted with RV1.
Switch the amp off and attach a load.
Only then switch the amp back on with the lamp protection still attached.
If you touch the inputs youshould hear some noise.
Still no dc on the outputs?
Your amp is working.
Now it's a matter to adjust the bias for a clean signal at the output and not to much current through these power transistors.

Usually only TR8 and 10 die...
Good luck
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Old 14th September 2013, 09:17 PM   #12
defec is offline defec  United States
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Tarzan, thanks for the detailed advice. I'll probably just replace the .33ohm resistors to be safe. I checked the transistors/diodes and only two are maybe off besides the outputs. I'll have to lift them to see if they are still exhibiting the same symptom. 400ohms between the base and either the collector or emitter in the reverse direction that normally gives a reading.

I'm actually excited to repair this. I'm glad that I kept it around.
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Old 29th November 2013, 07:16 AM   #13
defec is offline defec  United States
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I finally got around to replacing the outputs on this. Seems only them and the pico fuses went. Biased it to 5mv at idle and it seems to work okay now. Thanks guys for all your help.
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