Roland Cube 40 Repair advice needed

[Enzo]

Sure, go over to SSGUITAR.com forum, and in the amp discussion section at top, in the sticky area at the top of the page, one of the stickys is a book on solid state amps, written by Teemu K. (Who also posts here) He put it there for free download. I suggest you retrieve it and read it.

[rocknroller]

Just downloaded it!

[Enzo]

There is also a beginners section there near the lower end of the main menu, and there are some tutorial stickies there as well. You may or may not need them. But check them out.

[J M Fahey]

Excellent advice above.

Also search and download Jack Darr´s "Service Guitar Amplifiers" book, most of it refers to Tubes, some to SS, but many techniques are "universal" , such as measuring drifted resistors, shorted caps, etc.

[Dave H]

Still curious why it's more than double that of it's counterpart (which is also out of circuit)

So am I With the power transistors out of circuit and no load connected to the output Q10, R44, Q9, R54 are in series and therefore must have the same current. How can there be 1.8mA through R44 but 3.6mA through R54 when they are in series? Where is the extra 1.8mA through R54 coming from? The only place I can see is through a leaky bootstrap cap (C43). It's rated 25V with nearly 25V across it. I'd replace it with a 100u 35V cap.

[Jazz P Bass]

the emitter resistor failed causing the transistor fail, .

I don't buy this one.

If the emitter resistor failed, there would not be a path for the electrons to 'cause' the transistor to fail.

I would vote for the transistor failed first & that failure 'took out' the emitter resistor.

[J M Fahey]

So am I With the power transistors out of circuit and no load connected to the output Q10, R44, Q9, R54 are in series and therefore must have the same current. How can there be 1.8mA through R44 but 3.6mA through R54 when they are in series? Where is the extra 1.8mA through R54 coming from? The only place I can see is through a leaky bootstrap cap (C43). It's rated 25V with nearly 25V across it. I'd replace it with a 100u 35V cap.

Look at the full schematic instead of focusing on a small section and ignoring the rest: indeed both transistors are in series ... but each one has other elements in parallel, providing alternative paths.

In fact the correct statement is: "top and bottom circuit halves are in series, whatever current gets in through the +35V rail must come out through the -35V one"
Top and bottom half currents do NOT have R44 and R54 as the only possible path, far from it.

And even if output transistors are added, the circuit is very unsymmetrical (Darlington pair on top, Sziklai pair on bottom), so DC feedback circuit (it´s a giant Op Amp after all) will apply whatever voltage is necessary at different internal points, so *output* stays close to 0V DC.

There is another factor: IF you have an offset voltage (and you will, even if as low as 40/50 mV) AND the speaker is connected, it will "steal" some current through it.
Calculate , say, 40mV into 6.5 ohms (speaker DC resistance): 6 mA which come from the +35V rail and never reach the -35V one.

Believe me, I´m quite the Obsessive Compulsive (which in this Profession is an asset) , measured such inconsistencies, worried about them, and digged digged digged until I found the cause

[Dave H]

Look at the full schematic instead of focusing on a small section and ignoring the rest: indeed both transistors are in series ... but each one has other elements in parallel, providing alternative paths.

In fact the correct statement is: "top and bottom circuit halves are in series, whatever current gets in through the +35V rail must come out through the -35V one"
Top and bottom half currents do NOT have R44 and R54 as the only possible path, far from it.

I knew I was going to get it for that post

But what alternative paths are there for the R44, R54 current Juan? The power transistors are out, the speaker is disconnected so the (R44, R54) current must go through Q10, R44, Q9, R54 but R44 has 1.8mA and R54 3.6mA. Where is that extra 1.8mA coming from? If anything R44 should have more current than R54 because R44 has Q10 B+C current, R54 only has Q9 C current. We are looking for a 100% difference in current here so it can't be Q9, Q10 base current or component tolerance. The only other path is the 47k feedback resistor and there can't be 1.8mA flowing through there or it would drop 84V

[J M Fahey]

Are you sure the speaker is disconnected?
Or was disconnected when that measurement was taken?