Roland Cube 60 Cosm only working through headphones

[peter762033]

Hi,

I've got a Roland cube 60 cosm that is only working through the headphones.

It was acting strange for a while before it stopped working through the speaker. When I turned the amp on sound would only come through if I turned the volume up quite a bit and then it would suddenly kick in.

I've cleaned the input socket and headphone socket with contact cleaner and that didn't help. I then desoldered the headphone socket from the pcb to double check it wasn't muting the cab in some way but that didn't help either. Also I've run the speaker through another amp so that's working ok.

Any ideas on what could be causing the speaker not to work?

Thanks,
Pete

[pdf64]

Perhaps a muting or protection circuit is being operated / won’t disengage? It seems like you done all you can, in the absence of a schematic.

[peter762033]

Yep that was my thinking but not sure where to look. I found this service manual,

https://manualzz.com/doc/7341384/rol...service-manual

It has various schematics, where would you be looking?

Thanks for your help

[52 Bill]

If the manual that you linked is correct, the headphones are driven by a separate amplifier, so the headphone jack does not cut off the speaker in a traditional sense.

The extension speaker jack is wired in series with the internal speaker, so if the normally closed contact of that jack is open, the internal speaker will be cut off. Check and clean the ext. spk. jack.

Beyond that, the power amp ic chip would be suspect.

[peter762033]

Hi Bill thanks for your reply.

Ok I'll clean and check the external speaker jack and see if that does the trick. If not is there a way of checking the chip or is it a replace and see situation?

My knowledge in this area is limited but I'm keen to learn, can you recommend some good books/resources for learning electronics?

[DrGonz78]

This thread has so many great recommendations...
https://music-electronics-forum.com/...ad.php?t=41039

I like these...
http://www.tubebooks.org/technical_books_online.htm

The Electric Guitar Amplifier Handbook, Jack Darr >>https://drive.google.com/folderview?...kk&usp=sharing

Solid State Guitar Amplifiers, Teemu Kyttala >>http://www.jeanpierrepoulin.com/PDF/transistor.pdf

[peter762033]

Thanks for the recommendations much appreciated, that'll keep me busy!

I've had a go at cleaning the external speaker jack and running a speaker through it but no luck unfortunately.

I've been looking at the power amp chip which is a lm3886tf and doing some testing.



I've tested with a multimeter on the diode setting between pin 7 (Ground) and pin 8 (Mute) and there's no reading. Infact the only pins that get a reading are pins 1 and 5 (They both read 488). Could this suggest the muting portion of the chip is faulty and causing the fault?

[52 Bill]

Before you replace the output chip, let's take a step back and see what's going on.

Have you tested all of the internal fuses? I think I counted 5 secondary fuses in the power supply.

Have you tested the speaker itself?

Have you or can you take voltage readings of the power supplies? Remember that working on a live circuit can be dangerous to you and to the equipment that you are testing. If this is beyond your skill set, then it may be time to seek outside help.

There is a wire connector that connects the power amp board to the main circuit board. Is the wire and connectors in good condition and making good contact?

[peter762033]

Good idea, yep I tested the 5 internal fuses and they are all giving good readings. I tested the speaker with another amp and it worked perfectly.

I haven't taken readings from the power supply, is there a rough guide on how to do this?

I gave all of the connecting wires a look over and all seem solidly connected.

Thanks for your help

[g1]

Most of your supply voltages must be working as the headphone out works.
Pins 1,4, & 5 on the LM3886 are what you need to check (DC volts).