Repairing a Roland Cube Keyboard 60 Amplifier

[Mick Bailey]

PM sent.

[Mick Bailey]

Here it is;

[Sephardisound]

Great thanks for posting the pic Mick.

[Mick Bailey]

Just found this pic of a similar amp - post #5

https://music-electronics-forum.com/...ad.php?t=41927

[Sephardisound]

Ah yes this seems to be the same amp that I have looking at the PCB which is identical. Now if only I could understand what components are in the preamp then I could start looking at what to replace. I guess I could maybe replace the op amp itself?

[Mick Bailey]

I would avoid replacing anything unless you know it to be faulty. Especially if the function is unclear. Opamps can be used for amplification, or they can also be configured as a comparator and involved with switching, Looking at the surrounding components though I think it's being used as an amplifier. You could check to see if there's any significant DC on the output on pins 1&7 (with respect to ground). Another check with the opamp is to set your DMM to AC and see if there's any noise on the outputs. If there is, check the inputs for noise. A faulty opamp can either be noisy, have a DC output or both.

[Sephardisound]

hi guys, so replacing the opamp reduced the static-like noise that was coming form the amp. The only problem is the amp still doesn't have much volume. Can anyone offer me any insight there? If I insert an instrument into the effect return it's the same volume as when I turn up the volume control to max from the normal input.

I'm just stumped with what else to do. Tried to check electrolytics around the input area with an ESR meter and they seemed to be in range. Maybe electrolytic elsewhere? I just cant seem to work out what else to do

[J M Fahey]

You´ll have to inject some known voltage at the input, say 100mV 1kHz and follow it along the path

[Mick Bailey]

The main output appears to go to the headphone socket. If this is the case (it is with some Cube amps) there will be switching contacts that could have oxidized. It may be worth looking at the arrangement for the KC-100 to check the connections to your amp. You can even use clip leads to connect the speakers directly to the amps output, but if you do this make certain you don't short across the output. Another check is to measure the resistance from the wire-wrap pins to the actual cable. They can also oxidize and cause a high-resistance joint. I would ensure they're all good. Sometimes they need to be soldered up. I would measure form the tip of the pin to the destination at the other end, as pressure on the wrap during measurement can sometimes give an over-optimistic reading.

[Sephardisound]

Thanks Mick

So I tested and then completely removed the headphone socket - ie connected the PCB wire directly to the speaker itself to bypass the headphone socket arrangement. Amp still doesn't have much volume. Also checked for high resistance amongst the wire-wrap pins but all checked out normal.

Think I'm out of ideas now. I'd inject a signal and follow its path but sadly dont know how to follow the path without a schematic.

[Mick Bailey]

Where I don't have a schematic I firstly look to see if there are schematics of similar models that may offer a clue to the overall topology. Companies often have a certain way of doing things within a model range that can offer a clue to the signal path. So with Roland you may have an input buffer FET, then a preamp stage driving the tone stack, then a couple of amplifier stages that end up at the FX send. Sometimes the transistors are numbered in sequence, so Q1 is the first, Q2 second etc. What I'm looking to do initially is to visualise the amp sections. So there's the obvious pre/power amp split, and then if the preamp is the issue I look at the controls to see where I may be able to pick up the signal to see if it's getting to a volume or gain pot.

With you cube It looks to me that the tone stack is a midway point in the preamp and is likely to be a fairly conventional design where the output is picked off the treble wiper, which feeds the volume control. So I'd take a look there to see if I could establish if the signal is getting attenuated before or after the tone stack. Narrow the problem down and then trace the signal backward or forwards from a known point. The main thing is to establish that a component is faulty (or likely to be faulty) before replacing it. Unnecessary replacement can complicate fault-finding and sometimes a good original part can be replaced with a new but defective one. You also minimize the possibility of damaging the board by having a pad lift.

It's worthwhile downloading a copy of Jack Darr's book to pick up advice on the philosophy of troubleshooting.

[Sephardisound]

Hi guys, I'm back and now have a copy of the schematic which I was able to locate. If you could help me I'd be so grateful. I again cant attach anything as I appear to not have the same privileges with attachments but can send a copy of the schematic if someone could assist me?

Thanks again

[TomCarlos]

I am posting the Roland Keyboard Cube 60 schematic for Sephardisound.

[Sephardisound]

Thanks so much tom

So given my issue is a lack of volume with the amp and a background static like hiss (and given how much of a novice/idiot I am with my lack of electronics knowledge) can you guys steer me in the right direction to sort out my problem?

I’m guessing I start with the following:

1) Use the oscilloscope and start at the input socket – first by injecting a sine wave tone into it and ensuring a wave form shows on the scope’s screen.

2) Then I trace the circuit according to the schematic. I guess I follow the line of the schematic until it meets the transistors Q11 + Q13 and Q12+Q14. In terms of testing the signal flow here, I’m guessing I start with the scope probe at the base of the transistor, but do I follow the collector or the emitter as the path? Is it also possible to test transistors with the scope or do I need to pull one leg out and test with a multimeter?

If I’m wrong here can you guys let me know what other things I should check or what else I could do to diagnose the fault?
Perhaps with your expertise reading schematics you could educate me with what to look for in the diagram to ascertain the problem?

I’m sorry I’m so elementary but I’d be grateful for your help and hungry to learn.

Thanks again

[The Dude]

Are both inputs the same (hiss and low volume)? If so, the problem is likely either a power supply issue, or after the junction of R7 & C9 where the two channels join together. If you've proven the amp works and it is a preamp problem, the problem is also before C48. I think you said plugging into the main input got you decent output. If not, have you checked the speaker? Maybe try a known good speaker to make sure it's not a simple speaker issue. You seem to have the right idea on the signal tracing.