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Thread: Repairing a Roland Cube Keyboard 60 Amplifier

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    Repairing a Roland Cube Keyboard 60 Amplifier

    Hi guys, I'm brand new here so please go easy on me.

    I have been unable to find a schematic for this amp, although I noticed on this board a 100 watt version which appears to be a different circuit.

    My amp has very little volume and a lot of hiss but otherwise the amp works. I'm no electronics expert but I can read a schematic (if I had one) and have put together and soldered audio circuits before.

    So given my problem - the amp is noisy and has little volume - could someone steer me into the right direction with where to start looking? Ie would it possibly be the 2 main output transistors that most amplifiers have? As the amp works but just has low volume and hiss I'm guessing the output transistors may not be the problem?

    Should I be looking at the input stage and any Opamp or transistors in the beginning of the circuit first?

    Any help would be appreciated and my apologies if I'm such a novice.

    Thank you

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    Hi and welcome.

    Does your amp have an FX loop? If so, try patching it from the send to return to see if that improves the volume. Also, does it still hiss with all controls on zero?

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    Supporting Member TomCarlos's Avatar
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    In addition to Mick's suggestion, clean the pots with DeoxIT F5 (the one with a lubricant) and the jacks. Dirt or corrosion will make it harder for a signal to pass. Here is a video showing you how to do this but use the Deoxit product. The D5 is good for jacks, connectors, etc. The F5 has the lubricant - good for amp and guitar pots.

    Clean Pots Video

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    Supporting Member The Dude's Avatar
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    Agree with the above suggestions and add: Also check the supplies to the op amps pins 4 & 8 (+&- 15V-ish). If you are missing one of the supplies, you might get the symptoms described.

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    Thanks very much guys.

    After doing the tasks you've noted the following are the outcomes:

    1) Plugging a cable between fx loop doesn't change anything to the sound

    2) The noise seems to mostly go away when the treble is completely turned down. This seems to be the culprit of most of the noise, however I'm guessing as its high frequency this would be a common issue

    3) Have sprayed some D5 lubricant and clean into the puts and jacks. No change there.

    4) Pins 4 and 8 measure -14.25vdc and +14.25vdc respectively.

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    Supporting Member The Dude's Avatar
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    Here's a schematic for the Cube 60D. Is it similar to the amp you have in front of you?

    https://www.manualslib.com/manual/13...-Cube-60d.html

    I also have a schematic for a Cube 60, but it has no op amps, so I don't think that's the one.

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    Supporting Member TomCarlos's Avatar
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    Have you tried to insert a signal into the "Main In" jack? That would tell you if the Power Amp section is working properly. Conversely, you can take a cable from the "Pre Out" to another amp to hear what the signal sounds like. After that, you'd need to start looking at the signal as it travels through the amp.

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    Ok so it has a headphone out and played through headphones the same problem persists so I'm guessing the problem is in the preamp?

    Thank you The Dude, I checked through the manual you posted but the circuit appears different.

    so the problem I have ,and this may be a complete novice question is, how do I trace the signal if I dont have the right schematic? For that matter, how do repair guys fix things by tracing a signal with no schematic?

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    Supporting Member The Dude's Avatar
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    Can you post a pic of the board(s)? Maybe we can figure out which schematic to use.

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    Supporting Member TomCarlos's Avatar
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    Take a look at this thread - maybe it has the correct schematic? https://music-electronics-forum.com/...ad.php?t=46827

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    This schematic on the thread you posted TomCarlos is similar but seems to have more IC's to it and a different layout?

    I'd try to post a photo of the board if this is useful but I cant seem to work out how to attach photos to a thread?

    Thanks a lot

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    Supporting Member TomCarlos's Avatar
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    You might have a problem posting files if your account is new. So tell us if you see any numbers on the PC boards. I found that other thread by googling your amp and "music electronics forum." So there could be other past threads on this amp.

    As for tracing a signal without a schematic - on a solid state amp, that get's tricky. Before I would even try that, I would take a very close look at the pots, connectors, everything. Look for loose wires, ugly looking solder joints, anything where a mechanical connection might be failing.

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    Old Timer J M Fahey's Avatar
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    Quote Originally Posted by Sephardisound View Post
    This schematic on the thread you posted TomCarlos is similar but seems to have more IC's to it and a different layout?

    I'd try to post a photo of the board if this is useful but I cant seem to work out how to attach photos to a thread?

    Thanks a lot
    What some do is to create a small gallery at some picture hosting site, say Picasa, Flickr, perhaps Imgurl, etc., you upload all you want there, **remember to set it to "Public"** so everybody can see it, and post gallery link here.

    Problem is excellent Roland Cubes have been made for over 40 years now, always upgraded, and probably a dozen different versions by now.

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    Juan Manuel Fahey

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    Or PM me and I'll give you my email address so you can send me the pics and I'll upload them. Did you try inputting a signal as per post #7? A guitar plugged in should work ok.

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    Thanks Mick for offering to post the photo. It appears that even with sending you a PM I cant send photos. I guess one needs to earn their reputation here before getting access to certain privileges

    I did try putting a guitar into the power amp input (fx return) - it all sounded normal which may me think the problem is in the preamp. The board has markings for components but I just can't figure out what components are part of the preamp

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    PM sent.

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    Here it is;

    Click image for larger version. 

Name:	Cube60.JPG 
Views:	58 
Size:	797.5 KB 
ID:	54886

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    Great thanks for posting the pic Mick.

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

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

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    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?

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    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.

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    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

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    Last edited by Sephardisound; 08-28-2019 at 08:08 AM.

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    Old Timer J M Fahey's Avatar
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    You´ll have to inject some known voltage at the input, say 100mV 1kHz and follow it along the path

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    Juan Manuel Fahey

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    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.

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    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.

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    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.

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    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

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    Supporting Member TomCarlos's Avatar
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    I am posting the Roland Keyboard Cube 60 schematic for Sephardisound.
    Attached Files Attached Files

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    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

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    Supporting Member The Dude's Avatar
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    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.

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    Supporting Member TomCarlos's Avatar
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    Start again by taking another close look for obvious mechanical issues. Do you have any loose wires? Are the connectors seated nice and tight. Are the speaker wire connections tight? Do you have a solid connection at the terminals of the speaker? On the PC board, do you have any funky looking solder connections? Did you clean the pots and jacks with a chem cleaner? And have you tested the amp with another cable?

    Make sure you have the correct DC voltages around the amp. The schematic gives you some voltages. Look at those. Check the power supplies at C20 and C21. Check the emitter of Q101. Check the op amps (V+, Input and Output pins). Remove the speaker and check the output. You should not have any dc voltage on the output. Until you know this for sure, I would keep the speaker disconnected. Recheck just to make sure all is well.

    Be careful of the input signal. Do not overdrive the amp. Start with maybe 50mv rms... something like that. Juan suggested 100mv rms at 1Khz. I would check to see if you have signal at the Preamp out. If yes, work your way forward. If no, you go backwards.

    As for testing transistors, you can first look at the voltages on the transistors. You should have the typical .7 volt drop from the base to emitter. If you do not measure that and need to check a transistor, I would pull it. You can do a quick check with a multi-meter but I use the ATLAS DCA SEMICONDUCTOR COMPONENT TESTER AND ANALYZER. Easy to use. Watch a YouTube video for instructions.

    Start there and perhaps the others can chime in.

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