Repairing a Roland Cube Keyboard 60 Amplifier

[Sephardisound]

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

[Mick Bailey]

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?

[TomCarlos]

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

[The Dude]

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.

[Sephardisound]

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.

[The Dude]

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.

[TomCarlos]

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.

[Sephardisound]

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?

[The Dude]

Can you post a pic of the board(s)? Maybe we can figure out which schematic to use.

[TomCarlos]

Take a look at this thread - maybe it has the correct schematic? https://music-electronics-forum.com/...ad.php?t=46827

[Sephardisound]

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

[TomCarlos]

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.

[J M Fahey]

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.

[Mick Bailey]

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.

[Sephardisound]

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