roland cube 15 amp

[d.glover1]

problem with my roland cube 15 amp volume very low when volume control is up full what is the cause.dave

[bay4hotrod]

I'm going to add to this rather old topic if you guys don't mind since I'm dealing with the same symptom. I realize that the cube 15 isn't a highly desired amp, but I gave it to my daughter for her first and I don't know who she was impersonating (probably Kirk Hammet or one of her emo band guitarists), but she got wound up and knocked the amp off the, wellll anyway.

We got it disassembled, finding a burnt resistor on the main board (R75). Does anybody have a schematic for the Cube 15?

The only thing I've been able to find is this: http://www.ampix.org/albums/userpics...Schematics.pdf and the value assigned to R75 doesn't jibe with what little I can still see of the r-bands. According to the Schematic, R75 is a 6.8 ohm resistor- (blue/gray/gold). If I had to guess under a good magnifying glass I'd say Black/white/black - gold. Of course that's not right.
Could be blue/gray/? ; the 4th is definatley gold, whereas the 3rd most certainly does not look gold...

Anybody have a full parts list or schematic that concurs with 6.8?

Just for fun, the burnt one currenly measures 15.6

[nickb]

I'm going to add to this rather old topic if you guys don't mind since I'm dealing with the same symptom. I realize that the cube 15 isn't a highly desired amp, but I gave it to my daughter for her first and I don't know who she was impersonating (probably Kirk Hammet or one of her emo band guitarists), but she got wound up and knocked the amp off the, wellll anyway.

We got it disassembled, finding a burnt resistor on the main board (R75).[ATTACH=CONFIG]28595[/ATTACH] Does anybody have a schematic for the Cube 15?

The only thing I've been able to find is this: http://www.ampix.org/albums/userpics...Schematics.pdf and the value assigned to R75 doesn't jibe with what little I can still see of the r-bands. According to the Schematic, R75 is a 6.8 ohm resistor- (blue/gray/gold). If I had to guess under a good magnifying glass I'd say Black/white/black - gold. Of course that's not right.
Could be blue/gray/? ; the 4th is definatley gold, whereas the 3rd most certainly does not look gold...

Anybody have a full parts list or schematic that concurs with 6.8?

Just for fun, the burnt one currenly measures 15.6

6.8 ohms is the correct value. It's chosen to match the speaker's DC resistance. Whilst you should replace it, be sure to use a metal oxide flameproof type, I'll wager it's not the real issue. To honest just about anything could have happened in a big fall like that, momentary short to chassis, cracked PCB traces to name a couple of things even a damaged speaker. The going could get tough.

The useful easy tests you can do are:
Start by disconnecting the speaker for now, just to prevent further damage.
Next check the power supply voltages on C75 (approx +20V), C90 (approx -20V) , C86 (+8v) and C87 (-8V).
That being OK, check the DC voltage in pin 4 of IC6 - it should be zero.
Wire the speaker up to another amp and test it.
You could also measure pins 1 and 7 of all the NJM072 chips and check they are all zero.

[g1]

To add to the above, it's extremely difficult to guess what the colours were on a burnt resistor. However, resistors always burn to a higher value, so you know it was less than 15 ohms, so no problem believing it was 6.8 stock.
That resistor is part of the zobel network and doesn't really have to be there for the amp to run. It just smooths out the impedance that the amplifier is "seeing" at the speaker. It may have been burnt years ago, you would probably not notice. It could be burnt by feedback, or squealing from the amp.
Usually, when an amp is pulled over by the cord, there is physical damage at the input jack. In addition to the checks nickb suggested, have a look around the input jack for damage, cracked traces, or broken solder connections.

[bay4hotrod]

Thanks for the checklist nickb, get right on that..

Thanks for the addition g-one, now that I think about it, seems the headphone output is being routed through the speaker. I'll check for damage at that point as well.

Will get back with the results; hopefully I have a proper 6.8 around here somewhere!

[g1]

Yes, there are switches in the headphone jack that must close for the speaker to connect. When the phones are plugged in the speaker is thus disconnected.
What was the actual symptom after the "trauma" ?

[bay4hotrod]

Well, the same as the original poster. Volume all the way up is audible, but you have to listen closely. If headphones are plugged in, it's acceptable sound, but obviously not at capacity.

I got as far as realizing I'm fresh out of 6.8's. Best I have is a 10 so off to the resistor store later. I haven't had time to go through Nick's checklist, but I feel like a complete MMmoron after looking
closer at the schematic [I]I[I] posted. After enlarging the thing, R75 is clearly labeled lol! Oh and I wonder if Nick meant C80 instead of C90 (C90 was laid out but was never added to the board on this model)?

Anyhow, I'll get busy tomorrow. I shouldn't think that with the speaker disconnected, power on to test without the zobel resistor will adversely affect some simple voltage tests... I wondered about the double capacitors too - I'm guessing that smooths the filter? Don't worry, you are free to patronize me (a little haha) - it's been a number of years since I was any good at circuit analysis!

I'll be back after some info gathering on this thing

[nickb]

Indeed, I didn't look at the schematic carefully enough. Look at C75, C80, C89, C91.

Anyway, I think g-one may well be on the right lines - damage to the headphone jack since you are getting some signal.

[bay4hotrod]

Hello everybody, It always drives me mad when a topic isn't solved soooooooooooooo... It seems the little amp wasn't very high on the priority list. I finally sat down this morning and replaced the R75 with a 1 watt 6.8 metal oxide and the amp came right back to life. No, it doesn't make any sense to me either. According to the print, that leg shouldn't prevent output. I went over both boards pretty thorough too, and didn't find anything amiss. What bothers me, and will just have to stay that way for the time-being is, I swear the layout on the back of the main board didn't quite match the print; I think I remember seeing that the filter wasn't stand-alone.... but that was a few (ahem) months ago, and I didn't think about it again till it was back together doh.

Thanks to you guys above for your time.......now I get to work on the AT100 (again). Sometimes I wonder if that thing is trying to piss me off or just keeping me in practice lol!