Roland Cube 60D Humming

[paul_g_787]

Hi all.

First time posting here. Because I am genuinely stuck repairing my amplifier. A Roland Cube 60D solid state amplifier.   

The symptoms:
There is a very loud hum coming from the amp which sounds to me like 50Hz from the mains and it is at full volume. 
There is no audible signal from the guitar.   
None of the knobs, switches or jacks change the sound or adjust the volume of the hum in any way.

So I have found the schematic diagram and started with the power supply.

I am experiencing an AC voltage on all the +DC rails but no AC on the -DC rails. I am sure this is not right and might indicate the cause of the fault.   

I have checked the bridge rectifiers using the diode test mode on my multi-meter and can confirm none of the rectifier diodes are shorted or open circuit.

I have attached the power supply schematic with the voltages I can see labelled on it. Can anyone please suggest what to check next and where this AC signal might be getting onto both of the primary +DC rails?

All voltage readings are taken with respect to the center tap ground on the transformer which is connected to mains earth.

[paul_g_787]

Please note that the 86VAC reading on the output of BD1 bridge rectifier was mis-typed as 86VDC.

Please see corrected image.

[shakalnokturn]

Your AC voltage measurements look, errr..., nuts!
What reference point did you use for your measurements?

[paul_g_787]

I thought so too. That's why I am confused.

As stated at the end of OP the reference is the center tap / mains earth.

[shakalnokturn]

Oops, sorry I missed that...

Just in case check that the point you actually set your DMM ground lead to has continuity to the transformer centre tap, frame ground and DC ground.

[paul_g_787]

Hi, yes. The multimeter ground lead was crocodile clipped to the chassis and if you check the schematic, the chassis ground is connected to mains ground, and the mains ground is connected to the centre tap of the secondary windings. So all the grounds are the same.

I will take some more readings with a friends Fluke DMM tomorrow if I can borrow it, but mine is usually fairly accurate.

My theory:
As far as I can see, the  problem is that the AC voltage is leaking onto the two positive DC rails somehow. This I think is causing the problem with the hum because it is applying AC to the positive reference of the op amps and messing them up including the power amp IC.

[shakalnokturn]

I have looked at the schematic and seen that, earth, and secondary ground should be the same.
I still think something is wrong with your measurements though and would check that reality conforms to schematic...
You can't measure 7V AC on a 3.3V regulator and have a mainly working device... In that sort of case you should see/smell smoke.

[paul_g_787]

that is what I thought which is why I am confused There is no smoky smell and the digital part of the amp seems to work fine judging by the indicator LEDs.

Unless the regulator is switching on and off to protect itself perhaps? But it seems to be rectifying the AC too by the look of the voltage drop.

Do you think that both the bridge rectifiers could be leaking AC? Or perhaps a diode or tranny on the 'mute_ac' line?

[Synthtech]

Cracked tracks or solder joints on the filter caps?

[paul_g_787]

Quite probably solder joints, although I have had a good look with a magnifying glass and there are none that I can see obviously cracked or crusty looking. Also I have tried prodding each component with a pencil to check the joints and nothing seemed to change the signal, crackle or other such noise.

I have a LCR meter but I ideally do not want to start de-soldering everything as it is all SMD and some of the power supply caps caps are under a big blob of white concrete like glue which I cannot seem to shift.

Although if it were filter caps, thinking about it, how might the AC then be getting past the bridge rectifiers?

[Renate]

You're not going crazy.
Your meter is lying to you.

Some meters use a blocking capacitor on the AC ranges, some don't.
Yours uses a diode in the positive flowing direction and some sort of filtering/averaging circuit.
Proof?

Find a clean 12 VDC supply somewhere, put your probes on it in the normal direction
On the 20 VDC range it will read 12 VDC.
On the 20 VAC range it will read 20 VAC (or something).
Reverse the probes (still on the AC range), it will read dead zero.

[shakalnokturn]

So that's the explanation... The multimeter AC "polarity" would need swapping to read the AC on positive DC voltage rails.
I wasn't aware such designs existed, are those very cheap meters usually?

[SilverSolder]

[...]  Or perhaps a diode or tranny on the 'mute_ac' line?

That seems like a pretty good guess, intuitively.  Imagine what the amplifier IC would do if its "Mute" line was jiggled at 50Hz...

I would carefully check every component along the 'mute_ac' path.  Shorted diodes, dried capacitors, that kind of thing.

[paul_g_787]

That seems like a pretty good guess, intuitively.  Imagine what the amplifier IC would do if its "Mute" line was jiggled at 50Hz...

I would carefully check every component along the 'mute_ac' path.  Shorted diodes, dried capacitors, that kind of thing.

Now that has made me think1 Yes if this part of the circuit was faulty then the mute could be turning on and off very fast, and AC will probably get amplified by the poweramp and make the horrible sound I am hearing.


I think the purpose of the circuit at the top of the previous schematic is to delay the turn on of the op amps and the power amp for protection.


Now, bear with me, thinking out loud here.
If you look at the attached analogue schematic you can see the MUTE line is being used to turn on a bunch of NPN transistors (shown in green) which short the OP amp ICs to ground. This would mute the output from those op amps.

Then there is the P_MUTE (shown in blue). This is connecting to the mute pin of the LM3886 Power Amp IC.

According to the datasheet:
Pin 8 Open or at 0V, Mute: On
Current out of Pin 8 > 0.5 mA, Mute: Off

So I think that to bring this IC to mute it is being switched by the power supply to -12V, therefore current will flow out of pin8 and turn 'on' the IC.
So the purpose of the circuit at the top of the previous schematic I think is to delay the turn on of the op amps and the power amp for protection.

[SilverSolder]

Yes, the idea is probably to delay turn-on until the power supply has stabilized,  and perhaps also to quickly mute things when power is switched off.

Some products don't have this kind of circuitry and make a bump/noise in the speaker when turned on/off.  So it looks like the Cube 60 is well designed!

[paul_g_787]

So it looks like the Cube 60 is well designed!

Apart from it is broken lol

[paul_g_787]

You're not going crazy.
Your meter is lying to you.

Some meters use a blocking capacitor on the AC ranges, some don't.
Yours uses a diode in the positive flowing direction and some sort of filtering/averaging circuit.
Proof?

Find a clean 12 VDC supply somewhere, put your probes on it in the normal direction
On the 20 VDC range it will read 12 VDC.
On the 20 VAC range it will read 20 VAC (or something).
Reverse the probes (still on the AC range), it will read dead zero.

thanks you were right. I took all the measurements again with the meter both ways round and no AC voltage.
Tried with my friend's Fluke DMM and that doesn't have this problem.

Although my £4.99 meter is only 0.1V of from his. Go figure.

[paul_g_787]

So I tested all the components in the muting circuit. No shorted diodes or trannys. Resistors and caps all read in tolerance as far as I can see. The P_MUTE output seems fine at -12v which turns on the power amp.
I am not sure why the MUTE output is the 4.5v though. This is turning on all the transistors to mute all the the op amps in the pre amp circuit.  Probably why I have no sound from the guitar.

OK so see attached the power supply output measurements taken with the Fluke DMM. All measurements reference to ground as before. 
The mains input voltage at the time of taking the measurements was 244VAC.

As you can see the transformer winding are perfectly balanced on both sides and read the same on both sides of ground.
I cannot find a datasheet for the transformer but it is a ST-11785 (Roland part code SD000469) if anyone knows what it's output is supposed to be? To me it appears to be 2x30V and 2x15V

As you can then see after the rectifier BD2 on the bottom circuit the voltages are less balanced for some reason? Both rails are shifted by approx +1v relative to the labelled voltages. 

Then after the three regulators the output voltages are pretty much perfect.

Then looking up at the voltages after BD1. The positive voltage is around 8 volts too High! These rails are used solely to run the power amp. this definitely doesn't seem right. 

[Renate]

Step 1: This thing is humming loudly. Disconnect the speaker.
Step 2: Measure the voltage on the speaker output wire.
Step 3: If it were very negative see if your mute voltages all changed to non-muted. See if the -30V supply has jumped to -40V.
Step 4: Use the headphone jack and see if something works.

Edit: Note that 30V (transformer RMS) * 1.4142 (sqrt(2) peak factor) - 0.7V (silicon diode drop) = 41.7 VDC

[paul_g_787]

Step 1: This thing is humming loudly. Disconnect the speaker.

Thanks will try those steps but shouldn't there be a load on the amplifier? I have always been told to not power up amps without a load.

[Renate]

Old tube amplifiers with output transformers might have high flyback voltages without a load.
Old transistor amplifiers can usually tolerate no load without any problem.
New IC amplifiers have plenty of protection built in.

[paul_g_787]

Old tube amplifiers with output transformers might have high flyback voltages without a load.
Old transistor amplifiers can usually tolerate no load without any problem.
New IC amplifiers have plenty of protection built in.

Well I have no idea if this amplifier does have this kind of protection or not, or if it is working correctly so I will play it safe and leave the speaker connected.

Speaker shows 8.1Ω DC resistance so appears good.

I checked the voltages at the speaker terminals using the fluke meter and there is no AC signal but there is 16.5VDC
Fluke meter:                                             -16.5VDC 0VAC
Fluke meter in reverse polarity:                  16.5VDC 0VAC
Cheapo chinese meter:                           - 16.6VDC 0VAC
Cheapo chinese meter in reverse polarity: 16.6VDC 36VAC

[Renate]

So, as I guessed the amplifier is putting out -16VDC. It shouldn't.
You're putting 2 amps (16V/8ohm) through the speaker at 16V for 32W of heating.
I would not do that. You're cooking the speaker.

Either your amp IC is blown or else the DC feedback is seriously screwed.
You're not going to be able to troubleshoot that with the speaker cooking.
You're also dragging down the -40V supply so that you can't tell what's really going on.

If you feel really heartbroken running the amp without a load, put a light load on it, maybe 50 ohms.
You'll still need better than a 5W resistor.

Then go and check the -40V supply to see if it's recovered.
Check all the pins on the LM3886.

A 12" speaker costs more than a $5 IC.

You're also dissipating another 30W of heat on the amplifier IC too. Did you notice that?

[paul_g_787]

So, as I guessed the amplifier is putting out -16VDC. It shouldn't.
You're putting 2 amps (16V/8ohm) through the speaker at 16V for 32W of heating.
I would not do that. You're cooking the speaker.

Either your amp IC is blown or else the DC feedback is seriously screwed.
You're not going to be able to troubleshoot that with the speaker cooking.
You're also dragging down the -40V supply so that you can't tell what's really going on.

If you feel really heartbroken running the amp without a load, put a light load on it, maybe 50 ohms.
You'll still need better than a 5W resistor.

Then go and check the -40V supply to see if it's recovered.
Check all the pins on the LM3886.

Would a 100Ω 50W resistor be ok to use in place of the speaker? i have one spare.

It was the +30V DC supply that was reading too high at 39.4V not the -30V supply.

Should I try disconnecting the + and - 30V wires from the power amp PCB and see what the voltage on the power supply is then?

[paul_g_787]

Also, I forgot to mention, I tried the headphone jack. It starts silent, then is starts to hiss very quietly, the hiss gets louder and louder. Then there is a small 'pop' and it is silent again until the amp is turned off. Some capacitor somewhere I would say.

Will try headphones with an input.

Although looking at the schematic, the signal goes into a ADC which is connected to the CPU, then there is a DAC which connects to the headphone / line-out op amp.

Seems a little poorly thought out to me. Analogue - > digital  -> Analogue. Especially if one wanted to hook line out to a PA system, it would be  not as good as taking analogue straight out.