Roland Blues Cube BC-60 - v low output

[dave_n_s]

This older Blues Cube pre-amp section is in good shape, all working via effects send (no headphone out on this older one), but a barely detectable output via the speaker.

There isn't a steady DC voltage on the speaker, but on power up the cone does one cycle, finishing about where it started, making a healthy thump in the process.

I'd appreciate any troubleshooting steps to get me started

Thank you

[52 Bill]

Welcome to the place.

This older Blues Cube pre-amp section is in good shape, all working via effects send (no headphone out on this older one), but a barely detectable output via the speaker

And what about a signal sent back into the FX return?

[dave_n_s]

Welcome to the place.
And what about a signal sent back into the FX return?

Bill,

Apologies for not adding that as I had checked it. Connecting FX send from another amp to FX return on the Cube and connecting Cube send to return via an external cord both resulted in no output.

[52 Bill]

Bill,

Apologies for not adding that as I had checked it. Connecting FX send from another amp to FX return on the Cube and connecting Cube send to return via an external cord both resulted in no output.

Does your amp have a effect control? If you shake the amp with the reverb turned up can you hear the tank rattle through the speaker? Can you hear anything through the speaker, hiss, hum?

There is a fet mute at the input to the power amp on some models. Do you have a multimeter to check yours?

[dave_n_s]

Does your amp have a effect control? If you shake the amp with the reverb turned up can you hear the tank rattle through the speaker? Can you hear anything through the speaker, hiss, hum?

There is a fet mute at the input to the power amp on some models. Do you have a multimeter to check yours?

Hi Bill,

I appreciate your help.

It has an effect control. I'll carry out the check you propose on the reverb. I've got a decent Fluke DVM and a scope and can check the FET.

I've got the service manual and saw the FET as the first semiconductor in the output stage. What switches the FET? If that is stuck, it may be switching the FET open, perhaps? As far as I can tell from the circuit diagram, the FET is intended to mute the output until the power supply is stabilized on switch on and as soon as the amp is switched off, but I'm not an expert in circuit design.

[52 Bill]

It has an effect control. I'll carry out the check you propose on the reverb. I've got a decent Fluke DVM and a scope and can check the FET.

I've got the service manual and saw the FET as the first semiconductor in the output stage. What switches the FET? If that is stuck, it may be switching the FET open, perhaps? As far as I can tell from the circuit diagram, the FET is intended to mute the output until the power supply is stabilized on switch on and as soon as the amp is switched off, but I'm not an expert in circuit design.

Exactly, it is there to keep any noise during power up from being amplified by the power amp.

An fet normally is low resistance from source to drain. When a voltage is applied to the gate, the resistance increases. There is a positive voltage that is immediately applied to the gate that is slowly reduced as the timing circuit charges up. As a test, see what voltage you get at the gate of the fet at turn on and see if it drops out.

The effect control grounds out the FX signal or the straight signal. You could try and change its setting to see if there is any signal change.

The reverb circuit comes after the FX loop so if there is noise from the tank you know that the problem is somewhere before the reverb. If there is no tank noise then the problem is from the FX loop on to the power amp.

[J M Fahey]

I've got the service manual and saw the FET as the first semiconductor in the output stage. What switches the FET?

Please post schematic .

Otherwise it's just a guessing game

[dave_n_s]

Bill,

No noise from reverb tank other than the slightest hum if I disconnect and touch the return. The muting FET is reading 340 Ohms in circuit, powered or depowered, going open (> 1.5M briefly when power is switched on and off. That seems OK to me. Correct?

Left to my own devices, I'd now inject a signal into the FX return and try to follow it through the circuit, carefully as my transistor theory class is 30 years in the past and not used in the meantime.

Heading in the right direction or are there any short-cuts to the likely problem? I've checked the power transistors have +/-V respectively, about 40, and are not dead shorted (OV / 0.5 V on the other terminals)

Thanks again

Dave

Juan - the schematic is available on line here, to save cluttering the forum with attachments: http://www.synfo.nl/servicemanuals/R...VICE_NOTES.pdf

[g1]

The muting FET is reading 340 Ohms in circuit, powered or depowered, going open (> 1.5M briefly when power is switched on and off. That seems OK to me. Correct?

No, it should go high resistance when powered. It is either defective or not getting it's gate "turn off" voltage. You will probably find if you just remove it for a test, you will get your volume back.

[g1]

Ooops. I made that last statement before looking at the schematic. It appears this one has a series Fet, usually they are shorting the signal to ground. So in this case, removing it would not work. And it appears it should be low resistance when operating, so it seems this part of the mute circuit is good.

[52 Bill]

Left to my own devices, I'd now inject a signal into the FX return and try to follow it through the circuit, carefully as my transistor theory class is 30 years in the past and not used in the meantime.

Yes, great idea.

The fet seems to be okay and responding correctly. If the output transistor were shorted, you would be blowing fuses. And in fact the turn on thump is actually a good sign.

Keep trying to isolate the problem to smaller and smaller sections of the circuit. Inject a signal into the FX return and follow it through the rest of the amp. Because you didn't hear the reverb springs, then everything before the reverb section probably works.

If you don't have an oscilloscope, you can listen for the signal with an ac voltmeter or with an audio probe. Look for loose connections, cold solder joints, etc. In an amp this old I would first look at any electrolytic coupling caps.

[Mick Bailey]

You need to establish that the output IC is working. Is the +/- 24v present and good continuity to the power ground? Can you inject a signal into MAIN IN - even touching it with a probe should buzz. The muting circuit is controlled off the little circuit around Q12 - straight off the spec sheet for the IC. Removing Q12 will disable power amp muting if C25 is in good shape.

[g1]

There are 2 schematics in there. The 30 uses output IC, the 60 is discrete.

[Mick Bailey]

Ahhhh.... Basic Schoolboy error on my part.

[J M Fahey]

To test inside the power amp itself, pull C211 and touch with a thin screwdriver tip the node where it used to connect to R230, a convenient place may be the wire link shown in layout to left of R230 .
It should click, and if you touch the screwdriver metal with your finger you should hear hum on speaker.
If you want to double check, you can solder a piece of shielded wire to link (hot) and some nearby ground and inject audio.
Should be loud and clear through the speaker.

I think it will work fine, but let's clear the minefield before ordering the soldiers to advance.

If it works, then we can use the BC power amp as its own built in signal tracer