Roland Blues Cube BC-60 - v low output

[J M Fahey]

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

Well, it depends

There's schoolboys (the fresh out of the oven types):



and then there's schoolboys .... which have been around the block a few times by now



Which one will it be ?

[dave_n_s]

Juan,

Thank you.

I've now returned to the battlefield, no longer hors de combat after finally fighting off a nasty bug.

I did the the test after pulling C211 and it behaved as you suggested and then hooked up and audio source. I'm now listening to a cassette of the Kreutzer Sonata via the Blues Cube power amp and speaker. Bizarre!

That does raise questions about why nothing happened when connected to the effect return, so I'm going to start tracing backwards. However if you have a recommended next step, please let me know.

Dave

[J M Fahey]

Ok, now we use the confirmed working power amp and speaker as built in sound probe, how's that?
You solder to that point a short piece of simple single wire,( it will catch hum, no big deal), long enough (20/30 cm?) so the free end will reach anywhere in the preamp and loop jacks.

Solder a 0.1 or 0.047uF cap to the end, extend legs in opposite ways, if you want wrap it in a piece of tape, feed some music into the preamp, set all controls halfway and touch a couple places with the free capacitor leg (which now is your test probe) trying to find where audio reaches ane where it does not.

Suggested test points.
Since we will test more than the designated TP , I'll refer to them as "nodes".

* A node is whatever line (meaning a conductor) which connects 2 or more components, any leg, wire or connector pin in a node is electrically the same because they are all connected.

* No need to name all parts connected to a node, just 2 should be enough to identify it.

OK, you should have audio at nodes:

In preamp:

* R87/C42 meaning the bottom leg of R87 which touches the right leg of C42, drain of Q17, left leg of C44 and C51; as you see all those points arte connected and touching any of them with your audioprobe is the same, also just 2 define all the node.
Hope this is clear.

* C51/VR6

* wiper of VR6

* Q27/R193

and so on and on, until you reach the preamp output connector CN6, then continue on power amp in connector also CN6 until you reach C211 which you just pulled.

Somewhere along that path you lose signal.

When you find it, of course, then you start measuring, probing, looking around the suspect area.

Or you can follow standard "Internet Knowledge" practice:

"Replace the tube"
"replace all caps"
"it's the transformer, isn't it?"
* in a marathonic session lasting 72 hours non stop you replace all 1347 parts of the amp trying to play it safe

No, I guess I'd rather troubleshoot first

It can be as stupid (yet annoying and time wasting) as a poorly seated connector, go figure.

Or even worse, the footswitch or channel switching set to mute the channel that's open , with the enabled one "all on 0" ... because we are not interested in it.

Don't laugh, it happens all the time

[mozwell]

a lot of the pre amp circuitry is powered from +A volts, through rR235 a fusible resistor.
Check the + /- 36V DC are present and +A (30V ?) and -D (-16V) are also present before we go signal tracing.
Are all the connectors goings between boards fully pushed into the sockets on the board.... double check these are ok

[dave_n_s]

Thank you! Fixed

Juan and all,

Thank you ever so much.

I'm a really good system level troubleshooter, having about 30 years practice, but horribly out of currency at circuit level.

So your assistance was invaluable.


Here's the whole story as a piece of reference if anyone finds a similar problem.


Roland 90's BC60 with very faint output, no controls having any effect and nothing if plugged in via Return, but Send output was OK.

Suspected power amp or one of the effects jacks.

Following guidance, I proved the muting circuit and its FET was operating correctly.

Next step injected audio downstream of the FET (removed C211) and demonstrated power amp functional.

Juan then advised working backwards, tracing the signal from the amp input, injected into the known good section of power amp until I found a discontinuity.

That would have worked.

However with the advantage of having the thing in front of me and a known good power amp, I injected audio at the Return jack and got nothing. So I only had a small section of circuit to worry about. I'd got lucky! I then replaced C211 and the Muting FET and started tracing from the known good point, first the other side of the FET and then onto the pre-amp board and joy of joys, across the master volume pot - Silence! Re-soldered and all is well.

I've now got a very nice sounding little amp back in action and I can get rid of my VGA-7 - too big and I've no use for MIDI guitars at the moment. Anyone in Colorado want it???

Thanks again!

Dave

Ok, now we use the confirmed working power amp and speaker as built in sound probe, how's that?
You solder to that point a short piece of simple single wire,( it will catch hum, no big deal), long enough (20/30 cm?) so the free end will reach anywhere in the preamp and loop jacks.

Solder a 0.1 or 0.047uF cap to the end, extend legs in opposite ways, if you want wrap it in a piece of tape, feed some music into the preamp, set all controls halfway and touch a couple places with the free capacitor leg (which now is your test probe) trying to find where audio reaches ane where it does not.

Suggested test points.
Since we will test more than the designated TP , I'll refer to them as "nodes".

* A node is whatever line (meaning a conductor) which connects 2 or more components, any leg, wire or connector pin in a node is electrically the same because they are all connected.

* No need to name all parts connected to a node, just 2 should be enough to identify it.

OK, you should have audio at nodes:

In preamp:

* R87/C42 meaning the bottom leg of R87 which touches the right leg of C42, drain of Q17, left leg of C44 and C51; as you see all those points arte connected and touching any of them with your audioprobe is the same, also just 2 define all the node.
Hope this is clear.

* C51/VR6

* wiper of VR6

* Q27/R193

and so on and on, until you reach the preamp output connector CN6, then continue on power amp in connector also CN6 until you reach C211 which you just pulled.

Somewhere along that path you lose signal.

When you find it, of course, then you start measuring, probing, looking around the suspect area.

Or you can follow standard "Internet Knowledge" practice:

"Replace the tube"
"replace all caps"
"it's the transformer, isn't it?"
* in a marathonic session lasting 72 hours non stop you replace all 1347 parts of the amp trying to play it safe

No, I guess I'd rather troubleshoot first

It can be as stupid (yet annoying and time wasting) as a poorly seated connector, go figure.

Or even worse, the footswitch or channel switching set to mute the channel that's open , with the enabled one "all on 0" ... because we are not interested in it.

Don't laugh, it happens all the time

[J M Fahey]

Glad you found it

Plus you avoided (uselessly) replacing some 1347 parts, getting NOS tubes and having Mercury wind a custom transformer

[dave_n_s]

I should have added in my write-up of the solution that the reverb tank had an open circuit output transducer, which meant we were chasing one problem with the ability to be mislead by another, when using a tap on the reverb return connector as part of the "divide and conquer" troubleshooting principle.

It certainly pays in troubleshooting anything (aircraft systems being my career) to focus on what you know for certain and find ways of answering questions about things you don't know without resorting to swapping out parts for the heck of it.

That is why in this exercise finding something that certainly worked (the power amp) and certainly didn't (the pre-amp) set me on the course of finding this.