Carlsboro Cobra 110 schematic sought please!

[brymac]

I'm really struggling with this amp repair and could benefit from a schematic for (or one similar) please. I'm based in the UK, but don't hold that against me!

Many thanks,

Bryan

[J M Fahey]

Just curious, does "110" refer to supposed power or to it having a single 10" speaker?
In which case it might be quite similar to Carlsbro 45 (Cobra - Colt - whatever, that is just "a name") which IS online, I had it on a now dead hard disk.

Post a picture to know what are we talking about, manufacturers often change names and some silly details for commercial reasons but not much inside, we might find a close "brother".

Known Carlsbro SS amps come in 45 - 60 - 90 - 120 - 150 Watt flavours, never heard of "110" but of course itīs not impossible.

What is your actual problem?

[brymac]

Sorry - senior moment! It's Carlsbro Cobra 1100......... The problem…. - nearly 37 volts DC coming out of the speaker sockets! Having burnt out one vintage speaker as I tried it out on the bench, I’m not about to sacrifice another until it’s fixed!

The amp is from a local Church Hall, where my partner runs a Folk Dance Club.

Initial symptoms were loud cracking followed by a hum. I got sparks and smoke from the loudspeaker socket in situ. A component (or two) has failed causing this very destructive fault.

So far - two large black power caps are my no 1 suspects! replaced, as well as two smaller blue caps (although the originals tested OK when I got them out) I pulled one of the bridge rectifier diodes and it tested OK, so I put it back. I now have a pack of new ones, so while you are all considering this, I’ll replace them all. But no signs of any frazzled diodes.

What next??

Bryan

[pdf64]

So it’s a 4 channel mixer 100W PA amp, as per https://www.ebay.co.uk/itm/Carlsbro-...-127635-2958-0
Maybe an output device shorted.
If so, collateral damage is likely, lots of dead and walking wounded parts throughout the power amp.
Are the positive and negative supplies fuse protected? If so, have you checked them?

[brymac]

Yes - that's the one....

Both fuses popped; both replaced

[J M Fahey]

Sorry but thatīs not a Carlsbro any more.

They went bankrupt years ago, somebody bought the brand and imports cheap Korea/China mixers, that one is a most generic one, sold under 1000 different brands.

Itīs not a capacitor problem so donīt waste time and money but ablown power amp.

Post a clear picture showing it, maybe it uses something we can recognize, such as a TDA7294 or a Sanken module, in that case replacing it plus visibly burnt components will restore functionality.

If discrete transistors (what I suspect), you must troubleshoot it the normal way: measuring voltages, measuring suspect components, specially power and driver transistors but itīs an uphill battle, specially since no schematic is available.

If possible, show us a gut picture showing the power module.

I have repaired such amps with no schematic, by replacing blown power and drivervtransistors plus 3 or 4 extra parts BUT I consider myself lucky, an extra problem might have been still present somewhere else.

In a couple cases, specially if board catches fire, I plain junk the power amp and install my own, fed from same power supply, but then I have lots of them available on hand.
That said, often there is no physical space to fit a new one or itīs housed in a large PCB together with othyer modules, and I canīt separate them.

The schematics and experience we have with Carlsbro applies to the old UK made ones, not "mystery" Oriental ones.
Sorry.

[brymac]

Many thanks Juan - I am working though the output transistors - the usual 2SC3280/2SA1301 combination and may replace those to see if that fixes it.....

Bryan

[g1]

the usual 2SC3280/2SA1301 combination and may replace those to see if that fixes it.....

Bryan

They will probably just blow up if you just replace them without finding the source of the problem. Build yourself a lamp limiter to avoid blowing any of your new parts.

[The Dude]

^^^^^^Yes that, and please don't replace them just "to see if that fixes it". Test them. Troubleshoot the thing. Shot-gunning will get you know where fast.

[J M Fahey]

Meaning:

1) build a lamp limiter (as easy as modding an old desk or night table lamp, adding a safely mounted mains socket in series with 1 mains wire and using a 40W to 7W tungsten filament lamp, not a LED/CFL/etc. ) and plugging suspect amp there.

2) tun amp on, no speaker connected, all pots to zero or minimum, and measure 3 important voltages:

* +V , I expect around +40V or so.

* -V , same thing but negative.

* speaker out "hot". Expecting near zero Volts

Since we have no schematic, at least pot some power board bpicture, and label transistors with visible inscriptions they have (2Sc**** or whatever) or at least Q1 - Q2 - etc.

No need to remove anything before measuring first.

If bulb shines bright and you only measure, say, 1.5V rails, post it anyway, it IS a meaningful reading.

We continue after some answers

[brymac]

Many thanks - I have a lamp limiter, but also have my trusty variac!!!

Got the message about not just replacing and popping the two big transistors! It was more in desperation than hope!!

An eagle eyed member on the Discord forum has found some other iffy looking components on my photos (which this forum won't let me send - what am I doing wrong......a limit of 960kb??)

I'll keep you posted!

Thanks again!!

Bryan

[brymac]

I think I've cracked it!

Having pulled and tested all the large semiconductors, I pulled and tested the two L7915CV voltage regulators. One was outputting 14 volts when fed 14V DC (yes, I checked the data sheet first) the other half a volt...

New ones arriving tomorrow, together with two new o/p transistors (which I may not need...)

I'll keep you posted...!!

Bryan

[brymac]

Just been pointed out to me that one of the voltage regulators i a 7915 but the other is a 7815 - does that knock my test theory out of th park? Bryan

[Jazz P Bass]

"An eagle eyed member on the Discord forum has found some other iffy looking components on my photos (which this forum won't let me send - what am I doing wrong......a limit of 960kb??)"

Can you post a link to the Discord site so we can see the photos?

[brymac]

Sure - here it is:


https://discord.com/channels/7500354...64881220104222