Carlsboro Cobra 1100 repair.
 

I'm really struggling with this amp repair and could benefit from a schematic for it (or one similar) please.

Many thanks,

​​​​​​​Bryan

Re: Carlsboro Cobra 1100 schematic sought
 

Is it the Cobra 1100DSP (Chinese), or is it the Cobra 1100 (English)

Re: Carlsboro Cobra 1100 schematic sought
 

It's the Chinese one!!

Bryan

Re: Carlsboro Cobra 1100 schematic sought
 

Good job, I only have Chinese data. Here you go.

Kevin

Re: Carlsboro Cobra 1100 schematic sought
 

Ah no - this one is much older than that - see photo of similar one attached!

Bryan

Re: Carlsboro Cobra 1100 schematic sought
 

Contact Carlsbro? http://www.carlsbro.com/support/

Re: Carlsboro Cobra 1100 schematic sought
 

Like this one perhaps;
They are all very similar just more or less in the way of power transistors and power transformer etc.

Re: Carlsboro Cobra 1100 schematic sought
 

Thanks - Carlsboro are notorious for not replying to questions like this; they have moved on and have changed ownership several times.

Thanks for the Cobra 90 schematic - this one is very much pre-ICs though (but post valves - it's solid state.) I'll post some photos later this morning.

Thanks again,

Bryan

Re: Carlsboro Cobra 1100 schematic sought
 

OK - here we go....Here is my problem child - and the problem….!!!!

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 - see photos...

Top centre - two large black power caps are replacements (my no 1 suspects!)
Bottom left and right - two smaller blue caps are replacements (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 as you can see, no signs of any frazzled diodes.

What next??

Bryan

Re: Carlsboro Cobra 1100 schematic sought
 

Sparks and smoke on the OP suggests shorted transistor, DC on the OP, so pull & check all transistors for SC before powering up with a current limited bench supply with the big OP transitorss removed, then follow an AC signal through to the base/s of the OP Q's before re-fitting after checking emitter resistors. Doesn't look like there's many coupling caps suggesting it's DC coupled. Without a schematic I'd go though and check all diodes too, especially that bias diode on the heat sink.

Good luck, Andy.

Re: Carlsboro Cobra 1100 schematic sought
 

Thanks Andy...

Big output transistors out and tested - see photo. I'll test the others.....

I'm afraid that I'm not clever enough to send or follow an AC signal and don't know what "Q's" are!

Re: Carlsboro Cobra 1100 schematic sought
 

OK here goes…..final try before I bin it and tell them to buy a new one!!

The two big output transistors are a 2SC3280 (NPN) and a 3SA1301 (PNP)

Although both are identified as OK by my Chinese multi tester, but when I put the AVO across them, this is what I see:

NPN Base (-ve) to collector (+ve) (i.e. reversed so I would expect no conductivity) = 2.2K ohms

PNP - reversed about 0.9K ohm

See photos attached....

I’m afraid don’t know enough about transistors to know whether one or other is duff..

Small transistors pulled and tested and seem OK…

Last call for any views before I bin it!

Ta,

Bryan

Re: Carlsboro Cobra 1100 schematic sought
 

Your Chinese tester tells you that the transistor has gain, at a low voltage. When you give it the higher working voltage then it may breakdown. It’s worth checking the specs as they may have internal resistors that will give you odd results with a multimeter.
If you do replace them, or other parts, DO NOT CONNECT A SPEAKER, just check that the output is at zero, or close to, across the speaker terminals. It could still be anything, a resistor or leaking capacitor that is causing the faults.
Your picture no.2 seems to show a probably shorted transistor.

Re: Carlsboro Cobra 1100 schematic sought
 

Thanks - I certainly don't put anything other than a meter across the speaker terminals right now.....!!

Picture no 2 shows the NPN transistor with the meter across the base and emitter and the positive connected to the base. Wouldn't you expect full scale deflection on an AVO with that?

Re: Carlsboro Cobra 1100 schematic sought
 

And picture No 1 shows what happens with the same transistor when you reverse the leads...

Re: Carlsboro Cobra 1100 schematic sought
 

I test all my trannys with a DMM, wouldn't know about testing with an Avo, but I'm sure there's a Sticky thread somewhere on the forum that deals with testing trannies with both Avo and Dmm. AFAIK a shorted tranny would show low ohms between C & E as well as B maybe. A Q is shorthand for a transistor, from the way a lot of 70's Japanese amps labelled trannies; Q103, Q506 etc. Same as the old VT.

Right, you have the big OP Q's out, have you a bench power supply that can be current limited, what DC rails does it run off, +35 & -35v? With nothing connected test the PSU first on DC volts, then AC volts. What results do you get? Also do you have a scope?

Andy.

Re: Carlsboro Cobra 1100 schematic sought
 

Thanks Andy - I'm sorry that I'm not very good with the shorthand!! Although a child of the 50s (I'm 70) some of this has left me behind.... I know DMM is a Digital Multi Meter and have one of those. I looked up AFAIK - "as far as I know"!

Yes I have a bench power supply that has a current limiter knob on it. The AC input to the power board, which has the PSU and the power amp on it, is 3 pronged and measures 35V, so I guess that is the +35/0/-35 V AC. Are you suggesting running the board without the power transistors back in place? What am I looking for??

Sorry, no 'scope - never owned one and when I bought a modern one recently I didn't have a clue what I was doing!! (You may think the same with this, but I am VERY good at restoring and rebuilding record decks and older radiograms/record players - so not all is lost!)

I've found a UK supplier of a 2SC3280/2SA1301 pair for £10 and think I'll take the plunge. If it is still then kicking out 35V DC on the speaker output, I think I'll advise the church to buy a new amp.....

Many thanks for your patience -needless to say I never managed to find a schematic....!!!!!

Mods - is this thread now in the wrong place?

Bryan

Re: Carlsboro Cobra 1100 schematic sought
 

If your DMM has a diode test range you can test transistors as described in this thread:-

https://www.vintage-radio.net/forum/...ad.php?t=42194

Re: Carlsboro Cobra 1100 repair.
 

Thanks Graham - will do!

Re: Carlsboro Cobra 1100 repair.
 

I always used an AVO for checking transistor junctions, but I'm gradually getting used to using a DMM...I never use a transistor tester.

Those old solid state PA amps usually go on forever if they're not shorted out, so my very strong recommendation would be to thoroughly check and test the whole speaker installation in that church hall before even thinking about either fitting that repaired amplifier, or a new one back in the place and connecting up. I suspect a speaker wiring fault may/could have caused the original blow up, so definitely check it out just to be on the safe side.