Carlsbro Cobra 90 KB preamp circuit diagram

[Pitagi]

Hi, I have a Carlsbro cobra 90 KB the power stage was blown and after replacing the 2n3955 power transistors, I discovered that there was also a fault in the pre amp.

Does anybody know the value of R36 it has burnt out, I suspect the quad op has gone as well I could realy do with a diagram on the preamp. Please help if you know, thanks, Peter

[martin.sadler]

Carlsbro Cobra 90 KB preamp circuit diagram

Hi, I have a Carlsbro cobra 90 KB the power stage was blown and after replacing the 2n3955 power transistors, I discovered that there was also a fault in the pre amp.

Does anybody know the value of R36 it has burnt out, I suspect the quad op has gone as well I could realy do with a diagram on the preamp. Please help if you know, thanks, Peter

Carlsbro schematics are now almost impossible to come by. I don't have anything for the Cobra 90 but I do have a partial schematic for the Colt 45 pre-amp (which I worked out myself by simply eye-balling the circuit board - very time-consuming) - it's just possible they will have some similarities - manufacturers are not known for redesigning for the fun of it.

Please refer to attached and I hope it helps.

[J M Fahey]

Carlsbros have dissappeared. Incredible.
Everybody and his brother had one !!
I also guessthe only way is hand-drawing the schematics.
A couple *very* good and sharp pictures might help.
In such cases I take the picture component side, but put a strong light below so as to show the track shadows through it.
Sometimes they can be seen, sometimes not, depending on PCB material.
Fiberglass/epoxy is quite translucid.
You can also *scan* (yes, on a flatbed scanner) the PCB, both sides, so they are the exact same size, so they can be compared and superimposed (thanks Photoshop) , most boutique pedals are "reverse engineered" (vulgo: stolen) that way.
You can always go the pencil+paper+rubber+*lots* of time and patience way.

[martin.sadler]

Carlsbros have dissappeared. Incredible.
Everybody and his brother had one !!
I also guessthe only way is hand-drawing the schematics.
A couple *very* good and sharp pictures might help.
In such cases I take the picture component side, but put a strong light below so as to show the track shadows through it.
Sometimes they can be seen, sometimes not, depending on PCB material.
Fiberglass/epoxy is quite translucid.
You can also *scan* (yes, on a flatbed scanner) the PCB, both sides, so they are the exact same size, so they can be compared and superimposed (thanks Photoshop) , most boutique pedals are "reverse engineered" (vulgo: stolen) that way.
You can always go the pencil+paper+rubber+*lots* of time and patience way.

Yep, a strong light and magnifying glass is basically how I did it (I'm a child of the 50's so my eyesight is not what it was...).

[Pitagi]

Hi Martin, thanks for the reply, it seems the circuit is different, the 90 has 3 resistors 2 are 680r @ 4 watt and the other is a 1/4 watt burnt out, it then feeds a bias circuit of 2 15v zener diods there with a couple of 47uf caps. I know the guy who had it before me tried repairing it and reversed the i/p wire.

It is an old amp and I dont know weather it is worth recovering but being a service engineer in the past my design experience is limited. Thanks again, Peter

[Pitagi]

Hi thanks for the advice, I probably could draw the diagram but i have some burnt components and like you I am also a child of the very late fiftys and understand the eye sight problem but that is only scratching the surface, LOL. Thanks Peter

[martin.sadler]

Hi Martin, thanks for the reply, it seems the circuit is different, the 90 has 3 resistors 2 are 680r @ 4 watt and the other is a 1/4 watt burnt out, it then feeds a bias circuit of 2 15v zener diods there with a couple of 47uf caps. I know the guy who had it before me tried repairing it and reversed the i/p wire.

It is an old amp and I dont know weather it is worth recovering but being a service engineer in the past my design experience is limited. Thanks again, Peter

Ok, that's part of the power-supply circuit that I didn't bother to draw up as I wasn't having any problems with it. I don't have the amp in my possession any more but it's a very safe bet that the two 15v zeners/47uF caps provide regulation of the +/- 15v supplies to the op-amps and the two 680Rs @ 4 watts are droppers from the main +/- voltage supplies to the power amp - this is pretty well standard in most solid-state amps. I can't think what purpose the 1/4-watt resistor would serve off-hand - it may well not be related to the power supply though your description seems to indicate that it may be. Could you have a go at working out the schematic as described in previous messages? If all else fails, could you post some reasonably high-definition photo's of both sides of the circuit board?

When you say the previous owner "reversed the i/p wire" did you mean the signal input? I suspect not as reversing the signal input wouldn't cause any damage, just no signal. So what exactly did you mean?

Wait - I've just had an idea as to what the 1/4-watt resistor might be - it could be the current-limiter resistor for an led used as the "On" lamp. If the "i/p wire" you referred to above was one of the power-supply feeds, and depending on where in the circuit the led/current-limiter combination is connected, this might well cause a burn-out if it was accidentally connected to too high a voltage at some point. Are you 100% sure the "i/p wire" is now connected correctly?

The other thing you have to consider is - what caused the power stages to blow in the first place? Was it the ministrations of the previous owner, was the amp simply overrun causing it to overheat and go into thermal runaway (I suspect there's no temperature compensation or power-limiting incorporated in this design as you would typically see in expensive hi-fi gear) or is there some other fault lurking there waiting to bite......


If you can power the amp on in it's current state it would be good to know:

- is there any smoke or does anything (eg power transformer) appear to be getting very hot? If so, switch off asap!

- does the "On" lamp light up?

- What is the voltage across each of the two zeners (obviously should be +/-15v relative to chassis earth)

- What is the voltage across each of the two 680Rs? My guess would be anything from 10 - 30 volts if all is well.

- if there is no sign of life at all, have you checked that all the fuses (including the plug at the wall) are OK?

- can you apply a signal to the input of the power amp? If so, how does it sound?

[martin.sadler]

Hi Martin, thanks for the reply, it seems the circuit is different, the 90 has 3 resistors 2 are 680r @ 4 watt and the other is a 1/4 watt burnt out, it then feeds a bias circuit of 2 15v zener diods there with a couple of 47uf caps. I know the guy who had it before me tried repairing it and reversed the i/p wire.

It is an old amp and I dont know weather it is worth recovering but being a service engineer in the past my design experience is limited. Thanks again, Peter

I found the schematic for a Carlsbro Colt 120 in another thread on this forum (thanks to Ted). It has different values and component numbers but it looks as though it's similar to your 90b.

It has 2 x 560R/6watt instead of your 680R/4-watt but has two 15v zeners as part of the regulation for the +/15v supplies. However, I note there is also a single 15v zener and 47uF cap in the input stage of the power amp section which has a number of 1/4-watt resistors associated with it - it biases the long-tailed pair input stage and I suspect this is the bias circuit you mentioned, although it only consists of a single zener and 47uF cap. There are two other 15v zeners in the limiter section but I don't think these are the ones you're referring to.

I hope this helps.....

[martin.sadler]

I found the schematic for a Carlsbro Colt 120 in another thread on this forum (thanks to Ted). It has different values and component numbers but it looks as though it's similar to your 90b.

It has 2 x 560R/6watt instead of your 680R/4-watt but has two 15v zeners as part of the regulation for the +/15v supplies. However, I note there is also a single 15v zener and 47uF cap in the input stage of the power amp section which has a number of 1/4-watt resistors associated with it - it biases the long-tailed pair input stage and I suspect this is the bias circuit you mentioned, although it only consists of a single zener and 47uF cap. There are two other 15v zeners in the limiter section but I don't think these are the ones you're referring to.

I hope this helps.....

I've just got around to taking a look inside the C'bro B150 I have awaiting repair. Lo and behold, the power-amp matches the Colt 120 schematic exactly - I suspect yours might, too - or should at least be very similar.

The pre-amp board is different BUT - it has 2 x 680R/7-watt dropper resistors, connected to 2 x 15v zeners, providing the +/- 15v supplies for the op-amps, etc. Next to the 680Rs is a 180R 1/4-watt resistor which is in parallel with the led "On" lamp and thus provides a shunt to limit the current to the led - the 180R/led combination being in series with the 680R dropper thats connected to the +ve power rail. This is the first time I've seen this, most designers use a simple series current limiting resistor with the combination connected across the power rails. I now think it may well be this resistor that has burnt out in your amp - if someone accidentally shorted the +15v to ground this resistor would carry a large current and burn out pdq. The led may well have blown, too.

If both led and 180R have blown, you'll have no +15v supply so the pre-amp won't function.

If I'm right, you can simply replace the burnt-out resistor with a wire link to restore the +15v supply and hopefully you'll be up and running - but don't do this until you've confirmed all of the above.

BTW, the caps associated with the 15v zeners are 2 x 220uF, not 47uF as you quoted.

[Pitagi]

Hi Martin, looking closer at the schematic and I will get back, the led is ok and does power up, the resistor seems to be connected with the main volume pot and sits from one end of the audio out and the 4k7 volume pot. It must have damage to the power supply and audio circuit. Could you recomend a good way of checking the power stage? The preamp and power amp are joined by 1, 4 pin connector on 1 side is the power 30v and the other is the i/p to the power amp audio out from the preamp to the power amp. Thanks Martin, please bear with me if my reply seems to take a few days, regards, Peter

Ps
I have added a pic I hope you can see the contents

[martin.sadler]

[ATTACH=CONFIG]14483[/ATTACH]

Hi Martin, looking closer at the schematic and I will get back, the led is ok and does power up, the resistor seems to be connected with the main volume pot and sits from one end of the audio out and the 4k7 volume pot. It must have damage to the power supply and audio circuit. Could you recomend a good way of checking the power stage? The preamp and power amp are joined by 1, 4 pin connector on 1 side is the power 30v and the other is the i/p to the power amp audio out from the preamp to the power amp. Thanks Martin, please bear with me if my reply seems to take a few days, regards, Peter

Ps
I have added a pic I hope you can see the contents

Ok, the pic helps a lot, although a view of the underside would help to clinch it. However, from what you've said above it looks like the burnt-out resistor is in the audio path feeding the power amp. If the 4-way plug was reversed and the volume control set to minimum, this would have put approx. 60 volts dc across this resistor. On the B150 I'm currently working with the equivalent resistor is a 1k 1/4-watt, although this also has a 4u7 cap in series so no dc current can pass through it. From what I can see from your pic there is no series cap. Putting 60 volts across a 1k resistor would generate 3.6watts - bye-bye resistor! It's possible there's some damage to the pot and possibly the op=amps - let's hope not!

If you want to test the power amp, simply connect a low-level signal (output from a guitar is ideal - you want it to be a lot less than, say 100mV rms otherwise you'll deafen yourself - there's no volume control on the power amp. If you have some form of dummy load or power-soak/speaker attenuator I'd use this rather than a directly connected speaker. If there's an FX loop fitted to this amp and the sockets are directly connected to the power-amp input (as on the B150) you can simply plug a guitar into the FX Return socket. Do this with the pre-amp board disconnected.

If the power-amp checks out ok, turn your attention to the pre-amp.

Replace the burnt-out resistor with a 1k 1/4-watt. You should also check the volume pot for continuity . The best way to do this is to measure across the two outer-most pins of the pot - should obviously read 4k7. Then read from the centre pin to either of the outer pins. Should vary from 0 - 4k7 as you turn the knob. If the pot is suspect you need to replace it before proceeding.

Connect the 4-way connector - the right way round! If your amp is wired the same as the other Carlsbros I've seen, the signal connection is the right-most pin as shown in your pic. The pin next to this is 0v (ground), then -30 volts and finally +30 volts on the left-most pin. This all should be clearly marked on the pcb but this area is over-exposed in your pic. Don't worry too much if the +/- volts are not precisely 30v - anything in that general ball park is fine.

If everything else survived then you should now have a working amp. If not, then you have to decide how much more time and effort you want to expend on it. Worst case, all the op-amps may need replacing and some of the electrolytic caps may be damaged - having had reverse polarity on them.

Good luck, and please let me know how you get on.

Martin

[Pitagi]

Hi, Martin, I thought I replied but it seems there is no listing, anyway thanks for all your kind help, The power stage is ok and I applied a signal through and the o/p seemed ok, I put a 1k resistor in the preamp and it just wants to heat up, so I imagin that there is more damage, I will remove the quad op amp first and then see what happens. There is definately a short, I have some tricks up my sleeve when i was a test engineer so I will keep you informed. Thanks for your kind help it was very kind of you to help. Peter

[martin.sadler]

Carlsbro Cobra 90 KB preamp circuit diagram

Hi, Martin, I thought I replied but it seems there is no listing, anyway thanks for all your kind help, The power stage is ok and I applied a signal through and the o/p seemed ok, I put a 1k resistor in the preamp and it just wants to heat up, so I imagin that there is more damage, I will remove the quad op amp first and then see what happens. There is definately a short, I have some tricks up my sleeve when i was a test engineer so I will keep you informed. Thanks for your kind help it was very kind of you to help. Peter

Only too happy to help. Good news on the power amp.

The op-amp shown in your picture is a TL071 which is a single op-amp, not a quad. I've not seen Carlsbro use quad op-amps in any of their amps I've had through my workshop - they seem to like to keep the pcbs as simple as possible. They usually use a mixture of TL071s (single) and TL072s (dual), which have different pin-outs so are not interchangeable. Sorry if I'm stating the obvious here.

If you're lucky it may just be that the output stage of that last, visible TL071 is shorted. A quick test is to power-up again after removing this chip and see if the 1k resistor stays cool. Also check the voltages on the power supply pins of the other op-amps. They should be +/-15v. Anything lower (or higher!) than this and you still have problems.

Whenever replacing op-amps I recommend putting in sockets so that you can insert and remove chips without risk of heat damage. They're very cheap and save a lot of heartache.

One other thing. I'm curious as to what the pale blue wires are for in your picture - any clues from what you can see?

Best regards,

Martin

[hennep]

Hi Peter,
Sorry for replying to this old thread but it is difficult to find information for these old amps.
I can confirm that R36 is a 1k resistor.
My son has a "Carlsbro Cobra 90 BG" and he has blown the fuses on the amplifier board. Both fuses are 2.5A but there is 5A printed on the circuit board.
Now I'm not sure what value to use as replacement.
Can you tell me the value of the fuses on your amp. board please?

thanks,
Hennie

[J M Fahey]

My son has a "Carlsbro Cobra 90 BG" and he has blown the fuses on the amplifier board. Both fuses are 2.5A but there is 5A printed on the circuit board.
Now I'm not sure what value to use as replacement.
Can you tell me the value of the fuses on your amp. board please?

thanks,
Hennie

Does it blow fuses at power on, without signal, or it works properly at low volume, and blows at high power?
Also: follow the tracks, *where* are those fuses?
On the +/- DC power rails, in series with the transformer AC wires?
Please post a closeup picture showing them and what's printed on the PCB.