Trying to fix a broken Carlsbro Cobra 90 Series "Sherwood" amp

[Erik]

Hi all,

I have recently acquired the above amp. It has suffered some abuse though, inexplicably the master volume pot has been torn off together with some other internal components. The new Carlsbro company apparently has no schematics for old Carlsbro products anymore. I have little or no idea about electronics but if I could just figure out what components are needed then I think I should be able to solder them

Check it out:





I made a drawing so you can see better:



So: one resistor has one leg torn off, but it's still there so no problem. I need to know what kind of volume pot should be there, and the type of capacitors C42 and C43. I have no schematics and I am unable to find some on the Internet. Is there any hope? Anyone know anything about these amps? Any help is much appreciated!

[52 Bill]

Unless you find a copy of the schematic, you'll have to do some deduction to figure out what you need parts wise. The one resistor is a 1K (1,000 ohm). The color code reads brown/black/red/gold=1/0/00/5%.

The two missing caps will be a little harder to figure out, but my guess is that they will be something like 100uF-470uF rated at 25 volts. These are filter caps for the low voltage power supply. They are fed from the two Zener diodes Z1 and Z2. If you read the numbers on the diodes, it will allow you to look up the voltage rating of the diodes and with this voltage figure, you will know what the voltage rating for the two missing caps should be.

The volume pot will be more difficult to figure out, as it's value will depend on how it operates in the circuit. You will need to see if it used as part of the gain circuitry of the opamp that feeds it or if it is just a basic voltage divider. Once you know this, you will be able to guess at a value for the pot.

[plec.22]

some master pot's can be from..100k..220k..470k or even 1mg..try a 100k pot and see if that will sort this part of the circuit...

[Erik]

Thanks very much to both for the replies. As I said, I know very little when it comes to electronics, so please bear with some dumb questions

You will need to see if it used as part of the gain circuitry of the opamp that feeds it or if it is just a basic voltage divider.

Is there an easy way to find this out? There is an IC nearby (just visible in the photo above) that I guess is an opamp.

Also: can I ruin something by putting the wrong caps and pots? Will the amp be flat out dead or will it probably make some kind of a sound? I.e. if I put in a 100k pot and the circuit was designed for 220k, how will I be able to tell?

[plec.22]

Like Bill said the zener diodes are the low voltage supply for the from preamplifier stage..so the about filter caps as Bill say's will work...I've seen them as high as 1000uf but 470uf will do the job as that stage does not pull high current.. only about 100ma or so..

no you'll not damage the pre amp stage via putting the 100k.. try and see...

There maybe old posts on here that may have the circuit diagram have a look...

All the best foe now..

[52 Bill]

Is there an easy way to find this out? There is an IC nearby (just visible in the photo above) that I guess is an opamp.

No, you would need to reverse engineer the circuit to figure this value out. If you just try any value pot, the worst that can happen is that it damages something else in the circuit, which puts you back where you started.

Also: can I ruin something by putting the wrong caps and pots? Will the amp be flat out dead or will it probably make some kind of a sound? I.e. if I put in a 100k pot and the circuit was designed for 220k, how will I be able to tell?

The only critical thing here is to watch the polarity of the two caps that you need to replace. If they are installed backwards, they may explode on you.

If you put the wrong value pot in there and the amp works, there may be changes in the amp that you wouldn't notice, like a little less maximum volume or a little more distortion, etc.

If you replace the missing parts and power it up, it may work perfectly or it may not work at all, you just don't know until you get there. There may be other things that are wrong with that amp that have nothing to do with the missing parts. Or you may find the reason why the parts were removed in the first place.

[Erik]

Cool, thanks. I think I will look at the zener diodes and figure out the voltage ratings, then put in two suitable 470uf caps and a 100k pot, and see what happens. The amp is junk in its current state anyway so if I end up breaking it completely, no big deal. Best case scenario I will have a usable amp for pretty much free.

Or you may find the reason why the parts were removed in the first place.

I'm pretty sure it has just been subjected to violence. My best guess is the amp has suffered some sort of impact and the volume pot has gotten pushed in so far that it's broken off and taken the caps behind it with it. Or the pot broke off and people have stuck screwdrivers or whatever in there and managed to break stuff. It does not look like anything was deliberately removed, just torn off by force.

[plec.22]

Has a guide to help put the caps in the right way...look at a different amp with low voltage +/- supply and your see the correct way to install the cap's..

in dual rail voltage supply's your see the filter cap's.(4700uf's) facing different way's.. use this to help you place the small rail caps in the same lay out.. you can clip the black lead of your test meter to the amps metal case and switch it's setting to say 200vdc range and power up your amp and use your red meter lead to touch the zener diodes and see what reading there is...say 15vdc 18vdc...

[plec.22]

cos you said you've little know how in electronics...never solder with the amp powered up....if you've not got one buy a meter to help you test and record voltages..there's on line guides to aid you how to help make things clear for you...the thing about this site is there's tech's and hobbyist on here who will help were we can..

It's good to share...

[Erik]

Thanks so much for the help. Yeah, I will definitely turn the amp off It's actually possible to disconnect this circuit board entirely from the rest of the amp so I hope to be pretty safe from frying myself.

You said "4700uf" just now. Did you mean 470uf? Because that's what I've bought.

The polarity for the caps is printed on the circuit board, so hopefully I should have no problem there. Thanks again! I will report back and tell y'all if I got anything to work or burned down my house or what

[plec.22]

Them cap's you bought will do the job... some preamp lower supply have used 1000uf....Disconnecting and or removing the circuit board from the amp can be a little risky..because you may have lots of cables running from the board back the the power transformer. Just left out the board so you can fit the parts in and then refit the board in tight and power up... if you feel any thing getting hot or 'see' any smoke unplug quick.. but if you've fitted the bit's in the next step is to power up!!

if things are well and there's no burnt part's you many of saved this amp from the bin.... best of luck..

[Pitagi]

Hi I have just looked on my cobra 90 and the pot is 4k7 I hope it is the same as the cobra 90kb. is so do you know the value of r33. I hope it helps, regards, Peter