Carlsboro Cobra 110 schematic sought please!

[The Dude]

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

Yes. The 7915 is a negative voltage regulator. The input/supply pin must have negative voltage for there to be -15 output.

[The Dude]

Sure - here it is:


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

That link requires a login, so it won't help us. FWIW, you should be able to upload pictures Just compress them into smaller .jpeg format. 960kb should be plenty to upload decent pictures.

[Jazz P Bass]

Well.
That was interesting.
I enrolled in the Discord site, clicked the OP's link & ............
Wallah.
Nothing!

[DrGonz78]

Well the definition for discord is a disagreement between people. Why did they name their site that way?

[J M Fahey]

on my photos (which this forum won't let me send - what am I doing wrong......a limit of 960kb??)

1) download and install excellent Irfanview.
First the main program, then the plugins.
Pick the version which matches your computer computer (32 bit or 64 bit)

2) open image into Irfanview by right clicking on it and selecting "open with: Irfanview"
If not shown the first time, it will offer to "browse" so you find the proper directory, then pick Irfanview.exe

Or wen installing choose Irfanview as the preset viewer for .jpg, .gif, .png, etc. , all image formats.
Believe me, it is THAT good.

3) when open, click the Image menu, it will offer to (among many other things): Resize/Resample, and inside it: "Half" (size) which should easily be enough.
If needed, "half" once more.

4) for a practical example, I am reducing this way too large 3600 by 2700 pixel image


to "half" so 1800 by 1350





if needed, can repeat the operation.

[brymac]

Many thanks Juan.....I'll load photos onto drop box and post a link - it might be quicker!!

But - this is really baffling me….

I have replaced both power transistors, both voltage relulators, the 4 largest capacitors, all 4 diodes in the bridge rectifier and checked numerous components in and out of circuit. I have unplugged the front panel, thus limiting the problem to the AC input from the transformer (which is fine at 36v AC) onto the power board.

But - I am still getting 36V DC out of the speaker sockets!!!!!

What have I missed?????

[brymac]

This will take you to all the photos so far...


https://www.dropbox.com/sh/h7lis92aj...xgiggc7Va?dl=0

I am now working back from the speaker output and testing the components as I go...!!

Bryan

[Jazz P Bass]

Here are 2 Calboro schematics.
They both have 9 transistors as yours does.
You will have to do the homework to fill in the proper component designators.
As there will be differences in the particulars, these schemes should get you started.

Do you have working knowledge of how a basic Class AB output section works?
So far I have not seen any voltage measurements.
The output transistor bases should be at +.600 Vdc for the NPN & -.600 Vdc for the PNP.

What is the polarity of the Vdc on the output. That is a big hint as to where the issue is.

[J M Fahey]

In no particular order:

1) Thatīs a generic Chinese amp, sold under many brands, here *labelled* Carlsbro, meaning it will not match known Carlsbro amps except by chance.

2) those are monster 4000 by 3000 pixel images, 35Mb each when displayed, which will fit in no monitor known to Man, here Iīll reduce to "half" using Irfanview.
You are suggested to the same in the future before posting
And if too large, will halve again, or crop a smaller more useful section.

3) here I cropped to show power transistors and halved image .
2SC3280 is NPN, I expect around +500/600mV BE , almost +40V CE ...if all is fine of course.
2SA1301 is PNP , same thing but opposite polarity

[J M Fahey]

Are those transistors original or you got them from EBay or Alibaba or similar?
If the latter, they are highly suspect.

4) those TO220 sized transistors in the center of the board seem to be power transistor drivers, model unseen, PCB labels 1Q04 and 1Q05.

I had to halve the image twice

Google them, one will be NPN, the other PNP.
I expect similar voltages as in Power transistors above.

[dmeek]

Here is a schematic of the power amp

[brymac]

Brilliant - thank you!! Did you just draw it from the photos?? How do I get a larger better definition version please?

[brymac]

It's OK - larger version now printed out! Thanks again......

[J M Fahey]

Here is a schematic of the power amp

Wow! Impressive!!!!
Thanks for your patience and posting.
I had found service manuals for a couple "original" OEM amps, sold under the Brand: Phonic Power Pod "something" , same as this one, but more modern versions using an Op Amp as input stage, didnīt post them here to avoid confusing the situation even more.

I found it funny the Power Amp + Supply PCB also includes the reverb drive and return circuit, based on a dual Op Amp .... and which is unpopulated.



Instead they added a certainly much cheaper mini board using a single PT2399, connected to preamp board with a ribbon cable.



So this is a very successful amplifier, but brands confuse the matter and to boot they keep mutating.

I have repaired them with no schematics, but having the board on my bench for testing helps, not so much by "remote control" as in a Forum.

[brymac]

Many thanks again - it's now having some new eyes on it in the form of a colleague from thr Repair Cafe I run here in East Sussex UK - I'll keep you posted!!