Carlsboro Cobra 110 schematic sought please!

[brymac]

Although now being looked at by a colleague, for the sake of completeness, here is a set of voltage measurements I took before handing it to him. He ha since discovered that the +20 V reverts to -15V once the board is connected to the case......but there is still 36VDC coming out of the speaker sockets! He is on it.....

[brymac]

Oh - and of course that was done before I got the "real" schematic from our friend in Canada!

[brymac]

Thank you all - I was trying to explain to the small church here in the Sussex countryside that their amplifier was now the subject of expert advice from across the world and I think it was beyond their comprehension!!!!

One of the electrics team here at Chailey Repair Cafe now has it so it has a new set of eyes on it, I'm delighted that he has as a) he is much better at this stuff than I am and b) I'm off to Portugal for a week's holiday (remember those?) on Friday morning and have been living and breathing this amp for a week now. I knew it was time to hand it over when I started dreaming about it!!! The problem is now on 3 forums - here, Discord and the UK vintage radio and restoration forum. It has attracted a lot of attention, so I shall feel very foolish when it proves to be someting straightforward!!

If nothing else, this has connected me up with this forum and the remarkable range of knowledge across the world!!

Thanks again and I shall be watching progress closely!!

[DrGonz78]

I have plenty of repairs where even in my dreams I was still troubleshooting the amp. In fact I have had that moment when I awoke and fixed the amp right after dreaming. Pretty sure we all work on problem solving even when we are asleep.

[brymac]

Yup - hard to explain to my missus that I've been dreaming of vintage electronics though; she doesn't believe me!!!!

[brymac]

I am delighted to be able to tell you all that my colleague Des from Chailey Repair Cafe (which I run) has fixed it - here is his report, which comes with my thanks to you all for all your time and patience with this challenging problem.....!!!

Bryan



The amplifier was passed on to me the other day - and is now working, I'm pleased to say. I thought that it would be appropriate to pass on the details of the actual problem I found, for the benefit of all the readers of this thread and to anyone else searching with a similar problem.

First - enormous thanks to all who contributed with ideas and suggestions - reading through this thread was really helpful. I'm delighted by the competence and kindness shown in this forum.

Inevitably, the lack of an accurate circuit diagram slowed the repair down. I spent a while tracing out parts of the circuit. It's a class AB amplifier with (discrete) darlington pairs on the output, fed by two rails of +/- 40v or so. There are also +/- 15v power rails - the oddity (mentioned in this thread) with the -15v rail disappeared when the preamp and effects board was plugged in and that rail was no longer floating.

The key problem was then the +38v or so appearing on the speaker output, the junction between a pair of 0.5 ohm resistors and the emitters of the two output transistors (one pnp, the other npn). I pulled both output transistors - the +38v remained. A bit of circuit tracing suggested that this voltage was coming from the V+ rail via the TIP41C, being the other half of the npn darlington pair. Measurements on this transistor showed it was working and fully switched on - it's base being driven by a 2N5401. And that was the faulty transistor. A replacement returned the speaker output to very close to zero volts and a working amplifier (and much relief!).

Thanks again for all your help and support.

Best wishes

Des

[brymac]

And here it is in action......https://www.dropbox.com/s/mlh2w2kpnv..._2794.MOV?dl=0

Thanks again all,

Bryan

[J M Fahey]

GLAD you solved it.
And no, it was not a "silly" fault, rest assured, which showed far away from its point of origin, delights of Direct Coupling.
The actual bad part may be less than $1 , the real job being FINDING it

[brymac]

But the best bit has been that we have been able to call for help from across the world!!

Bryan