Carlsboro Cobra 1100 repair.

[Diabolical Artificer]

I have unplugged the front panel, thus limiting the problem to the AC input from the transformer

Don't understand that.

I'm no expert, never worked in electronics, no degree, self taught. What I've learned is swopping components only gets you so far, you have to be more logical and use the old noggin. So you have a reading of 36v DC on the OP, either your meter is faulty or it's coming from somewhere, this is electronics where things work to rules not Hogwarts or faeryland.

The OP stage looks to be complementary Graham, no cap. As the old song goes, the leg bones connected to the knee bone, the knee bone's connected to the other bit, so there's your OP terminal, what's it connected to? Usually in a tranny amp you have those big trannys, collector of the NPN to positive V, emitter to OP, OP terminal to PNP E, PNP Collector to negative rail, see - http://www.ecircuitcenter.com/Circui...l/pushpull.htm. It's a push pull amp.

Therefore you must have a shorted NPN OP tranny. I did recommend leaving those OP tranny's out until you know all the stuff they are connected to is ok, it could be you have killed them. Take the OP trannies out, have you still got +36v on the red speaker/OP terminal? Check that the black speaker terminal is connected to PCB ground. Now with big OP trannies out if you still have +36v on the OP, turn off amp. Set DMM to continuity, one probe to OP, one probe to +35v or whatever, does it beep?

The OP is connected to +35v how? I've attached two generic simple tranny amp schematics, your amp will be the same, pretty much, nothing complicated. The answer's there, use your noggin. The second schematic has a cap (C8) as Graham mentioned from OP trannies to OP, if it shorted as well as C5 maybe, result, positive voltage on OP. Drawing the OP circuit of your amp might be beneficial.

Andy.

[brymac]

Thanks Andy - I'm doing one better than that; I'm using somebody else's noggin!!!

One of the advantages of running a Repair Cafe with 36 skilled volunteers is that there are some exceptionally skilled electronics buffs amongst them, and one is taking it on today. But I will pass on your very welcome pearls of wisdom for which I am most grateful and may well be eventually in a position to report on a successful outcome!!

Thanks again,

Bryan

[brymac]

Here's the latest voltages - the -15 rail makes no sense at all!!

[Herald1360]

As you say, not a lot of sense there!

Someone will need to check it out bit by bit. I'd start by pulling the regulators to see if the unregulated side makes sense. The odd voltages suggest perhaps diode problems or a broken 0V connection somewhere. Or perhaps there's unwanted voltage being fed back down the +/- 15V rails from a fault somewhere else downstream.

If your Repair Cafe volunteer skill set is anything like the ones at which I volunteer, you'll get a fix pretty damned quick

It'll be interesting to find out what the problem was though by now it might simply not be the original one

[wave solder]

Why are those voltage readings not being taken with the negative probe on the 0V (GND)? The readings would then start to make a bit more sense.

[Diabolical Artificer]

Why are those voltage readings not being taken with the negative probe on the 0V (GND)

How do you know they weren't, did I miss something? It does sound like something odd is going off or an iffy ground or damage further on sounds probable.

Hope you get to the bottom of this,very curious. Andy.

[thermionic]

Trying to make out from the pictures of where those voltage regulators are from. I can see two tucked up near the edge of the pcb, but the two centrally mounted T220 devices should be driver transistors, surely?

Otherwise, what’s driving the output pair - that pair of small signal TO92 cased transistors?

Hmm....something’s not right.


SimonT.

[Richard675]

Isoltate/disconnect the outputs from the power supply to the rest of the board, then check the voltages.

I had a Fender solid state amp that produced weird readings on the supply rails. The fault was a shorted IC in the preamp.

[brymac]

Thank you all - the problem is now on 3 forums - Discord, the US music electronic forum and here. It has attracted a lot of attention, so I shall feel very foolish when it proves to be something straightforward!! I was trying to explain to the small church concerned that their amplifier was now the subject of expert advice from across the world and I think it was beyond their comprehension!!!!

As you will have gathered, Des from Chailey Repair cafe (which I run, so thanks for the compliment about RCs!) now has it. He's one of our electrics team. I'm delighted that he has it as a) he is much better at this stuff than I am and b) I'm off to Portugal for a week's holdoay (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!!!

Thanks again and I shall be watching progress closely!!

Bryan

[Peter F4VSA]

With most amps of that type the preamp is capacitively coupled to the power amplifier so faults there are vary unlikely to cause the output to rise to one of the rails. Preamp issues normally don't cause power amp failures unless they produce uncontrolled oscillations.

As a professional electronics designer I would agree that randomly changing components is not a good route to follow except perhaps to the power output pair.

The negative voltage level after the regulor being at a positive level is impossible as is the input voltage reading shown earlier. If it's the real suituation it sounds like shorts maybe by solder splashes or the mains transformer centre tap being disconnected.

I have as a hobby repaired 20 or 30 valve and transistor guitar amps and this sounds very weird to me.

Peter

[deswradio]

Originally Posted by brymac

As you will have gathered, Des from Chailey Repair cafe (which I run, so thanks for the compliment about RCs!) now has it.

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

[Station X]

Thanks Des. It's always good to read a proper report rather than a simple "It's now working" post.

[brymac]

Many thanks Des - this had reached the point where it had got beyond me and I am very fortunate to have been able to call upon my colleagues at Chailey Repair Cafe to bail me out! (not for the first time!)

And again - thank you all

Bryan

[Diabolical Artificer]

Was the 2N5401 shorted or just leaky? Also was the 7915 dead after all? Trying to picture the failure. Good to see this amp fixed, well done Des.

Andy.

[deswradio]

Originally Posted by Diabolical Artificer

Was the 2N5401 shorted or just leaky? Also was the 7915 dead after all?
Andy.

It was very leaky (emitter to collector) - the other junctions were fine, thus temporarily confusing me when fleetingly testing it with a meter while in circuit. My generic component tester reported the device was 2 diodes.

By the time I got my hands on the amp, the 7915 had been replaced. But the output looked odd until I reconnected the preamp/effects board - the 7915 had no load at all (output open circuit) when the main amp board was tested in isolation.

Best wishes

Des

[brymac]

And here it is in action:

https://www.dropbox.com/s/mlh2w2kpnv..._2794.MOV?dl=0

Thanks again all!!

Bryan

[Diabolical Artificer]

Nice jig, well done all.

[brymac]

POSTSCRIPT....

Amp reinstalled in church hall yesterday, tested and speakers rewired - they had not blown up! (phew!)

All tested and working perfectly…..

A good job - thanks to all of you who helped solve it!